https://doi.org/10.1007/s12633-023-02403-9 REVIEW PAPER
Silica‑Based Catalysts for Biodiesel Production: A Brief Review
Aneu Aneu1 · Remi Ayu Pratika2 · Hasanudin3 · Saharman Gea4 · Karna Wijaya1 · Won‑Chun Oh5
Received: 29 January 2023 / Accepted: 9 March 2023
© Springer Nature B.V. 2023
Abstract
Biodiesel, an environmentally friendly fuel, has been developed as a renewable fuel. It is accomplished through either a transesterification reaction with a base catalyst or a two-step reaction with an acid catalyst followed by a transesterification reaction with a base catalyst. A two-step reaction is performed on oil involving high levels of free fatty acids (FFA), such as used cooking oil or others. Silica is a material that has the potential to be developed as a heterogeneous catalyst based on its acidic and basic properties in biodiesel production. This review discusses the use of silica material and its modification as a catalyst to increase its acidity and/or basicity, which can increase its catalytic activity to produce biodiesel. Modification of silica as an acid catalyst, for example, in the sulfation process using sulfuric acid solution, has led to an increase in silica’s acidity and a half-reduction in the FFA content of used cooking oil. The silica catalyst in the esterification and/or transes- terification reactions shows good conversion of biodiesel. Loading base catalyst materials such as CaO, MgO, and KF onto silica has resulted in close to 100% conversion of an oil such as used cooking oil, corn oil, or Jatropha oil.
Keywords Modification of Silica · Acid-catalyst · Base-catalyst · Biodiesel
1 Introduction
Biodiesel is one of the fuels that has a similar chemical composition to fossil fuels and is gaining the attention of researchers as one of the most promising alternative fuels.
Biodiesel is made by chemically reacting to feedstocks such as oil with alcohol and a catalyst. As a result of this process, new chemical compounds were formed, known as methyl
esters or biodiesel. [1–4]. The calorific value, cetane num- ber, viscosity, and density of the methyl ester product are comparable to those of fossil diesel fuel [5, 6]. Biodiesel has advantages compared to petroleum diesel, including being safe, renewable, non-toxic, easy to decompose, hav- ing low sulfur content, and having better lubricating power [7, 8]. In addition, biodiesel is considered a sustainable fuel and environmentally friendly due to its clean, and carbon neutral properties [9, 10]. Raw materials commonly used to produce biodiesel include soybean oil [11], castor oil [12], vegetable oil [13], palm oil [14], used cooking oil [15], and corn oil [16]. It is obvious that using edible vegetable oils or fats as a high-quality feedstock is not competitive [11–14].
Non-edible oils or used cooking oils are promising because they do not compete with sources of food and are relatively inexpensive [17, 18].
Used cooking oil is the material obtained when cooking oil is used for a certain time at high temperatures. The main components contained in used cooking oil include triglycer- ides and C15-C18 or long chain fatty acids [19, 20]. Utiliza- tion of used cooking oil provides an advantage in reducing household or food industry cost, resulting in a lower pro- duction cost than using other vegetable oils. However, used cooking oil contains high levels of free fatty acids (FFA) [21–23] that can cause a saponification reaction when they
* Karna Wijaya [email protected]
1 Department of Chemistry, Faculty of Mathematics
and Natural Sciences, Universitas Gadjah Mada, Yogyakarta, Indonesia
2 Study Program of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Palangka Raya, Central of Kalimantan, Palangkaraya, Indonesia
3 Department of Chemistry, Faculty of Mathematics
and Natural Sciences, Universitas Sriwijaya, South Sumatera, Palembang, Indonesia
4 Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Sumatera Utara, North Sumatera, Indonesia
5 Department of Advanced Materials and Engineering, Hansoe University, Seosan-Si, Chungnam-Do, Hongseong, Republic of Korea
react with a base catalyst through a transesterification reac- tion. Saponification is a chemical reaction that results in the formation of soap or ester salts when free fatty acids react with a base catalyst (Fig. 1a) [24, 25]. This reaction can reduce the efficiency of the transesterification process in pro- ducing biodiesel products by increasing viscosity, making separation of the biodiesel products difficult. On the other hand, the presence of water is also able to hydrolyze methyl esters to form free fatty acids (Fig. 1b) [26, 27]. The preven- tion of the saponification reaction can be done by reacting with the oil through an esterification reaction with an acid catalyst [28]. According to Luo et al. [29], the presence of an acid catalyst can minimize the levels of free fatty acids and water in the oil, preventing saponification reactions during the conversion of methyl esters to biodiesel.
