experimental study of biodiesel synthesis

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• Open access
• Published: 15 September 2022

Optimization and kinetics studies of biodiesel synthesis from Jatropha curcas oil under the application of eco-friendly microwave heating technique: an environmentally benign and sustainable bio-waste management approach

• Kassian T. T. Amesho 1 , 2 ,
• Yuan-Chung Lin 1 , 3 , 4 , 2 ,
• Chin-En Chen 1 , 2 ,
• Pei-Cheng Cheng 1 , 2 &
• Vinoth Kumar Ponnusamy 5 , 6 , 2  

Sustainable Environment Research volume  32 , Article number:  41 ( 2022 ) Cite this article

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This study attempts to synthesize biodiesel as a green liquid fuel from Jatropha curcas oil (JCO) utilizing waste eggshell (WES) as an effective and excellent sustainable source of the heterogeneous catalyst under the application of environmentally benign microwave heating technique. After preparing the CaO-based catalyst, diverse characterization techniques such as X-Ray Diffraction, Energy Dispersive Spectroscopy, Scanning Electron Microscopy, Fourier Transform Infrared, and Brunauer–Emmett–Teller were employed for characterization measurements. Under different optimized conditions, a considerable high biodiesel yield of 92% was attained while employing the following reaction conditions; methanol to oil molar ratio of 9:1, 5 wt% catalyst loading, 165 min reaction time with a microwave power of 800 W, and a 65 °C reaction temperature. The developed catalyst had significantly retained its reusability up to the 5 th cycle of reuse. The catalysed transesterification process's activation energy of 38.5 kJ mol −1 demonstrated that the reaction is chemically controlled. Therefore, the WES has been utilized as a renewable base heterogeneous catalyst for novel biodiesel synthesis from JCO, which can be applied in diesel engines to lessen air pollution, specifically pollutant emissions from diesel vehicles. The results of this study are not for academic purposes only. They can also serve as models for industrial biodiesel production by exploiting bio-waste as catalysts and non-edible oils as feedstocks in microwave heating systems as environmentally friendly chemistry systems. In addition, our study uses non-consumable oil feedstock and bio-waste materials in an economical way to produce biofuel while contributing to environmental sustainability and sustainable bio-waste management. This approach extends to the existing state-of-the-art research.

1 Introduction

The fast depletion of fossil-fuel-based resources and global warming as a result of climate change ramifications are the major driving forces towards the development of renewable and more environmentally benign energy resources. The forever fast-growing population has also prompted this, along with booming industrialization across the globe, which have similarly led to a serious shortage, uncertainty and rigorous ecological impact of fossil fuel-based energy utilization [ 1 , 2 ]. In response to this, biodiesel has consequently been identified as one of the most promising green energy that can replace the unsustainable petrol diesel utilisation due to its vast environmental benefits [ 3 , 4 ].

Biodiesel as a green fuel has been broadly considered as an eco-friendly source of energy, owing to its advantage in producing less CO 2 , SOx, and hydrocarbon emissions, which contribute to environmental sustainability [ 5 ]. Conversely, over 95% of biodiesel is mainly synthesized from most notably edible oil crops (such as palm, rapeseed, soybean and sunflower), which compromise food security [ 6 ]. In view of this, various stakeholders such as government agencies, scientists, industries, and policymakers have aimed for the exploration of economical and non-edible oil-producing energy crops like Jatropha curcas for research.

Being an energy crop, J. curcas de-oiled seed cake can be used as a feedstock for the production of bio-oil via the pyrolysis process [ 7 ]. The J. curcas bio-oil has diverse potential for industrial applications. Among others, J. curcas bio-oil can also be used as bio dielectric fluids in the Electric Discharge Machining process. It is also reported that J. curcas bio-oil is an excellent substitute of the conventional hydrocarbon oils [ 7 , 8 ].

Meanwhile, shell waste, which is identified as inevitable food waste materials formed in food preparation or consumption, has become a global environmental problem, specifically concerning the issue of waste management. In this regard, eggshells waste is generated from various food industries such as egg processing companies, and enormous amounts of waste shells that are discarded to the landfills or incinerated, have virtually become a significant cause for organic pollution.

Eggshells are a classic example of food waste emanating from the food processing industry that still contains recyclable parts. By 2030, global egg production will increase to around 90 Mt. Because eggshells are considered worthless, most of this waste is usually disposed of in landfills without being converted into valuable materials. However, managing these wastes requires appropriate strategies that take into account the increasing cost of disposal, environmental concerns associated with the risk of spreading pathogens, odours, and where they are disposed of [ 8 , 9 , 10 ]. In addition, eggshells as an agro-waste have the potential to cause environmental pollution according to European Union regulations. This is because the large amounts of the discarded eggshells contribute to food wastage which causes significant damage to the environment by increasing global carbon footprint when buried, which is one of the prime greenhouse gases contributing to the global warming [ 11 ]. Therefore, it is essential to find an alternative technique to transform eggshells into valued materials for further utilization.

Essentially, eggshells are considered high-value natural materials as opposed to waste, considering their most efficient calcified shells, comprised of over 96% calcium carbonate (CaCO 3 ). Eggshells are predominantly rich in CaCO 3 that mostly appears in the mineral calcite form, a better-decontaminated and thermodynamically persistent form of CaCO 3 , which has a much lower level of contaminations. In view of this, eggshells could offer enormous opportunities for applications in the form of limestone (CaCO 3 ), if not lime (CaO), in a wide range of applications, through significant reusability as a substitute in the production of cement. In addition, eggshells signify an excellent substitute which is vastly plentiful, low-cost, and more sustainable source of filler for the rubber industry [ 4 , 6 , 11 ]. CaO, on the other hand, has a high bonding density with H 2 O and CO 2 , so it is easily degraded during the aqueous reaction [ 12 , 13 ]. CaO also demonstrated improved stability and catalytic efficiency in biodiesel synthesis processes due to its high basic strength, low cost and inadequate dissolvability in methanol. CaO is also the cheapest and most durable heterogeneous catalyst [ 14 ]. Due to these properties, CaO was obtained from eggshells [ 5 , 15 ], scallop shells [ 16 ], cockles shells (Anadara granosa) [ 17 ], and animal bones [ 18 ]. From this perspective, reusing eggshell waste for many purposes will benefit the environment and the economy. Furthermore, the reprocessing and recycling of waste eggshells (WES) integrates the fundamental principles of circular economy, which increases resource utilization efficiency through transforming waste or by-products into resources with both cost-effective and ecological beneficial impact [ 19 ].

Many researchers have been investigating the application of the microwave (MW) technique for the synthesis of biodiesel. It has been widely demonstrated that the MW heating system is one type of clean, fast, and suitable energy source that can expedite the procedure and enhance the selectivity of particular reactions [ 4 , 5 ]. Lin et al. [ 5 ] reported the exploitation of ionic liquid as a green catalyst for the transesterification procedure in the MW heating system, giving a quick way to biodiesel synthesis from J. curcas oil (JCO). The MW heating system is equally more energy-efficient or energy-saving and more economical than the conventional heating system [ 5 ].

In the light of the above-mentioned discussion, the present study attempts to promote the efficiency and synthesis of bio-waste derived CaO-based heterogeneous catalyst and transesterification of JCO under the application of environmentally benign MW heating technique as a sustainable bio-waste management approach. The developed technique can tremendously promote the integration of the circular economy concept through an efficient biorefinery model.

2 Experimental section

2.1 materials and reagents.

WES were acquired from a fast-food establishment in Taiwan. The non-consumable oil (i.e., JCO) as a raw material was provided by the Chinese Petroleum Corporation (CPC), Taiwan. Methanol and sodium hydroxide were procured from Burdick & Jackson (UNI-ONWARD, Taiwan). The methanol and sodium hydroxide used in our experimentations were of excellent analytical grade, and the purity of all solid chemicals were at least 99.5%. The feedstock was subsequently heated to remove the moisture content, whereas all other chemicals were used as received without further decontamination.

2.2 Catalyst synthesis

2.2.1 calcination of eggshells.

The collected WES were washed thoroughly with tap water to eliminate dirt, impurities, and organic materials. Subsequently, the WES were further rinsed using distilled water at least 2–3 times and, after that, stored in a hot air oven at the temperature of 110–140 °C, for approximately 24 h. At this stage, the desiccated eggshells were removed from the oven for further processing and reduced the materials to a fine powder due to continuous crushing in an agate mortar. Lastly, the calcination was conducted in a high-temperature furnace at 1000 °C for 4 h and consequently placed in a desiccator. Thus, the acquired product was CaO, which was kept in a cool, dark desiccator at a temperature in the range of 20–38 °C.

2.3 The transesterification procedures for the JCO

In this research work, an MW synthesis reactor (PreeKem APEX, Shanghai, China) fitted with an automated stirrer, and a capacitor was employed to enable MW reactions. Figure  1 shows the experimental system for this study. The stirrer was performed at 600 rpm with a hypnotizing (magnetic) midpoint. As a result, the eggshells were exploited to develop a heterogeneous catalyst, a product that has been calcined (CaO constituent), to achieve the waste products (discarded materials) reutilizing objective.

Schematic diagram of the experimental setup (microwave system structure). (Reprinted with permission from Lin et al. [ 5 ]). Copyright (2020), Elsevier [License Number: 5270740247480]

JCO and methanol were blended in the vessels, and subsequently, the catalyst was consequently supplemented to the vessel. The vessel was placed in an MW heating system. The samples were performed at several reaction intervals and temperatures during this time. Various catalyst loading (3–7 wt%), reaction time (120–210 min), methanol to oil molar ratio (MTOMR) (9:1), and a temperature reaction of (45, 55, 65, 75, and 85 °C) were investigated during the experiments. With the application of the Fourier Transform Infrared (FTIR) technique, the JCO conversion to biodiesel was thoroughly examined. After 32 scans, the FTIR spectra of the catalyst samples were determined to be in the range of 500–4000 cm −1 . These FTIR spectra were accordingly validated in addition to the documentation aimed at every sample. Krishnamurthy et al. [ 20 ] quantified fatty acid methyl esters (FAME) in microwave-mediated transesterification biodiesel using FTIR analysis and identified peaks that may be associated with the typical C = O stretching of the esters. These peaks were found in the 1800–1700 cm −1 spectral region and were familiar to both the FAME and refined oil spectra. Foroutan et al. [ 21 ] conducted FTIR analysis to determine the functional groups of the studied catalyst and produced biodiesel from edible waste oil using a calcium oxide@magnesium oxide nanocatalyst. Analysis was performed on 400–4000 cm −1 band range.

2.4 Determination of acid value and separation procedures

The JCO sample used under this investigation was found to have an acid value of 2.8 mg KOH g −1 and a saponification value of 195 mg KOH g −1 . Additionally, the JCO sample had a free fatty acid (FFA) value of 2.5 wt% with an average molecular weight of 861 g mol −1 and water content of 0.12 wt% [ 4 , 6 ]. JCO sample was found to have a pH value of 5.8.

Under this investigation, the method on the transesterification reaction was anticipated to have had happened when the quantity of methyl ester was above 90%. In view of this, the mixture and assortment at the concluding phase of the transesterification process were separated with the help of a centrifuge running at 2000 rpm for 10 min, which resulted in the composition of an upper phase characterized by methyl ester and consequently a reduced phase composed of glycerin. This has also progressed into disproportionate methanol during the methyl ester stage, which was vaporized through heating at 80 °C. Afterwards, the catalyst was efficiently withdrawn with the support of silica gel supplemented to the reaction. Therefore, biodiesel was subsequently achieved, and its content was determined with the support of Gas Chromatography (GC-6890, Agilent, USA).

2.5 Product evaluation

The evaluation procedures employed in our experiments were validated in compliance with Taiwan CNS15051 for determining the amount of methyl ester. Additionally, the methyl ester content during this study was determined using the Gas Chromatography method furnished with the flame ionization detector (GC-FID). The conditions used for GC-FID to analyze the products and other instruments and methods used for other analysis such as X-ray Diffraction (XRD), Energy Dispersive Spectroscopy (EDS), Scanning Electron Microscopy (SEM), FTIR, and Brunauer–Emmett–Teller (BET), are reported in detail in our previous study [ 4 , 5 ]. The content of methyl ester in this study was determined by employing the following Eq. ( 1 ) as reported in our preceding studies [ 4 , 5 ]:

where ΣA is the total of the peak area of FAME from C 14 to C 24:1 ; A EI is the peak area of the internal standard, methyl heptadecanoate (C 18 H 36 O 2 ); C EI is the absorption of methyl heptadecanoate (mg mL −1 ); V EI is the quantity of methyl heptadecanoate (mL −1 ); and m is the quantity of input biodiesel (g). The methyl ester yield is expressed using Eq. ( 2 ) as reported in our earlier studies [ 4 , 5 ]:

where C is the methyl ester content (%); W B is the quantity of synthesized biodiesel (g); and W oil is the quantity of the initial volume of JCO (g). Consequently, biodiesel production was investigated and measured as part of the preliminary capacity of JCO by weight. A similar approach was used and demonstrated in our previous work [ 4 , 6 ].

2.6 Physico-chemical properties of biodiesel from JCO

Table S 1 of the Supplementary Materials shows the summarized physico-chemical properties of the JCO biodiesel. The biodiesel physico-chemical characteristics are unique, predominantly its density, which can substantially influence the engine's performance [ 6 ]. This emanates from the evidence that the fuel density can effectively split up the fuel spray from the injector. For that reason, the density at 15 °C for JCO biodiesel is mainly found to be within the scope and appropriate range of 0.859–0.891 g cm −3 for fuel standard (ASTM D6751) or EN 14,214 standard [ 22 , 23 ].

It can also be observed from Table S 1 that the JCO biodiesel could have a level of acidity of 0.18 mg KOH g −1 and saponification number of 191 mg KOH g −1 that are under the normal standard range 0.5–370 for biodiesel in consistent with the ASTM standards (ASTM D6751-02). The iodine value is used to establish the iodine absorption values in g, which could be engrossed by 100 g oil. It helps to determine the amount of unsaturation of biodiesel, and thus it is beneficial to examine the oil durability [ 6 ]. It is also worth noting that a significant proportion of desaturation can lead to fuel polymerization due to the development of epoxide in the buildup of oxygen in twofold links. As it can be observed from Table S 1 , the JCO biodiesel has an iodine value of 69 g I 2 100 g −1 oil that falls further down the recommended upper threshold of 120 (EN 14,214 standard). This iodine value is comparable with the results reported by Kumar et al. [ 22 ] for JCO biodiesel (75 g I2 100 g −1 ). Additionally, other greater iodine concentrations of 119 and 122 g I 2 100 g −1 were recorded in the study conducted by Sarma et al. [ 24 ] and Nath et al. [ 25 ]. JCO biodiesel is reported to have a higher heating value (calorific value) of 40.5 MJ kg −1 [ 7 ].

3 Results and discussion

3.1 catalyst characterization, 3.1.1 xrd analysis.

We studied the eggshells before and after calcination using the XRD (HR-XRD D8 SSS, Bruker) analysis technique. Figure S 1 illustrates the XRD patterns of the uncalcinated and calcinated eggshells before (Fig. S 1 a) and after calcination (Fig. S 1 b). According to the results, the occurrence and strong peaks were located at a brag angle of (2 θ) at 29.7, 39.3, 43.2 and 47.6° for the recycled WES before calcination (Fig. S 1 a). However, strong peaks were equally noticed after calcination (Fig. S 1 b) and were detected an angle of ( 2 θ) at 29.8, 34.1, 39.4, 43.3, 47.6 and 48.5° in accordance with the bands of CaO [ 6 ].

In the earlier investigations, it has been discovered that the snail shells were calcined at 900 °C and indicated the presence of CaO with strong peaks of brag angle ( 2 θ) located at several points, coordinating from 32.2, 37.3 as well as 53.8°. Conversely, the uncalcined snail shell band was similarly described to have peaks 2 θ located at several points coordinating at 26.2, 33.2 and 45.9°, suggesting that snail shell component comprised of CaCO 3 [ 26 ]. Risso et al. [ 15 ] examined the calcination of waste shells, calcined for the duration of 4 h at 900 °C. The grinded clamshell powder had eventually been transformed into a fine white powder, firmly demonstrating that it was converted into CaO. By comparison, the bands of XRD assessment were discovered at an angle of ( 2 θ) located at several degrees (32.5, 37.6, 54.1 and 62.6°), and this attests to the presence of CaO [ 6 ]. Takeno et al. [ 14 ] described cubic crystals in calcined Silver Croaker’s stone, showing that the calcination temperature and time were adequate to decompose all the carbonates into oxides. CaO has been designated as one of the most suitable catalysts for the transesterification of soybean oil [ 27 ].

3.1.2 FTIR analysis

The FTIR analysis technique was applied to characterize eggshells before calcination (Fig. S 2 a) and after calcination at at 1000 °C for 2 h (Fig. S2b). According to the results, pre-calcination absorption bands were situated at various peaks at 1397.0, 862.9 and 710.3 cm −1 . These types of bands are essentially caused by carbonate's existence on the surface of the catalyst [ 4 ].

Lastly, the absorption bands after calcination were located at 3650.9, 1397.1, 873.9, and 710.1 cm −1 . The expanded matrix oscillations of the CaO bond are indicated by large, strong absorption peaks of approximately 873.9 and 710.1 cm −1 . Asymmetric expansion and contraction of C-O bonds in unknown carbonate species was associated with peaks of 1397.1 and 873.9 cm −1 . There is a large peak at 3650.9 cm −1 due to the creation of -OH bonds at Ca(OH) 2 after H 2 O adsorption on the metal surface by CaO. This means that water is not the cause of catalyst deactivation [ 16 ]. It may also be due to Ca(OH) 2 forming soaps in acidic oils, which reduces yields in subsequent runs. These FTIR results acquired in the current study are comparable with related results described in our previous work [ 4 , 6 ].

3.1.3 EDS analysis

In this study, the EDS analysis was conducted with a single goal to determine particular elements that might be present in the produced catalyst. In light of this, it was revealed (after calcination) that individual chemical elements such as C (6.5%), O (19.1), and Ca (74.4%) as constituents of the developed CaO-based eggshells catalyst as displayed in Table 1 .

The determined EDS results are consistent with the XRD. Rahman et al. [ 28 ] reported that the catalyst performance could be potentially strengthened by the complementary effect of different metal oxide-based catalysts. The catalyst constituents are relatively compatible with the banana peel ash recounted by Balajii and Niju [ 29 ]. SEM diagrams in Fig.  2 show the WES material structure, prior calcination (Fig.  2 a) and after calcination (Fig.  2 b) at high temperature. The specific surface area (Micromeritics, ASAP 2020) and chemical configurations of the developed catalyst were determined. It is evidently illustrated from Fig.  2 that the pore structures in the eggshell showed extensive, substantial disparities before (Fig.  2 a) and after calcination (Fig.  2 b).

SEM image of eggshell before ( a ) and after calcination ( b ) at high temperature

After calcination, there should be significant modifications in the shells' physical structures, signifying that the transformation of CaCO 3 into CaO has happened. SEM images of the developed CaO-based catalyst showed packs of appropriately structured cubic crystals with visible edges. These images are quite similar to the ones revealed by Nisar et al. [ 18 ] for calcined waste animal bones. The calcined bones were irregular, and some stuck together. For comparison, Sirisomboonchai et al. [ 16 ] developed nano-sized CaO-based catalysts from snail shells using SEM micrographs showing significant accumulation of catalyst particles due to a high specific area and a spherical structure without scattering morphology.

3.1.4 Brunauer–Emmett–Teller (BET)-Barrett-Joyner-Halenda (BJH) measurements

The dimensions for the surface area and the absorbency depiction of the developed catalyst were investigated using N 2 physisorption with a BET technique (AUTOSORB-1C Quantachrome). As for the adsorption of nitrogen gas employing the micrometrics equipment, the general pore capacity and average pore dimension of the developed catalysts were calculated using the BJH technique. The Hammett indicators were (acquired from Sigma-Aldrich, Kaohsiungy, Taiwan) used to measure the basic strength and the basicity of the catalyst samples. The outcomes are displayed in Table 2 . The analysis was conducted at a temperature range of 800–1000 °C. In this respect, the assessment via BET suggests that the developed catalyst’s surface area had considerably declined from 7.0 to 6.2 m 2  g −1 with a typical BJH adsorption pore measurement of pore size between 19 and 31 nm. Palitsakun et al. [ 30 ] reported a much lower surface area and pore volume measurements for CaO-T catalyst with a surface area of ​​1.95 m 2  g −1 and an average pore volume of 0.019 cm 3  g −1 , while the CaO-H catalyst had a surface area of ​​23.3 m 2  g −1 with an average pore volume of 0.201 cm 3  g −1 . The CaO-P catalyst was found to have a surface area of ​​1.76 m 2  g −1 and an average pore volume of 0.0155 cm 3  g −1 . Rahman et al. [ 28 ] have found that a solid catalyst's higher surface area can improve its catalytic performance.

3.2 Optimization of transesterification over the developed catalyst and exploring the optimum reaction conditions

3.2.1 effect of catalyst loading on biodiesel yield.

The catalyst effect substantially influences biodiesel yield, and thus, it is an indispensable parameter in optimising the transesterification reaction. On the other hand, when the percentage of free fatty acids (FFAs) in the feedstock is much high, the base catalyst can react with FFAs, resulting in saponification. This could make the product partition and purification process more complex and reduce the biodiesel yield [ 5 , 6 ]. Accordingly, various catalyst loadings on biodiesel yield were studied to examine their effect on biodiesel production yield. In this regard, the results in Fig.  3 a revealed that catalyst loading significantly influenced biodiesel yield. As shown in Fig.  3 a, the biodiesel yields increased when the catalyst loading was increased from 3 to 4%.

