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Review

Heterogeneous Catalysts from Food Waste for Biodiesel Synthesis—A Comprehensive Review

by
Violeta Makarevičienė
*,
Ieva Gaidė
and
Eglė Sendžikienė
Agriculture Academy, Vytautas Magnus University, K. Donelaicio Street 58, LT-44248 Kaunas, Lithuania
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(10), 957; https://doi.org/10.3390/catal15100957
Submission received: 8 September 2025 / Revised: 2 October 2025 / Accepted: 3 October 2025 / Published: 5 October 2025
(This article belongs to the Special Issue Applications of Catalysis in Organic Chemistry: Sustainable Catalysts for Sustainable Processes)
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Versions Notes

Abstract

The transesterification process of vegetable oil applied in biodiesel synthesis is catalytic. Industrial production uses chemical catalysts that are difficult to separate from the product, regenerate, and reuse, which is why there is a search for new catalysts that are of natural origin or obtained from various types of waste. Calcium oxide is widely used as a heterogeneous catalyst, and can be obtained from calcium carbonate. The article reviews the possibilities of using eggshells as a catalyst for biodiesel synthesis: the optimal calcination conditions, the efficiency of the obtained catalyst, the optimal transesterification conditions, and the influence of various factors on biodiesel yield. It also discusses the possibilities and conditions for regenerating the catalyst and reusing it. Another food industry waste containing calcium compounds is animal bones, from which an effective biodiesel synthesis catalyst can be obtained. Before use, the bones are also crushed and calcined. The article presents the conditions for catalyst preparation and catalytic activity, and the possibilities for its enhancement by incorporating other elements, as well as the dependence of ester yields on transesterification conditions. The process of catalyst regeneration and reuse is discussed.

Graphical Abstract

1. Introduction

The development of biofuel production and use is inevitable due to the need to reduce greenhouse gas emissions and mineral resource consumption, while implementing environmentally friendly, sustainable technologies. Biodiesel is produced and used in many countries of the world from various raw materials, which are characteristic of specific climatic conditions and prevailing vegetation types in these countries. For its production, vegetable oil and suitable oily waste are mainly used, which are cheaper sources of raw materials and are encouraged to be used in implementing the EU Green Deal goals related to the development of renewable energy and the efficient use of waste in all sectors of the economy.
During the synthesis of biodiesel, the transesterification reaction of triglycerides with short-chain alcohols takes place. In this process, biodiesel (fatty acid esters) is formed [1]. Methanol is most often used as the acyl receptor in the transesterification reaction. Under normal conditions (atmospheric pressure, room temperature, and light) the process occurs very slowly, so catalysts are used to increase the reaction rate. Homogeneous catalysts are most widely used, the most effective and cheapest of which are alkaline catalysts of chemical origin. Among the acidic catalysts, sulfuric acid, hydrochloric acid, and sulfonic acid are used [2]. They are more suitable when the oil contains a large amount of free fatty acids because they catalyze the esterification reaction. Although homogeneous catalysts are most widely used in industrial biodiesel production, their use has a number of disadvantages. Alkaline catalysts with free fatty acids form soaps, which are good emulsifiers, making it difficult not only to separate them from the synthesized biodiesel but also to separate the biodiesel, methanol, and glycerol phases after the reaction is completed. This reduces the biodiesel yield, making it difficult to clean the product of impurities. Problems arise when using acidic homogeneous catalysts due to the aggressiveness of the acids and corrosion of equipment. Heterogeneous catalysts help to avoid the above problems. Most heterogeneous catalysts are solid, and the transesterification process occurs on the active centers on their surface. After the reaction is complete, the biodiesel molecules desorb from the catalyst surface, making it easy to separate them from the reaction mixture. Soaps are not formed, and the product is easier to purify [3]. In addition, the energy-intensive glycerol purification step is not required. Solid acids and solid bases are used as heterogeneous catalysts in biodiesel synthesis. The main solid acid catalysts are zeolites, heteropoly acids, and pure or modified oxides of transition metals such as zirconium, molybdenum, silicon, and aluminum. These catalysts cause fewer corrosion problems and are more environmentally friendly. Acidic zeolites are characterized by a high porosity. Instead of sulfuric acid, which is used in homogeneous biodiesel synthesis, sulfated zirconia and organic sulfuric acids are used [4]. Solid bases include basic zeolites, metal carbonates, and oxides. Among solid base catalysts, CaO, MgO, SrO, BaO, CaCO3, MgCO3, SrCO3, and BaCO3 have good catalytic properties. They are inexpensive, easy to prepare, and do not cause corrosion. Metal oxides can be extracted from natural rocks. Before being used as a catalyst, they are specially prepared. Calcium or magnesium oxides are obtained thermally, and zinc oxide is obtained hydrothermally. Oxide mixtures are obtained through impregnation and heat treatment [5].
Metal oxide heterogeneous acid catalysts can also be used as catalysts for biodiesel synthesis. They can replace homogeneous acid catalysts, have a high catalytic efficiency, can be used with the high free acid content in feedstock and in high humidity, and can be regenerated and reused. Among such catalysts are silica-based catalysts, molybdenum, tungsten, tin, iron, and aluminum oxide catalysts, titania and zirconium-based catalysts, and mixed oxide-based catalysts [6]. Recently, there has been an increasing interest in MOF-derived oxide-based catalysts [7]. However, their industrial use is complicated due to the complex preparation process, leaching of their components in the reaction medium, potential secondary pollution during their production, and difficult disposal of spent catalysts.
In applying the principles of sustainable production and implementing the goals of the Green Deal, opportunities are being sought to use natural or waste raw materials in all industrial areas. In biodiesel production, the possibility of using waste oil is being studied, and chemical synthetic catalysts are being replaced with more environmentally friendly ones obtained from natural sources.
Natural minerals and wastes containing calcium compounds could be used as heterogeneous catalysts in biodiesel synthesis, which, when properly prepared, yield calcium oxide, which has one of the best catalytic properties among heterogeneous catalysts. The preparation process and efficiency of the widespread rock dolomite in the process of triglyceride transesterification have been extensively studied. Considerable attention has also been paid to the possibilities of using suitable wastes. Among these wastes, eggshells, which are a waste from the food industry and the amount of which is large in the world, have considerable prospects. About 86.67 million tons of eggs are consumed in the world [8]. Considering that about 10% of the weight of an egg is made up of its shell, about 8.67 million tons of shells are generated annually [9]. Another animal waste containing calcium salts is animal bones. Some of them are used in the feed industry as a source of calcium, but a higher value-added product would be a heterogeneous catalyst for biodiesel synthesis obtained from animal bones. The use of both eggshells and animal bones is valuable in that, by optimally utilizing suitable waste, it is possible to obtain an effective and cheap catalyst for the biodiesel industry, replacing the synthetic chemical homogeneous alkali catalysts currently used in biodiesel industrial production. This is proven by the results of scientific research. They are reviewed in this article.

