Efficient biodiesel production via solid superacid catalysis: a critical review on recent breakthrough

Abstract

Biodiesel produced from triglycerides and/or free fatty acids (FFAs) by transesterification and esterification has attracted immense attention during the past decades as a biodegradable, renewable and sustainable fuel. Currently, the use of solid superacid catalysts has proved a more efficient and “green” approach due to avoidance of environmental and corrosion problems and reduced product purification procedures. However, it is less viable economically because the reusability is low due to the lack of a hydrophilic/hydrophobic balance in the reactions that involve the use of inedible feedstock with a high water content. Therefore, this study gives a critical review on recent strategies towards efficient and “green” production of biodiesel via solid superacid catalysis. The strategies discussed include alkyl-bridged organosilica moieties functionalized hybrid catalysis to improve the hydrothermal stability of superacid catalysts, pre- and in situ water removal, and process intensification via temperature profile reduction. The strategies enabled well-defined porosity and an excellent hydrophobicity/hydrophilicity balance, which suppressed deactivation by water and glycerol.

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Peter Adeniyi Alaba

Peter Adeniyi Alaba was born and raised in Kwara State, Nigeria. He did his undergraduate study in Chemical Engineering at the Federal University of Technology, Minna. He received his MEngSc by research in Chemical Engineering from the University of Malaya under the supervision of Prof. Wan Mohd Ashri Wan Daud. He was appointed as a research assistant at the University of Malaya in 2013 till date. His research interests lie in rational design of tailored solid acid catalysts for efficient green energy production CFD, computational chemistry and environmental engineering. He has published more than 9 papers in reputed journals and has been serving as a reviewer for reputable journals since 2015.

Wan Mohd Ashri Wan Daud

Professor Wan Mohd Ashri Wan Daud received his bachelor degree in Chemical Engineering at Leeds University, Leeds, UK in 1991 and his master's degree in Chemical Engineering at the University of Sheffield, Sheffield, UK in 1993. He received his PhD degree in Chemical Engineering at the University of Sheffield in 1996. After nine years as an academic and scientist at the Faculty of Engineering, in 2005, he became Professor of Chemical Engineering. Since 2005 until now, he has worked as a Professor of Chemical Engineering at the University of Malaya, Malaysia. His research fields include energy, biomass conversion to bio-fuel, catalyst synthesis, polymerization and separation processes, and hydrogen storage. He has more than 131 publications in Web Science journals.

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1. Introduction

There is urgent need for alternative sources of energy due to the unrenewability, unsustainability and environmental threat posed by fossil diesel. Some studies have revealed that biodiesel has become the most interesting alternative.^1–3 The two major precursors of biodiesel are triglycerides and free fatty acids (FFAs), which are obtained from edible oil, nonedible oil,^4,5 animal fats and used vegetable oils.⁶ Previously, the most popular biodiesel produced was from edible oil. However, this is quite not economically viable due to the high cost of feedstock. Currently, nonedible oil has gained immense popularity in production of second generation biodiesel because of high biodiesel yield and low feedstock price.^7,8 However, inedible oils are characterized with trace salts, water and high FFAs. Biodiesel comprises a C₁₂–C₂₂ fatty acid monoalkyl esters (FAMEs) mixture, which has gained wide acceptability as “green” diesel due to its non-toxicity, biodegradability and sustainability. The remarkable properties of biodiesel also include higher lubricity, fewer emissions of carcinogenic particulate matter^9,10 and ease of handling, storage and transport compared to petrol-diesel.^11,12 Biodiesel is normally produced using homogeneous basic catalysts such as KOH, NaOH and other hydroxides.¹³ The preference for homogeneous basic catalysts to their acidic counterparts is basically due to their better activity, particularly for triglycerides with low FFA content.^14,15 Likewise, homogeneous acids such as H₂SO₄ and HCl are more suitable for triglycerides from inedible oil due to the presence of Brønsted acidity, which promotes electrolytic activation of substrates.¹⁶ However, the use of homogeneous catalysts is faced with the problem of long transesterification reaction times, corrosion, soap formation and separation of glycerol and water quenching, which requires large amounts of water. These generates large amount of wastewater.^17–20 The most common setback is separation cost, which covers more than half of the entire investment on the fuel industry and specialty chemical equipment.^21,22 In fact, separation cost majorly determines the economic viability of a process.²³

Heterogeneous catalysts are suitable eco-friendly alternatives because of their ease of separation from the reaction medium, corrosion reduction and reusability.^22,24 Their development could aid process design for continuous production of biodiesel to minimize purification costs. However, they give rather lower biodiesel yields due to drawbacks such as leaching of the active sites and mass transfer limitations.^13,17,25,26 The FFA content of the feedstock also affects the performance of heterogeneous basic catalysts because they form soap, which consumes the catalyst and blocks the pore, thereby poisoning it.²³

For better performance in the biodiesel production process, an efficient heterogeneous catalyst needs mesopores to minimize steric hindrance, higher acid strength and density as well as thermal and hydrothermal stability to minimize poisoning and leaching. Superacids are promising catalysts for biodiesel production because they simultaneously support both transesterification and esterification, thereby circumventing the threat posed by FFAs. Chemists synthesized them in both solid and liquid forms, and their amazing acid strength makes it easier to conduct problematic reactions under satisfactory experimental conditions. For instance, Misono and Okuhara²⁷ reported that activation of alkanes proceeds at low temperatures over a superacid catalyst. Conventional superacids such as liquid HF and AlCl₃ have been cited as hazardous to the environment; however, solid superacids are more attractive for industrial utilization. Solid superacid catalysts are indeed more environmentally benign in numerous industrial processes.^27–29