Effects of a catalyst loading, b reaction time, c methanol to oil molar ratio (MTOMR), and d reaction temperature on biodiesel yield. Conditions: MTOMR 9:1; catalyst loading 5 wt%; reaction time 165 min; 65 ºC; microwave power of 800 W

Notwithstanding, as the catalyst loading was increased to 5%, the biodiesel yield had consequently increased to more than 90%. Furthermore, when the catalyst loading was increased to 6%, biodiesel yield was subsequently observed not to be increasing any longer and started to decline from 91 to 87%. Other researchers highlighted that excessive catalyst loading might impede the mixing of methanol, oil and catalyst, which may further lead to the separation phase [ 4 , 6 ]. Therefore, 5 wt% catalyst loading was the best catalyst amount during this study.

3.2.2 Effect of reaction time

Figure  3 b shows the effect of reaction time on biodiesel yield. Different experimental conditions were as follows: MTOMR of 9:1, 5 wt% catalyst loading, 180 min reaction time, temperature 65 ºC and MW power of 800 W. As it can be observed from Fig.  3 b, the biodiesel yield had significantly increased as the reaction time continued to increase. Subsequently, this trend has resulted in a high biodiesel yield of 91% in 180 min. However, the biodiesel yield began to decline slightly as the time was further increased from 195 to 210 min. The possible elucidation of this result could be that as the reaction time was at 180 min, the reaction had probably already reached its equilibrium phase. In addition, the backward reaction might occur after reaching the equilibrium period because this reaction, in effect, holds a reversibility nature of the reaction and consequently declines the yield [ 28 , 31 ].

Alternatively, the highest conversion of JCO to biodiesel can be attributed to various factors such as high basic strength, high absorbency, and large surface area of the developed CaO-supported catalysts from WES materials. It is also worthy of highlighting that a very long reaction time could reduce biodiesel yield in such a way that the transesterification reaction will shift to the left and lead to the formation of soap [ 15 ]. Such soap formation can result from esters that could have probably been affected by hydrolysis by increasing reaction time further [ 31 ]. Palitsakun et al. [ 30 ] argued that methanol normally activates CaO. A small amount of CaO is converted to the parental Ca(OCH 3 ) 2 , which is more catalytically active than inactivated CaO. On the other hand, the FAME yield rose substantially from 54 to 88% in 80 min and remained steady as the reaction time intensified to 120 min, indicating that the system reached equilibrium.

3.2.3 Effect of MTOMR

The influence of MTOMR on biodiesel yield was investigated under the following conditions: MTOMR of 9:1, 5 wt% catalyst loading, 180 min reaction time, temperature 65 ºC and MW power of 800 W. MTOMR has a similar effect analogous to the loading of catalysts, specifically on the reaction rate and biodiesel yield in particular. As a result, the study on the transesterification reaction was conducted by adjusting the MTOMR while employing the enhanced catalyst loading (5 wt%) at 65 ºC (Fig.  3 c). It can be determined from the results in Fig.  3 c that as MTOMR increased from 7:1 to 9:1, a considerable increase in biodiesel harvest from 86 to 91% was notably observed. Despite that, as the MTOMR was increased to 10:1, there was an insignificant decrease in biodiesel yield (90%). When the MTOMR was further increased to 10:1, the biodiesel yields gradually decreased to 87% at 11:1.

Conversely, as the methanol concentration continued to increase, the catalyst’s elements of the catalyst and reaction agents were diluted by methanol, which could trigger a reverse reaction and result in declined biodiesel yield [ 6 ]. A comparable pattern of the results was affirmed in the study conducted by Nath et al. [ 25 ] and Basumatary et al. [ 7 ]. Their studies recorded that large amount to MTOMR above best reaction prerequisite can weaken the reaction assortments, causing submerging of catalyst’s active sites, thus declining their essential interactions for effective reaction and subsequently, the reaction rate and decreasing biodiesel yield product. Additionally, above the ideal MTOMR level, there is always a possibility of increasing biodiesel's hydrolysis to produce soap. Such an effect would trigger a substantial reduction in biodiesel yield [ 7 , 32 ]. In the present work, the most promising level for JCO transesterification to biodiesel was determined to be 9:1 of MTOMR with 5 wt% of CaO-supported catalyst generated from eggshell waste materials.

3.2.4 Effect of reaction temperature

The effect of temperature on the biodiesel yield was studied under the following diverse conditions: MTOMR of 9:1, 5 wt% catalyst loading, 180 min reaction time, temperature 65 ºC and MW power of 800 W. According to the results in Fig.  3 d, it was found that when the temperature was lower than 65 ºC, the biodiesel yield increased with increasing temperature and achieved the highest considerable yield at 65 ºC. The biodiesel yield was observed to be decreasing slightly as the temperature above 65 ºC. Various studies envisaged reaction of this nature to be thermic [ 7 ], whereas other researchers anticipated it to be marginally exothermic [ 32 , 33 ]. It is also imperative to highlight that the molecular activity increased with the increasing temperature, increasing the reaction rate. Hitherto, as the temperature continues to increase to 65 ºC, which is assumed to be the boiling temperature of methanol, the methanol will be evaporated and disappear from the reaction mix. Accordingly, the biodiesel yield was found declining. In addition, higher temperatures could speed up the saponification of triglycerides, which can cause adverse consequences on product yields [ 34 ].

3.3 Determination of transesterification process kinetics and activation energy

To investigate the transesterification reaction's kinetics, the effects of temperature and constant reaction time on the reaction were studied. The kinetics of transesterification reaction was investigated at various reaction temperatures of 45, 55, 65, 75 and 85 °C under the ideal operational conditions. As indicated in Table 3 , the linear regression equation at diverse reaction temperatures was attained, and the results are presented thereof. Regression equation was established, and it was observed to be linear with high regression coefficient (R 2  = 0.9994). The reaction's activation energy ( Ea ) was determined as 38.5 kJ mol −1 .

Additionally, a comparison of the activation energy for biodiesel production of the synthesized catalysts with several solid heterogeneous base catalysts from the previous studies has been made, signifying how the diverse Ea is attained by exploiting different feedstock (oils) in the transesterification reaction, as shown in Table 4 . Mazubert et al. [ 35 ] had reported activation energy of 9.7 kJ mol −1 while utilizing waste cooking oil (WCO) under an MW heating system. While using the same WCO as a feedstock and heterogeneous catalysts, Gupta and Rathod [ 36 ], Foroutan et al. [ 21 ] and Al-Sakkari et al. [ 11 ] have recorded high activation energy of 27, 46 and 49 kJ mol −1 . In comparison, Pavlovic et al. [ 19 ] reported very high activation energy (67; 58 kJ mol −1 ) as compared to the present study. Uzun et al. [ 37 ] used a conventional heating method and reported a very low activation energy of 12 kJ mol −1 .

3.4 Comparative study of the synthesized catalyst

The results showed a significantly high yield of 92% for biodiesel within 180 min reaction time within ideal requirements of 9:1 MTOMR and 5 wt% catalyst loading with 65 °C reaction temperature. The results in this study were compared to various preceding studies, specifically on waste biomass derivatives for catalysts of the heterogeneous nature that were utilized for the generation of biodiesel, as indicated in Table 5 . Biodiesel production from JCO while exploiting various solid base heterogeneous catalysts derived from different sources such as by Teo et al. [ 8 ] and Vyas et al. [ 38 ] have all recounted high reaction temperatures (360 and 120 °C, respectively) for the transesterification procedure with pretty much similar yields to the catalyst used under this investigation. They have reported 94 and 92% biodiesel yields after the 9 th and 5 th cycles, respectively.

In other studies, the synthesis of biodiesel while exploiting different waste biomass derived heterogeneous catalysts, Chavan et al. [ 39 ] and Teo et al. [ 8 ] have reported a diminished catalytic performance with lengthy reaction time (2.5 and 4 h). This can perhaps be ascribed to significant smaller surface areas of the various catalysts used. However, Pratika et al. [ 40 ] and Basumatary et al. [ 7 ] reported CaO-based heterogeneous catalysts with relatively outstanding catalytic performance (with 98 and 98% biodiesel yields) even though they may have low catalysts reusability or recyclability of 4 th and 3 rd cycles as compared to that of the catalyst utilized under this present study which was recycled 5 times. Teo et al. [ 8 ] have also described similar catalytic performance for JCO biodiesel production utilizing solid based heterogeneous catalysts and produced biodiesel yields in the range of 90–97%. Consistently, the current WES derived catalyst has displayed exceptional catalytic activity for the generation of biodiesel. In the current research work, EDS analysis has reported the existence of a high quantity of calcium in the WES catalysts as compared to other elements (Table S 1 ), and the Ca is an outstanding source of calcium oxide (CaO), which has been broadly utilized in several commercial enterprises as a heterogeneous catalyst. CaO has a fundamental role in the catalysis for the application of biodiesel production [ 21 , 23 ].

3.5 Reusability and leaching study of the catalyst

It is worth mentioning that the economic sustainability or viability of the transesterification procedure regarding biodiesel production is immensely influenced by the recyclability of a catalyst [ 23 ]. Reusability is one of the most essential and promising characteristics of the solid base heterogeneous catalysts that should be considered in the continuous reaction process for the low-cost biodiesel generation [ 26 , 27 ]. Figure  4 shows the results of the reusability study of the catalyst.

Reusability of the WES derived catalyst (CaO-based catalyst). Conditions: MTOMR 9:1; catalyst loading 5 wt%; reaction time 165 min; 65 °C; microwave power of 800 W

The synthesized catalyst was regained from the reaction mixture by separation and consequently rinsed with hexane and subsequently calcinated at 1000 °C for 4 h. Thus, the restored catalyst was acquired and immediately reused in successive 5 catalytic cycles using the same experimental and reusability approach. Furthermore, the recycled catalyst was ascertained to produce a more than 90% biodiesel yield in the 1 st cycle and stayed the same at 90%. Yet even in the 3 rd cycle of the five progressive catalytic runs, the recycled catalyst performed well with an excellent biodiesel yield of more than 80%. Conversely, the results were observed to have been drastically reduced to 72% after the 5 th cycle. This may be due to the contamination of the active site with CO 2 and H 2 O and, hence, it shortened lifespan of the developed CaO-based catalyst. As a result, the CaO-based catalyst developed in our study exhibits outstanding reusability in biodiesel fuel production with a significant reduction in yield up to 5 cycles. The degree of Ca leaching can affect the lifetime of the developed CaO catalyst, and biodiesel can be contaminated with leached Ca particles [ 19 ].

4 Economic and environmental cost–benefit analysis

Waste such as eggshells can be transformed into high value-added products, contributing to sustainable economic development and, more so, to sustainable bio-waste management. Undoubtedly, it costs about US$100,000 per annum to discard the eggshell waste produced by the average egg processing plant in the United States [ 10 ]. In contrast, the industrial scale of recycling WES to CaO-based catalysts can provide economic benefits that are at least five times higher than the cost of conservative disposal methods [ 8 ]. At the same time, by minimizing the exposure and likelihood of pathogens transmission, abbreviating discarding costs and synthesizing potential CaO-based heterogeneous catalysts for economical biodiesel production, great environmental benefits can be achieved that greatly support the sustainability of the anticipated technique [ 8 ]. One of the environmental benefits of reutilization of WES is that the eggshell is made up of calcium carbonate, which is a rich source of calcium. Eggshells are collected as waste from residences, hotels, and egg processing facilities, among other places. Eggshells are still difficult to dispose of in an environmentally acceptable manner because of their limited decomposition capacity, which generates pollution. To reduce microbiological species such as molds and bacteria, such shells can be dried at high temperatures (about 80 °C). Eggshell powder can then be made by powdering dried shells in a grinder [ 29 ]. Eggshell powder has been demonstrated in studies to be effective as a calcium supplement for plants suffering from blossom-end rot disease, such as tomatoes and berries [ 6 ].

5 Conclusions

This research has successfully synthesized an economical, eco-friendly, and highly efficient bio-waste-derived CaO solid base heterogeneous catalyst for low-cost biodiesel synthesis. The developed catalyst has demonstrated outstanding catalytic performance with a biodiesel yield of 92% under the measured, optimized reaction conditions. While the kinetic study parameters and Ea of the reaction were determined to be 38.5 kJ mol −1 , culminating that it is not an energy-intensive procedure. We hypothesized that the high coefficient of determinations (R 2 ) value of 0.9994 signified the prospect and excellent suitability of this model can be applied in future. The eggshell catalyst, produced from renewable and cheap raw materials under MW-assisted technology, is an excellent and environmentally friendly technique. These capabilities are required to develop high-performance heterogeneous catalysts that can be upgraded to a commercial scale in the future or to industrial applications for large-scale biodiesel production. The results of this study are not for academic purposes only. They can also serve as prototypes for the industrial production of biodiesel using non-consumable oils as feedstock and biowaste as catalysts for MW heating systems as environmentally friendly methods for sustainable bio-waste management approaches. In addition, this study expands on existing state-of-the-art research as we know about biofuel production using non-consumable oil feedstocks and bio-waste economically.

Availability of data and materials

The datasets supporting the conclusions of this article are included within the article and supplementary materials.

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Acknowledgements

This work was supported by the Ministry of Science and Technology (MOST), Taiwan, under the grant NSC 100-2221-E-110-015-MY2.

This work was supported by Ministry of Science and Technology (MOST) (Project No. NSC 100–2221-E-110–015-MY2).

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Kassian T.T. Amesho: Writing-original draft. Yuan-Chung Lin: Conceptualization, Funding acquisition, Methodology, Supervision. Chin-En Chen: Data curation, Investigation, Formal analysis/Software. Pei-Cheng Cheng: Data curation, Investigation. Vinoth Kumar Ponnusamy: Writing- review & editing. All authors read and approved the final manuscript.

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Additional file 1: table s1..

Physico-chemical properties of the Jatropha curcas biodiesel (ASTM D6751, EN 14214) from [1, 2]. Fig. S1. XRD analysis of eggshells (a) before and (b) after calcination. Fig. S2. FTIR analysis of eggshells before (a) and after calcination (b).

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Amesho, K.T.T., Lin, YC., Chen, CE. et al. Optimization and kinetics studies of biodiesel synthesis from Jatropha curcas oil under the application of eco-friendly microwave heating technique: an environmentally benign and sustainable bio-waste management approach. Sustain Environ Res 32 , 41 (2022). https://doi.org/10.1186/s42834-022-00151-w

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Yaasinta Lafasha Nabila Department of Chemistry, Faculty of Mathematics and Natural Sciences, Tanjungpura University, Jl. Ahmad Yani Pontianak, Kalimantan Barat 79124 Indonesia

Andi Hairil Alimuddin https://scholar.google.co.id/citations?user=iVxMuTkAAAAJ&hl=id Department of Chemistry, Faculty of Mathematics and Natural Sciences, Tanjungpura University, Jl. Ahmad Yani Pontianak, Kalimantan Barat 79124 Indonesia

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Synthesis of Biodiesel in Low-Grade Palm Oil using Geopolymer-ZnO Catalyst

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Recent advances in transesterification for sustainable biodiesel production, challenges, and prospects: a comprehensive review

Sabah mohamed farouk.

1 Chemical Engineering Department, Egyptian Academy for Engineering and Advanced Technology (EA&EAT), affiliated to the Ministry of Military Production, Km. 3 Cairo Belbeis Desert Rd., Cairo Governorate, 3066 Egypt

Aghareed M. Tayeb

2 Faculty of Engineering, Minia University, Misr Aswan Agricultural Rd., EL MAHATTA, Menia Governorate, 2431384 Egypt

Shereen M. S. Abdel-Hamid

Randa m. osman.

3 Chemical Engineering and Pilot Plant Department, National Research Centre (NRC), 33 El Bohouth St., Dokki, 12622 Giza Governorate Egypt

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Data available on request from the authors.

Biodiesel, a renewable and sustainable alternative to fossil fuels, has garnered significant attention as a potential solution to the growing energy crisis and environmental concerns. The review commences with a thorough examination of feedstock selection and preparation, emphasizing the critical role of feedstock quality in ensuring optimal biodiesel production efficiency and quality. Next, it delves into the advancements in biodiesel applications, highlighting its versatility and potential to reduce greenhouse gas emissions and dependence on fossil fuels. The heart of the review focuses on transesterification, the key process in biodiesel production. It provides an in-depth analysis of various catalysts, including homogeneous, heterogeneous, enzyme-based, and nanomaterial catalysts, exploring their distinct characteristics and behavior during transesterification. The review also sheds light on the transesterification reaction mechanism and kinetics, emphasizing the importance of kinetic modeling in process optimization. Recent developments in biodiesel production, including feedstock selection, process optimization, and sustainability, are discussed, along with the challenges related to engine performance, emissions, and compatibility that hinder wider biodiesel adoption. The review concludes by emphasizing the need for ongoing research, development, and collaboration among academia, industry, and policymakers to address the challenges and pursue further research in biodiesel production. It outlines specific recommendations for future research, paving the way for the widespread adoption of biodiesel as a renewable energy source and fostering a cleaner and more sustainable future.

Introduction

Fossil fuels are responsible for most of the world’s energy needs, endangering the environment. The consumption of fossil fuel products like coal and petroleum has increased because of ongoing globalization and industrialization (Touqeer et al. 2020 ). Around 580 TJ of energy is needed annually for the entire planet, and a staggering 80% of that energy is supplied by burning traditional fossil fuels. Greater energy use is required by expanding the population and improving accessibility, which increases the depletion of fossil fuel reserves (Martchamadol and Kumar 2012 ). Finding an alternate energy source, like biodiesel, is essential for improving energy security for economic development (Oh et al. 2002 ; Um and Kim 2009 ). Biodiesel is a renewable, sustainable, and biodegradable fuel with low greenhouse gas emissions (Sharma and Singh 2009 ; Lee et al. 2011 ). Oils from plants and animals, as well as other lipids such as triacylglycerides (TAGs), can be converted into biodiesel (Hoekman and Robbins 2012 ). The process of making biodiesel, called transesterification or alcoholysis, typically requires the use of catalysts such as acids, bases, and enzymes (Ong et al. 2014 ). Catalysts can exist in either a homogeneous or heterogeneous phase. A homogeneous catalyst undergoes alcoholysis in the same phase, often liquid, as the reactants. In contrast, a heterogeneous catalyst is in a different phase, often one that is not liquid, from the reactants (Ruhul et al. 2015 ). Nanomaterials have become increasingly important in enhancing the production of biodiesel. The weight of the catalyst, the reaction temperature, and the oil-to-alcohol ratio are all reduced when a nanocatalyst is used (Dhawane et al. 2018 ; Ghosh and Halder 2022 ). It has already been proven that nanocatalysts play a key role in increasing response velocity. Additionally, it significantly lowers the reactants’ activation energy. This study addresses the need to keep up with the rapidly changing field of biodiesel production. By conducting thorough surveys and reviews, researchers can stay informed about the latest advancements and challenges. This approach helps focus research efforts by identifying promising methodologies and comparing their performance. The production of biodiesel encounters several challenges, including costly feedstocks, environmental concerns, and the quest for improved efficiency. Through a meticulous review of relevant literature, researchers can identify these challenges and propose potential solutions. This review serves as a valuable resource, synthesizing information from different sources to provide a comprehensive understanding of the current state of biodiesel production technology. Policymakers and industry leaders rely on up-to-date insights to make informed decisions that shape the future of the biodiesel industry. Concise surveys and reviews effectively communicate the latest information, enabling informed decision-making. Additionally, this review promotes collaboration and knowledge sharing within the biodiesel research community, facilitating the development of new and better technologies. It also educates the public about biodiesel production and its potential benefits, potentially increasing support and encouraging the adoption of biodiesel as a sustainable alternative to petroleum-based fuels. Literary, and in detail, this paper offers a comprehensive review of biodiesel production, encompassing key topics such as feedstock selection, the transesterification process, the recovery and reusability of nanoparticles, the benefits and challenges of biodiesel use in engines, and the significance of techno-economic analysis. It begins with various feedstocks, including used oils, animal fats, algae, and edible and inedible vegetable oils, which are chosen for their potential to produce biodiesel. The review then highlights biodiesel as a promising alternative to fossil fuels, exploring its environmental benefits, renewability, and potential to reduce dependence on fossil fuels. The transesterification process, a crucial step in biodiesel production, is discussed in detail, covering reaction mechanisms, catalysts, influencing factors, and challenges associated with optimization. Additionally, various methods and strategies, including solid supports, magnetic separation, centrifugation, and surface modification, have been explored for the recovery and reusability of nanoparticles as catalysts in biodiesel production. These approaches offer promising prospects for enhancing the efficiency and sustainability of biodiesel synthesis while reducing costs and environmental impacts. The benefits of using biodiesel in engines, including improved lubricity, reduced emissions, and potential performance enhancements, are discussed. Lastly, this review emphasizes the importance of techno-economic analysis in assessing the financial viability and sustainability of biodiesel production, considering factors such as capital investment, operating costs, feedstock prices, and governmental policies. Overall, this comprehensive review serves as a valuable resource for researchers, engineers, and policymakers in the biodiesel industry.

Feedstocks for biodiesel production

Biodiesel can be synthesized from various raw materials, including vegetable oils, animal fats, algae, and waste cooking oil. Each feedstock has its own advantages and challenges regarding availability, cost, sustainability, and energy content. The purpose of using different raw materials is to diversify the sources of renewable energy and reduce dependence on fossil fuels. At least 80% of the current costs of producing biodiesel come from the feedstock (Sayed and El-Gharbawy 2016 ). Given the global food crisis, almost 95% of biodiesel produced worldwide is manufactured from edible oils, which are viewed as unnecessary (Azizian and Kramer 2005 ). As a result, making biodiesel from inexpensive, non-edible oil is the current trend (Balat 2011 ). A few feedstocks, including palm, jatropha, microalgae, coconut tallow, and used cooking oil, stand out for their high productivity in the manufacturing of biodiesel (Gui et al. 2008 ). Figure  1 represents some different feedstocks for biodiesel production.