2. Food Industry Waste—A Resource for a Heterogeneous Catalyst for Biodiesel Synthesis

2.1. Preparation of Eggshells for Triglyceride Transesterification

Eggshells contain about 94% calcium carbonate; the rest is made up of various minerals such as calcium phosphate, magnesium carbonate, and organic matter. Due to its composition, eggshells are a potential catalyst for biodiesel synthesis. In order for it to acquire catalytic properties, calcium carbonate must be decomposed into calcium oxide, which has a high catalytic activity. The decomposition takes place at high temperatures for a certain time so that all CaCO3 is converted to CaO. Most studies have been conducted with chicken eggshells, but guinea fowl eggshells have also been studied. The calcium oxide content, surface area, and basic strength of the prepared eggshells were tested. The eggshells were calcined at temperatures of 800–1000 °C [10,11]; the decomposition temperature of calcium carbonate is 825 °C. The optimal calcination time selected by scientists ranged from 2 to 5 h (Table 1).
Viriya-empikul et al. [12] and Khemthong et al. [10] calcined eggshells at 800 °C for 4 h and determined the CaO content to be 99.2% and 99%, respectively. Kara et al. [19] calcined chicken eggshells for 2 h at 850 °C and determined a 98.21% CaO content, and Gaide et al. [22] calcinated eggshell powder for 5 h at 850 °C, yielding an 89.01 ± 0.79% CaO content. Eggshells calcined at 900 °C for 2 h yielded a 97.1% CaO content [14], while, at the same calcination temperature and a 3 h duration, the CaO content was 99.20 % [17]. Some researchers calcined eggshells at 1000 °C for 4 h and found 98.5% CaO in guinea fowl shells and 98.0% in chicken shells. When examining different eggshells calcined under the same conditions, very similar CaO contents were found [11]. Therefore, it can be concluded that the calcination conditions have a greater influence than the type of egg. The CaO content in all calcined eggshells was high and ranged from 89.01% to 99.2%. When determining the BET surface area of the calcined eggshells, contradictory results were obtained. Ahmad et al. calcined waste eggshells at a temperature of 900 °C for 3 h and determined a BET surface area of 64.51 m2/g with a pore size of 9.28 nm [21]. Tan et al. calcined eggshells at 1000 °C for 4 h and determined a BET surface area of 54.6 m2/g, while Niju et al. calcined eggshells at 900 °C for 2.5 h and obtained a BET area of 3.7262 m2/g [13]. A smaller surface area was reported by other scientists. Rahman et al., after 4 h of calcination at 900 °C, reported a 2.98 ± 0.01 m2/g BET surface area [20], while Picker et al. indicate a 1.8 m2/g surface area for eggshells calcined at 900 °C for 3 h [16].
Some researchers have impregnated eggshells with metal salts to improve their catalytic properties. Rahman et al. studied eggshells calcined for 4 h at 900 °C and found that their BET surface area was 2.98  ±  0.01 m2/g, with a basic strength of 7.2 < H_ < 9.8. The calcined eggshells were soaked in a metal nitrate aqueous solution and calcined again for 4 h at 900 °C. The resulting Cu-CaO had a BET surface area of 11.92  ±  0.01 m2/g and a basic strength of 12.2 < H_ < 15. Zn-CaO had even better properties, with a BET surface area of 13.73  ±  0.01 m2/g and a basic strength of 15.0 < H_ < 17.2 [20]. The resulting metal oxides had a higher BET surface area and basic strength, but additional chemicals were used and energy consumption was doubled. Falowo et al. produced a bifunctional catalyst CaSO4/Fe2O3 from waste eggshells through the impregnation of calcined eggshell at 800 °C for 3 h with ferric sulphate at a ratio of 1:1 [24]. The catalyst composition contained 20.7% Ca, 18.5% Fe, 4.5% S, and 54.8% O.

2.2. Preparation of Animal Bones for Triglyceride Transesterification

Other organic food wastes that could be used as catalysts for biodiesel synthesis are waste animal bones. They contain about 40% calcium phosphate, which, upon calcination, can be obtained to produce calcium oxide, which catalyzes the transesterification reaction. At the same time, beta-tricalcium phosphate is formed. Calcined bone contains hydroxyapatite [Ca10(PO4)6(OH)2], which is highly porous and also has a large surface area, allowing the catalyst to disperse over it largely and effectively. Hydroxyapatite has good catalytic properties, which depend on the composition of the bones of different species of animals.
The conducted studies and their results show that using bones for biodiesel synthesis requires their appropriate preparation. Most authors indicate that the bones must first be washed with hot water or an alkali solution, dried at a temperature of 70–100 °C, crushed, and calcined at a high temperature. Calcined bones below 650 °C do not exhibit catalytic activity. Calcined bones above 900 °C lose catalytic efficiency due to a decrease in the active surface area. Smith et al., studying bovine waste calcination, found that the decomposition of calcium carbonate occurs at temperatures of 635–865 °C. Above 950 °C, there were no diffraction peaks corresponding to the CaCO3 phase in the sample that was calcined [25]. That is, all calcium carbonate was decomposed into CaO, and IR analysis showed that all organic impurities were removed from the bones. A goat bone calcination process was carried out at 800–1000 °C. XRD results showed that, at 900 °C, sharper peaks indicating crystallinity were observed, and the smallest crystallite size of 41.47 nm and the largest surface area of 90.65 m2/g were obtained by calcining at 900 °C [26]. In addition, the catalyst calcined at 900 °C had a better porous structure and a maximum pore volume of 0.05 cm3g−1. Calcined bones are basic catalysts; their basic strength is from 10 to 15 H- [27]. The surface properties of the catalysts obtained from animal bones were also studied by Jazie et al. [26]. They found that increasing the calcination temperature to 900 °C increases the surface area, total pore volume, and crystallite size (Table 2). An even larger surface area was obtained by Farooq et al. [28] after calcining waste animal bones at 1000 °C.
However, Smith et al. studied the use of bovine bones for the transesterification of soybean oil and found that the highest number of esters was obtained by calcining the bones at 650–750 °C for 6 h [25]. They concluded that the CaO present in the calcined bones is responsible for catalytic activity. However, these results contradict the observations of Jazie et al. [26], who used waste animal bones calcined at higher temperatures for biodiesel synthesis. Using bones calcined at 800 °C, they obtained a biodiesel yield of 80%, while, using bones calcined at 900 °C, they achieved a yield of 95%, and, using bones calcined at 1000 °C, the yield was lower and reached 82%. This is explained by the high-temperature sintering of the catalyst.
The suggested calcination time for bones is about 4 h. A considerable part of the studies was carried out using fish bones, which, in some cases, showed good catalytic properties and high ester yields. However, compared with the activity of other calcium-containing catalysts derived from natural raw materials, the catalysts derived from bones showed a lower activity in the transesterification reaction with methanol: more of them were needed to obtain high yields of fatty acid methyl esters.