Recently, several authors tested heteropoly acids, inorganic metal oxides as well as sulfonic and sulfuric acid based resins as solid superacid catalysts for transesterification of triglycerides and esterification of FFAs.^24,30–32 Sulfated inorganic metal oxides exhibit stronger super acidity compared to 100% sulfuric acid (H₀ ≤ −12).³³ This is because metal oxides are chemically stable, environmentally benign, and possess remarkable acid–base and redox properties.¹⁶ One of the most popular sulfated metal oxide catalysts is sulfated zirconia (SZ). SZ has been proved as an efficient superacid for both esterification and transesterification.^34–36 Kiss et al.³⁷ performed a comparative study on various solid acids, including ion exchange resins, zeolites and inorganic metal oxides for lauric acid esterification using different alcohols. They reported that SZ exhibited the best performance because zeolites are limited, being microporous, whereas low thermal stability is the bane of ion exchange resins. SZ is a promising catalyst for several industrial processes of commercial importance due to its super acid strength.^38–40 Saravanan et al.^40,41 also reported the outstanding performance of SZ in esterification of caprylic acid and myristic acid at mild temperatures and low catalyst loading.

Other sulfated metal oxides with remarkable performances include titania, silica and a combination of both. Recently, several authors have investigated the use of sulfated silicas as catalysts for esterification and transesterification.^24,42,43 The report of Ropero-Vega et al.⁴⁴ shows that sulfate ion addition to titania by incipient impregnation of ammonium sulfate inculcated super-acidity to titania. This engenders remarkable activity when used to esterify fatty acids.

Ordered mesoporous silicas and aluminosilicates such as MCM-41, HMS, SBA-15 and USY zeolite are viable supports due to their high specific surface area (SSA) and hierarchical mesoporosity.^43,45–48 The major problems attributed to such materials are hydrophobicity and lower catalytic activity as compared to microporous zeolites.⁴⁹ However, functional groups that are more reactive are added to improve the surface hydrophobicity as well as reactivity. Testa et al.⁴² comparatively studied glycerol acetylation over different solid acid catalysts. They reported that mesoporous sulfated silica showed superior performance with respect to reactivity.

However, the use of solid superacid catalysts for biodiesel production can be affected by the presence of a substantial amount of water due to the robust interaction between acid sites and water molecules, which promotes hydrolysis reactions. This condition is typical of inedible raw materials, which are the most appropriate raw materials for 2nd generation biodiesel. Most of the non-edible raw materials such as sewage sludge⁵⁰ and rendered animal fats⁵¹ consist of up to 50% FFA and water. Several attempts have been made towards the synthesis of a better solid superacid catalyst that could overcome this drawback. One of them is an organosulfonic acid-functionalized solid catalyst. The studies of Melero et al.^52–54 reveal that this type of functionalized solid acid exhibit high catalytic activity and stability during biodiesel production. They further used bi-dimensional NMR (solid-state) spectroscopic analysis to study the sulfonic acid sites interaction with water in hydrophobic reaction media to reveal their robust hydrophilic nature. Consequently, this jeopardized their catalytic activity in reactions that are water sensitive.

Several authors have reviewed the application of solid superacid catalysts for biodiesel production, focusing on the mechanism of solid acid catalysts, kinetics for transesterification and esterification reactions and the most effective parameters for the catalyst.^25,55–58 However, this study gives a review on the improvement of solid superacid catalysts in biodiesel production, which includes the detailed solid superacid catalyst synthesis, the super acidity characterization and functionalization with alkyl-bridged organosilica moieties as an improvement strategy. Furthermore, the biodiesel production process intensification was discussed towards a more careful control of hydrolysis–esterification–transesterification reaction mechanism.

2. Kinetics and mechanism of biodiesel production with solid superacid catalysts

Solid super acids support both esterification and transesterification reactions. Transesterification reactions in the presence of an alcohol proceed in three consecutive reversible reaction steps. The first is transformation of triglycerides to diglycerides, followed by formation of monoglycerides. Monoglycerides are then converted to glycerol. Each step yields one molecule of methyl ester from glycerides, as shown in Fig. 1. The reaction stoichiometry shows that a mole of triglyceride requires 3 mol of methanol to form a mole of glycerol and 3 mol of methyl ester.³⁶ However, for a reasonable conversion, the methanol/oil ratio should be more than 3. Therefore, the typical reaction condition employs a methanol/oil molar ratio of ≥5 at a temperature range of 60 to about 200 °C over a catalyst loading of ≥0.5 wt% for about an hour or more.

Fig. 1 Triglyceride transesterification with alcohol (a) of fatty acids and R₄ represents the alcohol alkyl group. (b) Overall reaction.

Several authors investigated the kinetics of transesterification of triglycerides with alcohol.^40,59,60 López et al.⁵⁹ investigated solid superacid catalyzed transesterification of triacetin with methanol, comparing the activity of sulfated zirconia (SZ) with H₂SO₄ as catalysts on a rational basis. The turnover frequency of the two catalysts was found to be similar, which indicates that both SZ and H₂SO₄ follow a similar reaction pathway. This is probably because they possess a similar acid strength.