The choice of raw materials has a significant impact on the production of biodiesel in terms of yield, quality, cost, and environmental sustainability. The fatty acid profile of the raw material plays a crucial role in determining the properties and performance of biodiesel. Oils with high levels of saturated fatty acids produce biodiesel with better cold flow properties, while oils with high levels of unsaturated fatty acids produce biodiesel with better oxidative stability (Suzihaque et al. 2022 ). FFA content significantly impacts the reaction rate and biodiesel yield. Higher FFA content requires pre-treatment steps, such as esterification, to reduce acidity and prevent soap formation, which can hinder the reaction and reduce biodiesel purity (Javidialesaadi and Raeissi 2013 ). The amount of water in the raw material can also affect the transesterification reaction. Excess water can hydrolyze the fatty acid methyl esters produced during the reaction, reducing biodiesel yield. Therefore, feedstocks with low water content are preferred (Atadashi et al. 2012 ). The overall quality of the raw material, including its purity and the presence of impurities, can influence biodiesel production. Impurities like phospholipids, gums, and waxes can interfere with the reaction and complicate downstream processing (Manzanera et al. 2008 ). The environmental impact of raw material sourcing should be considered. Using waste cooking oil or non-edible oils reduces the competition for land with food production and minimizes the environmental footprint of biodiesel production. The price of the raw material is a major factor affecting the economics of biodiesel production. Utilizing inexpensive feedstocks, such as waste cooking oil or locally produced oils, can significantly reduce production costs. The choice of raw material may necessitate adjustments in processing conditions, such as reaction temperature, catalyst concentration, and reaction time, to optimize biodiesel yield and quality. Byproducts generated during biodiesel production, such as glycerol, can be utilized for various applications, adding value to the overall process and reducing waste. The sourcing of raw materials can have social and economic impacts on local communities. Promoting the use of locally available or waste-based feedstocks can stimulate agricultural economies and create employment opportunities (Srivastava et al. 2018 ; Suzihaque et al. 2022 ). In conclusion, the selection of raw materials for biodiesel production requires a careful assessment of their fatty acid profile, FFA content, water content, overall quality, environmental sustainability, cost-effectiveness, and potential for byproduct utilization. By considering these factors, biodiesel production can be optimized for yield, quality, cost, and environmental sustainability. Devaraj et al. ( 2020 ) created biodiesel from used frying oil. At 75 °C, 1 wt% catalyst concentration, 1:6 oil-to-methanol molar ratio, 350 rpm, and 90 min, 97% of the possible biodiesel output was produced. In the pilot plant, biodiesel was also produced under these process conditions, with a 97% yield. Hamed et al. ( 2021 ) converted Afia waste cooking oil (AWCO) into biodiesel fuel. At the ideal working conditions of 60 °C reaction temperature, 3 h reaction time, and 0.4 catalyst concentration, the maximum conversion and yield of biodiesel are 97.54 and 94.935%, respectively. Roy et al. ( 2021 ) reported on the conversion of used frying oil into biodiesel to manage liquid waste. The optimum conditions for the transesterification of WFO were 1:16 oil to methanol by weight, 3 wt% catalyst, 65 °C reaction temperature, and 35 min reaction time. FAME conversion (99.5%) and 96% yield are achieved at this optimal reaction setting. Karmakar and Halder ( 2021 ) researched the transesterification reaction used in supercritical settings to produce biodiesel fuel from fish waste oil. According to calculations, the experimental yield of biodiesel generation under ideal conditions was 94.6%. Al Hatrooshi et al. ( 2020 ) created the fatty acid methyl ester from waste shark liver oil (WSLO). At a methanol-to-WSLO ratio of 10.3 M, a reaction duration of 6.5 h, a temperature of 60 °C, and a catalyst concentration of 5.9 wt%, acid-catalyzed WSLO transesterification achieved 99% FAME conversion. A biodiesel yield of 99.73% was achieved at 60 °C for 1.5 h using a 1.2% catalyst and a 6:1 methanol:oil ratio. In a batch-stirred reactor, Nisa et al. ( 2020 ) conducted an experiment to produce biodiesel from the microalgae Spirulina sp. using 1 wt% (w/w) of palm oil as a co-solvent for methanol and potassium hydroxide. At 60 °C, with a methanol-to-palm oil molar ratio of 10:1 and a palm oil-to-microalgae weight ratio of 5:1, the best biodiesel production of 85.28% was generated. Olubunmi et al. ( 2020 ) converted the bio-oil from dairy scum waste into biodiesel. Using a 9:1 methanol-to-oil molar ratio, 40 min for the reaction, 65 °C for the reaction temperature, and 300 rpm for the mixing speed, the maximum biodiesel yield of 94.8% was achieved. In India, Jain and Sharma ( 2010 ) utilized Jatropha oil for the manufacture of biodiesel under the ideal conditions of a 3:7 (v/v) methanol-to-oil ratio and a 1% (w/w) catalyst concentration for mixing at 400 rpm with H 2 SO 4 and NaOH. The transesterification of pretreated JCO yielded the highest yield of 90.1%. Abdulrahman ( 2017 ) made a biodiesel fuel from used cooking oil and chicken fat. The highest conversion to ester was attained at a methanol-to-oil ratio of roughly 7:1 at 60 °C and an 83% yield, also Al-Mawaali et al. ( 2023 ) utilized discarded animal fats and used cooking oil as a cheap source of feedstock to produce biodiesel. With NaOH acting as a catalyst, the discarded frying oil produced the highest yield of synthesized biodiesel (80.6%), followed by a mixture of waste cooking oil and animal fats (79.3%). Finally, using multiple raw materials as feedstock for biodiesel synthesis helps to diversify energy sources, reduce dependency on fossil fuels, and promote sustainability. Every feedstock source has benefits and drawbacks, and the choice of feedstock is influenced by factors like cost, sustainability, availability, and the needs of the biodiesel production process. Continued R&D efforts are aimed at enhancing the efficiency, cost-effectiveness, and sustainability of biodiesel production from diverse feedstocks.

Feedstock preparation

• Cleaning and drying: The feedstock is cleaned to get rid of pollutants, water, and debris. It could entail processes like sedimentation or filtration. The feedstock is dried to eliminate any last traces of moisture after cleaning (Sait et al. 2022 ).
• Pretreatment: Pretreatment of feedstocks may be necessary in some situations to enhance their quality and appropriateness for the manufacture of biodiesel. Degumming, acid esterification, or neutralization are some examples of pretreatment techniques used to get rid of pollutants, gums, and free fatty acids (Singh et al. 2011 ).

The most popular method of treatment for reducing FFAs is acid esterification. One efficient method for lowering FFAs is glycerolysis, also known as glycerol esterification (Elgharbawy et al. 2021 ). Hayyan et al. ( 2011 ) treated the high FFA percentage in palm oil using sulfuric acid. They used H 2 SO 4 by 0.75 wt%, a methanol-to-oil molar ratio of 8:1, 60 min of reaction time, and 60 °C to successfully reduce the FFA content from 23 wt% to less than 2 wt%. Kara et al. ( 2018 ) examined how various methanol-to-oil molar ratios affected the final FFA% throughout the esterification reaction while maintaining constant values for other parameters. Within 160 min of the reaction, the ideal conditions were reached at a molar ratio of 15:1. The highest conversion of 92.6% was achieved, while the FFA content decreased from 21 to 1.5%. Sadaf et al. ( 2018 ) investigated how three acids HCl, H 2 SO 4 , and H 3 PO 4 affected used cooking oil containing 2.75 wt% FFAs. At 60 °C and a 2.5:1 methanol-to-oil molar ratio, the FFA dropped to 0.33 wt%, indicating that H 2 SO 4 was the most effective catalyst. Sousa et al. ( 2010 ) treated castor oil with a non-catalytic glycerolysis procedure with a glycerol-to-oil mass ratio of 1:1 and an FFA of 2.5 wt% for 2 h at 120 °C. They were able to reduce the FFAs from 2.5 to 0.2 wt%.

Triglyceride content in different feedstocks

Triglycerides are the primary component of biodiesel and are present in various amounts in the feedstocks used in the process. Mono- and diglycerides, free fatty acids, phosphatides, sterols, fatty alcohols, fat-soluble vitamins, and other compounds are among the minor constituents. The amount of triglycerides in the feedstock has a significant impact on the output and caliber of biodiesel produced. Most dietary fats and oils are mostly composed of triglycerides.

Vegetable oils

Various vegetable oils, including canola, sunflower, soybean, and palm oils, are frequently utilized as feedstocks in the manufacturing of biodiesel. High concentrations of triglycerides, which are made up of three fatty acid chains joined to a glycerol backbone, are found naturally in these oils. Vegetable oils, therefore, have a high triglyceride concentration, usually more than 90% by weight. Vegetable oils are ideal feedstocks for the synthesis of biodiesel because of their high triglyceride content. Because refined vegetable oils convert pure triglycerides (TG) to FAME at a high rate and quickly, they are the ideal feedstock for producing biodiesel.

Animal fats

Fish oil, tallow, lard, and chicken fat are examples of animal fats that can be utilized as feedstocks to produce biodiesel. Comparing animal fats to vegetable oils, the former usually have a lower triglyceride concentration, usually between 70 and 85% by weight. The species of the animal, its nutrition, and the rendering techniques all affect the variation in triglyceride content. Animal fats can still be effectively turned into biodiesel even when their triglyceride level is slightly lower.

Waste oils and greases

The manufacturing of biodiesel can use recycled waste cooking oils and greases from food processing industries, restaurants, and other sources as feedstocks. Depending on their quality and place of origin, waste oils and greases might include varying amounts of triglyceride. The quality and conversion efficiency of biodiesel can be impacted by the presence of contaminants or elevated quantities of free fatty acids in these feedstocks.

Algae and microorganisms

The capacity of algae and specific microorganisms to accumulate large concentrations of lipids (fatty acids and triglycerides) makes them viable feedstocks for the generation of biodiesel. Depending on the species and growing environment, triglycerides can make up a large portion of algal oils, anywhere from 20 to 60% by weight. Table ​ Table1 1 represents the triglyceride content of different feedstocks.

Table 1

Biodiesel as a promising alternative source of biofuel

Among the emerging alternatives, biodiesel has garnered significant attention as a promising biofuel with the potential to reduce reliance on petroleum-based fuels and mitigate environmental impacts. The non-toxic, biodegradable fuel known as biodiesel is made from leftover cooking oil, animal fats, or vegetable oil, and presents several advantages over conventional diesel. Firstly, its renewable nature alleviates concerns over depleting fossil fuel resources. Unlike petroleum diesel, which is extracted from finite underground reservoirs, biodiesel can be produced from continuously replenished sources, ensuring long-term sustainability. Secondly, biodiesel boasts superior environmental credentials compared to diesel. Its burning emits significantly lower degrees of particulate matter, nitrogen oxides, sulfur oxides, and carbon monoxide, all of which contribute to air pollution and its detrimental effects on human health and the environment. Additionally, biodiesel’s lower greenhouse gas emissions make it a promising alternative for reducing the transportation sector’s contribution to climate change (Demirbas 2009 ). Moreover, biodiesel offers economic benefits, particularly for regions with abundant feedstock sources. Local production of biodiesel can stimulate agricultural economies, reduce reliance on imported petroleum, and create employment opportunities. Additionally, biodiesel’s compatibility with existing diesel engines eliminates the need for expensive infrastructure overhauls, facilitating a smoother transition toward a more sustainable fuel source (Fukuda et al. 2001 ). Despite its promise, biodiesel faces certain challenges that need to be addressed for its widespread adoption. One challenge lies in its economic viability. The production cost of biodiesel is currently higher than that of petroleum diesel, primarily due to feedstock costs and processing expenses. However, advancements in production technologies and economies of scale are expected to reduce biodiesel costs over time.

Another challenge involves the availability of suitable feedstocks. While various plant oils, such as soybean, palm, and jatropha, can be used for biodiesel production, concerns have arisen regarding land-use competition for the food industry and the potential environmental effects of broad-scope palm oil plantations. Sustainable feedstock sourcing strategies, such as utilizing waste cooking oil and cultivating oil crops on marginal lands, are crucial to addressing these concerns (Knothe 2010 ; Singh and Singh 2010 ). In conclusion, biodiesel presents a compelling alternative to conventional diesel, offering a combination of sustainability, environmental benefits, and economic potential. Addressing the current challenges related to feedstock availability and production costs will pave the way for biodiesel’s wider adoption and its significant contribution to a more sustainable energy future.

Innovations in biodiesel applications

Biodiesel, a renewable and cleaner-burning alternative to conventional diesel fuel, has gained significant attention and widespread applications in recent years. Its versatile nature allows for various uses across different sectors. In transportation, biodiesel can be blended with petroleum diesel to power vehicles, reducing emissions of greenhouse gases and air pollutants. Moreover, biodiesel finds applications in industrial settings, where it serves as a substitute for petroleum-based fuels in machinery and equipment. The growing interest in sustainable energy solutions has spurred research and development efforts, leading to advancements in biodiesel production techniques, feedstock selection, and engine compatibility. Recent studies (Mobin et al. 2022 ; Tripathi et al. 2023 ) have explored the potential of advanced feedstocks, such as algae and waste oils, to enhance biodiesel production efficiency and reduce environmental impacts. These advancements in biodiesel applications and technology are crucial steps in achieving a more sustainable and greener energy future. Figure  2 shows some of the applications of biodiesel.

Biodiesel applications

Transesterification in biodiesel production

Alcohol and lipids react chemically to produce fatty acid alkyl esters, a process known as transesterification. Triglycerides and alcohol are transesterified to produce FAAE and glycerol. Triglycerides and alcohols combine to form diglycerides in the first stage, which are then converted to monoglycerides and glycerol, each of which yields an alkyl ester (Thangaraj et al. 2019 ). Among the factors influencing biodiesel yield in transesterification are time, temperature, type and concentration of the catalyst, kind of feedstock oil, and alcohol-to-oil ratio. It is possible to reverse the transesterification process. To shift the reaction’s equilibrium in favor of the product’s production, an excess of alcohol is therefore necessary. Alcohols with short chains, long chains, and cyclic chains are all used in this process. The availability, polarity, better reactivity, and inexpensive price of methanol and ethanol, however, make them popular choices (Avhad and Marchetti 2015 ), Fig.  3 illustrates the characteristics and features of the transesterification process used to make biodiesel, and Fig.  4 shows a schematic representation of the biodiesel synthesis path through the transesterification process.

The characteristics and features of the transesterification process used to make biodiesel

Schematic representation of the biodiesel synthesis path through the transesterification process

Triglycerides are changed into diglycerides, monoglycerides, and finally glycerol by a sequence of chemical events called transesterification, which is the process used to produce biodiesel. Alkali catalysts are usually used in a single phase of this process. However, a two-step procedure can be required if the feedstock has large concentrations of water or free fatty acids (FFAs). Fatty acid esters, or acid-catalyzed alcoholysis, are the initial step that turns FFAs into fatty acid esters. The process of transesterification, which turns the fatty acid esters into biodiesel, comes next. Figure  5 displays the schematic diagram for the manufacture of biodiesel in both one and two steps.

Biodiesel manufacturing can be done in two steps ( A ) or one step ( B ) (Fattah et al. 2014 ; Mofijur et al. 2014 )

Chemical or biological catalysts can be used in transesterification reactions that are catalyzed. The classification is shown in Fig.  6 . There are both homogeneous and heterogeneous chemical catalysts. Base or acid catalysts are included in the homogeneous catalyst. The heterogeneous catalyst is made up of nano-, biomass waste-based, base, and acid–base functionalities (Thangaraj et al. 2019 ). The choice of any catalyst is influenced by the following factors: oil quality, FFA content, operating conditions, necessary catalyst activity, cost, and availability (Tacias-Pascacio et al. 2019 ). The advantages and disadvantages of different catalysts are presented in Table  2 .

Different catalysts used in biodiesel production (Rizwanul Fattah et al. 2020 )

Table 2

Pros and cons of utilizing various catalysts in the process of transesterification

Homogeneous catalysis

Homogeneous catalysts are the ones that are most frequently used in the production of biodiesel because they are simple to use and require less time to complete a reaction. When dissolving a homogeneous catalyst, a solvent that is in the same phase as all the reactants is usually utilized (Rizwanul Fattah et al. 2020 ). Bhuana et al. ( 2020 ) used leftover beef tallow and methanol as solvents, in a KOH catalyst to create biodiesel. Ethanol effectively functioned as a co-solvent, lowering reaction time by 61.11% and functioning as a low-polarity active ester exchange agent, which prevented soap formation and increased yield by 3.08%. Karmee et al. ( 2015 ) looked at the transesterification of algal oil using methanol as the solvent. Under identical reactional conditions, HCl outperformed the homogeneous H 2 SO 4 catalyst. Rice bran oil was esterified by Arora et al. ( 2015 ) utilizing sulfuric acid as a uniform catalyst. Studies have been done on how the oil-to-methanol molar ratio (1:5 to 1:30), catalyst concentration (0.15 to 1.0 wt%), and reaction temperature (45 to 60 °C) affect the conversion of FFA. Siddiqua ( 2015 ) transesterified palm oil to create biodiesel. Further observation shows that a mixture of methanol and NaOH at a reaction temperature of 55 °C produced the highest production. Abdulsalam ( 2023 ) assessed the process of turning thevetia peruviana seed oil into biodiesel using two different catalysts (NaOH and KOH). Separate NaOH and KOH pellets were dissolved in methanol to create different catalysts with different amounts of sodium and potassium methoxide, respectively. The NaOH catalyst’s conversion yield was 81.2%, 80.4%, and 89%, whereas the KOH catalyst’s conversion yield was 96.8%, 88.4%, and 84.0. Through the calcination of leftover pineapple leaves, de Barros et al. ( 2020 ) created a unique catalyst. After a 30-min reaction period at 60 °C, 4 wt% of catalyst, a molar ratio of 1:40 for oil to methanol, and an oil-to-biodiesel conversion of above 98%, a high catalytic activity was detected. This activity is likely connected to the 85-wt% presence of alkali/alkali metals (K, Ca, and Mg). Kasirajan ( 2021 ) used two-step procedures to produce biodiesel from Chrysophyllum albidum seed, a non-edible source. Transesterification is performed on oil from Chrysophyllum albidum seeds after esterification employing a homogeneous catalyst of H 2 SO 4 . The maximum oil-to-biodiesel conversion was 99.2 wt% when the ideal situations were achieved, which included a 1:9 oil-to-methanol molar ratio, 1 wt% KOH, 500 rpm, and 40 min at 65 °C. Belkhanchi et al. ( 2021 ) showed that transesterifying used frying oils (UFO) at 18 °C for 60 min of reaction in the presence of methanol using MeOH/UFO 6:1 mol proportion and 1 wt% of KOH yields the best conversion of UFO. Jain et al. ( 2023 ) gave a description of the homogeneous base catalyst-based single-step transesterification process for producing biodiesel from waste cooking oil that contains high levels of free fatty acids and algal oil. According to the findings, a biodiesel yield of 92% may be achieved under optimal conditions, which include a 1.5% catalyst (w/w), a methanol to oil ratio of 21:1, a time of 110 min, and a temperature of 50 °C. Saeed et al. ( 2021 ) investigated S. elongata algal for biodiesel creation. To evaluate transesterification to FAME, zeolitic catalysts, KOH, and HCl were used. KOH produced the maximum biodiesel yield (99.9%), which was obtained under the ideal reaction conditions of a 1.0% catalyst, 60 °C, 4 h, and a 1:4 volume ratio between oil and methanol. Table ​ Table3 3 highlights some of the recently published research on the use of several homogeneous catalyst types for biodiesel synthesis, various feedstock sources, experimental setups, and biodiesel yields.

Table 3

Different types of homogeneous catalysts used for biodiesel synthesis

Heterogeneous catalysis

Heterogeneous catalysts go through different phases or states than reactants. According to Melero et al. ( 2009 ), these are the catalysts that often generate active sites when reacting with their reactants. Greater oil/alcohol ratios and greater temperatures than in homogeneous catalysis are the primary disadvantages of this catalysis. The catalyst’s improved reusability and ease of separation and purification are other advantages. Mohamed et al. ( 2020 ) prepared by quickly pyrolyzing rice straw, a heterogeneous catalyst (RS-SO 3 H) was created. The yield of biodiesel was 90.37%. in ideal conditions: 20:1 methanol: oil molar ratio with a 10% catalyst at 70 °C for 6 h. Choksi et al. ( 2021 ) created a solid acid catalyst using the sulfonation carbonization process from a palm fruit bunch. After that, the catalyst was put through esterification and transesterification processes to produce biodiesel. Utilizing a 4% catalyst, a 21:1 methanol-to-oil molar ratio, and a 60 °C temperature, an optimal yield of 88.5 wt% methyl ester was obtained in 180 min. Aghel et al. ( 2019 ) wanted to improve a pilot-scale microreactor that used kettle limescale to turn used cooking oil (WCO) into biodiesel. The produced biodiesel had a maximum conversion of 93.41% at 61.7 °C, a catalyst concentration of 8.87 wt %, a methanol-to-oil 1.7:3 volumetric ratio, and 15 min. Bhatia et al. ( 2020 ) developed a heterogeneous catalyst to initiate the transesterification of used cooking oil by pyrolyzing waste cork. The greatest conversion (98%) for the heterogeneous catalyst produced at 600 °C occurred at alcohol:oil ratios of 25:1, catalyst loadings of 1.5% w/v, and temperatures of 65 °C. Sahani et al. ( 2019 ) used a solid-base catalyst called barium cerate in the transesterification procedure to produce biodiesel from Karanja oil. To synthesize perovskite barium cerate with maximum phase purity, the calcination temperature was optimized. At 1.2 wt% catalyst, 1:19 oil-to-methanol molar ratio, 65 °C, 100 min, and 600 rpm, karanja oil methyl ester with 98.3% conversion was obtained. Kamel et al. ( 2019 ) utilized the fig leaves that had undergone calcination, KOH activation, and activation. The highest conversion to biodiesel (92.73%) was obtained from fig leaves treated with KOH under ideal conditions (2 h of heating, a 6:1 alcohol/oil molar ratio, 1% catalyst, and 400 rpm). Singh et al. ( 2023 ) produced biodiesel from Jatropha curcas oil using the transesterification technique and calcium oxide. The results of the experiment demonstrate that at a methanol/oil ratio of 12:1, 65 °C, 3 h, and a catalyst concentration of 5 wt%, a biodiesel yield of 81.6% was produced. Carbon spheres were the heterogeneous acid catalyst that Nata et al. ( 2017 ) utilized. A maximum yield of 87% was achieved at 60 °C and 1 h when WCO was used as the feedstock to make biodiesel utilizing a C–SO 3 H acid catalyst. Du et al. ( 2019 ) converted Scenedesmus quadricauda algal oil into biodiesel using a cobalt-doped CaO catalyst. Cao was obtained from eggshells and calcined at 400, 700, and 900 °C. Todorović et al. ( 2019 ) conducted research on canola oil-based potassium-supported TiO 2 for biodiesel generation. At 55 °C for 5 h, with a 6 wt% catalyst and a 54/1 methanol/oil, the highest biodiesel output of > 90% was discovered. Salinas et al. ( 2012 ) created a carbon-based MgO catalyst for castor oil transesterification utilizing the sol–gel method. With a 96.5% biodiesel output at 6 wt% catalyst loading and a 12:1 ethanol/oil ratio at 75 °C for 1 h, the MgO/UREA-800 demonstrated remarkable catalytic activity. Gardy et al. ( 2019 ) made a strong, magnetic core–shell SO 4 /Mg–Al–FeO 3 heterogeneous catalyst with the use of surface functionalization, encapsulation, and stepwise coprecipitation. Utilizing the synthesized catalyst, the transesterification reaction was carried out with the highest possible yield of 98.5% at 9:1 methanol/WCO, 95 °C, and 5 h. Table ​ Table4 4 highlights some of the recently published research on the use of several heterogeneous catalyst types for biodiesel synthesis, various feedstock sources, experimental setups, and biodiesel yields.