2.3. Efficiency of Eggshells in the Triglyceride Transesterification Process

Table 3 presents a comparison of the results of research by scientists using eggshells as a heterogeneous catalyst in biodiesel production. The CaO content of calcined eggshells was higher than 89%, but the shells loaded during the transesterification process varied greatly, ranging from 1.5 wt% [11,15] to 10 wt% [12] and 15 wt% [10]. A fatty acid methyl ester (FAME) content higher than 96%, which is specified in European standard EN 14214, was obtained using both 2% and 15% calcined eggshells. Oyelaran et al. obtained a 96% FAME yield using 1.5% calcined guinea fowl eggshells [11], while Khemthong et al. obtained a 96.7% ester yield using 15% calcined eggshells by performing transesterification for 4 min with 900 W of microwave power [10].
Transesterification reactions are most efficient when the process temperature is close to the boiling point of alcohol (Table 3). Most researchers have used methanol and synthesis temperatures of 55–65 °C, except for Picker et al. [16], who obtained soybean oil methyl ester yields of 98% at 25 °C, and Rahman et al. [23], who obtained a 94.5% waste palm oil methyl ester yield at 150 °C. Erchamo et al. compared the transesterification efficiency of waste cooking oil with methanol and methanol–ethanol (8:4) mixtures under the same reaction conditions: 2.5 wt% eggshell, 2 h, 12:1. A higher FAME yield of 94% was obtained when methanol was used; the yield using the methanol–ethanol mixture reached 92% [34]. Gaide et al. studied the use of 1-butanol and calcined eggshells in the transesterification of rapeseed oil, obtaining an ester content of 98.78%, using a process temperature of 110 °C (close to the boiling point of 1-butanol), a catalyst loading of 7.41 wt%, a duration of 11.81 h, and a molar ratio of 11.3:1 [22].The molar ratio of alcohol to oil has a significant impact on the transesterification process; the minimum theoretical ratio for the reaction to occur is 3:1. The transesterification reaction is reversible. To obtain a higher FAME yield, excess alcohol is taken; most often in industrial processes the molar ratio of methanol to oil reaches 6:1. If the ratio is too high, reversible reactions may begin and the transesterification will not be effective. The lowest molar ratio of 5:1 was determined for the transesterification of waste cooking oil, obtaining an ester yield of 89.94% [24]. The highest molar ratio of 30:1 methanol to oil was selected for the transesterification of chicken fat with methanol, obtaining a yield of 90.2% [33]. Some researchers have obtained high FAME yields of up to 96% using a 6:1 molar methanol-to-oil ratio [14,16], but others have used higher ratios of over 10:1 to obtain high yields. Yaşar studied the use of CaO and calcined eggshells in the transesterification of rapeseed oil and the effect of the molar ratio of methanol to oil. When the molar ratio of methanol to oil was 9:1, the biodiesel yield was 96.81% using CaO as a catalyst, and a biodiesel yield of 95.12% was obtained using the same molar ratio and eggshells as a catalyst. Although some researchers have used a 30:1 molar ratio of methanol to oil to obtain high yields of FAME, the results reported by Yaşar were contradictory. When the molar ratio of methanol to oil was increased from 9:1 to 10:1, it was observed that the product yield decreased [29].
The nature of the eggshells does not affect the catalytic efficiency. When comparing the use of different types of equally calcined guinea fowl and chicken eggshells in the transesterification of calabash oil with methanol, the obtained ester yields of 96 and 95% were not significantly different; the synthesis was carried out under the same conditions (64 °C, 1.5 wt% eggshell, 2 h, 12:1) [11].
Synthesis conditions are important for transesterification, but the efficiency of the process is determined by the obtained ester yield or ester content. Many studies have shown that the ester yield is greater than 90 wt%. Only a few scientists point to the low yield of FAME. Kumar et al., using cooking waste transesterified with methanol (molar ratio 6:1) and catalyst eggshells, reached a 20.1% ester yield at 50 °C within 2 h [32]. Kamaronzaman et al. obtained a 45.52% waste cooking oil methyl ester yield within 2 h when the temperature was 65 °C, with a chicken eggshell loading of 5 wt% and a methanol-to-oil molar ratio of 20:1 [31]. The marine fish waste methyl ester yield was 86.5% when the chicken eggshell loading was 5 wt%, the molar ratio was 25:1, and the duration was 107 min at 57 °C [36].
Many researchers report FAME yields greater than 95%. Correia et al. obtained a 99% sunflower oil methyl ester yield, with a reaction time of 2 h, a temperature of 60 °C, a calcined chicken eggshell loading of 2 wt%, and a molar ratio of 10.5:1 [17]. Kara et al. also obtained a high sunflower oil methyl ester yield of 96% (60 °C, 3 wt%, 3 h, 9:1) [19]. Yaşar studied the transesterification of rapeseed oil with methanol, and a FAME yield of 95.12% was obtained when the waste eggshell loading was 4 wt%, at 60 °C, 9:1, and a time of only 1 h [30]. Wei et al. obtained the same yield of soybean methyl ester when the reaction was carried out at 65 °C for 3 h, at 3 wt% and at 9:1 [29].
By optimizing the transesterification process of rapeseed oil using calcined chicken eggshells as a catalyst and different alcohols, a fatty acid methyl ester content of 97.79% was determined and a fatty acid butyl ester content of 98.78% was obtained [22,35], while the EN14214 standard requires at least 96.5%.
Ahmad et al. used waste eggshells for the direct transesterification of microalgae Chlorella pyrenoidosa with methanol, obtaining an ester yield of 93.44% in 3 h, at 60 °C temperature, 2.06 wt% catalyst loading, and a ratio of 30:1 of methanol to algal biomass [21].
Niju et al. studied the catalytic properties of differently processed waste eggshells and compared them with commercial CaO. One way to prepare eggshells is to calcine them at 900 °C to obtain “Egg shell-CaO-900”. Another way is to calcine the shells at 900 °C, then boil them in water at 60 °C for 6 h; the solid is filtered and dried in a hot air oven at 120 °C for 24 h, then dehydrated by calcining at 600 °C for 3 h to convert the hydroxide form to the oxide form “Egg shell-CaO-900-600”. The catalysts obtained were used for the transesterification of waste frying oil at 65 °C, with a catalyst content of 5 wt%, duration of 1 h, and methanol-to-oil molar ratio of 12:1. Commercial CaO yielded 67.57% biodiesel, eggshell-CaO-900 yielded 79.62%, and eggshell-CaO-900-600 exhibited the highest catalytic activity and achieved a high biodiesel conversion of 94.52%.
Rahman et al. have investigated the use not only of calcined eggshells, but also of eggshells that are calcined and impregnated with metal nitrate catalysts [20,23,37]. After performing the transesterification of eucalyptus oil with methanol using CaO obtained from eggshells impregnated with copper nitrate and zinc nitrate, the ester yield was determined to be 90.6% and 93.8%, respectively (synthesis conditions: 65 °C, 5 wt%, 2.5 h, and 6:1). When using pure CaO at the same conditions, the yield reached 70%.
A rubber seed oil methyl ester yield of 80.2% was obtained when calcined eggshells were used as the catalyst within 1.5 h, and a FAME yield of 94.12% was obtained using catalyst-impregnated Zn-CaO shells within 2 h (synthesis conditions: 55 °C, 5 wt%, and 12:1) [37]. Waste palm oil transesterification with methanol at 150 °C, using 7 wt% Zn-CaO from eggshells within 3 h at a molar methanol-to-oil ratio of 12:1, yielded 94.5% [23]. Rahman et al. concluded that impregnated eggshells are a more efficient catalyst than calcined eggshells alone. However, other researchers obtained no worse ester yields from calcined shells without additional treatment. Using a bifunctional catalyst from waste eggshells, CaSO4/Fe2O3, for the transesterification of waste cooking oil with methanol, only an 89.94% ester yield was obtained at 70 °C, with a catalyst loading of 4 wt%, an alcohol–oil molar ratio of 5:1, and a reaction duration of 45 min [24].