Similarly, the esterification reaction is reversible. It occurs between the FFA and alcohol over an acid catalyst to produce water and ester:⁶¹

R₄–OH + R–COOH ⇔ R–COO–R₄ + H₂O (1)

Peters et al.⁶¹ studied the kinetics of esterification of acetic acid with butanol over solid acid catalysts. The kinetics parameters were evaluated using the quasi-homogeneous model based on a first order reaction. They reported that the activity of solid acid catalysts per proton depends on their hydrophobicity, porosity and Brønsted acidity. This report corroborates with the investigation of Wilson and Lee.⁶² Functionalization with sulfate ions enhance both the Lewis and Brønsted acidity.⁶³ Effective distribution of covalently bonded sulfur complexes (S=O), which act as electron-withdrawing species, increase the strength of the Lewis acid. Therefore, increase in the sulfate content leads to an increase in catalytic activity.

Both the Lewis and Brønsted acid sites promote esterification and transesterification of trigycerides.^57,63–65 However, Lewis sites are more active promoters of transesterification reactions in the absence of water. Water tends to poison the Lewis sites. The mechanism starts from absorption of substrate over both Lewis and Brønsted acid sites of the catalyst to form a protonated reactant intermediate, as illustrated in Fig. 2 and 3. The Brønsted sites acts as an H⁺ donor, whereas the Lewis sites accept the electron pair. The intermediate reacts with alcohol in the bulk (nucleophilic attack) to form a tetrahedral intermediate. This is followed by proton migration and breakage of the tetrahedral intermediate.⁵⁷

Fig. 2 Transesterification reaction pathway over: (a) Brønsted acid sites (H⁺); (b) Lewis acid sites (A⁺).

Fig. 3 Esterification reaction pathway over (a) Brønsted acid sites (H⁺); (b) Lewis acid sites (A⁺).

3. Solid superacids

3.1. Functionalized heterogeneous catalysts

Functionalization of heterogeneous catalysts, such as clays, carbons, zeolites, mesoporous silica and metal oxides, to form a solid superacid has become an active research area over several decades due to their interesting catalytic performance.^66–69 Functionalization is aimed at incorporating superacidity into the reference material for enhanced catalytic activity. Although metal oxides themselves possess both Lewis and Brønsted sites that are capable of biodiesel production, their Brønsted acidity could be enhanced by functionalization with proton-donating inorganic compounds.⁷⁰ These inorganic compounds include a sulfonic group, sulfuric acid, ammonium sulfate, an aqueous ammonium metatungstate solution and tungstophosphoric acid.^44,67,71,72 Prominent among these are the sulfate precursors because of the superacid attributes of sulfate ions, which are capable of inducing polarization in nearby OH groups. This sulfate structure shows that each sulfur atom is bonded to two tri-coordinated oxygen atoms in a special scheme.⁷³ This exhibits C_2v symmetry, which forms a bridge and chelate structure (Fig. 4).⁴⁴

Fig. 4 Sulfate structures: (a) coordinated with C_2v chelate and (b) C_2v bridge.⁴⁴

The S–O–M bonds show the presence of Brønsted acid sites due to cleavage with water in the chelate structure, whereas the bridge structure ((M–O)₂S(=O)–OH) indicates that S(V) is the oxidation level of sulfur.³³

Metal oxide such as TiO₂,²⁴ SiO₂,⁴³ Nb₂O₅,⁶¹ ZrO₂,⁶¹ SnO₂,⁷⁴ Al₂O₃; mixed oxides such as SnO₂–Al₂O₃,⁷⁵ TiO₂–SiO₂,³³ ZrO₂–Al₂O₃;³³ hybrid silica such as amorphous, HMS, SBA-15, MCM-41,⁴⁵ and aluminosilicates have been functionalized successfully with sulfate groups. These have shown remarkable performance in production of biodiesel and specialty chemicals. Testa et al.⁴³ studied the catalytic performance of sulfated hybrid silicas for acetic acid esterification with butanol. The report shows that the samples produced exhibited splendid catalytic activity and are remarkably stable despite a decrease of sulfate ion concentration upon recycling. The catalytic performance of sulfated metal oxides is a major function of quantity of sulfate ions deposited onto the surface of the oxide. Increase in the sulfate ion content leads to an increase in the acid strength. Factors such as sulfate ion precursor, amount, calcination temperature and preparation method can influence the deposition of sulfate ions on a metal oxide.

de Almeida et al.²⁴ investigated the effect of sulfate concentration used in the preparation of sulfated titania on its catalytic performance. They reported that an increase in sulfate concentration enhanced the textural properties such as pore volume and average pore diameter, specific surface area and the sulfate ion incorporation. Fig. 5 displays the Fourier transmission infrared (FTIR) spectra of sulfated titania prepared with different concentrations of sulfuric acid. This reveals the direct proportionality of sulfate ion concentration to the band intensities of the sulfate vibration between 980 cm⁻¹ and 1300 cm⁻¹.

Fig. 5 FTIR spectra for catalysts TS-5, TS-10 and TS-20. Where TS is sulfated titania and TS-10 means TiO₂/H₂SO₄ = 10.²⁴

Ropero-Vega et al.⁴⁴ comparatively studied the synthesis of sulfated titania using ammonium sulfate and sulfuric acid as sulfate precursors and evaluated their performance in the esterification of oleic acid. Their report showed that larger amounts of sulfate ions were linked to the surface of the titania with ammonium sulfate than with sulfuric acid. This is because sulfation with ammonium sulfate ([TiO₂/SO₄²⁻–(NH₄)₂SO₄-I]) incorporated both Lewis and Brønsted acid sites, whereas sulfuric acid treatment ([TiO₂/SO₄²⁻–H₂SO₄-IS]) deposited only Lewis sites. Consequently, sulfation with ammonium sulfate gives superior catalytic performance. Fig. 6 presents the model for development of Lewis and Brønsted acid sites on sulfated titania. The inductive effect on S=O bonds could remarkably increase the acid strength. Although [TiO₂/SO₄²⁻–H₂SO₄-IS] exhibits a higher porosity (0.278 cm³ g⁻¹ and 5.7 nm) relative to [TiO₂/SO₄²⁻–(NH₄)₂SO₄-I] (0.188 cm³ g⁻¹ and 4.2 nm), the later shows a better catalytic performance compared to the former. This is attributed to the higher acidity of [TiO₂/SO₄²⁻–(NH₄)₂SO₄-I] (1.1 mmol SO₄ g⁻¹) compared to [TiO₂/SO₄²⁻–H₂SO₄-IS] (0.1 mmol SO₄ g⁻¹).