Table 4

Different types of heterogeneous catalysts used for biodiesel synthesis

Enzyme-based catalyst

Enzyme-based catalysts are produced from living things that speed up reactions while maintaining the stability of their composition (Amini et al. 2017 ). Extracellular lipases are the enzymes that have been isolated and processed from the microbial broth. In contrast, intracellular lipase remains inside the cell or in its walls of production (Gog et al. 2012 ). One drawback of employing extracellular enzymes as catalysts is the expense and difficulty of the separation and purification procedures (Rizwanul Fattah et al. 2020 ). The efficiency of the bio-catalyzed transesterification process is influenced by the enzyme’s source and the process variables (Aransiola et al. 2014 ). Enzymatic biodiesel production also has the advantages of being simple to remove, operating at a temperature between 35 and 45 °C, producing no byproducts, and allowing catalysts to be reused (Christopher et al. 2014 ). For the transesterification of low-grade fish oil, Marín-Suárez et al. ( 2019 ) used Novozym 435 lipase; the greatest FAEE yield was 82.91 wt% after 8 h, 35 °C, an excess of ethanol, and 1% catalyst. Novozym 435 can be used for 10 continuous cycles with a maximum activity decrease of 16%. Jayaraman et al. ( 2020 ) studied used cooking oil enzymatic transesterification with the use of pancreatic lipase to make methyl ester. The best reaction conditions were discovered to be methanol as the alcohol 3:1 M ratio, 1.5% enzyme concentration (by weight of WCO), 4 h reaction duration, 60 °C, and 88% yield after numerous attempts. Fatty acid methyl ester (FAME) was produced by Choi et al. ( 2018 ) produced FAME from the oil in rice bran by just adding methanol. The 83.4% yield was reached after 12 days under ideal conditions.

Nanocatalysts in transesterification

Nanocatalysts have garnered significant interest in the production of biodiesel. Because of their special qualities, which include a large active surface area, high reusability, better catalytic efficiency, high biodiesel conversion, and sustainability, nanocatalysts can be superior to conventional catalysts (Qiu et al. 2011 ). Since they are easily removed from the final products and retain their catalytic activity even after being reused several times, nanocatalysts are widely sought (Ahmed et al. 2023 ). There are numerous ways to create nanocatalysts. Among the techniques are microwave combustion, chemical vapor deposition, impregnation, and gas condensation (Quirino et al. 2016 ; Ambat et al. 2018 ). Some of the latest works on nanocatalysts for the transesterification reaction are listed in Table  5 .

Table 5

Various nanocatalysts in biodiesel production

Metal-oxide nanocatalysts

The most popular nanocatalysts are those based on metal oxide, and they play a crucial role in maximizing the synthesis of biodiesel. Nanoparticles that will be employed for transesterification catalysis have been created using the oxidized forms of numerous different metals, including Mg, Zn, and Ca (Pandit et al. 2023 ). Jamil et al. ( 2021 ) created highly efficient barium oxide using catalysts made of molybdenum oxide. Optimal conditions include 12 methanol/oil, 120 min, 65 °C, and a 4.5wt% catalyst. The best yield was achieved under these conditions, which resulted in a 97.8% yield. Sahani et al. ( 2020 ) produced biodiesel with a transesterification reaction involving used cooking oil and a mixed metal oxide catalyst made of Sr–Ti. Methanol as the alcohol in an 11:1 M ratio, 1% catalyst, an 80-min reaction period, and a temperature of 65 °C with 98% FAME conversion were found to be the best reaction conditions. In a study conducted by Tayeb et al. ( 2023 ), the production of biodiesel using a CaO catalyst through the transesterification of WCO was investigated. The study determined the optimal reaction parameters to be a WCO/methanol molar ratio of 1:6, a 1% CaO nanocatalyst, a reaction temperature of 70 °C, and a reaction duration of 85 min, which resulted in a 97% biodiesel yield.

Carbon nanocatalysts

Nanocatalysts are created from carbon materials, including graphene and reduced graphene oxides (Nizami and Rehan 2018 ). Due to their diverse structural, mechanical, thermal, and biocompatibility qualities, carbon nanocatalysts are good catalysts and have advantageous applications in electrocatalytic devices such as fuel cells and other electro-processing systems. CNTs are often manufactured from graphite sheets that have been wound into cylinder forms. They have a large surface area, measure in nanometers, and are incredibly biocompatible (Rai et al. 2016 ).

Zeolite nanocatalysts

Large exterior surface areas and the hydrophobic nature of nanozeolites increase enzyme access to the substrate. Natural zeolite materials are far less frequently used in commercial industries than synthetic-based products. Commercially available synthetic zeolites such as ZSM-5, X, Y, and beta are used primarily in the production of biodiesel (Abukhadra et al. 2019 ). Using zeolites from NaY, KL, and NaZSM-5, Wu et al. ( 2013 ) produced CaO catalysts that were utilized to catalyze the transformation of methanol with soybean oil. In comparison to pure CaO, the activities of synthesized catalysts were studied. It was discovered that after being supported by zeolites, the CaO catalyst’s activity improved, with the CaO/NaY catalyst showing the greatest performance. Using the CaO/NaY catalyst, methanol-to-soybean oil 9:1 molar ratio at 65 °C with a reaction period of 3 h, and a 3% catalyst were used to produce a 95% biodiesel yield. Firouzjaee and Taghizadeh ( 2017 ) synthesized a CaO/NaY-Fe 3 O 4 nano-magnetic catalyst that was employed for the generation of biodiesel. The ideal methanol-to-oil molar ratio is 8.78, the catalyst loading is 5.19% (30% CaO loaded on the surface nanomagnetic zeolite), and the reaction period is 4 h. The maximum methyl esters obtained are 95.37%.

Distinct behavior of nanocatalysts during biodiesel production

Nanocatalysts have been widely used in biodiesel production due to their high catalytic activity, low cost, and environmental friendliness. The properties of nanocatalysts can vary depending on the preparation method, which can affect their catalytic performance. For example, the size, shape, and surface area of the catalyst particles can influence the reaction kinetics and yield of biodiesel. Recent studies have investigated the effects of different preparation methods on the properties of nanocatalysts for biodiesel production. The preparation method and calcination temperature are important factors that can affect the properties and catalytic performance of nanocatalysts for biodiesel production. Further research is needed to optimize the preparation methods and properties of nanocatalysts to improve the efficiency and sustainability of biodiesel production. We can offer general insights into the variations of nanocatalysts throughout the biodiesel production process, focusing on the following aspects.

Catalyst types: Different generations of nanocatalysts may involve distinct types of materials. For instance, first-generation nanocatalysts might include basic materials, while second- or third-generation may involve more advanced materials like metal oxides, zeolites, or other nanostructured materials.

Particle size: Advances in nanotechnology enable the control of particle size in nanocatalysts. The particle size can significantly impact catalytic activity. Smaller particle sizes may provide larger surface areas and enhanced catalytic efficiency.

Functionalization: The functionalization of nanocatalysts with specific groups or ligands can vary across generations. Functionalization can influence the catalyst’s selectivity and stability during biodiesel production.

Reusability and stability: Reusability and recovery are the two main advantages of using heterogeneous nanocatalysts in the production of biodiesel. The nanocatalyst is recovered and utilized again at each stage of these processes, which include many cycles of producing biodiesel. Nanocatalysts are often recovered via chemical means. The intended product and any byproduct may be easily and quickly recovered from the reaction mixture thanks to heterogeneous catalysts. This type of catalyst eliminates the need for a washing step. The esterification method using nanocatalysts was proposed to have several benefits, including speedier mixing of the reactants and catalyst and easy and rapid separation from the reaction mixture (Pandit et al. 2023 ).

Synthesis methods: The methods used to synthesize nanocatalysts may evolve, affecting their structure and properties. Recent advancements might include greener synthesis approaches or techniques that enhance the reproducibility of catalysts.

In addition to the aspects mentioned, the surface chemistry of nanocatalysts can also vary across generations, affecting their catalytic behavior during biodiesel production. The surface chemistry of nanocatalysts can be modified through various methods, such as surface functionalization, doping, or coating, to tune their catalytic activity, selectivity, and stability. For instance, surface functionalization with organic molecules or inorganic ions can enhance the catalyst’s selectivity for specific reactions or improve its compatibility with the reaction medium. The use of nanocatalysts in biodiesel production also presents some challenges, such as the aggregation, fouling, and leaching of active species. These issues can lead to a decrease in catalytic activity and selectivity, as well as an increase in production costs. To address these challenges, researchers are exploring various strategies, such as surface modification, stabilization techniques, and immobilization methods, to improve the stability and reusability of nanocatalysts. In summary, the distinct behavior of nanocatalysts during biodiesel production is influenced by various factors, including catalyst type, particle size, functionalization, surface chemistry, synthesis methods, and stability. The optimization of these factors can lead to more efficient, selective, and sustainable biodiesel production processes. However, further research is needed to fully understand the underlying mechanisms and to develop new generations of nanocatalysts with enhanced performance and stability.

Transesterification reaction mechanism (alcoholysis)

A large variety of exchange reactions involving oils, fats, and other reactants may be explained by the reaction mechanism. This comprises three processes: (1) transesterification, a rearrangement that yields monoglyceride, diglyceride, or other esters; (2) acidolysis, which involves exchanging fatty acids to produce specific fatty acid products; and (3) alcoholysis, which produces methyl esters in reactions with monohydric alcohols and monyl glycerol in reactions with polyhydric alcohols. Natural vegetable oils, animal fats, and food industry waste oil may all be utilized as source materials for transesterification, a process that produces biodiesel. Methanol, ethanol, propanol, butanol, and pentanol are among the alcohols that can be utilized for transesterification. Because it is a cheap, short-chain, strong polar raw material that reacts rapidly with fatty acid glycerides, methanol is the most widely used of them. Also freely soluble in methanol are base catalysts. A catalytic agent in this reaction might be an acid, base, or enzyme. Base catalysts consist of carbonate, NaOH, KOH, and potassium and sodium alkaloids. Acid catalysts might be hydrochloric, phosphoric, or sulfuric acids. The enzyme lipase is a good catalyst for the esterification of alcohols to fatty acid glycerides. Figures  7 , ​ ,8, 8 , ​ ,9, 9 , and ​ and10 10 represent continuous reversible processes for transesterification reactions; every reaction yields a distinct type of alcohol (Kang et al. 2015 ; Sait et al.  2022 ; Li et al. 2020 ; Oyekunle et al. 2023 ).

Continuous reversible processes of transesterification reactions

Acid-catalyzed alcoholysis reaction mechanism

Base-catalyzed alcoholysis reaction mechanism

Enzyme-catalyzed alcoholysis reaction mechanism

Kinetic modeling’s significance for process optimization

Kinetic models of chemical processes are powerful tools for reactor design. The kinetic models are very helpful in choosing the best reaction conditions (temperature, pressure, mixing rate, etc.) for chemical or biochemical transformations in reactors or bioreactors. This maximizes the formation of desired products with the least material investment and financial resources. This also holds true for the many techniques used to produce biodiesel, such as homogeneous, heterogeneous, enzyme catalysis, and others. One of the most important stages in the development of chemical processes for industrial applications is thought to be carefully thought-out experimental research and the subsequent creation of a kinetic model (Trejo-Zárraga et al. 2018 ). Portha et al. ( 2012 ) were able to decrease the extra ethanol used in the transesterification reaction in a continuous mode. By adjusting the temperature of the second reactor and adding methanol in stages, they were able to enhance the system’s overall performance, as demonstrated by the results of their simulation. Using triolein as a model chemical, the authors conducted experiments and discovered that it was beneficial to convert diglyceride and monoglyceride in the second reactor and the majority of triolein in the first. Additionally, their calculations suggested that to improve reaction rates at this point, it would be prudent to raise the temperature in the second reactor. Additionally, the authors computed internal concentration profiles using a reactor model that included the kinetic model. They discovered the limiting phenomenon in the overall transformation. To get a deeper comprehension of the rates of output and the inhibitory patterns seen in the transformation scheme, a kinetic model may also be strategically employed (Firdaus et al. 2016 ). For instance, a reaction scheme for the enzymatic creation of biodiesel might consider many more reaction stages and, consequently, a greater number of parameters. This adds difficulty to the kinetic model creation process, but once this model is solved, it may be utilized to construct an enzyme-catalyzed reactor and eventually optimize the process. The use of kinetic models, which can faithfully replicate the process at various reaction conditions, is helpful in the field of research and process improvement as it offers guidelines for additional experimental work and helps eliminate potentially fruitless experimental trials. Additionally, models may be utilized to foresee how composition will affect the final product’s quality. A model might forecast, for instance, how the feedstock’s water content or FFA may impact the reaction conversion and, in turn, the biodiesel’s production and quality.

Kinetics of transesterification

Few studies have dealt with kinetic modeling; most of the heterogeneous catalysis research has been on the manufacture and utilization of catalysts. To achieve reaction conditions with inherent kinetics and minimal effects, efforts have been focused on using tiny solid particles. It has been discovered that most heterogeneous transesterifications adhere to a pseudo-first-order model. For instance, Kaur and Ali ( 2014 ) discovered that the ethanolysis of Jatropha curcas L. oil, which were catalyzed by 15-Zr/CaO-700, adhered to a pseudo-first-order rate law. The Koros-Nowak test proved that the transit impacts were insignificant. Lukić et al. ( 2014 ) also discovered a first-order reversible rate law under ideal circumstances for the transesterification of sunflower oil. Table ​ Table6 6 lists some kinetic modeling studies of heterogeneous transesterification.

Table 6

List of some kinetic modeling studies of heterogeneous transesterification

Characterization methods for the assessment of produced biodiesels

Characterization methods for the assessment of produced biodiesel include various analytical techniques to evaluate the quality and properties of biodiesel. These methods are essential for ensuring that biodiesel meets the required standards and specifications for use as a sustainable and efficient alternative fuel source. The American Society for Testing and Materials (ASTM) is a prominent organization that provides authoritative guidelines for biodiesel testing and characterization methods.

The most common characterization methods for assessing produced biodiesel include the following.

Fatty acid methyl ester (FAME) analysis: FAME analysis is a fundamental method for biodiesel characterization, involving the determination of the fatty acid methyl ester content in biodiesel. This analysis is typically performed using gas chromatography (GC) or high-performance liquid chromatography (HPLC) to quantify individual FAME components, which provides valuable information about the biodiesel’s composition and purity.

Viscosity measurement: Viscosity is a crucial parameter for biodiesel quality assessment, as it affects the flow behavior and performance of the fuel. Dynamic viscosity measurements are commonly conducted to determine the resistance of biodiesel to flow under specific conditions, offering insights into its suitability for use in engines and transportation applications.

Oxidation stability testing: Biodiesel’s resistance to oxidation is an important characteristic that influences its shelf life and storage stability. Various methods, such as the Rancimat test and the PetroOXY test, are employed to assess the oxidation stability of biodiesel by measuring its susceptibility to oxidative degradation over time.

Cold flow properties analysis: The cold flow properties of biodiesel, including cloud point and pour point, are critical factors affecting its performance in cold weather conditions. Characterization methods such as differential scanning calorimetry (DSC) and automated cloud and pour point analyzers are utilized to determine these properties, ensuring that biodiesel remains operational at low temperatures.

Acid value determination: The acid value of biodiesel indicates its acidity level, which can impact engine components and fuel system integrity. Acid value determination involves titration methods to quantify the amount of free fatty acids present in biodiesel, enabling the assessment of its corrosiveness and potential impact on engine performance.

Calorific value measurement: Calorific value, also known as heating value, represents the energy content of biodiesel and is crucial for evaluating its combustion efficiency and heat output. Bomb calorimetry is commonly used to measure the calorific value of biodiesel, providing essential data for assessing its energy potential as a fuel source.

Sulfur content analysis: Sulfur content determination is essential for ensuring compliance with environmental regulations and assessing the environmental impact of biodiesel combustion. Techniques such as X-ray fluorescence (XRF) spectroscopy or ultraviolet fluorescence analysis are employed to measure sulfur levels in biodiesel samples.

Glycerol content quantification: Glycerol content in biodiesel must be monitored to ensure compliance with quality standards and prevent potential issues related to fuel stability and engine performance. Analytical methods like gas chromatography coupled with flame ionization detection (GC-FID) are utilized for the accurate quantification of glycerol in biodiesel products.

These characterization methods collectively provide comprehensive insights into the chemical composition, physical properties, stability, and environmental impact of produced biodiesel, supporting quality control measures and regulatory compliance within the biofuel industry.

The recent development in biodiesel production

Biodiesel, a renewable and sustainable alternative to conventional diesel fuel, has seen significant developments in recent years. These advancements have focused on improving the efficiency of biodiesel production processes, expanding feedstock options, and enhancing the overall sustainability of biodiesel as a viable energy source. One notable recent development is the use of advanced catalysts in biodiesel production. Catalysts play a crucial role in the conversion of vegetable oils or animal fats into biodiesel through a process called transesterification. Researchers have been exploring various catalysts, such as solid acid catalysts, enzyme catalysts, and heterogeneous catalysts, to improve reaction rates, reduce energy consumption, and enhance biodiesel quality. These catalysts offer advantages like higher conversion rates, milder reaction conditions, and easier separation of the catalyst from the product (Garcia-Silvera et al. 2023 ). Another significant development is the utilization of non-traditional feedstocks for biodiesel production. While conventional biodiesel feedstocks include soybean oil and rapeseed oil, researchers have been investigating alternative sources such as algae, waste cooking oil, and non-food crops like jatropha and camelina. Algae have gained attention due to their high oil content and ability to grow in various environments. The use of non-traditional feedstocks helps to reduce competition with food production and enhances the overall sustainability of biodiesel (Garg et al. 2023 ). Furthermore, efforts have been made to improve the sustainability of biodiesel production by reducing its environmental impact. This includes optimizing production processes to minimize water and energy consumption, reducing greenhouse gas emissions, and implementing waste management strategies. Additionally, researchers have been exploring the concept of “second-generation” biodiesel, which involves utilizing waste materials, such as agricultural residues and lignocellulosic biomass, to produce biodiesel. This approach not only reduces waste but also maximizes resource utilization (Makepa et al. 2023 ).

Biodiesel engine performance and emissions

Compared to petrodiesel fuel, burning biodiesel releases fewer particulates, carbon monoxide, and unburned hydrocarbons. Since biodiesel is produced using natural resources, its sulfur content is relatively low, which means that when it burns in an engine, it releases less sulfur dioxide into the atmosphere (Rayati et al. 2020 ). All biodiesels and their blends have shown the capacity to enhance gas turbine performance while lowering emissions of carbon dioxide, carbon monoxide, nitrogen oxide, and hydrocarbons under a range of operating conditions. To employ fuels in an engine, one must be aware of their characteristics for combustion. Although fossil fuel-based diesel fuel may not be entirely replaced by biodiesel, it can aid in achieving balanced energy utilization. One benefit is that biodiesel may be used in contemporary engines with little modification. Older vehicles with natural rubber gasoline lines, however, require a few modifications. Rubber fuel lines must be replaced since they will crack when used with biodiesel. On the other hand, an oil or gasoline dilution in the fuel system is possible in a modern vehicle with a DPF (diesel particulate filter). The ability of gasoline to lubricate the fuel injection system is believed to be crucial for diesel engines. The use of diesel–biodiesel mixes can thereby enhance their general lubricity. Additionally, the lower sulfur level of today’s diesel fuel could affect its lubricity because the compounds that provided lubrication are no longer present (Veza et al. 2022 ).

Techno-economic analysis

Techno-economic analysis (TEA) plays a crucial role in assessing the economic feasibility and viability of biodiesel production processes. It involves evaluating the overall costs, revenues, and profitability of biodiesel production, considering various factors such as feedstock costs, capital investment, operational expenses, and market prices. Recent studies have employed TEA to analyze and optimize biodiesel production processes, providing valuable insights for decision-making and process design. One example of TEA in biodiesel production is a study conducted by Zhang ( 2021 ), which evaluated the techno-economic performance of different feedstocks and process configurations for biodiesel production. The analysis considered factors such as feedstock availability, conversion efficiency, capital costs, operating costs, and market prices. The study highlighted the importance of feedstock selection and process optimization in achieving cost-effective biodiesel production. Another study by Tasić ( 2020 ) performed TEA for manufacturing biodiesel from used cooking oil. The analysis included the estimation of capital and operational costs, energy consumption, and environmental impacts. The study demonstrated the economic feasibility of waste cooking oil-based biodiesel production and identified critical parameters affecting the overall economics of the process. Furthermore, a study by Atabani ( 2020 ) conducted TEA for biodiesel production from microalgae. The analysis considered various scenarios, including different cultivation systems and conversion technologies. The study assessed the economic viability of microalgae-based biodiesel production, considering factors such as biomass productivity, lipid content, capital investment, and operational costs. These recent studies emphasize the importance of TEA in evaluating the economic aspects of biodiesel production. By considering a comprehensive range of factors, TEA provides valuable insights into the cost-effectiveness, profitability, and sustainability of biodiesel production processes, helping guide decision-making and process optimization.