2.4. Efficiency of Animal Bones in the Transesterification of Triglycerides

Table 4 presents a comparison of the results of research by scientists using animal bones as a heterogeneous catalyst.
As can be seen from the data presented in Table 4, the catalyst obtained from calcined animal bones has a lower catalytic activity than the catalyst obtained from eggshells. High FAME yields, as required by the EN 14214 standard for biodiesel fuel, can only be obtained by using a large amount of catalyst. In some cases, using only 20% of the catalyst, a yield of methyl esters exceeding 96% was obtained. Corro et al. [42] obtained a 96% FAME yield using 20% calcined cow bones and a 12:1 molar ratio of methanol to oil for 8 h, but the process was performed at 70 °C, and Obadiah et al. [44] obtained a 96.78% FAME yield using 20% calcined sheep bones, but a higher methanol-to-oil ratio of 18:1 was used. With a lower amount of catalyst, the ester yield did not reach 95%, even with a large excess of methanol. However, according to other authors [26], using as much as 20% of the catalyst obtained through the calcination of animal bones, only a 94% FAME yield is obtained at the same molar ratio of methanol to oil, at a lower process temperature and a higher bone calcination temperature. A significantly lower amount of catalyst was used by Suwannasom et al. [38], who obtained a 96% FAME yield using 3% of the catalyst obtained by calcining chicken bones and a relatively low molar ratio of methanol to oil, which reached 3:1. However, in this case, the process was performed at a temperature of 80 °C.
The molar ratio of methanol to oil is one of the most important factors influencing the yield of esters. The data presented show that this molar alcohol-to-oil ratio, when using bones as a catalyst, is significantly higher than required by stoichiometry and used in conventional biodiesel production. Compared to the use of eggshells, a larger excess of methanol is also required. When using calcined bones, a methanol-to-oil ratio of 12:1 and higher was used, and the best results were obtained using a molar ratio even of 18:1. Most of the results showed that, using this molar ratio, the FAME yield is still less than 96%. Sulaiman et al. [40] obtained only a 90% yield, Buasri et al. [42] obtained a 94% FAME yield, and Jazzie et al. obtained a 94% yield [26]. Only Obadiah et al. [44] reported a 96.78% FAME yield using a molar methanol-to-oil ratio of 18:1.
However, some authors have reported FAME yields higher than 96% using a small excess of methanol. Suwannasom et al. [38] used a 3:1 molar ratio of methanol to waste cooking oil and obtained a 96% FAME yield using a 3% catalyst obtained from chicken bones in 3 h. A high yield was also obtained by Chakraborty et al. [27]: a 97.73% FAME yield was obtained in 5 h using a 6.27:1 molar ratio of methanol to soybean oil and a 1.01% catalyst obtained from waste fish scales.
The process duration is indicated by many authors to be quite short, on average 4 h, and, in addition, by using microwaves, the process duration can be significantly shortened. It is clear from the data presented that the yield of the transesterification process depends on four independent variables: the amount of catalyst, the molar ratio of methanol to oil, the process duration, and the temperature. Although increasing the temperature accelerates the chemical reactions, a temperature higher than the boiling point of alcohol is undesirable due to alcohol losses from evaporation and more difficult working conditions, so many authors conducted their studies at 60–65 °C.
The obtained research results show that calcined bones alone do not exhibit a very high catalytic efficiency in the transesterification of oil with methanol because a large amount of catalyst and excess methanol are required to obtain a biodiesel yield of over 96%, as required by the EU standard EN 14214. In order to increase the activity of the catalyst obtained from animal bones, various methods have recently been investigated to increase the surface area or basic strength of the catalyst. The efficiency of the modified bone catalysts and the transesterification conditions are presented in Table 5. Chingakham et al. [48] proposed a hydrothermal treatment method for processing animal bones and found that it is possible to obtain a 96% FAME yield using only 2.5% calcined animal bones hydrothermally treated at 200 °C at a 12:1 methanol-to-oil ratio in a relatively short time of 2 h. Nissar et al. [49] used calcined animal bones modified with KOH (10%) for the transesterification of Jatropha curcas oil and found that a 6% catalyst was sufficient to obtain a 96.1% FAME yield in 3 h, but at a temperature of 70 °C.
Studies have been conducted using KOH and K2CO3-supported catalysts on calcined bones, and it was found that the biodiesel yield increases with an increasing number of loaded materials. Calcined chicken bones impregnated with Li/Zn showed good results. Using 4% of such a catalyst, a yield of as much as 98% waste canola oil methyl esters was obtained using an 18:1 methanol-to-oil molar ratio [54] within 3.5 h. Chen et al., obtained a 96.4% FAME yield using the catalyst of 8% calcined pig bones impregnated with K2CO3, when the methanol-to-oil molar ratio was 9:1 [52]. An even higher FAME yield of 98% was obtained by Alsharifi and Znad [54] using 4% calcined chicken bones impregnated with Li/Zn within 3.5 h using a 18:1 methanol-to-oil molar ratio. Other modification methods did not show a significant increase in catalyst activity.
The data presented show that the highest FAME yield was obtained by modifying calcined bones with KOH and impregnating them with Li/Zn. It can also be observed that using modified bones requires a slightly lower amount of catalyst than using unmodified calcined bones. In addition, a higher yield is obtained by using a lower methanol-to-oil ratio than in the case of the unmodified bone catalyst.