Fig. 6 Schematic of the Brønsted and Lewis acid sites present in the sulfated titania in chelate form.⁴⁴

Usai et al.⁴⁷ compared the effect of two different organic sulfate precursors (propyl-sulfonic acid and phenyl-sulfonic acid) on mesoporous silica (SBA-15) for interesterification of a vegetable oil. They observed that phenyl-sulfonic acid functionalized SBA-15 showed outstanding catalytic performance due to its remarkably higher acidity compared to propyl-sulfonic acid. This report corroborates with previous findings.⁷⁶

Calcination temperature is of vital importance in synthesis of sulfated oxides because calcination stabilizes the properties and produces the active sites of the materials. The optimum calcination temperature varies from one metal oxide to another. Khder et al.⁶³ studied the effect of calcination temperature on the formation of sulfated tin oxide for acetic acid esterification with amyl alcohol. Their report showed that an increase in calcination temperature up to 550 °C leads to an increase in sulfate content and corresponding increase in acidity, which culminated in an increased catalytic activity. However, a further increase in calcination temperature up to 750 °C resulted in decreased catalytic performance due to a decrease in acidity. This could be attributed to the removal of Brønsted acid sites and addition of Lewis acid sites at temperature above the optimum since better activity is essentially promoted by Brønsted acid sites. Moreover, Yang et al.³³ reported 450 °C as the optimum calcination temperature for sulfated titanium–silicon for acetic acid conversion. This is attributed to the highest number of acid sites and specific surface area produced at the said optimum temperature.

Similarly, acid strength of the sulfated metal oxide depends on the electronegativity of the metal element. Proton release is more facile for metal element with a lower electronegativity. Considering Zr, Ta, Ti and Nb, their electronegativities are 1.33, 1.50, 1.54 and 1.6, respectively. This corroborates with the report of Li et al.,⁷⁷ which shows that the order of the Brønsted acidity of the metal oxides as SO₄²⁻/ZrO₂ > SO₄²⁻/Ta₂O₅ > SO₄²⁻/Nb₂O₅ > SO₄²⁻/TiO₂. Furuta et al.⁷⁴ investigated the efficiency of sulfated tin oxide and zirconia for n-octanoic acid esterification with methanol. They reported that the acid strength of sulfated zirconia is lower than that of sulfated tin oxide, which enables better catalytic activities for sulfated tin oxide. This report is concomitant with previous findings.^74,78,79 Table 1 presents more reports on the activities of sulfated metal oxides.^{34,52,80–91}