Heterogeneous catalysts are catalysts that have a different phase than the reactants. This catalyst is non-corrosive, non- toxic, easy to separate, low purification costs, and environ- mentally friendly. A heterogeneous catalyst contains Lewis sites and Brønsted bases at the center of activity that can supply electrons or act as proton acceptors from the reac- tants, resulting in a high percentage of biodiesel products [30, 31]. Heterogeneous catalysts are classified as heteroge- neous acid catalysts such as sulfated metal oxides (sulfated zirconia or sulfated silica) and heterogeneous base catalysts such as alkaline earth metal oxides, including calcium oxide or calcium methoxide [32, 33].
Silica is a material that can be used as a heterogeneous acid or base catalyst because it has acidic and basic proper- ties, making it suitable for biodiesel production. Its acidity can be employed to reduce free fatty acids from oil in an esterification reaction, while its basicity is needed to convert the methyl esters to biodiesel [34]. Furthermore, silica is material that has good temperature stability and reactivity and non-toxic and a high purity [35, 36]. In this article, we will discuss the use of silica and its modifications to increase its acidity and basicity and their applications in the produc- tion of biodiesel, which is prepared by transesterification,
or in two steps: esterification using oil with a high level of free fatty acid, followed by a transesterification reaction to produce biodiesel.
2 Catalyst for Biodiesel
The base-catalyzed biodiesel conversion process, coupled with the acid-catalyzed process, becomes possible when the feedstock is oil that has a high FFA level. In an esteri- fication reaction, used cooking oil containing high levels of free fatty acids should be treated with an acid catalyst before being transesterified with a base catalyst [37]. H2SO4, HCl, or H3PO4 as homogenous catalysts have all been uti- lized in esterification reactions and show good activity and selectivity. The drawback of these catalysts is that they are corrosive, reactive, less stable, and difficult to recover [38].
To overcome this problem, the use of heterogeneous acid catalysts was selected and extensively researched due to their low risk of contamination, good stabilization, ease of sepa- ration, and accessibility [38, 39]. In addition, when using a high FFA level, heterogeneous acid catalysts are typically used to prevent the formation of the saponification reaction [40, 41]. The drawbacks of the advantages and disadvan- tages of the homogeneous and heterogeneous catalyst due to their properties in many reaction applications are shown in Fig. 2.
The type of heterogeneous acid catalyst commonly used in organic reactions, including esterification, is the sulfated metal oxide catalyst (SO4/MxOy). This catalyst is com- monly referred to as a “superacid solid catalyst” due to its superior acidity and its use in various organic reactions [42, 43]. Types of sulfated metal oxide catalysts include sulfated zirconia (SO4/ZrO2) [34], sulfated titania (SO4/TiO2) [39], and sulfated silica (SO4/SiO2) [35]. The type of heteroge- neous base catalyst that is often used is CaO, which has a strong basicity [44–46]. Producing biodiesel with CaO- MnO2, CaO-ZrO2, and CaO-CeO2 catalysts results in high
Fig. 1 The reaction mechanism of free fatty acid and homoge- neous catalyst (a); and methyl ester and water (b)
catalytic activity and biodiesel conversion rates of 85–90%
[47]. Esterification of oleic acid using CaO impregnated with TiO2 catalyst has also resulted in up to 96% biodiesel products [48].
Pratika et al. [39] reported the modification of TiO2 to improve its properties. The acid catalyst preparation was done by impregnation of sulfate on titania (SO4/TiO2). This catalyst was used to reduce the FFA level of Jatropha oil.
The results show the decrease in FAA level reached 49.09%.