2.5. Regeneration and Multiple Reuse Possibilities of Catalysts Derived from Eggshells

The use of heterogeneous catalysts is attractive because they are easy to separate from the reaction medium and can be regenerated and reused. This reduces the cost of biodiesel production and reduces environmental pollution. Catalyst reuse is an important indicator of the efficiency of the biodiesel production process. Some researchers have used catalysts repeatedly without additional treatment, while others have regenerated them. The obtained results are presented in Table 6.
Khemthong et al. found that the used waste eggshell was separated by centrifugation and reused for the next reaction test without pretreatment or regeneration, and can be used five times for the transesterification of palm olein [10].
The results of Picker et al. showed that the catalyst can be reused ten times with soybean oil (after the 9th recycling, the FAME yield decreased from 95 to 75 wt%) and five times with waste cooking oil (after the 4th recycling, the FAME yield decreased from 93 to 62 wt%). The authors explain the difference between the number of cycles where the catalyst can be reused with soybean oil and waste cooking oil, which can be explained by the fact that the free fatty acids in used cooking oil destroy the catalytic activity of CaO faster than soybean oil [16]. Based on the results of the research by Picker et al., it can be seen that the reuse of the catalyst is greatly influenced by the oil from which biodiesel is produced.
Other researchers have treated the spent catalysts before reuse. After each cycle, the chicken eggshell was separated from the reaction mixture and re-calcined at 700 °C for 2 h. The catalyst was used for five cycles, and it was found that a 90% FAME yield was obtained in the first cycle and a 75% yield was obtained in the fifth cycle [18].
Researchers found that an eggshell-derived catalyst could be reused 13 times for the transesterification of soybean with methanol without an apparent loss of activity by calcining it at 1000 °C after each cycle. After being used more than 13 times, the eggshell-derived catalyst gradually lost its activity. The eggshell catalyst was completely deactivated after being used more than 17 times [37]. Kara et al. found that an eggshell catalyst could be used up to eight times in transesterification reactions without an apparent loss of activity. After each cycle, the spent catalyst was dried for reuse. The catalytic activity remained between 80% and 96%. After that, the catalyst gradually lost its activity after being used more than five times. The catalyst was completely inactivated after being used more than 10 times. However, the catalyst can be regenerated by calcination [19].
Niju et al. washed the catalyst separated from the reaction mixture with methanol after each cycle and re-calcined it at 600 °C for further use. The results show that a high biodiesel yield exceeding 90% was achieved in all six cycles [13]. Correia et al. separated the chicken eggshell catalyst between each cycle by centrifugation, then washed it twice with methanol, dried it for 30 min at 100 °C, and stored it in a desiccator. Before reuse, the catalyst was re-calcined (900 °C for 3 h). After each use of the catalyst, its activity decreases, as does the ester yield to 99.00 ± 0.02 wt.% (1st run), 86.14 ± 0.02 wt.% (2nd run), and 78.26 ± 0.04 wt.% (3rd run) of FAME [17]. Ahmad et al. used the waste shell catalyst for seven cycles. After each cycle, the catalyst was separated from the reaction mixture by centrifugation, then washed with hexane and dried overnight in an oven [21]. Other researchers washed the catalyst with hexane and calcined it before reuse. Tan et al. re-calcined the used and hexane-washed catalyst at 700 °C. The biodiesel yield was still about 75% after the 5th cycle [15]. Scientists who studied the potential of using calcined and Zn-impregnated eggshells (Zn-CaO) for the transesterification of eucalyptus oil [20] and waste palm [23] found that, after washing the catalyst with hexane (95.99%) and calcination at 900 °C for 4 h after each cycle, it can be used six times; with each cycle, a lower ester yield is obtained, and the catalytic properties of the catalyst decrease. Falowo et al. found that a bifunctional catalyst from eggshells (CaSO4/Fe2O3) can be used four times; the biodiesel yield decreased from 89.49% to 81.4% and, after the fifth cycle, the yield reached 76.5% [24]. After reviewing the results of the scientists’ research, it is not possible to conclude the best/most effective catalyst treatment for reuse. Because some researchers used the catalyst for the same number of cycles without additional treatment, others obtained the same results obtained by calcining it at a high temperature.

2.6. Regeneration and Multiple Reuse Possibilities of Catalysts Derived from Animal Bones

Researchers have investigated the use of calcined bones in biodiesel synthesis and have analyzed the possibilities of their regeneration and reuse. The results of the studies are presented in Table 7. From the data presented, it is clear that, when using animal bones or their waste calcined at a temperature of 800–900 °C for biodiesel production, the catalysts are separated after use, washed with solvents from the products formed during the transesterification process, and dried.
Among the solvents studied were methanol, ethanol, hexane, and acetone. In rare cases, an additional calcination process was studied, since this is associated with additional energy consumption. When using calcined bones or their waste, the activity of the catalysts is, on average, sufficient for 4–5 cycles of their use. It was found that, when washed with solvents and dried, the catalyst can be used at least four times. However, some authors have obtained good results without additional catalyst treatments. Suwannasom et al., using a waste chicken bone catalyst, found that it was effective after four cycles [38], while Corro et al. observed that the catalytic activity of a catalyst obtained from cow bones calcined at 800 °C remained for ten cycles without using any treatment procedures [42]. Chakraborty et al., using calcined fish scales, showed that the catalyst was active for six consecutive experimental runs under optimal process conditions without a remarkable deactivation without additional treatment [27].
The authors studied the regeneration of the catalyst by washing with methanol and found that the catalyst can be used for 4–6 cycles. Jazie et al. report that calcined goat bones can be reused six times after washing with methanol [26], while Nisar et al. found that calcined animal bones modified with KOH remained active for four cycles when washed with methanol. Chingakham et al. found that calcined animal bones hydrothermally treated at 200 °C and washed with methanol yield a 96% FAME retention until the 5th cycle of the transesterification reaction [48], while Voli et al. used a centrifuged and methanol-washed catalyst obtained from calcined sheep bones impregnated with ash powder, with only a slight decrease in catalytic efficiency over five cycles [51].
Some authors suggested washing with ethanol for catalyst regeneration. Khan et al., after each cycle, cleaned an ostrich bone catalyst with pure ethanol and desiccated it at 100 °C for 3 h in a laboratory oven [47]. The catalytic activity remained consistent for four repeated runs, with minor yield loss. Using a calcined quail waste head catalyst under the same conditions with ethanol, Khan et al. observed that the activity of the catalyst was found to be very consistent for five repeated runs without a considerable yield loss [46]. Obadiah et al. investigated the reuse of a catalyst by washing it with acetone [44]. The catalyst obtained from calcined waste animal bones was separated from the reaction mixture, washed with diethyl water and acetone, and dried. The catalyst thus prepared was effective for five cycles.
Some authors used hexane washing for catalyst regeneration. However, in this case, the catalyst was separated from the reaction medium by centrifugation, washed with hexane, and then calcined. Tan et al. calcined at 1000 °C and found that a calcined chicken and fish bone catalyst can be reused up to four times without a significant drop in catalytic activity [46]. Faroog et al. applied hexane washing and used a lower calcination temperature. It was found that, when washed with n-hexane and calcined at 400 °C, the catalyst remained active for four cycles of use [28].