Challenges, perspectives, and further research

The homogeneous catalyst has been thoroughly examined, and the literature has addressed several issues. However, heterogeneous catalysts are a very new field of study, and there is now a lot of research being done in this area. The literature has documented many obstacles regarding these catalysts:

• It has been said that the primary issues with heterogeneous catalysts are instability, reduced reaction rate, and short catalyst life.
• It has been reported that solid-base catalysts are FFA, CO 2 , and water sensitive. Through saponification, they destroy the catalyst and render it inactive.
• Because water hydrolyzes the ionic group in solid acid catalysts, leaching and product contamination have been documented.
• It has been documented that during enzymatic transesterification, methanol causes lipase inhibition.
• In the case of nanocatalysts, good performance requires increasing the reaction time under very moderate working conditions. To attain normal reaction times, however, harsh working conditions must be used, which raises energy consumption.

Future research should pay attention to the following recommendations:

• To create novel catalysts with enhanced catalytic performance, more research into waste-derived catalysts is required.
• The creation of highly selective and active heterogeneous catalysts that may be used in industrial settings at a reasonable cost.
• Investigating novel catalyst supports with a network of linked pores of the right size and a selected surface area.
• Investigating the use of waste or biomass as a catalyst source to lower related costs and increase sustainability for solid catalysts that are sold commercially.
• Improving synthesis catalyst production procedures and treatment stages to move the technology from a lab to an industrial setting.
• Maintaining the high basic strength of the synthesis catalyst while improving the shape and sensitivity to FFA and water.
• Additional research is needed into the manufacturing of industrial enzymatic biodiesel as a guaranteed choice for the future.
• To give relevant information about the suitable catalyst, future research on the catalytic mechanism of partial catalysts should examine the features in depth.
• Ways to recover and reuse nanocatalysts that are both economical and energy efficient.
• Kinetic studies of the biodiesel production reaction using the synthesized catalyst should be carried out.

This extensive review delves into the various aspects of biodiesel production and its promise as a sustainable alternative for a greener energy future. The significance of feedstock selection and preparation is emphasized, with effective techniques discussed for optimizing biodiesel production efficiency and quality. Biodiesel has emerged as a versatile and promising alternative for transportation, industrial processes, and energy generation, demonstrating its potential to reduce greenhouse gas emissions and dependency on fossil fuels. The key process of transesterification is thoroughly examined, encompassing the utilization of diverse catalysts, including homogeneous, heterogeneous, enzyme based, and nanomaterials. The unique characteristics and performance of nanomaterials in transesterification are highlighted, offering prospects for enhanced efficiency and selectivity. Understanding the reaction mechanism and kinetics of transesterification is crucial for optimizing the production process. Kinetic modeling is identified as a valuable tool for process optimization, enabling better control and improved production efficiency. Methods for assessing the quality and properties of produced biodiesel are discussed, highlighting the importance of accurate characterization to meet quality standards and ensure compatibility with engine systems. Recent developments in biodiesel production showcase progress in feedstock selection, process optimization, and sustainability. However, challenges related to engine performance, emissions, and compatibility remain obstacles to wider biodiesel adoption. Future research should focus on addressing these challenges through innovative engine technologies, improved fuel formulations, and effective emission control strategies. Techno-economic analysis provides insights into the economic feasibility of biodiesel production, considering factors such as feedstock costs, process efficiency, and market demand. Ongoing analysis and assessment are essential for ensuring the commercial viability and scalability of biodiesel production. In conclusion, biodiesel presents a promising sustainable solution, but its advancement requires continuous research, development, and collaboration among academia, industry, and policymakers. Addressing challenges, pursuing further research, and implementing the recommendations outlined in this review will contribute to the widespread adoption of biodiesel as a renewable energy source, paving the way for a cleaner and more sustainable future.

Author contribution

All authors contributed to the study conception and design. Data collection and analysis were performed by Sabah Mohamed Farouk, Aghareed M. Tayeb, Shereen M. S. Abdel-Hamid, and Randa M. Osman.

Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

Data Availability

Declarations.

Not applicable.

The authors declare no competing interests.

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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RSM-based comparative experimental study of sustainable biodiesel synthesis from different 2G feedstocks using magnetic nanocatalyst CaFe 2 O 4

• Published: 22 December 2022
• Volume 26 , pages 3097–3126, ( 2024 )

Cite this article

• A. Saravanan 1 ,
• Ajith J. Kings 2 ,
• L. R. Monisha Miriam 3 &
• R. S. Rimal Isaac 4  

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Today’s demand of energy in the world of automobile provokes the researchers to strive for the easily available and cheapest renewable source of energy. Biodiesel has become one of the booming renewable sources in the world to mitigate the atmospheric pollution and the demand of fossil fuels. Oils are chosen based on their fatty acid content, availability and sustainability. A magnetic nanocatalyst CaFe 2 O 4 has been employed in the transesterification process and is characterized by various progressive techniques to confirm its compatibility. The locally available, nonedible oils such as cotton seed oil, rubber seed oil and pungai seed oil have been taken for this experimental work for efficient and sustainable biodiesel production. Multi-variant central composite design has been employed to enhance the influencing process parameters in biodiesel conversion. Each feedstock produced more than 95% of the yield which consumed very little amount of methanol and catalyst in a short period of time. In order to ensure a quick reaction and smooth stirring, the temperature is kept at 70 °C (beyond the boiling point of the solvent). The chromatography analysis was used to describe the end product samples which revealed the right proportion of saturated and unsaturated fatty acids at the proper level, resulting in better oxidation stability and combustion properties. Moreover, density, viscosity, cetene number, iodine value and other essential properties were analysed and found to be within the standards specified by EN and ASTM for use in automotive applications without modifying the engine.

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Data availability

All the figures and tables contain original experimental data which are obtained from St. Xavier’s Catholic College of Engineering, Nagercoil, and Government Polytechnic College, Vanavasi, Noorul Islam centre for Higher Education, Kumaracoil, and Manonmaniam Sundaranar University, Tirunelveli, Tamil Nadu, India.

Abbreviations

Calcium ferrite

Fourier transform infrared spectroscopy

X-ray powder diffraction

Thermogravimetric/differential thermal analyser

Brunauer–Emmett–Teller analysis

Scanning electron microscope

Atomic force microscopy

Central composite design

Fatty acid methyl ester (%)

Potassium hydroxide

Sodium hydroxide

Calcium oxide

Magnesium oxide

Strontiun oxide

Manganese (II) oxide

Molybdenum oxide

Zirconium dioxide

Iron(II,III) oxide

Response surface methodology

Analysis of variance

Gas chromatography mass spectrometry

Rubber seed oil

Cotton seed oil

Pungai seed oil

Free fatty acid (%)

Unsaponified matter (%)

Joint committee on powder diffraction standards

Density functional theory

Rubber seed oil biodiesel

Cotton seed oil biodiesel

Pungai seed oil biodiesel

Aminopropyl triethoxysilane magnetite nanoparticles

Calcium oxide/gold

Magnesium ferrite

Magnesium aluminium oxide

Titanium oxide

Nanoparticles

American Society for Testing and Materials

European Standards

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Saravanan, A., Kings, A.J., Miriam, L.R.M. et al. RSM-based comparative experimental study of sustainable biodiesel synthesis from different 2G feedstocks using magnetic nanocatalyst CaFe 2 O 4 . Environ Dev Sustain 26 , 3097–3126 (2024). https://doi.org/10.1007/s10668-022-02761-1

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Retrofit of a marine engine to dual-fuel methane–diesel: experimental analysis of performance and exhaust emission with continuous and phased methane injection systems.

1. Introduction

2. materials and methods, 3.1. comparison between continuous and phased ng injection systems, 3.2. effect of diesel injection strategy on combustion and emissions, 3.2.1. effects of diesel injection strategy on df combustion process and fuel consumption, 3.2.2. effect of diesel injection strategy on df exhaust emissions, 3.3. effect of diesel injection pressure on combustion and emissions, 3.3.1. effects of diesel injection pressure on df combustion process and fuel consumption, 3.3.2. effect of diesel injection pressure on df exhaust emissions, 3.4. evaluation of optimum point compared to diesel references, 4. conclusions.

• Phased injection resulted in a reduction in fuel consumption, compared to continuous mode, of between 8% and 11% for the single-injection strategy; similarly, using SOGAV results in a fuel consumption reductions from 7% to 9.7% and 5.4% to 7.2% in the cases of the double and triple-injection strategies, respectively.
• Even advancing the diesel SOI main, the DF combustion centre (MFB50) is about 8 CAD later than in the FD reference case, due to the difference between diffusive FD and premixed DF combustion mechanisms. A slight improvement is obtained by adopting multi-injection strategies that reduce the induction time.
• Despite the difference in MFB10 and MFB50, their combustion duration (2xMFB 10–50 ) curves are almost overlapped, indicating a faster NG combustion in the late stage in the case of single injection.
• At 100 MPa diesel injection pressure, independent of injection strategies, minimum fuel consumption was achieved between −35 and −30 CAD ATDC, with improvements of between 14.0 and 14.5% compared to the FD reference.
• At 100 MPa diesel injection pressure, DF NO x emissions were consistently higher than the FD reference case, ranging between 12.5 and 17.2 g/kWh, with increases of 20.3% and 65% compared to the FD reference case (10.4 g/kWh), respectively.
• DF HC emissions were an order of magnitude higher than in the diesel case due to the different combustion mechanism. A slight reduction in HC emissions was found at the most advanced SOI main , independent of the injection strategy.
• DF engines can significantly reduce smoke emissions. Compared to the FD reference (1.43%), at the most advanced diesel SOI main , DF test points exhibited minimal levels close to zero, independent of the injection strategy.
• Increasing diesel injection pressure reduces the DF combustion duration: at 80 MPa injection pressure, DF combustion duration was higher than in the FD reference case; at 120 MPa, this behaviour was inverted.
• Despite the improvements in the early stage of combustion, which were linked to the better atomization of diesel spray, and consequently triggering the combustion of NG, increasing injection pressure did not result in a corresponding reduction in fuel consumption, with the exception of the single-injection strategy. This behaviour can be ascribed to a too-short duration of the main injection, when pressure is increased, which deteriorated the NG combustion stability during the early stage of combustion. The minimum BSFCeq was 219.1 g Diesel_eq /kWh, achieved with triple injection at 80 MPa injection pressure, with a reduction of about 20.7% compared to the FD reference (276.3 g Diesel_eq /kWh).
• Higher injection pressures corresponded to an increase in NO x , while HC and CO were reduced.
• Even though DF combustion provided a reduction in BSFC, with a consistent reduction in CO 2 emissions of up to 28.5%, the reduction in GWP was only 2.9%, due to the increase in HC emissions. This is still a promising result, considering that the optimization of DF combustion systems can significantly reduce the emission of methane hydrocarbons in the exhaust, thus making more effective the reduction in CO 2 in terms of GHG emissions.

Author Contributions

Data availability statement, acknowledgments, conflicts of interest, abbreviations.

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Marchitto, L.; De Simio, L.; Iannaccone, S.; Pennino, V.; Altieri, N. Retrofit of a Marine Engine to Dual-Fuel Methane–Diesel: Experimental Analysis of Performance and Exhaust Emission with Continuous and Phased Methane Injection Systems. Energies 2024 , 17 , 4304. https://doi.org/10.3390/en17174304

Marchitto L, De Simio L, Iannaccone S, Pennino V, Altieri N. Retrofit of a Marine Engine to Dual-Fuel Methane–Diesel: Experimental Analysis of Performance and Exhaust Emission with Continuous and Phased Methane Injection Systems. Energies . 2024; 17(17):4304. https://doi.org/10.3390/en17174304

Marchitto, Luca, Luigi De Simio, Sabato Iannaccone, Vincenzo Pennino, and Nunzio Altieri. 2024. "Retrofit of a Marine Engine to Dual-Fuel Methane–Diesel: Experimental Analysis of Performance and Exhaust Emission with Continuous and Phased Methane Injection Systems" Energies 17, no. 17: 4304. https://doi.org/10.3390/en17174304

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• Published: 24 August 2024

Green synthesis of trimetallic CuO/Ag/ZnO nanocomposite using Ziziphus spina-christi plant extract: characterization, statistically experimental designs, and antimicrobial assessment

• Ayman K. El-Sawaf 1 , 2 ,
• Shahira H. El-Moslamy 3 ,
• Elbadawy A. Kamoun 4 &
• Kaizar Hossain 5  

Scientific Reports volume  14 , Article number:  19718 ( 2024 ) Cite this article

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• Environmental sciences
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• Nanoscience and technology

In this study, Ziziphus spina christi leaves was used to synthesize a trimetallic CuO/Ag/ZnO nanocomposite by a simple and green method. Many characterizations e.g. FTIR, UV–vis DRS, SEM–EDX, TEM, XRD, zeta-size analysis, and DLS, were used to confirm green-synthesized trimetallic CuO/Ag/ZnO nanocomposite. The green, synthesized trimetallic CuO/Ag/ZnO nanocomposite exhibited a spherical dot-like structure, with an average particle size of around 7.11 ± 0.67 nm and a zeta potential of 21.5 mV. An extremely homogeneous distribution of signals, including O (79.25%), Cu (13.78%), Zn (4.42%), and Ag (2.55%), is evident on the surface of green-synthetic nanocomposite, according to EDX data. To the best of our knowledge, this is the first study to effectively use an industrially produced green trimetallic CuO/Ag/ZnO nanocomposite as a potent antimicrobial agent by employing different statistically experimental designs. The highest yield of green synthetic trimetallic CuO/Ag/ZnO nanocomposite was (1.65 mg/mL), which was enhanced by 1.85 and 5.7 times; respectively, by using the Taguchi approach in comparison to the Plackett–Burman strategy and basal condition. A variety of assays techniques were utilized to evaluate the antimicrobial capabilities of the green-synthesized trimetallic CuO/Ag/ZnO nanocomposite at a 200 µg/mL concentration against multidrug-resistant human pathogens. After a 36-h period, the tested 200 µg/mL of the green-synthetic trimetallic CuO/Ag/ZnO nanocomposite effectively reduced the planktonic viable counts of the studied bacteria, Escherichia coli and Staphylococcus aureus , which showed the highest percentage of biofilm reduction (98.06 ± 0.93 and 97.47 ± 0.65%; respectively).

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Introduction.

Nanotechnology is an area of technology that studies, applies, and develops materials at the nanoscopic scale, which typically ranges from 1 to 100 nm 1 . To manufacture nanoparticles, chemical or physical processes are commonly employed. Nevertheless, both processes are challenging to scale up and require large amounts of energy, in addition to potentially dangerous substances 2 . These hazardous substances persist at the interface between nanomaterials, affecting the biocompatibility of nanomaterials. Biological biosynthesis is therefore a well-known solution for these widely used approaches 3 . The green synthesis technique has effectively utilized a wide range of candidates, including bacteria, fungi, algae, and plants. Metallic nanomaterials can be produced by reducing metal ions with the help of the bioactive compounds found in the extracts of these candidates. Because of their large surface-to-volume ratio, these nanomaterials are widely used in many different engineering and materials science domains, including medical, optical, biotechnological, microbiological, electronics, and environmental 4 .

Several areas in Egypt are home to the common medicinal plants Mentha spp., Ziziphus spina-christi, and Ocimum basilicum, which are highly valued for their anti-inflammatory, antioxidant, antibacterial, and anticancer properties 1 , 2 , 3 . Ziziphus spina-christi , a member of the Rhamnaceae family, is commonly referred to as Sidr. Together with a number of recently introduced exotic plants, it is an important cultivated tree and one of the few surviving natural tree species in Arabia. The genus Ziziphus is well-known for its therapeutic uses as an immune system booster, hypotensive, anti-inflammatory, antimicrobial, antioxidant, and liver-protective agent. Furthermore, there have been reports that the Z. spina-christi extract protects against aflatoxicosis. Mentha spp., also known as mint (genus Mentha , family Lamiaceae ), is widely used as a spice component for many different types of food worldwide and is also regularly used to make herbal tea. Because the essential oils of mint include antibacterial and antioxidant qualities, it is widely known that the leaves of the plant are still employed in traditional medicine to treat digestive problems. According to phytochemical analyses of mentha plants, the main components of the leaf extracts that reduced and stabilized nanoparticles were phenolic, flavonoid, steroid, and terpenoid 8 , 9 , 10 . The herb sweet basil ( Ocimum basilicum ) has small, pink-tinged, or white blooms and elliptic, bright green leaves, it also has a strong scent.

Many bioactive molecules, including flavonoids, alkaloids, phenolic compounds, sterols, saponins, tannins, and fatty acids, are rich in different plant-based extracts 11 . In the green synthesis of nanoparticles, these molecules act as reductants, capping agents, and stabilizing agents, keeping the resultant nanoparticles stable and preventing them from aggregating completely 12 . According to a previously published study, the crude extract of Ziziphus-Spina Christi leaves (Sider) was utilized in the green production of zinc oxide nanoparticles (38.177 nm) at the hexagonal wurtzite phase 6 . Graphene oxide was phyto-reduced in the other study employing various doses of Ziziphus spina-christi aqueous extract 13 . Furthermore, other prior investigations reported a fast and safe synthesis of selenium-doped zinc oxide nanoparticles (50 nm) in spherical shape utilizing aqueous leaf extract ( Mangifera indica ), which showed strong antimicrobial properties 14 . The green synthesis of a safe, stable, and trimetallic nanocomposite containing Cu, Ag, and Zn was achieved previously using an aqueous leaf extract of Catharanthus roseus 15 . Previously, Ocimum basilicum L. seed extract was used to generate an Ag-doped ZnO-MgO-CaO nanocomposite in a manner that is environmentally friendly 16 . Furthermore, MgO and CuO/MgO nanoparticles were produced via a green approach using an extract from the Opuntia monacantha plant 17 . Moreover, a content-based extract from the plant powder ( Ocimum basilicum ) was used in a green method for the biological synthesis of MnO 2 nanoparticles and MnO 2 @eggshell nanocomposite 18 . Also, Calotropis gigantea leaf extract was used to generate the green-manufactured binary ZnO-CuO nanocomposites, which show promising antimicrobial properties against skin-related infections 11 . Meanwhile, silver nanoparticles (Ag NPs) that are cost-effective and environmentally friendly were produced using the aqueous extract of Ziziphus spina-christi leaves for treating Fusarium wilt disease 19 . Besides, Poly(HEMA-co-FAOEME)/ZnO nanocomposites were generated as an antimicrobial agent by biosynthesizing ZnO nanoparticles using Mentha plegium L. extract 20 . ZnO, MgO, CuO, and their composite mixed oxide nanoparticles were previously manufactured utilizing a green approach and leaf extracts of medicinal plants e.g. Pisonia grandis R.Br. 21 . The antimicrobial and pro-healing abilities of silver, copper, and zinc oxide nanoparticles are widely recognized. Because of their photo-oxidizing and photo-catalytic effects on biological species, these nanoparticles are safe and biocompatible nanomaterials 22 .

To prove their antimicrobial properties, these nanoparticles may interact chemically as well as physically. As a result of these nanomaterials' interactions with microbial cells, reactive oxygen species (ROS), H 2 O 2 , and ions are released under photoinduced conditions 23 . Conversely, depending on the examined nanomaterials, physical interaction may exhibit biocidal impacts through cellular internalization, breakdown of the cell membrane, or forceful damage 24 . Furthermore, it has been suggested that ZnO-Ag NCs have the strongest capacity to break down microbial cell membranes and interact with vital DNA elements such as phosphorus and sulfur, inhibiting DNA replication 23 , 25 , 26 . The higher specific surface area to volume ratio of the nanoparticles led to the formation of more ROS, which is dependent on binding and interacting with the cell membrane and aggregating in the lipid layer 23 . The negative charge of super oxides and hydroxide ions allows them to enter microbial cells. Eventually, this may result in the cell wall breaking down, releasing its contents, and finally causing cell death 27 . Damaged electrostatic interactions lead to the mortality of pathogens when the negative charge on the surface of the cell membrane catches the positive charge on emitted ions from nanomaterials 26 , 28 . Our results can be explained by the green, synthesized trimetallic CuO/Ag/ZnO nanocomposite's high infusibility and ability to generate more released ions e.g. (Ag +1 , Zn +2 , and Cu +2 ions) 23 . Additionally, the released ions penetrated the host cell by binding to surface proteins on the cell wall. Afterward, the microbe's cells died as a result of the altered metabolism 29 .

As environmental problems throughout the world become more urgent, scientists are looking into the possibility of using nanomaterials to address these problems. Scientists have recently focused their attention on nanomaterials and nanocomposites developed from plant extracts. Our group was drawn to the trend of using plant extracts to produce nanocomposites on a large scale. When compared to alternative fabrication methods, biosynthesized nanoparticles are less costly, non-toxic, and very stable. Biomolecules produced by plants are extensive and can be used to generate nanomaterials for a variety of biological applications.

To the best of our knowledge, no previous reports of a trimetallic CuO/Ag/ZnO nanocomposite that was produced environmentally employing a Ziziphus spina christi leaf extract, have been published to date. Thus, different experimental designs, such as the Plackett–Burman and Genichi Taguchi procedures, were also targeted in this work to commercially optimize green-synthesized trimetallic CuO/Ag/ZnO nanocomposite as a strong antimicrobial ingredient.