Thus, the oil with a low FFA level was then reacted in trans- esterification reaction using a base catalyst (CaO/TiO2), yielding up to 79.68% biodiesel. Ore et al. [34] published a similar study in which they discovered that biodiesel can be produced in two steps. Esterification reactions using SO4/ ZrO2 catalyst successfully reduced the FFA level of low- grade crude palm oil up to 85.90%, and transesterification of low grade crude palm oil (LGCPO) after esterification resulted in a 69.52% biodiesel product.
Table 1 summarizes the use of a sulfated metal oxide catalyst in the production of biodiesel from used cooking
oil. The use of sulfated zirconia [40] and sulfated titania [47] catalysts to convert used cooking oil produced biodiesel products of 93.6% and 97.1%, respectively. The distinction in product conversion could be attributed to the total acidity and selectivity of sulfated titania being higher than those of a sulfated zirconia catalyst [49, 50]. Another sulfated metal oxide catalyst, like SO42−/Fe-Al-TiO2 [51] and SO42−/SnO2- SiO2 [52], also shows good catalytic activity in biodiesel production. Active site in SO42−/Fe-Al-TiO2 catalyst gives it has good acidity properties after the sulfation process. Over- all, the utilization of a sulfated metal oxide catalyst in the transesterification of used cooking oil into biodiesel shows an excellent conversion of > 80%.
3 Mesoporous Silica
Silicon dioxide (SiO2) is a crystalline or amorphous metal oxide material with good adsorption properties, high mechanical and thermal stability, and easy modification with
Fig. 2 The difference properties of nano-catalyst, homogeneous catalyst, and heterogeneous catalyst [37]
Table 1 Summary of transesterification to produce biodiesel with sulfated metal oxide catalyst
M = mole ratio oil to methanol, C = catalyst weight, R: reaction time
Catalyst Feedstock Reaction
condition Biodiesel
conversion (%) SO42−/ZrO2 [40] Used cooking oil C: 3%, M: 1:9, R: 4 h 93.6
Ti(SO4)O [47] Used cooking oil C: 1.5%, M: 1:9, R: 3 h 97.1
SO42−/Fe-Al-TiO2 [51] Used cooking oil C: 3%, M: 1:10, R: 2,5 h 96 SO42−/SnO2-SiO2 [52] Used cooking oil C: 5%, M: 1:15, R: 1 h 81.4
other materials to improve performance [53, 54]. Silica can come from various sources, both natural and synthetic. Natu- ral sources of silica include rice husk ash [55] and natural sand [56]. The synthetic silica is sourced from the precursors of tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate (TMOS), and sodium silicate (Na2SiO3) [57]. SiO2 is one of the metal oxide catalysts that has similar properties to ZrO2 and TiO2 [58]. Modification of silica to increase its catalytic activity shows good performance in many reactions [59].
However, the disadvantage of modifying a silica catalyst is that its surface area is reduced after the process. Ethanol conversion to diethyl ether (DEE) using silica modified with sulfuric acid solution has been investigated by Wijaya et al.
[60]. Sulfation process causing the decrease in surface area of silica (772.15 to 236.09 m2/g), but successfully produce liquid product conversion of 66.75% with DEE of 9.54% at temperature reaction of 225 °C, while the SiO2 only pro- duces 1.93% of DEE product. Nadia et al. [61] reported the synthesis of a monodisperse hierarchical porous silica cata- lyst using tetraethyl orthosilicate (TEOS) and NaHCO3 as a template. This catalyst was employed in the hydrocracking process to convert used frying oil to biofuel. The result of the conversion was a liquid product of 26.3% with a selectiv- ity gasoline fraction of 6.22% and a diesel fraction of 19.3%.
The synthesis of this catalyst also successfully increased the acidity of SiO2 (from 4.60 to 8.93 mmol/g) but decreased the surface area of SiO2 from 644.95% to 432.37%.