2.7. Comparison of Preparation Conditions and Efficiency of Catalysts Obtained from Eggshell and Animal Bones

A comparison of the preparation and efficiency of catalysts in the transesterification reaction is presented in Table 8. Considering that not all researchers provide catalyst characteristics, it is not possible to compare them and distinguish the catalysts with the greatest influence on the activity. It is also very difficult to compare the influence of catalysts on the biodiesel yield depending on the conditions of catalyst preparation because the transesterification process is influenced by as many as four independent variables that interact with each other and affect the biodiesel yield. In order to assess only the influence of the catalyst, the process should take place under the same conditions, but the authors conducted experiments with different molar ratios of alcohol and oil and process durations, although the temperature was kept the same.
The data presented in the table show that the applied calcination temperature had a small effect on the calcium content in the catalyst. At both 800 °C and 1000 °C, most of the CaCO3 decomposed into CaO. After conducting XRD diagrams of calcined eggshells, scientists found that the samples are well calcined, and most of the catalyst consists of high-purity CaO with a large alkaline concentration (with a large basic strength and a variety of basic site concentrations) [10,14]. In thermally calcined eggshells, the appearance of groups at 3642 and 3458 cm–1 was attributed to the -OH stretching vibration of Ca(OH)2 and H2O, respectively. The presence of these substances was related to humidity [14,15,18]. The BET demonstrated that the area of the calcined chicken- and ostrich-eggshell catalysts was higher than that of the pure commercial calcium oxide and that before the calcination process. From here, it can be concluded that calcination has a positive effect on the performance of the catalysts due to the increase in BET surface area [15]. At 800 °C and higher calcination temperatures, the morphology, studied using a Scanning Electron Microscope, showed that the particles become smaller, like in a raw eggshell [18]. Powder X-ray diffraction data suggested that the commercial bovine bone is composed of highly crystalline calcium carbonate, CaCO3, in the form of calcite, and poorly crystalline hydroxyapatite, Ca10(PO4)6(OH)2 (denoted as HAp). Calcination at temperatures between 650 and 850 °C resulted in some carbonate decomposition to form CaO. At calcination temperatures of 950 °C and above, the PXRD patterns implied that the CaCO3 → CaO conversion was complete, resulting in the absence of diffraction peaks corresponding to the CaCO3 phase. IR measurements suggest the presence of organic constituents in the commercial bovine bone, although these were removed upon calcination [25]. Jazie et al. determined that the XRD pattern of calcined animal bone at 900 °C shows sharper peaks, indicating a better crystallinity, and concluded that the calcination process has eliminated the collagen and organic compounds from the animal bone and did not affect the molecular skeleton of the hydroxyapatite [26].
The basic strength obtained from animal bones and eggshells also differed minimally, i.e., the catalysts were characterized by similar basic properties. However, the catalysts obtained from animal bones were characterized by a significantly larger surface area, which should increase the catalytic activity, but such a significant increase in the biodiesel yield was not observed. It can be concluded that other parameters also influenced the biodiesel yield.
When examining the possibilities of reuse, the same problem is encountered because various methods were applied for regeneration, starting from washing and ending with calcination. However, the catalyst obtained from animal bones was easier to regenerate and reuse for more cycles than the catalyst obtained from eggshells. Most of the authors only washed and dried the animal bone catalysts or did not apply any regeneration methods. Such catalysts were used for an average of 4–5 cycles, but they were also used for 10 cycles. Calcination at a temperature of 1000 °C did not increase the possibilities of reuse; it is indicated that they were used for only four cycles. Meanwhile, when using the catalyst from eggshells, most of the authors calcined it at a temperature of 600–1000 °C before reuse in order to be able to use it for 4–6 cycles. Only when calcining at a temperature of 1000 °C did the catalyst manage to be used for 13 cycles.

3. Conclusions

This review summarizes the possibilities of using kitchen waste—eggshells and animal bones—in biodiesel synthesis. These wastes contain calcium compounds which, upon calcination, are converted into calcium oxide, catalyzing the transesterification reaction. It was found that the optimal calcination temperature for eggshells and animal bones is 900 °C, with a duration of 4 h. Eggshells treated in this way are characterized by a large surface area and high basic strength. The catalytic activity of eggshells and animal bones can be increased by modifying them with other elements. It is proposed to impregnate the calcined waste with Li/Zn, K2CO3, KOH, or FeSO4 and perform hydrothermal treatments. Biodiesel yield depends on four variables: temperature, catalyst content, methanol content, and time. Most authors have conducted studies at or near the boiling point of methanol. The catalyst content using eggshells has varied from 2 to 15%, but most of them indicate that high ester yields are obtained using 2–7% calcined eggshells. Similar trends have been observed by researchers using calcined bones: good results have been obtained using both 3% and 20% catalysts. The highest FAME yields were obtained from 6:1 to 12:1 molar methanol-to-oil ratios using calcined shells as a catalyst. A higher molar ratio from 12:1 to 18:1 was required when using calcined bones. The optimal process duration is short and reaches 2–4 h. By modifying calcined catalysts, a higher FAME yield is obtained under milder process conditions. The highest FAME yield was obtained by modifying calcined bones with KOH and impregnating Li/Zn- and Zn-modified eggshells. Several methods are proposed for catalyst regeneration: without additional treatment, only drying, washing with solvents, and drying or washing with solvents and calcination. Some authors indicate that the catalyst can be reused only after drying, while others suggest additional calcination. It is indicated that the catalyst can be reused for 4 to 13 cycles.

4. Future Prospects

Summarizing the results, it can be stated that food waste—eggshells and animal bones—can be used in biodiesel synthesis as heterogeneous catalysts. However, it must be prepared accordingly—crushed and calcined at high temperature. The processes are energy-intensive. Although the waste does not cost anything, its processing increases the cost of the catalyst. Therefore, it is necessary to evaluate these costs and their impact on the cost of biodiesel and the economic benefits of waste catalysts. No one has provided such a detailed analysis yet. Not all authors analyze the dependence of the catalyst’s efficiency on its properties, such as surface area, pore diameter, and basic strength. Often, the information is contradictory; therefore, it is necessary to perform additional experiments in order to obtain accurate, unambiguous results.
There is also no information on the energy and material costs of catalyst regeneration, which is necessary for assessing the possibilities and economic prospects of catalyst reuse, as well as the impact of regenerated catalysts on the quality of biodiesel. Additional research and analysis are needed for this. In recent years, the possibilities of using modified eggshells and animal bones have been studied more extensively. Studies have been conducted on the incorporation of various elements into calcined catalysts, and their catalytic efficiency has been evaluated. However, the results showed that modified catalysts do not exhibit significantly higher activity; in order to obtain a high FAME yield, a relatively large amount of catalyst and excess alcohol are required. So far, no one has studied how much such a modification would cost and how it would affect the cost price of biodiesel. Therefore, it is appropriate to continue developing research in this direction. Recently, more and more attention has been paid to waste-free production. Efforts are being made to use all the waste generated during the process to obtain useful products. This is provided for by the green course promoted in EU countries. When implementing the sustainability principle, the assessment of the life cycle of processes and products is very important. Life cycle assessments allow us to determine and compare the environmental impact of different products or processes and choose those with a less negative environmental impact. The life cycle indicators of the preparation of waste catalysts have not yet been studied, nor has the unstudied biodiesel synthesis process using them. It is not clear whether the use of catalysts obtained from waste actually provides environmental benefits.