Table 1 Some recent sulfated metal oxide for biodiesel production

Catalyst	Feedstock	Reaction conditions	Activity	Recycling	Ref.
SO4^2−/Zr	Used frying oil (esterification)	Methanol-to-oil molar ratio = 5 : 1; 200 °C; 5 h; catalyst loading 2 wt%	Conv. (%) = 85	About 25% loss of activity after 5 successive cycles	61
TiO2/SO4^2−–(NH4)2SO4, TiO2/SO4^2−–(NH)SO4-IS	Oleic acid (esterification)	Methanol-to-acid molar ratio = 10 : 1; 80 °C; catalyst loading 2 wt%	Conv. (%) = 58.7–1 h; conv. (%) = 29.7–1.5 h	—	27
ZrSO4–SiO2		Methanol-to-oil molar ratio = 1.2 : 1; 110–120 °C; 1 h; catalyst loading 5 wt%	Conv. (%) = ∼80		62
ZS (sulfated zirconia)	Oleic acid (esterification)	Methanol-to-acid molar ratio = 40 : 1; 60 °C; 12 h; 0.5 g catalyst	Yield (%) = 90		63
Chlorosulfonic zirconia	Oleic acid (esterification)	Methanol-to-acid molar ratio = 8 : 1; 100 °C; 12 h; catalyst loading 3 wt%	Yield (%) = 100		57
Sulfated LaO/HZSM-5, sulfated LaO, HZSM-5	Oleic acid (esterification)	Methanol-to-acid molar ratio = 5 : 1; 100 °C; 7 h; catalyst loading 10 wt%	Conv. (%) = 100, conv. (%) = 96, conv. (%) = 80 with methanol-to-acid ratio = 20 : 1;		64
Fe(HSO4)3	Waste oil (esterification)	Methanol-to-oil molar ratio = 15 : 1; 205 °C; 4 h; catalyst loading 1 wt%	Yield (%) = 94		65
SO2/ZrO–TiO/La^3+	Acid oil (esterification)	Methanol-to-oil molar ratio = 15 : 1; 200 °C; 2 h; catalyst loading 5 wt%	Yield (%) = 96.24		66
SZ	Caprylic acid (esterification)	Methanol-to-acid molar ratio = 1 : 2; 75 °C; 2 h; catalyst loading 0.08 g ml^−1	Conv. (%) = 60	Lost activity by 50% after 6 cycle	67
SZ	Soybean oil	Methanol-to-oil molar ratio = 20 : 1; 150 °C; 2 h; catalyst loading 5 wt%	Conv. (%) = 99.8	Not stable	68
SZ, STO (sulfated tin oxide)	n-Octanoic acid (esterification)	Methanol-to-acid molar ratio = 4.5 : 1; 134 °C; 20 h; catalyst loading 4 g	Conv. (%) = 100, conv. (%) = 95	—	46
SZ, STO	n-Octanoic acid (esterification)	Methanol-to-acid molar ratio = 4.5 : 1; 175 °C; 20 h; catalyst loading 4 g	Conv. (%) = 99, conv. (%) = 100	—	16
STO		Methanol-to-acid molar ratio = 12 : 1; 230 °C; 8 h; catalyst loading 2 wt%	Conv. (%) = 90		69
STO		Methanol-to-acid molar ratio = 6 : 1; 200 °C; catalyst loading 3 wt%	Conv. (%) = 80		70
SZ		Methanol-to-acid molar ratio = 20 : 1; 120 °C; catalyst loading 5 wt%	Conv. (%) = 98		71
STO	Acetic acid (esterification)	Methanol-to-acid molar ratio = 2 : 1; 250 °C	Conv. (%) = 92.5	—	36
SZ	Myristic acid (esterification)	Methanol-to-acid molar ratio = 10 : 1; 60 °C; 7 h; catalyst loading 0.5 wt%	Conv. (%) > 98	No significant decrease after 5 cycles	23
15SA-SBA-15-p	Carboxylic acids (esterification)	Methanol-to-acid molar ratio = 5 : 1; 50 °C; 5 h; catalyst loading 3 wt%	Conv. (%) = 50	No significant decrease after 5 cycles	72
SZ	Triacetin (transesterification)	Methanol-to-acid molar ratio = 6 : 1; 60 °C; 6.5 h; catalyst loading 2 wt%	Conv. (%) = 50	—	32
TiO2/SO4	Soybean oil, castor oil (transesterification)	Methanol-to-oil molar ratio = 6 : 1; 120 °C; 1 h; catalyst loading 1 wt%	Conv. (%) = 40, conv. (%) = 25		11
TiO2/SO4^2−	Oleic acid (esterification)	Methanol-to-oil molar ratio = 6 : 1; 80 °C; 3 h; catalyst loading 1 wt%	Conv. (%) = 82.2		27

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Mesoporous silica could also serve as a support for superacid catalysts because its mesostructure is adjustable and the hydrophobic/hydrophilic balance can be improved by functionalizing with diverse groups of sulfonic acid^54,92 to enhance their reusability.

In general, solid superacids are hydrothermally stable but they become less stable when used with low quality feedstock, which consist of large amounts of FFAs and water. The reusability can be enhanced by the method discussed below (Section 3.3).

3.2. Sulfonated carbon-based catalyst

Sulfonated carbon-based materials such as biomass have enjoyed wide application in biodiesel production because of their ability to absorb large quantities of hydrophilic functional groups such as –SO₃H, –OH and –COOH.^93,94 Incorporation of SO₃H onto a flexible carbon fiber of processed biomass materials gives the materials easy access to the SO₃H containing acid sites. Despite the small surface area reported for sulfated carbon-based catalysts (<30 m² g⁻¹), they exhibit a remarkable catalytic performance. For instance, in our recent study,²⁹ we prepared sulfonated carbon-based materials using palm fronds and spikelet powder as the carbon precursors and sulfuric acid as the sulfate source. The synthesized materials exhibited a better catalytic performance for esterification of used frying oil than some earlier reported solid acid catalysts (Table 2). The use of a sulfonated carbon-based catalyst leads to a more economical process because it requires mild reaction conditions such as a low temperature (100 °C), low catalyst amount (2 wt%) and a methanol-to-oil ratio of 5 : 1, which gives a FFA conversion of 98%.