Materials and methods

A variety of human pathogens, such as Escherichia coli (ATCC 10536), Klebsiella pneumoniae (ATCC 10031), Staphylococcus aureus (ATCC 25923), Bacillus subtilis (ATCC 11774), Candida albicans (ATCC 10231), and Candida krusei (ATCC 6258), were used to assess the antimicrobial efficacy of the green synesthetic nanocomposites. All human pathogens were received from GEBRI, SRTA-City, Alexandria, Egypt. Fresh leaves of Mentha, Ocimum basilicum, and Ziziphus spina hristi were collected from the New Borg Al-Arab City farms in Alexandria, Egypt.

Preparation of Ziziphus spina-christi plant extract

Three well-known herbal plants i.e. ( Mentha, Ziziphus spina-christi), and (Ocimum basilicum) were gathered locally for this study. Greenish-yellow leaves were collected from these plants and washed twice under running water before being thoroughly cleansed with distilled water to remove any extra remaining dirt. After carefully cleaning every leaf with a white cloth, the leaves were allowed to air dry for three hours. It was ground into a fine powder after dried for 72 h at 60 °C. Finally, 10 g of dry powder and 100 mL of double-distilled water were added to a 250-mL Erlenmeyer flask and shaken at 200 rpm at 70 °C for 30 min to extract the components. To eliminate any last bits of tiny plant debris from the recovered material, centrifugation at 6000 rpm revolutions per minute was performed using Whatman No. 1 filter paper. The filtered extract was stored at 4 °C for future experimental use 16 , 30 , 31 .

Quantification of main leaf extract constituents

With a minor modification, the Folin-Ciocalteu method was utilized to determine the total phenolic content 32 . In brief, 40 μL of plant extract was mixed with 1.8 mL of 2N Folin-Ciocalteu for 5 min at room temperature (25 °C). The resulting mixture was subsequently mixed with 1.2 mL of a 7.5% sodium carbonate solution and allowed to react for one hour in the dark at ambient temperature, where the absorbance at 765 nm was finally measured. The standard component was gallic acid (y = 0.6812x + 0.0314, R 2  = 0.9975), and the sample’s total phenolic content was expressed in milligrams of gallic acid equivalents (mg GAE/g). Rutin, a slightly modified standard substance, was utilized with the aluminum chloride method to evaluate the total flavonoid concentration 33 . Overall, 1 mL of extracts and blank (H 2 O) were mixed with 3 mL of potassium acetate (0.1 mol/L) and 2 mL of aluminum chloride solution (0.1 mol/L). After allowing the mixture to react for 20 min, 70% aqueous ethanol (v/v) was added to dilute it to a final volume of 10 mL. The standard curve was (y = 1.4715x + 0.0364) (R 2  = 0.9998). The absorbance was finally measured at 510 nm, and the results were reported as rutin equivalents (mg RE/g). The total protein content was calculated using Bradford’s (1976) method and expressed as mg/g fresh weight (FW). To prepare one gram of fresh plant tissues for protein and enzyme extractions, three milliliters of 25 mM Tris–HCl buffer (pH 6.8) and 3% polyvinylpolypyrrolidone were homogenized at 4 °C. Protein analysis was completed using the supernatant after the resulting mixture was centrifuged for an hour at 13,000 rpm at 4 °C. The concentrations in mg/g FW were determined using standard curves for each reducing sugar 34 . The extract was centrifuged at 12,000 rpm, and the supernatant was kept in the dark for a whole day to determine anthocyanins 35 . After that, an absorbance measurement at 550 nm was taken for all the samples. After computing the total anthocyanin content using a coefficient of attrition of 33,000 mM/cm, the result was expressed as µg/g final weight.

Green synthesis of trimetallic CuO/Ag/ZnO nanocomposite

A 250-mL Erlenmeyer flask was filled with 50 mL of each diluted plant extract (50%) and was agitated for 30 min, while 0.1M AgNO 3 , 0.1M Cu (NO 3 ) 2 .3H 2 O, and 0.1M Zn (CH 3 COO) 2 .2H 2 O were titrated gradually at a time. The solution was then continually stirred at 80 °C, while the pH was adjusted to (5.5, 7.0, 14) using 2M NaOH solution. A precipitate that was dark brown developed after this reaction, was stirred for two hours. The precipitate that was produced was centrifuged for 15 min at 12,000 rpm, and after being repeatedly cleaned to remove contaminants with distilled water and ethanol, the pelt was dried for two hours at 80°C. A mortar and pestle were used to grind the dried pelt into a powder, and dry weights were estimated for each plant extract. UV–visible spectroscopy (Shimadzu, Japan) was also used to identify the absorbance bands and band gaps to confirm that the nanocomposite was synthesized utilizing each of the extracted plants.

Bioassay survey

An agar-well diffusion method was used to determine the green synthesized nanocomposite’s antimicrobial sensitivity in vitro, against a range of multi-drug-resistant human pathogens. The investigated pathogens were cultured in individual culture inoculated in sterile nutrient broth containing (0.5% peptone, 0.5% NaCl, and 0.3% yeast extract). The inocula was then incubated for 24 h at 37 °C. After incubation, the individual culture suspension was utilized for the bioassay survey. A sterile well cutter was used to cut 5-mm-diameter wells on Muller Hinton agar medium (0.2% beef extract, 0.15% starch, 1.75% casein, and 1.7% agar). Subsequently, 0.1 mL of every pathogen was spread out on the agar plates, and 50 μL of the nanocomposites that synthesized at distinct pH levels (pH 5, 7.0, and 14) were added to the hollows. 20 μL of Ziziphus spina-christi extract was used as a control. The inoculation culture plates were held at 4 °C for 5 h before incubation for 48 h at 37 °C. After incubation, the inhibitory zones that formed were measured in millimeters.

Characterization of green synthesized nanocomposite

FTIR spectra were investigated using a JASCO-410 spectrometer (JASCO, Easton, MD). To further explore the surface morphology of the green synthetic nanocomposite, a SEM (Qattro, Thermo-Scientific, USA) JSM-6510LV, USA was utilized. The transmission electron microscope was additionally utilized for analyzing the nano structural features (TEM, JEM-2100F, JEOL: Japan). Thermal stability of synthesized nanocomposites was assessed using (TGA, DTA, and DSC) was estimated at 29–1000 °C utilizing a DSC-TGA device model (SDTQ 600, USA) under a N2 atmosphere (flow rate of 100 mL/min and a heating rate of 10°C/min). Furthermore, (Horiba, SZ-100, Kyoto, Japan) specimen was used to investigate the green synthetic nanocomposite’s zeta potential using dynamic light scattering (DLS). A temperature of 25 °C was maintained during a 20-min dilution and dispersion process in an ultrasonic bath for examining the green synthetic nanocomposite in a DLS machine.

Statistical optimization to maximize the yield of green-synthesized nanocomposite

Two successive experimental designs were used in this work to maximize the yield of nanocomposite’s green-synthetic reaction. The factors affecting the green synesthetic reaction were assessed using the Plackett–Burman and Taguchi designs, such as concentrations of plant extract (F1), concentrations of precursors (F2), ratio of precursors (F3), reaction agitation (F4), reaction temperature (F5), reaction pH (F6), and incubation period (F7).

Plackett–Burman design (PBD)

In several investigations, this design was utilized to evaluate the rate of green synesthetic reaction and the overall yield of dry-weight nanocomposite 36 , 37 , 38 . Green-synthetic reaction variables are used in these qualitative and quantitative screening procedures to identify the ideal parameters for maximizing the dry weight of nanocomposite products. The yield weight of green synthetic trimetallic CuO/Ag/ZnO nanocomposite was found to be affected by seven factors, which included concentrations of plant extract, concentrations of precursors, ratios of precursors, reaction agitation, temperature, reaction pH, and incubation time. These factors were selected based on previous experiments (data not shown). As indicated in Table 1 , these factors were examined at two different levels: the highest (1) and the lowest (− 1).

The response was calculated using the average green synthesized nanocomposite dry weight, and each experiment was conducted twice. A first-order polynomial model serves as the foundation for mathematical modeling of PBD, as shown in Eq. ( 1 ). In this case, Y is the dry weight of nanocomposite that were biosynthesized (response), β 0 denotes the model intercept, β i is the linear coefficient, and X i is the number of independent variables.

Furthermore, Eq. ( 2 ) was used to calculate the efficiency of each variable. In this equation, M v represents the variable main effect, Mv + and Mv− are the cell dry weights in trials where the independent variable was present at high and low levels; respectively, and N is the number of trials divided by two.

Minitab® 18.1 software was utilized to generate a set of 12 trails for statistical analysis and graph charting. All independent variables were evaluated for their impact on the response using analysis of variance (ANOVA), with a significance level of P < 0.05. The fitness of the equation was assessed using the multiple correlation coefficient (R 2 ) and adjusted R 2 .

Taguchi technique: To generate a valid result, the Taguchi technique was built up in several steps: choosing important components, creating an accurate matrix, analyzing statistical data, and finally validating using the best values. The goal of this work is to use several criteria to determine the maximum cell dry weight of the overall dry-weight nanocomposite yield (Table 2 ). The L27(3^7) Taguchi orthogonal array design was used for this optimization technique (7 factors, 3 levels, and 27 runs). An orthogonal array (signal-to-noise ratio, or "S/N") is created by first identifying the factor levels (inner array) using numbers like 1, 2, 3 etc. These levels are then compared to different combinations of noise factors in the outer array. The S/N ratio is expressed in decibels (dB).

Once the average of produced cell dry weights and the signal-to-noise (S/N) ratio (the larger the better group) are determined for each process condition as designed, the F test and ANOVA are used to examine the significance of all factors and their relationships at levels using the MINITAB 18 software. At the end of the process, a confirmation test was conducted to compare the experimental value with the results that were achieved using Taguchi's method. The S/N ratio is expressed in decibels [dB] and calculated using Eq. ( 3 ), where n is the number of observations and Y is the observed data (dry-weight nanocomposite yield).

Moreover, the predicted S/N ratio was estimated, where n is the number of parameters, S/Nm is the total mean S/N ratio, and S/Ni is the mean S/N ratio at the optimum level using Eq. ( 4 ).

Antimicrobial efficiency bioassays

The CLSI standard 39 , was followed in the agar well-diffusion method, biofilm inhibition assay, and time-kill experiment employed to evaluate the antimicrobial efficiency of green synthesized CuO/Ag/ZnO nanocomposite.

Agar well-diffusion technique

The studied human pathogens were cultured in a nutrient broth medium (0.5% peptone, 0.3% yeast extract, 0.2% beef extract, and 0.5 NaCl) to achieve the 0.5 McFarland turbidity standards. On nutrient agar plates, 100 µL of microbial cultures were spread out using sterile cotton swabs. For this experiment, three different dosages of the selected green synesthetic nanocomposite designated as (A): 50 µg/mL, (B): 100 µg/mL, and (C): 150 µg/mL were prepared. Immediately after using a sterile 6-mm cork-borer to drill each well, 50 µL of the evaluated green synesthetic nanocomposite dosage was added. After that, the agar plates were incubated for 24 h at 37°C. A ruler was used to measure the inhibitory zone, or the clean zone, in millimeters (mm) around each well 40 .

Biofilm inhibition assay

The broth microdilution method was used to estimate the minimum inhibitory concentration of the examined green synesthetic nanocomposite by calculating the lowest concentration at which no discernible growth appeared 41 . To measure MIC, multiple dosages of the selected green synesthetic nanocomposite, ranging from 50 to 250 µg/mL were generated. The tested pathogens were cultured individually in nutrient broth medium at 37 °C and 150 rpm to generate the pre-inoculums. To obtain (2 × 10 5 ) CFU/mL, each pathogen was separately inoculated into a fresh nutrient broth, which was then incubated at 37 °C and 150 rpm. The optical density (OD) at 600 nm was measured over a 6-h incubation period to determine a microbe's exponential phase. Aseptically, 100 µL of each dosage of the tested green nanocomposite was put into 900 µL of these planktonic cultures to generate treated cultures. Moreover, green synesthetic nanocomposite-free cultures were employed for developing untreated (control) cultures. The microbiological turbidity was measured spectrophotometrically to evaluate the inhibitory effects of the tested chemical compounds. The percentage of anti-biofilm in each sample was determined by using Eq. ( 5 ) to compare OD of the treated culture (T) to the corresponding untreated culture (U).

Time kill-kinetics assay

Macro-broth dilution method was employed, to reach the early logarithmic stage; every one of such pathogens was inoculated separately into nutrient broth medium and incubated for 6 h at 37 °C while being agitated at 150 rpm. The inoculum from each microbial culture (5 × 10 8 CFU/mL) was then transferred to 9 mL of freshly made nutrient broth medium. The green synesthetic nanocomposite (1 mL) was then added at a concentration of 200 µg/mL. A control growth system was prepared for each pathogen, omitting the tested formulation. After that, these tubes were shaken constantly at 150 rpm and 37 °C for the rest of the period of incubation. Under aseptic conditions, the samples were routinely taken at many time intervals (0, 6, 12, 18, 24, 30, 36, 42, and 48 h). After diluting the samples with sterile saline, 100 µL of the mixture was swabbed onto nutrient agar plates. During the incubation periods, the number of visible colonies was counted and reported as CFU/mL. The percentage of the pathogen's cells' biofilm reduction exposed to the tested green synesthetic nanocomposite with each control was determined using the logarithm of the counted colonies (Log 10 CFU/ml) for each time interval (Eq.  6 ). Furthermore, by determining the lowest dose that eliminated at least 99.9% of the initial microbial cells, the minimum bactericidal concentration (MBC) was determined.

Statistical analyses

The results of antimicrobial efficacy tests were performed in triplicate, and the mean ± standard deviation (M ± SD) was utilized to describe the results. Tukey’s multiple comparison post hoc test was utilized in the Minitab 19 program ( MINITAB version 19.1 ) to compute a one-way analysis of variance ( ANOVA ) and confirm statistical significance. A 95% confidence interval was used for statistical significance (p < 0.05).

Results and discussion

Biosynthetic of green-synthesized trimetallic nanocomposite.

Basically, nanostructures can be synthesized using sol–gel processing, hydrolysis/condensation, and wet chemical processing. These techniques are mostly costly, require exact experimental parameters (temperature, pressure, energy, and timeframe), and require toxic traditional chemicals. However, green synthesis of nanostructures is attracting a lot of interest currently, due to many significant advantages including simpler, cheaper, and more eco-friendly technique. The most promising method of synthesis is " green synthesis ," which is achieved by employing either plant extracts or specific microbes (bacteria, fungi, algae, etc.). Many studies on the synthesis of various metals nanoparticles including Zn, Mn, Cu, Au, and Ag, which have been carried out in recent years with a focus on different types of biological systems 42 . In pharmaceutical formulations, medicinal plant extracts are utilized for their bioactive ingredients, helping in the reduction and capping of metal ions through the synthesis of nanostructures 43 . For instance, zinc nitrate ionization in an aqueous solution produced Zn 2+ , which was subsequently reduced to Zn + by a phytochemical present in the extract (functional as reducing, capping, and stabilizing agents). Chemicals containing phenolic groups and hydroxyl groups may hydrolyze and generate nanostructures. Among the several kinds of metallic nanoparticles, Ag, CuO, and ZnO have attracted the attention of many scientists, due to their many applications in different scientific sectors 44 , 45 , 46 . These nanoparticles have been extensively employed in antimicrobial, antioxidant, and photocatalytic applications 44 . On the connections between these metals in plant extract-based nanocomposites, however, there is currently no information available. Due to the synergistic effect of their respective qualities, this combination usually improves the material's properties 47 .

Thus, our work contributes to the effort to find a novel material with remarkable physiological characteristics that has been produced using green techniques. Polyphenols (including flavonoids and saponins), alkaloids, proteins, phenolic acids, sugars, and terpenoids all of which are found in various plant parts—help reduce and stabilize metal ions to produce nanostructures 48 . Thus, the use of plant extracts not only saves energy, time, and steps while reducing the use of toxic chemicals, which protects the environment and human health, but also enhances the efficacy and properties of nanoparticles in the pharmaceutical and medical fields by retaining active chemical molecules on their surfaces 49 . The green synthesis of nanocomposites in aqueous plant extracts is suggested to be influenced by a variety of bioactive molecules, including proteins, polyphenols, and polysaccharides 50 . So, the examined plant extracts were evaluated by determining their constituents. A set of methods was used to test specific components of the various leaf materials of Mentha , Ocimum basilicum , and Ziziphus spina christi before the green synthesis of CuO/Ag/ZnO nanocomposites, were developed. Table 3 initially reports the results of the determinations for total protein, reducing sugar, anthocyanin, phenol, and flavonoids. According to phytochemical investigations, the main constituents of the tested leaf extracts that contributed to the stabilization and reduction of nanoparticles were protein content, reducing sugar, flavonoids, phenolics, and anthocyanin. The results showed that these constituents were richest in Ziziphus spina-christi , Ocimum basilicum , followed by Mentha spp . The results showed that the Ziziphus spina christi extract consisted of high levels of flavonoids (26.60 ± 2.25), total phenolic compounds (35.69 ± 5.38), reducing sugar (2.84 ± 0.22), anthocyanin (5.92 ± 0.05), and total protein (2.96 ± 0.27).

These aromatic plant extracts (reductants) were then titrated under shaking conditions with the precursors composed of (0.1M AgNO 3 , 0.1M Cu(NO 3 ) 2 .3H 2 O, and 0.1M Zn(CH 3 COO) 2 .2H 2 O) together to generate a green synesthetic trimetallic nanocomposite. The reaction color changed from reddish yellow (aromatic plant extracts) to dark turbid brown, indicating that the extracts of Mentha spp. (Fig.  1 IC), Ziziphus spina-christi (Fig.  1 IIC), and Ocimum basilicum (Fig.  1 IIIC) generated a green synthetic nanocomposite (Fig.  1 I, II , and IIIN). An essential technique for figuring out the electronic structure, optical activities, and physico-chemical characteristics of nanoparticles is UV–visible (UV–vis) spectroscopy 51 . The classification of nanoparticles in the size range of 2–100 nm was found to be adequate for absorption of wavelengths 200–800 52 . The absorption edge of our green synthetic nanocomposite was estimated using a spectrophotometer scanning a range of 200–500 nm, and the results were compared with the extracts employed in each case. The real absorbance was graphed from 0 to 4.0 au. using the Origin Pro software (v. 8.0, OriginLab Co., Northampton, MA, USA) to generate fitted curves.

Findings of green synthesized nanocomposite generated from various aromatic plant extracts. UV–vis plots for the prepared nanocomposite, compared to the examined plant extracts: Mentha spp. ( I ), Ziziphus spina-christi ( II ), and Ocimum basilicum ( III ). Photos of the extract plants ( C ) and the yield nanocomposite ( N ). The chart depicts the dry weights of nanocomposites generated from various aromatic plant extracts at different pH levels ( IV ). The color of the resulting green nanocomposite synthesized with Ziziphus spina-christi at various pH levels ( V ).

The real green synthetic nanocomposite consistently displays distinct absorption peaks at 220 nm (Fig.  1 I), 240 nm (Fig.  1II ), and 260 nm (Fig.  1III ), in addition to 320 nm when its wavelength is compared to the extract's peaks. A green-generated trimetallic Cu, Zn, and Ag nanocomposite utilizing Catharanthus roseus leaf extract has shown similar results elsewhere 53 . According to reports, the absorption bands for Cu, Zn, and Ag nanocomposites have been identified at 220, 270, and 370 nm; respectively 15 . In addition, an experiment revealed the existence of zinc ions in the crystal lattices, which caused the lattices to shift, especially in consideration of their extension. The absorbance peak's strength increases as a result of this alteration after ZnO doping with Cu 54 . Furthermore, a rise in the absorbance band at 220 nm confirms the presence of copper oxide (CuO) 54 . This peak is found in a similar range by numerous other investigations that demonstrated the generation of CuO NPs. Zinc oxide (ZnO) has a further separate peak at 270 nm. Other investigations have shown that absorbance peaks at 230 and 270 nm in copper-doped ZnO nanoparticles suggest the presence of ZnO 40 , 48 , 49 , 50 . The production of Ag nanoparticles is shown by the absorbance band at 370 nm 15 . The aqueous extract of Berberis vulgaris leaf and root was used to generate nanoparticles of silver, which showed a broad peak in 380–400 nm area 53 . The results of the current study are fully consistent with all the outcomes.

Different parameters, including pH, temperature, reaction duration, and reactant concentration, can be used to optimize the green synthesis of nanoparticle morphological characterization 33 , 51 , 52 , 53 . Most of these environmental elements that influence nanoparticle synthesis should be identified. Consequently, these aspects can be efficiently addressed to maximize the yield of industrial fabrication of metallic nanoparticles 53 . The reaction's pH has significant effects on the nanoparticles' structure 61 . To be more precise, temperature and pH have an impact on how nucleation centers develop. In order to maximize the synthesis of metal nanoparticles, it is crucial to adjust the pH level since this results in the automatic growth of nucleation centers 62 . Moreover, the size and structural composition of the nanoparticles have been found to be significantly impacted by the pH of the solution 53 . Therefore, the green synthetic nanocomposite was generated at different pHs to determine which plant extract produced the heaviest dry weight of nanocomposite. To generate a green synesthetic nanocomposite of a trimetallic nanocomposite, these aromatic plant extracts (reductants) are separately adjusted at different pHs (5.5, 7, and 14). Then, the precursors that were used are added gradually and equally. For all studied aromatic plant extracts, the optimum response was seen at a pH between 5.5 and 7, as Fig.  1IV illustrates. Moreover, the heaviest dry weight of the generated nanocomposite was obtained using the Ziziphus spina-christi extract at all applicable pHs (Fig.  1V ). In brief, the largest dry weight of green synthetic nanocomposite was measured at pH-5.5 (0.29 mg/mL), followed by pH-7 (0.25 mg/mL), and pH-14 (0.05 mg/mL) was the lowest. With the exception of other extracts in all applicable screening analyses, the Ziziphus spina-christi extract produced the heaviest dry weight of the formed nanocomposite. Therefore, in all additional investigations, the Ziziphus spina-christi extract was selected for the green-generated nanocomposite.