Mesoporous silica is a type of silica-based catalyst mate- rial that has a highly ordered pore structure, which allows for the efficient diffusion of reactants and products during the reaction. Mesoporous silica also has a high surface area, which provides sample space for the active sites that cata- lyze the reaction [62–64]. Furthermore, mesoporous silica materials have attracted attention with their attractive char- acteristics, namely stability, controlled morphology, and porosity [65, 66]. The type of mesoporous silica commonly used are silica-based (mesoporous silica), such as SBA-15 [67] or modified mesoporous silica using various types of molds [68]. Surfactant impressions such as cethyl trimethyl ammonium bromide (CTAB) [69], cethyl trimethyl ammo- nium chloride (CTACl) [70], and non-surfactant molds such as urea [71]. In its utilization as a carrier material that is able to increase the surface area, it turns out that the presence of mesoporous silica is also able to increase its acidity, which has an impact on increasing its catalytic activity [72].
The conversion of palm fatty acid distillate into bio- diesel with mesoporous silica as a catalyst supported by sulfated titania has been reported by Manga et al. [73].
The mesoporous silica was synthesized from ludox HS40 solution and a surfactant called CTAB. The conversion of palm fatty acid distillate (PFAD) into biodiesel successfully produced a biodiesel product of 98.9% with a 5% catalyst weight and a 3.5 h reaction time. The biodiesel product,
based on American Standard Testing Material (ASTM) testing, demonstrates that the performance of biodiesel ful- fills ASTM D-6751–02 standard requirements, with a den- sity of 871.3 kg/m3 at 15 °C and a kinematic viscosity of 4.6555 mm/s2 at 40 °C.
4 Silica Based Acid Catalyst
Silica is a metal oxide material that is widely used as a het- erogeneous catalyst. Silica has a high surface area and Lewis acid sites on its surface, which makes it possible to serve as a catalyst, especially in the esterification or transesterification reaction. [74, 75]. The acidity of a heterogeneous catalyst is one of the significant factors that can affect its activity and selectivity, including esterification [39, 76]. Previous researchers have attempted to increase the catalytic activity of silica in various ways, including the addition of anions from H2SO4 or (NH4)2SO4 solutions [77]. This process is thought to produce Brønsted acid sites on the surface of the silica, thus increasing the total acidity of the SiO2 [78, 79].
In addition, it improves the thermal stability and porosity of silica, increasing its activity and selectivity [80, 81]. Sul- fated silica (SO4/SiO2) is one of the heterogeneous sulfated metal oxide catalysts that are formed when silica reacts with sulfate solution. To form Brønsted and Lewis acid sites, the O atom from the sulfate ion, which acts as a ligand, donates a lone pair of electrons and forms a coordination bond with the Si4+ cation (from SiO2). Sulfated silica has two oxygen atoms from S–O bonds (H2SO4) that are bonded to Si from SiO2 [82, 83]. Figure 3 shows the structure of sulfated silica.
Wijaya et al. [30] studied the preparation of sulfated silica for biogasoline from used cooking oil. A gravimetric method using ammonia solution was used to calculate the acidity on the catalyst’s surface. The acidity of SiO2 was 6.03 mmol/g. The sulfation process with various sulfu- ric acid solutions of 1 to 4 M caused the acidity of SiO2 to increase, and the SS2 catalyst (H2SO4 2 M) was the
Fig. 3 The structure of the sulfated silica catalyst
catalyst with the highest acidity of 10.33 mmol/g. The thermal stability of the SiO2 was also improved after the sulfation, and this catalyst increased the liquid product of biogasoline to 72.47%, with the gasoline fraction of 51.71% and the diesel fraction of 5.89%. Another acidity test of a sulfated silica catalyst with the same concentra- tion of sulfate (1 to 4 M) was investigated by Wijaya et al.
[60] by the gravimetric method using pyridine solution.
The result shows that SiO2 has an acidity of 2.08 mmol/g.
The SS-2 catalyst was found to have a good acidity after sulfation, with an acidity of 2.22 mmol/g. These findings confirmed that the optimum sulfate concentration for syn- thesizing the sulfated silica catalyst was 2 M, with higher concentrations resulting in a decrease in total acidity due to the maximum distribution of sulfate on the silica’s surface.