Table 2 Some recent sulfonated carbon-based catalyst for biodiesel production

Catalyst	Preparation method	Feedstocks	Reaction conditions	Activity	Recycling	Ref.
Sulfonated vegetable oil asphalt and petroleum asphalt derived solid acids	Sulfonation of an incompletely carbonized vegetable oil asphalt and petroleum asphalt	Waste oil containing 50 wt% of oleic acid and 50 wt% of cottonseed oil (esterification and transesterification)	Methanol-to-waste oil molar ratio = 20.9 : 1; catalysts loading 0.3 wt% (related to waste oil)	Waste oil conv. (%) = 95 (only having the reaction proceeded at 220 °C; 5 h); waste oil conv. (%) = 98 (coupling reaction and separation; 3 h; 140 and 220 °C for the first and second step)	Recoverable	78
SO3H-organic matter	Sulfonation of incompletely carbonized microcrystalline cellulose powder	Oleic acid, triolein (esterification and transesterification)	Methanol-to-acid molar ratio = 26 : 1; 95 °C; 4 h; catalyst loading 4 wt% (esterification); methanol-to-triolein molar ratio = 62 : 1; 130 °C; 700 kPa; 5 h; catalyst loading 7.3 wt% (transesterification)	Esterification yield (%) = 100, transesterification yield (%) = 98	Esterification and transesterification can be reused 10 and 5 times without decrease in activity	79
OMC–H2O2–SO3H	Sulfonation of H2O2-treated OMC	Palmitic acid, oleic acid, stearic acid (esterification)	Methanol-to-acid molar ratio = 20 : 1; 80 °C; 2 h	Conv. (%) = 80	No obvious activity loss after 5 successive cycles	80
Sulfonated D-glucose derived sugar catalyst	Sulfonation of incompletely carbonized D-glucose	Palmitic acid, oleic acid, stearic acid, (esterification)	Methanol-to-acid molar ratio = 10 : 1; 65 °C; 5 h; catalyst loading 5 wt%	Yield (%) > 95	Stable after fifty cycles of successive re-use	81
Sulfonated palm fond, sulfonated palm spinket	Sulfonation of incompletely carbonized palm residues	Used frying oil (esterification)	Methanol-to-oil molar ratio = 5 : 1; 150 °C; 6 h; catalyst loading 1 wt%	Conv. (%) = 78, conv. (%) = 91	About 5% loss of activity after 8 successive cycles	77
Sulfonated C. inophyllum seed cake	Sulfonation of incompletely carbonized C. inophyllum seed cake	C. inophyllum seed oil (esterification and transesterification)	Methanol-to-oil molar ratio = 30 : 1; 180 °C; 6 h; catalyst loading 7.5 wt%	Conv. (%) = 99	Catalyst regeneration is achievable	82
Sulfonated carbon nanohorn	Sulfonation of incompletely carbonized carbon nanohorn	Palmitic acid (esterification)	Methanol-to-oil molar ratio = 33 : 1; 64 °C; 6 h; catalyst loading 3 wt%	Yield (%) = 93	The active sites were not deactivated after 3 cycles	82
Sulfonated corn straw	Sulfonation of incompletely carbonized corn straw	Oleic acid (esterification)	Methanol-to-acid molar ratio = 7 : 1; 60 °C; 4 h; catalyst loading 7 wt%	Yield (%) = 98	—	84
Sulfonated starch, cellulose, sucrose, D-glucose	Sulfonation of incompletely carbonized	Waste cooking oil (esterification and transesterification)	Methanol-to-oil molar ratio = 30 : 1; 80 °C; 8 h; catalyst loading 10 wt%	Yield (%) = 95, yield (%) = 88, yield (%) = 80, yield (%) = 76	Very stable. Only about 7% loss of activity after 50 cycles	85

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Dawodu et al. also reported an outstanding performance for the sulfonated carbon-based catalyst prepared from C. inophyllum cake, which gives a C. inophyllum oil conversion of 96.6%. However, this is less economical when compared with our work because of the methanol-to-oil ratio of 30 : 1 and 7.5 wt% catalyst amount at 180 °C for 5 h. Table 2 shows more recent studies on the utilization of carbon-based materials as solid acid catalysts for biodiesel production.^95–102 The remarkable performance is due to the incorporation of large amounts of sulfate groups to the bulk of the carbon material by strong hydrogen bonds.^29,103,104 After regeneration by decantation, washing, and drying, the catalyst still maintained its performance after 8 cycles without significant leaching of its strong acid sites. This shows that sulfonated carbon-based catalysts show remarkable stability.

However, there is need for improvement in reusability in water and FFA rich reaction media.

3.3. Alkyl-bridged organosilica moieties functionalized hybrid catalysts

For rational design of efficient catalysts for biodiesel production, it is essential to consider improving the acid sites' environment such as hydrophobicity,^61,105 alleviating mass transfer limitations as well as increasing the density and access to acid sites on the catalyst surface. Hydrophobicity promotes preferential adsorption of hydrophobic reacting species on the catalyst surface and prevents strong adsorption of hydrophilic products such as water and glycerol, thereby reducing the deactivation propensity of the active sites.^93,106 Furthermore, enhanced surface area as well as pore size and volume helps to alleviate mass transfer limitations and increase the density and access to acid sites.^67,72,93 These design specifications are simultaneously obtainable by functionalizing solid acids with alkyl-bridged organosilica moieties to form an organic–inorganic hybrid catalyst.^62,67,93,107 Alkyl bridged silica moieties are obtained by in situ incorporation of organosulfonic acid groups throughout the silica framework with bridging alkyl groups.

These polymer-oxides and moieties are hydrophobic in nature and are capable of incorporating excellent porosity and removal of the surface hydroxyl groups to maintain the hydrophobic/hydrophilic balance of the catalyst.^93,108 Organic–inorganic hybrid catalysts include H₃PW₁₂O₄₀–Ta₂O₅/Si(Et/Ph)Si, H₃PW₁₂O₄₀–ZrO₂/Si(Et/Ph)Si, H₃PW₁₂O₄₀/Ta₂O₅–Si(Me/Ph), H₃PW₁₂O₄₀/ZrO₂–Si(Me/Ph) and SO₄²⁻/ZrO₂–SiO₂ (Et/Ph).⁹³ These hybrid catalysts are remarkably stable and easy to regenerate using dichloromethane to wash rather than employing thermal treatments as usual, thereby preserving the organic functionalities, Keggin unit and structural integrity of the catalysts.⁹³

To successfully achieve a well-designed superhydrophobic hybrid catalyst of this type with a 2D hexagonal p6mm, 3D interconnected wormhole-like pore morphologies and 3D cubic Im3m, it is imperative to vary parameters such as preparation conditions, surfactant type, Si/Zr molar ratio, and organic silica precursor.^93,109 High organic content hybrid materials could be synthesized by co-condensation, a periodically ordered mesoporous organosilica (PMO) approach, using a special sol–gel and by a one-step templated sol–gel condensation.^93,107,110 All the approaches, except the one-step template sol–gel condensation, lead to a reduction in surface area and pore volume because of poor loading control, loss homogeneity and acid site leaching associated with a post-grafting approach.¹¹¹ The one-step template sol–gel condensation approach incorporates the hydrophobic functionalities in situ to produce mesoporous hybrid materials with better homogeneity of active species, which preserves the active phase from leaching, thereby improving the catalyst stability.^112–114 The catalytic activity of these materials is discussed in Section 5.1.