An antimicrobial survey is carried out utilizing the green synthetic nanocomposite, which is prepared using Ziziphus spina-christi extract at all applicable pHs. When compared to the free extract (Co), the growth of the evaluated human pathogens was impacted by every nanocomposite created, as demonstrated by the plate photographs (Fig.  2 ). In brief, the widest inhibitory zone widths (Fig.  2 D) were detected at pH 7 against Bacillus subtilis (14.21 ± 1.56 mm) and Staphylococcus aureus (13.96 ± 2.33 mm). ANOVA and Tukey post-hoc tests were used to assess the mean values of the computed inhibitory zones to statistically identify the more effective versions. To find significant mean differences, Fig.  2 E then displays Tukey 's test means for each paired comparison. The adjusted confidence intervals are computed using the Tukey simultaneous tests on a 95% scale. At pH 7 intervals, the green synthesized nanocomposite is devoid of the zero line. This indicates that there are statistically significant differences between the green synthetic nanocomposite at pH 7 and the control group and other tested pHs. The results show statistically significant antimicrobial properties for the tested green synthetic nanocomposite at pH 7.

Antimicrobial effects of green synthesized nanocomposite utilizing Ziziphus spina-christi extract at all applicable pHs ( A ): pH-5, ( B ) pH-7, and ( C ) pH-14, in comparison to ( Co ): control against ( i ) Escherichia coli , ( ii ) Klebsiella pneumoniae , ( iii ) Staphylococcus aureus , ( iv ) Bacillus subtilis , ( v ) Candida albicans , and ( vi ) Candida krusei using agar-well diffusion analysis. Photos of antimicrobial plates are shown, as well as a chart of the computed inhibition zones ( D ) and simultaneous Tukey tests for mean difference using Tukey–Kramer post-hoc analysis ( E ).

Characterization of green synthesized CuO/Ag/ZnO nanocomposite

TEM imaging of green synthesized nanocomposite is observed with an accelerating voltage of 100 kV. Figure  3 I shows the dense, spherical dot-like structure of the green, synthetic trimetallic CuO/Ag/ZnO nanocomposite. This proves the effective development of trimetallic CuO/Ag/ZnO nanocomposite, which is produced in an environmentally friendly manner. The particle size measured on TEM images is used to visualize the real size of nanoparticles, and the result is an average particle size of 7.11 ± 0.67 nm with a narrow particle size dispersion. SEM investigation shows the film surface morphology, which can be characterized as a porosity-free, soft, smooth planar structure (Fig.  3II ) . Previous studies also used the green chemistry method using extract of Ocimum basilicum L., to generate Ag/doped ZnO-MgO-CaO nanocomposite (59 nm) and spherical and triangular-shaped Ag/doped MgO-NiO-ZnO nanocomposite (30–44 nm); respectively 12 , 56 . Furthermore, the spherical-shaped of ZnO-Ag nanocomposites (26.02 ± 1 nm) were formed by utilizing a novel, simple, cost-effective, and safe method that involved the utilization of Stenotaphrum secundatum extract 13 . Likely, a green technique is employed in a prior study to prepare Ag-doped ZnO nanoparticles (60 nm) utilizing Tridax procumbens leaf extract. These nanoparticles show synergistic antimicrobial properties against a variety of human pathogens 8 . As seen in Fig.  3III , EDX mapping verification at multiple sites demonstrates the presence of signals with a highly homogenous distribution on the surface of green synthesized nanocomposite, including O (79.25%), Cu (13.78%), Zn (4.42%), and Ag (2.55%). The study's findings verify that CuO, Ag, and ZnO nanocomposite are effectively synthesized using green techniques. Prior to this, the normal stoichiometric ratio that was employed to generate the trimetallic nanoparticles was not followed, resulting in a compositional atomic ratio of (1:1.46:1.05) of (Cu:Ag:Zn). This could have been brought about by differences in the surface energy of the nanoparticles or by the specific crystallographic orientation of the metal atoms 15 . A further vital characteristic is the ability to measure charge on a surface. The molecular weight of large molecules dissolved in water can be determined using Zeta-potential analyzer. Zeta potential levels rely on a number of factors, including chemical composition and roughness 64 . Zeta potential is a measure of the strength of charge on the surface of particles 64 , 65 . The stability of an emulsion or nanosuspension can be predicted based on the absolute value of the zeta potential. In order to stabilize the nanocrystal formation (electrostatic repulsion), a high absolute value of zeta potential needs to be achieved. Higher zeta potentials of the nanomaterial suspension predicted the formation of a more stable, non-aggregating particle dispersion. Previous studies found that a suspended particle is deemed stable if its zeta potential is either higher than + 30 mV or lower than − 30 mV 65 , 66 . According to earlier studies, particles will agglomerate when zeta potential values get closer to 0 mV 67 ; nevertheless, for values larger than ± 20 mV, the particles will remain stable and suspended 65 . The green synthetic trimetallic CuO/Ag/ZnO nanocomposite has a zeta-potential of 21.5 ± 5.53 mV, as shown in Fig.  3 IV. The large absolute zeta potentials (> 20 mV) of our developed green synthetic nanocomposite suggested long-term stability by reducing vesicle aggregation, indicating that it was stable in a liquid state.

TEM image (I) , SEM image (II) , TEM–EDX analysis (III) , and Zeta potential pattern (IV) of green synthesized trimetallic CuO/Ag/ZnO nanocomposite.

Furthermore, the thermal analysis of green synthesized trimetallic CuO/Ag/ZnO nanocomposite is characterized using TGA, DTA, DSC profiles (Fig.  4 ). DSC data provides a detailed description of the phase transition of tested nanocomposite. The Tg value is a crucial parameter to describe the stability of the lyophilized nanocomposite. The transition temperature and associated enthalpy drop have an impact on the stability of drug pharmacokinetics. A more tightly constructed nanocomposite is suggested by a greater transient enthalpy. DSC panel indicates that green synthetic trimetallic CuO/Ag/ZnO nanocomposite's transition temperature varied between 100 and 200 °C (Fig.  4 I). The characteristic endothermic peaks appear at approximately 118.84 °C, 138.44 °C, and 200.41 °C. These are caused by the release of absorbed water, the breakdown of organic molecule function groups, depolymerization, and decomposition, as well as the dehydration, phase conversion, and full combustion of the organic residue 68 , 69 . The green synthetic trimetallic CuO/Ag/ZnO nanocomposite's DTA curve (Fig.  4 II) displays three exothermic peaks at 112.75, 130.13, and 194.78°C and three endothermic peaks at 118.15, 137.89, and 201.31°C. The heat degradation process is shown in seven phases on the TGA curve in a smooth, stepwise manner (Fig.  4 III). While weight losses of 11.03, 4.12, 2.37, 4.404, 1.89, 2.404, and 6.288% accompanied the breakdown of green synthetic trimetallic CuO/Ag/ZnO nanocomposite, are detected at 79.67, 121.41, 141.83, 211.09, 259.49, 405.12, and 493.92°C. The green synthetic trimetallic CuO/Ag/ZnO nanocomposite loses weight in the initial stages due to the evaporation of adsorbed water molecules and humidity. Because of the breakdown of green synthetic trimetallic CuO/Ag/ZnO nanocomposite matrix, the largest weight losses (> 85.92%) occur at temperatures between 0 and 250 °C, because of CuO/Ag/ZnO is crystallinity-related, the final breakdown (10.57%) takes place between 260 and 500°C.

Characterization of green synthesized trimetallic CuO/Ag/ZnO nanocomposite's DSC ( I ), DTA ( II ), and TGA ( III ) curves with FTIR ( IV ) spectrum of green synthesized nanocomposite (black spectrum), and the extract of Ziziphus spina christi (red spectrum).

FTIR spectra of Ziziphus spina christi extract and CuO/Ag/ZnO nanocomposite specimens are shown in Fig.  4 IV. The spectrum of CuO/Ag/ZnO nanocomposite exhibits peaks at around ν 700–400 cm −1 ; in contrast to the extract bonds of Ziziphus spina christi spectrum. This can be attributed to the interactions that Ag has with metal oxides like ZnO or CuO. The green synthesized spectrum of CuO/Ag/ZnO nanocomposite shows distinct peaks at ν 630 cm −1 and peaks at approximately ν 500–420 cm −1 , which are related to the stretching vibrations of CuO and Zn–O, respectively 37 , 38 , 39 . Air humidity most likely influenced the sample measurement. A spectra band of ν 3600–3500 cm −1 is where O–H bond occurs 70 . Consequently, the signal at ν 3478 cm −1 is associated with inter-hydrogen bonding that is present in both the plant extract and nanocomposite spectra represents –OH groups and water molecules. The stretching frequency of the extract's phenolic O–H, which serves as a reducing and capping ligand, is responsible for the broad peak at ν 3354–1606 cm −1 . The stretching of carbon dioxide O=C=O bonds is also responsible for the peak at ν 2351 cm −1 . The additional clear peak is especially visible at ν 1520 cm −1 which is the vibrational frequency of a C=O bond and may indicate the presence of organic residues. Furthermore, there is a peak at ν 1427 cm −1 , which could be related to the O–H bonds in carboxylic acid bending. The stretching vibration of C=O polyphenols may be explained by strong peak at ν 1392 cm −1 . Aromatic C–O and N–H stretching vibrations from phenolic groups were responsible for the strong peaks at 1268 and ν 1076 cm −1 ; respectively. The stretching of C–O bonds in primary alcohols is connected to the peak detected at ν 1113 cm −1 . Overall, FT-IR spectrum of CuO/Ag/ZnO exhibits that; it is coated with active phytoconstituents, mainly O–H, C=O, and C–N residues of alkaloids and phenolic derivatives. To stabilize the resulting CuO/Ag/ZnO nanocomposite, O–H, C=O, and C–N residues might form bonds with metals by covering their surfaces and decreasing agglomeration 69 , 71 . Many studies have reported that a variety of biomolecules found in the Ziziphus spina christi extract are responsible for the stabilization and reduction of the green synthesized nanocomposite. The existence of several functional groups linked to active phytochemicals such as phenolic acids, flavonoids, aromatic compounds, etc . is shown by FTIR analysis of trimetallic Zn/Cu/Ag NCs that are synthesized from the leaf extract 72 . These groups have been suggested to be responsible for the generation of the trimetallic nanocomposite as well as the reduction of metal precursors and subsequent stabilization 61 . The phytochemicals in the extract, primarily flavonoids and phenolic acids, may decrease metal ions by donating electrons, resulting in the generation of metal nanoparticles. Furthermore, this may prevent the particles from aggregating by binding to the surface of the nanoparticle, forming a barrier that reduces surface energy and stabilizes the particles. Further oxidation of the nanoparticles could be inhibited by the carboxyl and hydroxyl groups binding to the metal ions on their surface, protecting the structural integrity of the particles 11 , 51 . Our FTIR results clearly show the presence of flavonoids and phenolic acids, which are responsible for the development of the green synthetic trimetallic CuO/Ag/ZnO nanocomposite.

Statistical optimization of the yield of green synthesized nanocomposite

In general, green synthesized nanocomposites show promise as antibacterial and anticancer agents for safer, more effective, and inexpensive medications or drug delivery systems. The various sizes, forms, dispersions, and stability of the generated nanocomposites are associated with the presented metabolites 40 . Green synthesized procedures for nanocomposite, in particular, have a number of delicate factors 63 , 64 . Several factors that influence the yield shape and size control include the concentrations of plant extract and precursors, as well as the ratio of precursors to other reaction parameters, including temperature, pH, agitation, and incubation time 54 , 65 . Worldwide, scientific investigations are being carried out to find out more about how temperature affects nanoparticles 33 , 54 . The main element that alters the size, shape, and degree of synthesis of the nanoparticles is the temperature 36 . Temperature-dependent modifications can be made to the synthesized nanoparticles' rod, spherical, octahedral platelet, triangular, and spherical shaped structure. Additionally, when the temperature improves, the reaction response rate increases the nucleation center development 54 , 65 . Conversely, the most important variable influencing the yield, size, and shape of nanoparticles generated during the synthesis of green nanoparticles is the reaction time 3 , 62 . According to EL-Moslamy et al., reported that reaction time is critical to produce various nanoparticles and nanocomposites. Therefore, three primary parameters that influence a nanoparticle's shape and structure are temperature, pH, and reaction time 53 , 54 . Until now the utilization of Ziziphus spina christi extract to optimize the conditions of green synthesized trimetallic (CuO/Ag/ZnO) nanocomposite statistically according to regulated conditions remains unexplored. In this study, a two-step experimental strategy known as Plackett–Burman and Taguchi designs is utilized to analyze the parameters influencing the green synthesized reaction to maximize the nanocomposite's green synthesized yield.

Plackett–Burman design

The best parameters for maximizing the dry weight of nanocomposite solutions are identified by using this qualitative and quantitative screening method employing green-synthesized reaction variables. The chosen experiments are utilized to identify the essential elements for the green synesthetic nanocomposites, determine the appropriate ratio, and create a mathematical model, that could be applied to the prediction procedure. The 12 experiments involved screening several components of green-synthetic reaction and exploring each one at two different levels: high (+ 1) and low (− 1), together with a dummy factor used to assess the experiment's standard error. The experiments are completed, and green synthesized nanocomposite’s dry weights are recorded (Table 4 ). Excel 2016 and Minitab 18 are the tools utilized for statistical analysis and graph plotting. As indicated by Table 4 the nanocomposite's highest dry weight was 0.78 mg/mL (run 12) and 0.65 mg/mL (run 8); in contrast, the lowest dry weight is 0 mg/mL that recorded at runs 5 and 7.

The effect of each independent variable on the response is ascertained by analysis of variance ( ANOVA ), where P < 0.05 was deemed statistically significant. Table 4 shows the results of equation's fitness evaluation using the multiple correlation coefficient (R 2 ) and adjusted R 2 . In the overall design, the p value indicates the significance of each independent variable. Larger t-values and smaller p-values (prob > F < 0.05) are associated with greater coefficient influence on the response. The model's overall performance is also estimated using the coefficient of determination (R 2 ) and the adjusted-R 2 (adj-R 2 ) value, which ideally should agree with R 2 value (less than 2%). A stronger model with better response prediction is indicated by R 2 value closer to 1 75 , 76 . The presented data shows model R 2 and adj-R 2 values for the bio-fabrication reaction of the green synthesized nanocomposite, which are 98.58%, and 96.10%; respectively (Table 5 ). According to these findings, the model can account for 98.58% of response data variability, with a 1.42% chance that noise is to blame for the variation. Additionally, a high adj-R 2 value showed that the model was precise and that there is a strong correlation between the experimental and anticipated findings. ANOVA summary typical of experimental Plackett–Burman tests indicated that the model was highly significant, due to the low probability value (p value ~ 0.05). Regarding the green synesthetic nanocomposite's dry weight (mg/mL), each of these components showed an acceptable adjustment (Table 5 ).

As shown in Fig. 5 I, II, and IV nanocomposite's yield is affected by the minimized values of precursor concentrations (F2), precursor ratio (F3), reaction agitation (F4), and reaction temperature (F5) factors, alongside the maximized values of plant extract concentrations (F1), reaction pH (F6), and incubation period (F7). The production efficiency of green synthesized nanocomposites is statistically significantly impacted by all evaluated parameters. As illustrated in Fig.  5 V concentrations of plant extract (F1), concentrations of precursors (F2), ratio of precursors (F3), reaction agitation (F4), reaction pH (F6), and incubation time (F7); are the main factors that influence the production efficiency of green synthesized nanocomposites, more so than reaction temperature (F5). Figure  5 I illustrates the principal impacts of every variable under investigation on the nanocomposite's dry weight. These main effects describe the average differences for each variable between its low and high values. Except for F2, F3, F4, and F5 factors, which vary dramatically between high and low levels, suggesting their impact on amplifying the response at low levels. As a factor rises from a low to a high level, the response always increases when the major effect of the factor is positive (F1, F6, and F7). Because it predicts the maximum dry weight of the nanocomposite using optimal parameters (Fig.  5 III) to determine individual effectiveness, the optimizer tool in MINITAB 18.0 was utilized to solve Eq. ( 7 ). Equation ( 7 ) indicates that a first-order polynomial model that serves as the starting point for the mathematical modeling of the PBD is used to verify the reaction by calculating the average dry weight of green synthesized nanocomposite. The green synthesized nanocomposites are verified by means of the ideal conditions expected for the green reaction, and the results are compared with those recorded under the baseline settings. By using this optimization process, the nanocomposite's dry weight increases from 0.29 to 0.89 mg/mL, i.e. a 3.06-fold increase.

Model summary of the factorial regression for the green synthetic nanocomposite (g/ml) for the investigated variables: plant extract concentrations ( F1 ), precursor concentrations ( F2 ), precursor ratio ( F3 ), reaction agitation ( F4 ), reaction temperature ( F5 ), reaction pH ( F6 ), and incubation period ( F7 ) via the following parameters: the main effect plot ( I and II ), the standardized effect using normal plot ( IV ), Pareto chart of the standardized effects ( V ), and a response optimizer with a maximum outcome and optimal values for these variables ( III ) of each variable.

Taguchi statistical method

A cost-efficient and attractive tool for optimizing and generating excellent industrial production processes has been developed by Genichi Taguchi model. According to the requirements of the experiment, several arrays included in Taguchi's model can be employed. Orthogonal arrays (OAs) have designs indicated by Ln (mP), where n signifies the total number of sections, m denotes the number of parameter levels, and P is the total number of parameters 69 , 70 . This work is the first to employ the Taguchi experimental design to statistically optimize the conditions of green synthesized trimetallic CuO/Ag/ZnO nanocomposite qualities using Ziziphus spina christi extract. Taguchi's L27 (3^7) orthogonal array design is utilized to optimize the yield of a green trimetallic CuO/Ag/ZnO nanocomposite. The yield and S/N ratio values of green trimetallic CuO/Ag/ZnO nanocomposite are determined by conducting 27 trials with seven parameters classified according to L27 (3^7) OA design, as indicated in Table 6 . The ideal combination of the responses of green trimetallic CuO/Ag/ZnO nanocomposite is designed by experimentation using Taguchi model. To identify the best combination of the evaluated factors, the data is examined using statistical techniques, including regression analysis and ANOVA . The biggest and smallest yields of green trimetallic CuO/Ag/ZnO nanocomposite values (0.04 and 1.42 mg/mL) are demonstrated in experimental No. 25 and No. 12; respectively. Table 6 displays the structure of Taguchi's orthogonal robust structure, as well as the measurement outcomes. The quality feature that deviates from the intended value is measured using S/N ratio data obtained from Taguchi method. S/N ratios vary based on the green trimetallic CuO/Ag/ZnO nanocomposite yield values (Table 6 ). The S/N ratio and green trimetallic CuO/Ag/ZnO nanocomposite yield values determined by Taguchi's equation (Eq.  3 ) are displayed in Table 6 .

The mean S/N ratio for each parameter level is reported, and Table 6 displays the S/N response table for yield of the green trimetallic CuO/Ag/ZnO nanocomposite. Both an ANOVA and an F-test can be used to assess the experimental data (Table 7 ). Our chosen model suits the experimental data well, as evidenced by its R 2 of 97.36%. So, both the model and its parameters were highly significant (P < 0.0001). The model's F-value stands at 100.33, and the significance F-value is 1.19 E−13 (Table 7 ).

It is shown that, the suggested model is adequate by the residuals found above and below zero line of the residual plot. A straight-line distribution is seen in the residual plots, which suggests the model fits the results effectively (Fig.  6 ). The end confidence level (%) and P-values of each factor indicated that F4, F5, and F6 are significant factors, followed by F7, F2, F1, and F3 (Fig.  6III ). This orthogonal array model is represented by equation No. 8, which also explains the yields of green trimetallic CuO/Ag/ZnO nanocomposite and the relationships between each of the seven elements. As illustrated in Fig.  6 I, the final rankings have the largest S/N ratio value (bigger is better) based on the ANOVA analysis of S/N ratio value and the factor level calculation of the main impact for this dry weight (Table 7 ). For the key effects obtained all through the optimization trial runs, a primary impact graphic was drawn (Fig.  6II ).

Characteristics of Taguchi's experimental results: ( I ) the larger-the-better main effects plot for S/N ratios; ( II ) the main effects plot for means of the production efficacy of green synesthetic nanocomposite; ( III ) the p-values and confidence level (%) of each factor in the yield of green synesthetic nanocomposite, and (IV) Schematic diagram of the green synthetic trimetallic CuO/Ag/ZnO nanocomposite employing 25% diluted Ziziphus spina-christi extract (pH = 5) as reducing/capping agent and 0.25 M AgNO, Cu(NO 3 ) 2 .3H 2 O, and Zn(CH 3 COO) 2 .2H 2 O as precursors. The green synthesized trimetallic CuO/Ag/ZnO nanocomposite coated with active phytoconstituents, mainly O–H, C=O, and C–N residues of alkaloids and phenolic derivatives.

To get the highest yield of the green synthesized trimetallic CuO/Ag/ZnO NCs, this approach recommends the optimal combination of the investigated parameters. For green trimetallic CuO/Ag/ZnO nanocomposite, S/N ratio indicates that the following are the ideal conditions: F1 at level 1, F2 at level 1, F3 at level 1, F4 at level 3, F5 at level 3, F6 at level 1, and F7 at level 3. The last stage is to predict and confirm the improvement of quality profile using the ideal level of design parameters after the optimal level has been determined. According to Eq. ( 4 ), it is possible to compute the predicted S/N ratio using design parameters at their optimal level. The pH of 25% diluted Ziziphus spina-christi extract is adjusted to 5 to achieve the highest dry weight possible for green trimetallic CuO/Ag/ZnO nanocomposite. This extract is then titrated slowly using (0.25M AgNO 3 , Cu(NO 3 ) 2 .3H 2 O, and Zn(CH 3 COO) 2 .2H 2 O), which are prepared at 1:1:1 ratio. This reaction is incubated at 50 °C and agitated for 3h at 200 rpm, after this titration phase (Fig.  6 IV). The green trimetallic CuO/Ag/ZnO nanocomposite is optimized statistically under controlled conditions using Ziziphus spina christi extract, resulting in an estimated yield of 1.65 mg/ml and a predicted S/N ratio of roughly 7.79 dB. Lastly, the comparison of data using Placket Burman strategy and Taguchi approach shown that it is feasible to efficiently raise and enhance the yield of green synthetic trimetallic CuO/Ag/ZnO nanocomposite. Compared to Plackett Burman strategy and basal condition, the maximum green synthetic trimetallic CuO/Ag/ZnO nanocomposite yield (1.65 mg/mL) may be increased by 1.85 and 5.7 times; respectively by applying Taguchi strategy.