Author Contributions

Conceptualization, V.M. and E.S.; methodology, I.G.; software, I.G.; formal analysis. I.G.; investigation, I.G. and. E.S.; resources, V.M.; data curation, V.M.; writing—original draft preparation, V.M.; writing—review and editing, V.M.; visualization, I.G.; supervision, V.M.; project administration, V.M.; funding acquisition, E.S. All authors have read and agreed to the published version of the manuscript.

Funding

This article is one of the results of the activities carried out within the project ‘Development of the Bioeconomy Research Center of Excellence’ (BioTEC). This project has received funding from the Ministry of Education, Science and Sports of the Republic of Lithuania and Research Council of Lithuania (LMTLT), agreement No S-A-UEI-23-14. The funding programme is the ‘University Excellence Initiative’ (No V-940).

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Table 1. CaO content, the basic strength, and surface area of calcined eggshells.
Table 1. CaO content, the basic strength, and surface area of calcined eggshells.
Origin of ShellsPreparationCaO Content (%)Surface Area (m2/g) and
Basic Strength (H_)		Reference
Waste eggCalcined at 800 °C for 4 h.99 [10]
ChickenCalcined at 800 °C for 4 h.99.2 [12]
Waste eggCalcined at 900 °C for 2.5 h. 3.7262 m2/g
9.8 < H_ < 12.2		[13]
Calcined at 900 °C for 2.5 h, refluxed in water at 60 °C for 6 h, and filtered at 120 °C overnight. The solid product was calcined at 600 °C for 3 h. 8.6401 m2/g
12.2 < H_ < 15.0
EggshellCalcined at 900 °C for 2 h.97.1 [14]
Chicken The cleaned eggshells were dried overnight in an oven at 100 °C and calcined at 1000 °C for 4 h. Surface area 54.6 m2/g[15]
EggshellCalcined at 900 °C for 3 h. Surface area 1.8 m2/g[16]
ChickenCalcined at 900 °C for 3 h99.20  [17]
ChickenCleaned eggshells were dried at 100 °C for 24 h and reduced to a mesh size of 120. Calcined at 900 °C for 3 h.97.86 [18]
ChickenDried at 100 °C
for 24 h and ground into a powder form. Calcined at 850 °C for 2 h.		98.21 [19]
ChickenCalcined for 4 h at 900 °C. 2.98  ±  0.01 m2/g
7.2 < H_ < 9.8		[20]
Calcined at 900 °C for 4 h. CaO was dissolved in distilled water. Zinc nitrate aqueous solution was added and stirred for 4 h, filtered, and dried at 120 °C for 4 h. Calcined again at 900 °C in a muffle furnace for 4 h. 13.73  ±  0.01 m2/g
15.0 < H_ < 17.2
Calcined for 4 h at 900 °C for 4 h. CaO was dissolved in distilled water. Copper nitrate aqueous solution was added and stirred for 4 h, then filtered at 120 °C for 4 h. Calcined again at 900 °C for 4 h. 11.92  ±  0.01 m2/g
12.2 < H_ < 15
Waste eggWashed by distilled water and then dried at 60 °C. Washed with hot distilled water at 120 °C for 24 h. The finely crushed powder was calcined at 900 °C for 3 h. 64.51 m2/g with pore size 9.28 nm[21]
Guinea fowlCalcined at 1000 °C for 4 h.98.500 ± 0.31 [11]
Chicken98.010 ± 0.22
ChickenCalcined at 850 °C for 5 h.89.01 ± 0.79% [22]
Zn-CaOCalcined for 5 h at 900 °C and impregnated with ZnNO3. 13.51 ± 0.01 m2/g[23]
Waste eggshells and ferric sulphateCalcined at 800 °C for 3 h and impregnated with ferric sulphate at a ratio of 1:1.Ca (20.7%), Fe (18.5%), S (4.5%), and O (54.8%) [24]
Table 2. Effect of calcination temperature on catalyst properties [24].
Table 2. Effect of calcination temperature on catalyst properties [24].
Calcinations Temperature (°C)Surface Area (m2/g)Total Pore Volume (cm3/g)Crystallite Size (nm)
8004.01730.01689557.24773
90090.65230.05099541.47434
10001.20080.00277395.0028
Table 3. Comparison of optimum conditions for biodiesel production when eggshells are used as a heterogeneous catalyst.
Table 3. Comparison of optimum conditions for biodiesel production when eggshells are used as a heterogeneous catalyst.
Eggshell OriginOilTemperature (°C) Shell Loading (wt%)Reaction Duration (h)Alcohol to Oil
Molar Ratio (mol/mol)		Ester Yield,
(Ester Content *) (wt%)		Reference
ChickenSoybean653 39:195[29]
ChickenPalm olein 6010212:1>90[12]
Waste eggsPalm olein 154 min with 900 W microwave power18:196.7[10]
Waste eggs Waste frying655112:179.62[13]
Chicken 60346:197.75 ± 0.02[14]
ChickenWaste cooking651.5212:194[15]
ChickenSoybean255.8116:198[16]
97
ChickenSunflower602 210.5:199[17]
ChickenSoybean57.57310:192.32[18]
WasteRapeseed604 19:195.12[30]
ChickenSunflower60339:196[19]
ChickenWaste cooking655220:145.52[31]
WasteChlorella pyrenoidosa602.06 330:1 methanol to algal biomass93.44[21]
WasteWaste cooking50526:120.1[32]
Guinea fowlCalabash641.5 212:196[11]
Chicken 95
ChickenChicken fat652 3.4130:190.2[33]
WasteWaste cooking602.5212:194[34]
92 (Methanol—ethanol (8:4)
ChickenRapeseed646.89.4810.93:197.79[35]
ChickenRapeseed1107.41 11.8111,3:198.78
(1-butanol)		[22]
ChickenMarine fish waste575 1.78
(107 min)		25:186.5[36]
CaO
Chicken		Eucalyptus 655 2.56:170.0[20]
CaO-CuO91.6
CaO-ZnO93.8
WasteRubber seed5551.512:180.2[37]
Zn-CaO
from eggshells		294.12
Zn-CaO. from eggshells Waste palm1507 312:194.5[23]
Bifunctional catalyst from waste eggshells CaSO4/Fe2O3Waste cooking704 45 min
(0.75 h)		5:189.94[24]
* Ester Content.
Table 4. Comparison of the performance of a natural catalyst produced from animal bones.
Table 4. Comparison of the performance of a natural catalyst produced from animal bones.
Catalyst OriginFat/OilCalcination Temperature (°C)Catalyst Amount (wt%)Molar Ratio of Methanol to Oil (mol/mol)Duration (h)Temperature (°C)Methyl Ester Yield (%)Reference
Chicken boneWaste cooking oil 900 515:14 89.33[28]
Chicken boneWaste cooking oil 80033:138096[38]
Fish bonePalm oil 900412:16.116594.3[39]
Fish bonePalm oil 9001018:126590[40]
Pork boneJatropha Curcas oil900418:15 min. microwave (800 W) 94[41]
Cow bone Waste cooking oil8002012:187096[42]
Acech cow boneCastor oil 900612:146558.7[43]