4. Super acidity characterization

Solid superacids are characterized by various techniques such as ammonia temperature program desorption (NH₃-TPD), infrared spectroscopy (FTIR) and pyridine adsorption. Moreover, a titration method with a probe using Hammett indicators and potentiometric titration is used.⁴⁴

The titration method with a probe using Hammett indicators also determines the acid strength distribution and surface acidity of heterogeneous catalysts. Hammett indicators are uncharged bases that are converted to conjugate acids by proton transfer. The probe reagents for the titration include water, pyridine, ethyl alcohol and n-butylamine. While n-butylamine (pK_a ≤ +10.6) and pyridine (pK_a ≤ +5.3) are easily absorbed onto the active sites, ethyl alcohol (pK_a ≤ −2) and water (pK_a ≤ −1.7) are scarcely absorbed.¹¹⁵ The most popularly used of these probe reagents is n-butylamine because it possesses the highest pK_a value. The degree of hydration and exchangeable cations of a sample strongly determines its surface acidity. Touillaux et al.¹¹⁶ discovered that water molecules absorbed on montmorillonite dissociate 10⁷ times faster than ordinary water. This is traceable to the acidic nature of the exchangeable cation in the absorbed water, which donates a proton as follows:¹¹⁷

Mⁿ(H₂O) → (M–OH)ⁿ⁻¹ + H⁺.

The ability of a solid sample to donate a proton (pK_a) is expressed by Hammett and Deyrup as

where pK_a represents the activity of the hydrogen ion from the surface acid; [BH⁺] and [B] are the activity coefficients of the acidic and basic sites formed by the indicator, respectively.¹¹⁸ If the reaction proceeds by the transfer of an electron pair to the surface, then the activity function is given by
where [AB] represents the neutral base concentration that reacted with the electron pair acceptor, A or Lewis acid.

Prior to the titration, the sample is prepared by heating at 200 °C for 1 h under vacuum condition according to Yang et al.³³ Ropero-Vega et al.⁴⁴ proposed preparation by dispersing the sample in dry benzene. After preparation, the sample is titrated with a probe prior to addition of the Hammett indicators vapor. The color change of the indicators with their respective pK_a's is presented in Table 3.¹¹⁹ The amount of acid sites for both Lewis and Brønsted sites are determined by the mole of the sites per gram of sample after the validation of acid strength. The acid strengths for solid acids are in from H₀ = 6.8 to as low as −8.2. Samples with an acid strength estimated greater than H₀ = −11.93 are said to be superacids. This corresponds to the strength of H₂SO₄ (100%). However, titration using Hammett indicators is not suitable for dark or colored samples due to the difficulty in observing a color change.

Table 3 Basic indicators used for the measurement of acid strength¹⁰¹

Indicator	Color	Color	pKa	[H2SO4]%
	Base form	Acid form
a The indicator is liquid at room temperature and acid strength corresponding to the indicator is higher than the acid strength of 100 percent H2SO4.
Neutral red	Yellow	Red	6.8	8 × 10^−8
Methyl red	Yellow	Red	4.8	—
Phenylazonaphthylamine	Yellow	Red	4	5 × 10^−5
p-Dimethylaminoazobenzene	Yellow	Red	3.3	3 × 10^−4
2-Amino-5-azotoluene	Yellow	Red	2	5 × 10^−3
Benzeneazodiphylamine	Yellow	Purple	1.5	2 × 10^−2
Crystal violet	Blue	Yellow	0.8	0.1
p-Nitrobenzeneazo-(p′-nitro-diphenylamine)	Orange	Purple	0.43	—
Dicinnamalacetone	Yellow	Red	−3	48
Benzalacetophenone	Colorless	Yellow	−5.6	71
Anthraquinone	Colorless	Yellow	−8.2	90
2,4,6-Trinitroaniline	Colorless	Yellow	−10.1	98
p-Nitrotoluene	Colorless	Yellow	−11.35	^a
m-Nitrotoluene	Colorless	Yellow	−11.99	^a
p-Nitrofluorobenzene	Colorless	Yellow	−12.44	^a
p-Nitrochlorobenzene	Colorless	Yellow	−12.7	^a
m-Nitrochlorobenzene	Colorless	Yellow	−13.16	^a
2,4-Dinitrotoluene	Colorless	Yellow	−13.75	^a
2,4-Dinitrofluorobenzene	Colorless	Yellow	−14.52	^a
1,3,5-Trinitrotoluene	Colorless	Yellow	−16.04	^a

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On the other hand, FTIR-pyridine adsorption is said to be more reliable in classifying and quantifying the acidity properties of solid acids because of it is has good resolution, sensitivity and ability to determine chemical species.^118,120 It classified the acid sites into Brønsted and/or Lewis and also classifies the acids based on acid strength. The samples are prepared by annealing and pressing them into a thin self-supporting wafer. This is followed by evacuation at 400 °C for 2 h in vacuum to purge the species that are strongly bonded to the sample and activate it prior to absorption. Furthermore, pyridine adsorption takes place at room temperature for about 10 min, after which the sample is desorbed at 200, 300, 400 and 700 °C. The desorption temperatures correspond to weak, mild, strong and super acidity. The band intensities at 1450 cm⁻¹, 1540 cm⁻¹ and 1438 cm⁻¹, which correspond to Lewis and Brønsted acid sites and surface hydroxyl determine the concentration of each site.^33,121,122 Furthermore, the band at 1488 cm⁻¹ represents a mixture of Lewis and Brønsted acid sites.⁴⁰