The characterization of sulfated silica catalyst has been studied by Aneu et al. [35]. The FTIR analysis revealed appearance peaks at 1100 cm−1, corresponding to the S–OH stretching vibration (HSO4− ion), confirming the presence of sulfate after sulfation. The diffractogram analysis shows the broad peak at 2θ of around 22° that confirmed the amorphous character of SiO2. There is no difference in crystalline structure after sulfation. The influ- ence of temperature calcination, on the other hand, is caus- ing the catalyst’s crystallite size to increase. Because of the presence of SO4 that covered the silica surface’s pores, the sulfated silica catalyst has a mesoporous character, and its porosity, including its surface area and pore diameter, was smaller than that of silica. The catalytic activity of the sulfated silica catalyst in the esterification reaction to decrease the FFA level of used cooking oil successfully decreased the FFA content to 49%.
Aneu et al. [83] synthesize porous sulfated silica from]
synthesize porous sulfated silica from TEOS using two heat- ing methods: conventional and microwave heating. Micro- wave treatment with electromagnetic irradiation is preferred as a thermal treatment for catalyst synthesis due to its lower energy consumption and shorter time. The microwave method required 1.5 h to form the gel, whereas the con- ventional method required twice as long (3 h). Because of the increased rate of crystallization caused by microwave irradiation, the crystal size of the catalyst was smaller when using microwaves than when using conventional method.
In addition, this method caused the morphology of the cata- lyst to become clearer and softer, and the surface area of the sulfated silica catalyst with microwave heating is higher than that with conventional heating methods. The esterifica- tion reaction studied by examining the influence of catalyst weight, time of reaction, and methanol to oil ratio on reduc- ing the FFA content of used cooking oil showed the optimal decrease in FFA level (45%) with catalyst loading of 5%, methanol ratio of 1:23, and time of 60 min.
5 Nanosilica Catalyst for the Biodiesel Production
Nano-sized catalysts have attracted the attention of many researchers. A very large surface area of nanoparticles provides benefits on reaction rate because of its reactant movement. Different components and materials like alu- minium, iron, titanium dioxide, and silica all have been utilized as catalysts in nanoscale form in the past dec- ades. The understanding of how the physical properties of nanoparticles influence the reactant movement can lead to the possibility of designing and developing nano catalyst which are exceptionally dynamic, profoundly particular, and exceptionally tough [84].
Nanosilica-based catalysts have been utilized in biodiesel production. Nanosilica-based catalyst is claimed as a catalyst with high surface area, high thermal stability and porosity.
Furthermore, in many studies, nanosilica plays a role as a support material to build better properties [85]. Narayanan and Pandey [86] prepared nanosilica as support material for lipase catalysts. This study was simulated mathematically by developing a multiparameter software package and subse- quently verified through pilot plant tests (experiments). Due to the use of nanoparticles, the effectiveness factor is close to unity and consequently, the global rate f transesterification is more or less equal to the intrinsic rate. This enhances the performance efficiency of the bioreactor.
Saxena et al. [87] synthesized nanosilica with induc- tively coupled plasma (ICP) method which was used as the medium to grow two benthic marine diatoms Chaetoceros sp. and Thalassiosira sp. This species of algae was used as biodiesel feedstock. This study compared both normal silica and ICP nanosilica. The result presented that the use of ICP nanosilica was able to enhance the production of biodiesel. Rafiee et al. [88] prepared nanosilica from rice husk. This highly pure amorphous silica in nanoscale pos- sesses high surface area and high activity and reusability.
This nanomaterial was used as the support for 12-tung- stophosporic acid (H3PW12O40). From these previous studies, silica in nanoscale showed a great property to be applied in various reaction, not only in biodiesel synthesis.
This condition is still possible to explore to design catalyst with desired properties.
6 Biodiesel Production Using Silica and its Modification
Silica-based materials are widely used in the production of biodiesel production based on its properties that has a high surface area. These materials can facilitate the
transesterification reaction between triglycerides and alco- hols through the transesterification reaction. The trans- esterification reaction produces fatty acid methyl esters (FAME), which are the primary component of biodiesel.
The use of silica-based catalysts in biodiesel production offers several advantages such as environmentally friendly, ease of separation and recovery, improved reusability, and reduced waste generation. In summary, silica-based mate- rials are an important class of catalysts for biodiesel pro- duction, and ongoing research is focused on optimizing their properties to improve the efficiency, selectivity, and sustainability of the biodiesel production process [89, 90].