Sheep bonePalm oil8002018:1 6596.78[44]
Waste animal bonePeanut oil9002018:146094[26]
A mixture of equal parts of chicken and fish bonesUsed cooking oil10001.9810:11.546589.5[45]
Quail waste beaksCanola oil900712:146589.4[46]
Quail waste beaksRapeseed oil900712:146591[46]
Quail waste beaksWaste cooking oil900712:146591.7[46]
Waste ostrich boneWaste cooking oil 1000515:146090.56[47]
Waste fish scale Soybean oil 9001.016.27:156597.73[27]
Table 5. Comparison of the performance of modified catalysts produced from animal bones.
Table 5. Comparison of the performance of modified catalysts produced from animal bones.
CatalystFat/OilCatalyst Amount (wt%)Molar Ratio of Methanol to Oil (mol/mol)Duration (h)Temperature (°C)Methyl Ester Yield (%)Reference
Calcined animal bone modified with KOH (10%)Jatropha curcas69:137096.1[49]
Calcined goat bone modified with KOH (6%)Waste cooking oil69:156584[50]
Calcined animal bone hydrothermal treated at 200 °CHonge oil2.512:126596[48]
Calcined sheep bone impregnated with ash powderMustard oil105:166590.4[51]
Calcined pig bone impregnated with K2CO3Palm oil89:11.56596.4[52]
Calcined pig bones impregnated with CaO-CeO2Palm oil119:136584.4[53]
Calcined chicken bone impregnated with Li/ZnWaste canola oil418:13.56098[54]
Calcined fish bone supported with polyvinyl alcoholPalm oil1020:1 6580.4[39]
Table 6. Regeneration and reuse of the catalyst obtained from eggshells.
Table 6. Regeneration and reuse of the catalyst obtained from eggshells.
CatalystPreparation of the Catalyst for ReuseCyclesReferences
Calcined chicken eggshellSeparation from reaction mixture; calcination at 1000 °C.13[37]
Calcined waste eggshellSeparation from the reaction mixture by centrifugation, without pretreatment or regeneration.5[10]
Calcined waste eggshell (CaO-900-600)Separation from the reaction mixture by filtration, washing with methanol, and calcination at 600 °C.6[13]
Calcined chicken eggshellSeparation from the reaction mixture by centrifugation, washing with n-hexane, drying, and calcination at 700 °C.5[15]
Calcined eggshellSeparation from the reaction mixture by centrifugation, without pretreatment or regeneration.9 (soybean oil)
4 (waste cooking oil)		[16]
Calcined chicken eggshellSeparation from the reaction mixture by centrifugation, drying at 100 °C, and re-calcination at 900 °C for 3 h.4[17]
Calcined chicken eggshellSeparation from the reaction mixture and re-calcination at 700 °C for 2 h.5[18]
Calcined chicken eggshellSeparation from the reaction mixture and drying. 8[19]
Calcined waste shellSeparation from the reaction mixture by centrifugation, washing with hexane, and drying overnight in the oven.7[21]
Zn-CaO from eggshellsSeparation from the reaction mixture by filtration, washing with hexane, and calcination at 900 °C for 4 h.6[20]
Calcined eggshell impregnated with ferric sulphate at a ratio of 1:1 4[24]
Zn-CaO from eggshellsSeparation from the reaction mixture by filtration, washing with hexane, and calcination at 900 °C for 4 h.6 [23]
Table 7. Regeneration and reuse of the catalyst obtained from animal bones.
Table 7. Regeneration and reuse of the catalyst obtained from animal bones.
CatalystRegenerationReusabilityReference
Calcined chicken bone Recovering, after each run, washing with n-hexane, and calcination at 400 °C for 2 h. 4 cycles [28]
Calcined waste animal bones Separation from the reaction mixture, washing with distilled water and acetone, and drying in an oven at 50 °C.5 cycles[44]
Calcined goat bones Separation by centrifugation, washing with methanol, and drying. 6 cycles[26]
Calcined Chicken and fish bones Separation by centrifugation, washing with hexane, and treatment at 1000 °C.4 cycles[45]
Calcined quail waste headWashing with pure ethanol and drying at 100 °C.5 cycles [46]
Calcined ostrich bones Washing with ethanol and drying at 100 °C for 3 h. 4 cycles[47]
Calcined fish scaleWithout treatment.6 cycles[27]
Waste chicken boneWithout treatment.4 cycles [38]
Cow boneWithout treatment.10 cycles [42]
Calcined animal bone modified with KOHDrying; washing with methanol.4 cycles [49]
Calcined animal bone hydrothermally treated at 200 °CRecovering; washing with methanol.5 cycles [48]
Calcined sheep bone impregnated with ash powderRecovering by centrifugation, washing with methanol, and drying at 110 °C. 5 cycles[51]
Table 8. Preparation conditions and properties of catalysts.
Table 8. Preparation conditions and properties of catalysts.
OriginCalcination TemperatureCaO Content (%)Basic Strength (H_)Surface Area (m2/g)Biodiesel Yield (%)
Eggshell800<99 >89.94
85089–98.21 92.32–98.78
90097.1–99.207.2 < H_ < 17.21.8–64.5171.0–97.75
100098.01–98.5 54.6>90
Animal bones 800 4.017380–96.78
900 10 < H_ < 1590.652358.7–97.73
1000 1.200889.5–97.73
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Makarevičienė, V.; Gaidė, I.; Sendžikienė, E. Heterogeneous Catalysts from Food Waste for Biodiesel Synthesis—A Comprehensive Review. Catalysts 2025, 15, 957. https://doi.org/10.3390/catal15100957

AMA Style

Makarevičienė V, Gaidė I, Sendžikienė E. Heterogeneous Catalysts from Food Waste for Biodiesel Synthesis—A Comprehensive Review. Catalysts. 2025; 15(10):957. https://doi.org/10.3390/catal15100957

Chicago/Turabian Style

Makarevičienė, Violeta, Ieva Gaidė, and Eglė Sendžikienė. 2025. "Heterogeneous Catalysts from Food Waste for Biodiesel Synthesis—A Comprehensive Review" Catalysts 15, no. 10: 957. https://doi.org/10.3390/catal15100957

APA Style

Makarevičienė, V., Gaidė, I., & Sendžikienė, E. (2025). Heterogeneous Catalysts from Food Waste for Biodiesel Synthesis—A Comprehensive Review. Catalysts, 15(10), 957. https://doi.org/10.3390/catal15100957

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