Antimicrobial potency of green trimetallic CuO/Ag/ZnO nanocomposite

Human infections with antibiotic-resistant microbes are a major cause of death worldwide 79 . Accordingly, a number of antimicrobial nanostructures have been generated recently 80 . So, our study investigated the antimicrobial qualities of different doses of optimized yield of green synthesized trimetallic CuO/Ag/ZnO nanocomposite. Initially, the antimicrobial activity of evaluated doses (50, 100, and 150 µg/mL) is evaluated using agar-well-diffusion method (Fig.  7 ). The inhibitory zone widths of tested doses of green trimetallic CuO/Ag/ZnO nanocomposite against multidrug-resistant human pathogens are determined. Generally, the largest inhibitory zone widths are recorded by using different doses of the green trimetallic CuO/Ag/ZnO nanocomposite against Gram -negative bacteria (Fig. 7 i,ii), and Gram- positive bacteria (Figs. 7 iii, and 8 iv), followed by yeast cells (Fig. 7 v,vi). There are differences in the affected doses for each human pathogen that has been studied, as seen in Fig.  7 I. The results of Table 8 demonstrate that Escherichia coli that are treated with 150 µg/mL of green trimetallic CuO/Ag/ZnO nanocomposite shows the largest inhibitory zone widths (20.68 ± 3.54 mm), followed by Klebsiella pneumoniae (19.22 ± 1.41 mm), and Staphylococcus aureus (17.14 ± 1.98 mm). Additionally, Bacillus subtilis (15.39 ± 3.52 mm), Candida albicans (14.32 ± 2.54 mm), and Candida krusei (13.29 ± 4.22 mm) show the narrowest inhibitory zones, when exposed to 150 µg/mL of green trimetallic CuO/Ag/ZnO NC. Ag-ZnO nanocomposites (75 nm) generated from fenugreek leaf extract at a dosage of 20 mg/mL are found to have antimicrobial properties against several human diseases in a previous study. In the agar diffusion method, the inhibition zone diameter for Escherichia coli is 12.5 ± 0.707 mm, for Staphylococcus aureus is 13.5 ± 0.707 mm, and for Candida albicans is 10.5 ± 0.707 mm 81 . But herein, Escherichia coli treated with 150 µg/mL of green trimetallic CuO/Ag/ZnO nanocomposite demonstrate the highest inhibitory zone widths (20.68 ± 3.54 mm), followed by Staphylococcus aureus (17.14 ± 1.98 mm) and Candida albicans (14.32 ± 2.54 mm). Our results show that Ag-ZnO nanocomposites have less impact on S. aureus and E. coli . Other reports on the antimicrobial abilities of Ag, ZnO, and CuO nanoparticles generated from various plant extracts have also been reported previously 11 , 74 , 75 , 76 . Numerous nanoparticles with antibacterial qualities have also been demonstrated in other studies, which include silica, iron oxide, copper oxide, magnesium oxide, titanium dioxide, silver, zinc oxide, and cerium dioxide. The capacity of nanomaterials to limit microbial development depends on the layers of the pathogen's cell wall or membrane structure. The synthesized nanostructure's size, shape, and core–shell morphology, which provide a high surface-area-to-volume ratio, also have an impact on the proliferation of microbes 17 , 82 , 85 .

Antimicrobial efficacy results for tested doses of green trimetallic CuO/Ag/ZnO nanocomposite labeled ( A : 50 µg/mL, B : 100 µg/mL, C : 150 µg/mL) against various multidrug-resistant human pathogens ( i : Escherichia coli, ii : Klebsiella pneumoniae, iii : Staphylococcus aureus, iv : Bacillus subtilis, v : Candida albicans, and vi : Candida krusei ). Photographs depict an Agar-well diffusion investigation. Chart displays the computed inhibition zones ( I ), box-plot graph ( II ) displays the inhibitory value distributions corresponding to the tested doses; and simultaneous results for analyzing the overall group's difference ( III ) via Tukey–Kramer post-hoc analysis. Means that don't have the same letter differ greatly.

Reduction in biofilm generation of the tested human pathogens using a biofilm inhibition assay. Chart shows the percentage of biofilm reduction ( I ), box-plot graph shows biofilm reduction value distributions corresponding to drug dosages via Tukey–Kramer post-hoc analysis ( II ), and simultaneous Tukey results appearing the overall group's difference ( III ).

The ANOVA , and Tukey–Kramer post-hoc analysis is employed to demonstrate the inhibitory value distributions that correspond to tested doses. Furthermore, data about the correlation between tested products and antimicrobial effectiveness is grouped using statistical clustering. So, the inhibitory effect distributions that match tested treatments are displayed on comparable interval and box plot graphs (Fig. 7 II, III). The Tukey–Kramer post-hoc results show that, out of all the treatments that are assessed, 150 µg/mL of green trimetallic CuO/Ag/ZnO nanocomposite have the highest anti-biofilm value. A boxplot that displays the significant mean differences is produced for each paired comparison using the means of Tukey's test. As can be seen in the box-plot graph (Fig.  7 II), there are significant antimicrobial variations among all tested doses of green trimetallic CuO/Ag/ZnO nanocomposite. Especially, 150 µg/mL of green trimetallic CuO/Ag/ZnO nanocomposite have the highest inhibitory values based on Tukey–Kramer post-hoc results. On 95% scale, the modified confidence intervals are computed using Tukey simultaneous tests. Due to the absence of zero line in the intervals for formulation with the highest efficacy, 150 µg/mL of green trimetallic CuO/Ag/ZnO nanocomposite, whose mean values are shown in Fig.  7 III, shows significant differences. All these results indicate that 150 µg/mL of green trimetallic CuO/Ag/ZnO nanocomposite have the strongest antimicrobial properties, compared to all doses that are tested.

Spectrophotometric antibiofilm assay is used to assess the antimicrobial efficacy of green trimetallic CuO/Ag/ZnO nanocomposite with several doses ranging from 50 to 250 µg/mL, against all tested human pathogens. The percentage of biofilm reduction is utilized to determine doses' in vitro efficacy to prevent pathogen growth (Fig.  8 ). The antimicrobial chart depicts 200 µg/mL dose's strong antagonistic antimicrobial effects against all pathogens tested (Fig.  8 I). Additionally, tested Gram -positive have the highest antimicrobial effect more than tested Gram -negative, and yeast cells. The highest percentage of antibiofilm after treatment with 200 µg/mL of green trimetallic CuO/Ag/ZnO nanocomposite are of 98.31 ± 0.98, and 97.68 ± 1.11% that are recorded against tested Gram -positive pathogens e.g. Bacillus subtilis , and Staphylococcus aureus; respectively (Table 9 ). Additionally, the modest percentage of antibiofilm are recorded against Escherichia coli (92.45 ± 1.41%), Klebsiella pneumoniae (91.07 ± 1.09%), Candida albicans (90.99 ± 0.87%), Candida krusei and (89.59 ± 0.15%), as seen in Table 9 . To statistically ascertain whether doses are more effective, the mean values of computed antibiofilm percentages are assessed using ANOVA and Tukey post-hoc test , Fig.  8 II, III. Furthermore, the correlation data between tested doses and antimicrobial effectiveness is grouped using statistical clustering. So, the inhibitory effect distributions that match the tested treatments are displayed on comparable interval and box plot graphs. A boxplot displays the significant mean differences and is produced for each paired comparison using the means of Tukey's test. Additionally, out of all doses that are assessed, 200 µg/mL of green trimetallic CuO/Ag/ZnO nanocomposite have the highest anti-biofilm value (Fig.  8 II). On a 95% scale, the modified confidence intervals are computed using Tukey simultaneous tests. There are narrow statistical differences between the recorded antibiofilm percentages intervals of 150–200, and 150–250 µg/mL doses of green trimetallic CuO/Ag/ZnO nanocomposite (pass through the zero line), as seen in Fig.  8 III. Due to the absence of zero line in the intervals for 200–250 µg/mL dose shows significant differences. So, the recorded MICs for all tested human pathogens range from 150 to 200 µg/mL (Table 9 ). An additional investigation 16 , examined the antimicrobial potential of green binary ZnO/CuO nanocomposites (irregular rod-shaped particles 7.52 nm in size) produced from Calotropis gigantea against drug-sensitive human pathogens ( Staphylococcus aureus and Escherichia coli ), multi-drug-resistant human pathogens ( Klebsiella pneumoniae , Pseudomonas aeruginosa , and methicillin-resistant S. aureus ). For S. aureus, its MICs varied between 5 and 2.5 mg/mL. Furthermore, for E. coli, P. aeruginosa, K. pneumoniae, and MRSA , the MIC values were 0.625, 0.15625, 0.625, and 0.15625 mg/mL, respectively. Therefore, our outcomes are extremely proficient, compared to earlier studies.

The 200 µg/mL of green trimetallic CuO/Ag/ZnO nanocomposite that demonstrates the highest degree of anti-microbial activity, is further focused for more antimicrobial exploration. The 200 µg/mL's time-kill kinetics are studied for every pathogen as part of time-kill analysis. Additionally, the log 10 CFU/mL levels and quantitative reduction of biofilm for each examined pathogens (treated, and untreated cells) are listed in Table 10 . The comparability of all studied human pathogens treated with 200 µg/mL of green trimetallic CuO/Ag/ZnO nanocomposite with the corresponding untreated cells are shown in Fig.  9 . As seen, there are differences in log 10 CFU/mL measurements within all tested human pathogens. Gram -positive bacteria show a significant decline in planktonic viable counts after 18 h (Fig. 9 iii, iv), however Gram -negative bacteria (Fig. 9 i, ii) and yeast cells (Fig. 9 v, vi) show a similar decline after 24 h. Among the studied bacteria, Escherichia coli , and Staphylococcus aureus show the highest percentage of biofilm reduction (98.06 ± 0.93, and 97.47 ± 0.65%; respectively), and its planktonic viable counts are effectively diminished by the tested 200 µg/mL of green trimetallic CuO/Ag/ZnO nanocomposite after 36-h period. However, the planktonic viable counts of Candida albicans (95.42 ± 1.78%) is subsequently successfully reduced after a 36-h interval (Table 10 ). The time-kill assay is also utilized to ascertain the length of 200 µg/mL of green trimetallic CuO/Ag/ZnO nanocomposite that is necessary to completely eradicate the pathogens' biofilm. The biofilms of treated Gram -positive bacteria reveal 0% CFU/ml after 52 h; however, the biofilms are destroyed by the treated yeast cells and Gram -negative bacteria after 72 and 96 h; respectively. Lastly, a promising green trimetallic CuO/Ag/ZnO nanocomposite has the potential to be used as an antimicrobial substance to suppress different human pathogens that are resistant to antibiotics.

Growth rate reduction in cell viability for all multidrug-resistant human pathogens ( i : Escherichia coli, ii : Klebsiella pneumoniae, iii : Staphylococcus aureus, iv : Bacillus subtilis, v : Candida albicans, and vi : Candida krusei ) treated with 200 µg/mL of green trimetallic CuO/Ag/ZnO nanocomposite, as well as the untreated cells during the incubation period.

The main mechanism causing the antimicrobial effect is an interaction between the pathogenic microbes' cell wall receptors and the surface of the generated nanomaterials 33 , 54 . The green trimetallic CuO/Ag/ZnO nanocomposite might have direct contact with the negatively charged microbial membrane through ions released, due to surface oxidation, complicated porosity, or electrostatic interaction. According to Noohpisheh et al., there is a strong interaction between metallic silver and semiconductor zinc oxide that splits the cell membrane and increases antimicrobial activity 73 . Numerous investigations have indicated nanocomposites have superior antimicrobial properties, compared to their individual nanoparticle counterparts 81 . Based on previous studies, the green synthesized trimetallic CuO/Ag/ZnO nanocomposite damages the microbial wall, penetrates the cytoplasm, and causes cell death; because it generates superoxide and hydroxy, which alter membrane protein as well as enzyme activity 53 , 54 , 66 , 73 . This green synthesized trimetallic CuO/Ag/ZnO nanocomposite should therefore be used in food packaging and surgical tool coatings to prevent microbial infection and strongly inhibit the growth.

Conclusions

In conclusion, the industrial green-synthesis of trimetallic CuO/Ag/ZnO nanocomposite was achieved by the utilization of an eco-friendly, straightforward approach that involved the extraction of sustainable resources of leaves from Ziziphus spina christi . Results of FTIR and phytochemical analysis showed that this extract included large concentrations of proteins, reducing sugar, anthocyanin, flavonoids, and phenolic compounds. Subsequently, the highest feasible dry weight for the green synthetic trimetallic CuO/Ag/ZnO nanocomposite was obtained by adjusting the pH of 25% diluted Ziziphus spina-christi extract (reductants) to 5. The precursors composed of (0.25M AgNO 3 , Cu (NO 3 ) 2 .3H 2 O, and Zn (CH 3 COO) 2 .2H 2 O) were subsequently prepared at a (1:1:1) ratio and gradually titrated into this plant extract. Furthermore, distinct statistically experimental designs ( Plackett Burman and Taguchi methods) were used to scaling-up the yield of green-produced trimetallic CuO/Ag/ZnO nanocomposite. Finally, the highest green synthesized trimetallic CuO/Ag/ZnO nanocomposite yield (1.65 mg/mL) might be enhanced by 1.85 and 5.7 times; respectively, by using the Taguchi approach in comparison to Plackett–Burman strategy and basal condition. In vitro, trimetallic nanocomposites have been applied to eliminate multi-drug-resistant human pathogens to determine their antimicrobial capabilities. All examined human pathogens were found to have MICs ranging from 150 to 200 µg/mL. The biofilms of treated Gram -positive bacteria showed 0% CFU/mL after 52 h. However, the treated Gram -negative bacteria and yeast cells totally eradicated the formation of biofilms after 72 and 96 h; respectively. Overall, the green synthesized trimetallic CuO/Ag/ZnO nanocomposite on a large scale has novel opportunities for the generation of antimicrobial agents that are both very stable and effective for inhibiting and preventing the microbial growth, moreover its eco-friendly generation from plant extracts.

Data availability

The datasets used and/or analyzed during the current study available from the corresponding authors (A.K. EL-Sawaf and E.A. Kamoun) on reasonable request.

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Acknowledgements

This project is sponsored by Prince Sattam Bin Abdulaziz University (PSAU) as part of funding for its SDG Roadmap Research Funding Program Project number PSAU-2023/SDG/44.

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Shahira H. EL-Moslamy: Experiments, data acquisition and analysis, interpretation of data, and wrote the original draft; Elbadawy A. Kamoun: Design of the work, draft revision, revised the final draft. Ayman El-Sawaf: Characterization and funding; and Kaizar Hossain: Data acquisition and analysis. All authors have critically reviewed and approved the final draft and are responsible for the content and similarity index of the manuscript.

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El-Sawaf, A.K., El-Moslamy, S.H., Kamoun, E.A. et al. Green synthesis of trimetallic CuO/Ag/ZnO nanocomposite using Ziziphus spina-christi plant extract: characterization, statistically experimental designs, and antimicrobial assessment. Sci Rep 14 , 19718 (2024). https://doi.org/10.1038/s41598-024-67579-5

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Physical Chemistry Chemical Physics

Computational study on the mechanism for the synthesis of the active pharmaceutical ingredients nitrofurantoin and dantrolene in both solution and mechanochemical conditions.

A combination of density functional theory (DFT) calculations and microkinetic simulations is applied to the study of the condensation in acidic media between N-acyl-hydrazides and aldehydes to produce the active pharmaceutical ingredients (API) nitrofurantoin and dantrolene. Previous experimental reports by some of us had shown that the use of ball milling conditions led to a reduction in reaction time which came associated with significant reduction of waste. This result is reproduced by the current calculations, which, moreover, provide a detailed mechanistic explanation for this behavior.

• This article is part of the themed collection: Fundamental Basis of Mechanochemical Reactivity

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D. M. Galeas, I. Tolbatov, E. Colacino and F. Maseras, Phys. Chem. Chem. Phys. , 2024, Accepted Manuscript , DOI: 10.1039/D4CP01613K

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An Experimental Study of Optimization of Biodiesel Synthesis from Waste Cooking Oil and Effect of the Combustion Duration on Engine Performance

The use of waste cooking oil (WCO) as a reagent for biodiesel synthesis ensures their transformation from harmful products into beneficial ones. The possibility of safe use as pure fuels or mixtures with diesel is a promoting and an environmental friendly alternative. This strategy is very encouraging especially for countries which have not enough space to produce vegetable oils. However, the researches in this field (WCO biodiesels) are still rare. In this work, we have synthesized biodiesel from WCO using the transesterification technique, then experimental investigations have been carried out on a four cylinder-direct injection diesel, engine equipped with a turbocharger on a test bench, according to the International norm ISO 27.020. In a first time, effects of different blends of methyl-ester/diesel in different proportions (B00, B10, B20 and B30) on engine behavior were studied and compared with petroleum diesel. In a second time, B20 blend was investigated but with variation of injection timing compared to original settings (as set by the engine manufacturer), on the same engine and following the same testing procedure. Experimental results showed that engine performances decreased with increasing amount of methyl ester in the fuel mixture. Moreover, it is found that advanced injecting B20 fuel by 2 crank angle degrees compared to that of the original injection timing, gives better performance without penalty on pollutant emissions (smoke opacity). The use of B20 accompanied with the advanced injection timing lead to a significant power increase (up to 4%) as well as an increase in torque (up to 2.8%) on conventional diesel engines compared to diesel. Emissions such as Smoke opacity remained close to the original values (without variation of injection timing).

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In the present experimental research work, used vegetable oil methyl ester (UVOME) is produced through transesterification of used vegetable oil using methanol in the presence of two different catalyst such as sodium hydroxide (NaOH) and Potassium hydroxide (KOH). Experimental investigations have been carried out to examine the combustion and emission characteristics in a direct injection transportation diesel engine running with

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The compression ignition (CI) engines are most efficient and robust but they rely on depleting fossil fuel. Hence there is a speedy need to use alternative fuels that replaces diesel and at the same time engine should yield better performance. Accordingly, honge oil methyl ester (BHO) and cotton seed oil methyl ester (BCO) were selected as an alternative fuel to power CI engine in the study. In the first part, this paper aims to evaluate best fuel injection timing (IT) and injector opening pressure (IOP) for the biodiesel fuels (BDF). The combustion chamber (CC) used for the study is toriodal re-entrant (TRCC). The experimental tests showed that BHO and BCO yielded overall better performance at IT of 19° before top dead centre (bTDC) and IOP of 240 bar. In the second part, the effect of number of holes on the performance of BDF powered CI engine was studied keeping optimized IT and IOP. The six-hole injector with 0.2 mm injector orifice diameter yielded better performance compared to other injectors of different holes and size tested.

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Alternative fuels have received much attention due to the depletion of world petroleum reserves and increased environmental concerns. The desire to reach higher efficiencies, lower specific fuel consumption and reduced emissions in modern engines has become the primary focus of engine researchers and manufacturers over the past three decades. Thus processed form of waste cooking oil (Biodiesel) offers attractive green alternative fuels to compression ignition engines. Biodiesel used in the experiment is a methyl ester of free fatty acid made from waste cooking oil (WCO).The fuel properties of biodiesel are very similar to the diesel fuel, so it can work in existing infrastructure of conventional diesel engine without any modification in the engine.The present work investigates and compares the engine performance parameters such as brake power and brake specific fuel consumption and emission characteristics such as CO, CO 2, HC and NO x emissions of direct injection Kirloskar diesel engine using various blends of waste cooking oil biodiesel and diesel. The biodiesel blends B10, B15, B20, B25, B35, B50, B75 and B100 were tested and B20 was found as the optimum blend based on the experimental results as it had properties comparable to diesel and lower emissions than diesel. From the results of investigation it was found that, there has been a decrease in brake power with an increase in brake specific fuel consumption for all blends of biodiesel over the entire load range when compared to the diesel fuel. In the case of engine exhaust gas emissions; lower HC, CO and higher CO 2 and NO x emissions have been found for all biodiesel blends when compared to diesel. Moreover, reductions in sound level for all biodiesel blends have been observed when compared to diesel.B20 was found as the best fuel as it showed lower emissions, better properties and economy than diesel.

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https://www.ijert.org/experimental-investigation-of-castrol-oil-methyl-esters-as-biodiesel-on-compression-ignition-engine https://www.ijert.org/research/experimental-investigation-of-castrol-oil-methyl-esters-as-biodiesel-on-compression-ignition-engine-IJERTV2IS2105.pdf The methyl esters of vegetable oils, known as biodiesel are becoming increasingly popular because of their low environmental impact and potential as a green alternative fuel for diesel engine and they would not require significant modification of existing engine hardware. Methyl ester of castor (CME) derived through transesterification process. Experimental investigations have been carried out to examine properties, performance and emissions of different blends (B10, B20, and B40) of castor in comparison to diesel. The use of alternatives for the fossil fuels, such as castor, jatropha, pongamia, etc. is very essential. It has been found that biodiesel plays an important role in the automobile industry now a day. This work aims at reducing the cost of the fuel consumed by blending the biodiesel from castor oil with diesel with different proportions and testing the performance of blended diesel. Initially the engine has to run by diesel and the following characteristics of Brake Power, Total Fuel Consumption, Indicated Power, Mechanical Efficiency, Brake Thermal Efficiency, and Volumetric Efficiency are calculated. Then same procedure is followed for biodiesel blend with varying the proportion by 20%, 40%, 70%, and then by 100% biodiesel. The Characteristics are obtained and compared with diesel in a graph. Based on the performance analysis the efficiency of B-40 just decreases by 2.62% then of pure diesel, which gives an inference that the biodiesel is an acceptable alternative fuel.

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