One of the modifications of silica to improve its perfor- mance as a catalyst is by adding a base catalyst to increase its basic properties [91]. Types of heterogeneous catalysts, such as CaO with high basicity, have better activity and selectiv- ity in many reactions, including transesterification [92, 93].
Moradi et al. [94] studied the preparation of a CaO/SiO2 catalyst via the sol–gel method for the transesterification of corn oil. The optimum CaO/SiO2 catalyst composition that obtained the biodiesel product up to 85.6% was a cata- lyst loading of CaO based on SiO2 of 70% at a calcination temperature of 650 °C. Another transesterification reaction using CaO/Silica catalyst to convert waste cooking oil into biodiesel was reported by Lani et al. [91]. A reaction with a 3% catalyst loading, a methanol ratio of 1:15, 90 min of reaction time, and a temperature of 60 °C produced biodiesel with a 90% conversion.
Base catalyst with similar basicity to CaO, such as SrO or MgO, exhibit high conversion of biodiesel products. This type of metal oxide catalyst has a cation with a Lewis base site and a Brønsted base at its activity center, which can sup- ply electrons or act as a proton acceptor from the reactants [95, 96]. In addition, in biodiesel production, these cata- lysts show excellent catalytic activity [97]. The use of MgO/
SiO2 catalyst in the study of the effect of various alcohols in the conversion of rubber seed oil to biodiesel has been reported by Pandiangan et al. [98]. With a ratio of 3:1 oil to alcohol (ethanol, propanol, and methanol) and a 10% MgO/
SiO2 catalyst, the great alcohol with the highest reactivity was methanol, yielding 90.1% biodiesel. Meanwhile, etha- nol and 2-propanol produce biodiesel at 73.3 and 63.2%, respectively.
Tangy et al. [99] studied biodiesel production through the microwave method with a SrO@SiO2 catalyst, which effectively produces biodiesel up to 99.4 wt% with an irradiation reaction time of only 10 s. The reusability of the catalyst with 10 reactions repeated reduces the prod- uct of biodiesel to 95 wt%. This result confirmed that the SrO@SiO2 catalyst has good stability and catalytic activ- ity. Higher conversion of the biodiesel was influenced by the presence of SrO, which increases the active site and surface area of SiO2. A similar study using microwave
irradiation as a method to produce biodiesel from microal- gae using a SrO/SiO2 catalyst by Naor et al. [100] reported a shorter reaction irradiation time (2 s) and a biodiesel conversion value of 99.9%. After the reusability test, with up to six reactions repeated, the catalyst still produced a high biodiesel conversion (97.9%).
Another solid base catalyst is potassium fluoride. Potas- sium fluoride (KF) shows better catalytic activity than KCl, KBr, KI because F− acts as a base site in the catalyst [101, 102]. On the other hand, when compared to alkali fluoride (NaF, CsF), potassium fluoride (KF) showed bet- ter catalytic activity due to the higher basicity of KF [103].
Potassium fluoride has been widely used as a catalyst in various reactions, one example is the transesterification reaction. However, when there is a decrease in the amount of KF used, the product of biodiesel conversion decreases.
This is related to the reduced content of the active sub- stance KF as a catalyst [104, 105, 105, 107, 108].
Aneu et al. [83] investigated the increase in total acidity with increasing KF solution concentration by preparing a KF/SiO2 base catalyst with various KF solutions (0.5–2 M) and temperature calcination at 450–600 °C. preparation of a KF/SiO2 base catalyst KF/SiO2 2.0 was the catalyst with the highest total basicity (1.21 mmol HCl/g). After the calcination at various temperatures, KF/SiO2 2.0–550 (with a KF concentration of 2 M and a calcination temper- ature of 550 °C) shows the highest basicity of 1.64 mmol/
HCl. Transesterification of used cooking oil with KF/SiO2 2.0–550 catalyst at a catalyst loading of 5%, a ratio of oil to methanol of 1:23, and a reaction time of 60 min success- fully produced biodiesel of 54.13% with methyl oleate as a major methyl ester. Modification of silica to increase its acidity and basicity as well as its catalytic activity in the process of converting oil to biodiesel has been reported by previous researchers. The summary of the use of silica and its modification as a catalyst in biodiesel production can be seen in Table 2.
The use of silica material in various modified forms as a catalyst has demonstrated an important role in its function to increase product conversion, particularly in biodiesel production. Increasing the basicity of silica by adding other high-basicity materials such as CaO, MgO, SrO, Na, and others results in excellent biodiesel production close to 100%, whereas using lower basicity materials such as SnO2 and KF results in lower biodiesel production. Its findings revealed that basicity (the presence of Lewis base sites and Bronsted base sites) plays an important role in increasing silica's catalytic activity. Furthermore, the type of oil used influences the final biodiesel product. The use of used cooking oil [85] and corn oil [95] in the produc- tion of biodiesel using a CaO-SiO2 catalyst yields differ- ent results. The conversion of biodiesel from used cooking oil is greater than that of corn oil, which is due to the
higher methyl ester content of used cooking oil and dif- ferent reaction conditions.
7 Conclusion
The use of silica material in a variety of applications has evolved. Silica's acidic and basic properties, large surface area, good chemical stability, and environmental friendli- ness make it an excellent catalyst substance. Many scien- tists have been studying silica's catalytic activity. A sul- fated silica catalyst, for example, exhibits high conversion in the production of biofuels, diethyl ether, biodiesel, and others. In the production of biodiesel, the modification of silica, for example, through sulfation with sulfuric acid to be used as an acid catalyst in the esterification reaction, shows good effectiveness in reducing the content of free fatty acids (FFA) in oils containing high levels of FFA, such as used cooking oil. Furthermore, the addition of CaO, MgO, SrO, or KF to silica to increase its basicity has succeeded in converting various types of oil through a transesterification reaction and producing up to 100%
biodiesel product.
Acknowledgements This work was supported by Rekognisi Tugas Akhir (RTA) Project of Universitas Gadjah Mada.
Author Contribution Aneu: Conceived and designed the analysis and wrote the original draft. Remi Ayu Pratika: Collected the data, Performed the analysis, Wrote and reviewed the original draft. Karna Wijaya: Conceived of the presented idea, Performed the analysis. Hasa- nudin: Collected the data. Saharman Gea: Collected the data. Won- Chun Oh: Conceived and designed the analysis.
Funding This work was supported by Rekognisi Tugas Akhir (RTA) Project of Universitas Gadjah Mada under the Contract Number: 5722/
UN1.P.III/Dit-Lit/PT.01.05/2022.
Data Availability All data generated during this review is included in this published article.
Declarations
Competing Interests The authors declare that they have no known competing financial interest or personal relationship that could have appeared to influence the work reported in this paper.
Consent to Participate Informed consent was obtained from all indi- vidual participants included in the study.
Consent for Publication All authors agreed to publish this study at silicon journal.
Conflict of Interest The authors have no conflict of interest to declare that are relevant to the content of this article.
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Table 2 Summary of silica and its modification as a catalyst for biodiesel production
M = mole ratio oil to methanol, C = catalyst weight, R: reaction time
Catalyst Feedstock Reaction
condition Biodiesel
conversion (%) CaO-SiO2 [91] Used cooking oil C: 3%, M: 1:15, R: 90 min 90
CaO/SiO2 [94] Corn oil C: 6%, M: 1:16, R: 8 h 85.6
MgO/SiO2 [98] Rubber seed oil C: 10%, M: 1:3, R: 6 h 90.1
SrO@SiO2 [99] Used cooking oil C: 3%, M: 1:5, R: 10 s (microwave) 99.4 SrO/SiO2 [100] Microalgae C: 1%, M: 1:10, R: 2 s (microwave) 99.9
KF/SiO2 [83] Used cooking oil C: 5%, M: 1:23, R: 60 m 54.13
Na/SiO2 [109] Jatropha oil C: 6%, M: 1:15, R: 45 min 99
SnO2/SiO2 [110] Soybean oil C: 5%, M: 1:24, R: 5 h 81.7
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