NH₃-TPD determines the acid strength distribution and amount of acid sites in the solid acid samples. In comparison with other analytical methods available, NH₃-TPD is becoming increasingly used to probe the surface acidity of solid acid samples.¹²³ However, the technique is somewhat tedious and time consuming towards repeated operation for more sample analysis. Moreover, Wang et al.¹²⁴ developed a high throughput strategy using a multistream mass spectrometer screening (MSMSS) procedure. This technique shows an outstanding improvement over the conventional method because it can be used to analyze up to 10 samples in around 6 h. Gorgulho et al.¹²³ comparatively studied the effectiveness of titration and TPD techniques. They reported that both methods give closely related results; however, the quantitative results of TPD are closer to that of the elemental analysis. The titration method is less effective in quantifying weak acid sites.

5. Biodiesel production via solid superacid catalysis: more efficient strategies

Owing to the fact that biodiesel production could also involve water as a reactant, solid superacid catalysts produced by functionalization with sulfate ions are prone to deactivation due to the leaching of active sites (sulfate ions). Therefore, it is expedient to decipher a more effective way of preventing or reducing the side effects (leaching of active sites) of the moisture content of the feedstock or/and of the product. The following section discussed in detail the recent advances towards the more efficient biodiesel production from highly acidic feedstocks.

5.1. Alkyl-bridged organosilica moieties functionalized hybrid catalysis

Alkyl-bridged organosilica moieties are highly valued water tolerant materials for hydrophobization of sulfated mesoporous silica catalyst^54,111 to improve the chemical stability of the catalyst towards efficient biodiesel production.

Xu et al.^125–127 investigated the catalytic activity of hybrid catalysts synthesized by functionalizing Ta₂O₅ with both a Keggin-type heteropolyacid and alkyl-bridged organosilica moieties. The H₃PW₁₂O₄₀ loading was varied from 3.6% to 20.1% in a one-step sol–gel hydrothermal route using a triblock copolymer as the surfactant. The catalyst showed a remarkable performance in the transesterification of soybean oil and tripalmitin and in the esterification of myristic acid and lauric acid. The catalyst is also proficient towards simultaneous transesterification and esterification under mild operating conditions. The report also shows that incorporating both hydrophobic and superacid functionalities within the Ta₂O₅ surface enhances the catalytic performance. Lastly, the hybrid catalysts were said to be reusable after three runs of catalytic reaction. Melero et al.^54,111 also studied the superhydrophobicity of a SBA-15-based hybrid catalyst obtained from SBA-15 modification with both arene-sulfonic acid and trimethyltrimethoxysilane (TMS) for biodiesel production from acidic crude palm oil. They reported that the alkyl-free sulfonated hybrid catalyst demonstrated low hydrophobicity when 1 wt% water was added to the reaction medium, leading to a vivid reduction in biodiesel yield. This weakens the active site environment, making it vulnerable to attack by glycerol, which is a polar by-product, thereby hindering the hydrophobic substrates from accessing the active sites. Furthermore, addition of more water up to 10 wt% leads to acid catalyzed hydrolysis, thus increasing the FFA content of the produced diesel, which makes it substandard. However, further modification with TMS alleviates blockage of active sites, so the catalytic performance improved remarkably.

To better manage water in biodiesel production, the study of Long et al.¹⁰⁵ dwells extensively on the synthesis of a polymer-oxide hybrid catalyst based on sulfate nonporous silica for hydrolysis of esters. The catalyst was synthesized by the formation of a polymer brush and subsequent functionalization with sulfonic acid. This gives a well-designed hybrid material with remarkable hydrolytic stability as well as access to the active site. They reported that the catalyst is as active as p-toluenesulfonic acid and more active than Amberlyst 15 in the hydrolysis of ethyl lactate. However, the major obstacle to its recyclability is that the polymer chains gradually detach from the oxide support and exclusion of SO₃H from the catalyst surface. Table 4 presents more reports on the activities of alkyl-bridged organosilica moieties.^128–131

Table 4 Some recent sulfonic acid modified mesoporous organo-silica for biodiesel production

Catalyst	Preparation method	Feedstocks	Reaction conditions	Activity	Recycling	Ref.
Pr SO3H-MM-SBA-15	Post-synthesis	Palmitic acid, glyceryl trioctanoate (esterification and transesterification)	Methanol-to-palmitic acid, molar ratio = 4 : 1; methanol-to-glyceryl, trioctanoate molar ratio = 30 : 1; 60 °C; 6 h; catalyst, loading 0.3 wt%	Conv. (%) of, palmitic acid = 55; conv. (%) of glyceryl trioctanoate = 2.5	No data about, deactivation, behaviour	110
Propyl-SO3H SBA-15, Me/Arene-SO3H SBA-15, arene-SO3H SBA-15	Co-condensation, post-synthesis, co-condensation	Soybean oil containing, 20 wt% of oleic acid (esterification and transesterification)	Microwave irradiation; 1-butanol-to-oil molar ratio = 6 : 1; 190 °C; 15 min; catalyst, loading 5 wt%	Yield (%) = 38, yield (%) = 58, yield (%) = 56	The activity level of the 2nd run was 85–90% of the fresh catalyst activity	111
SBA-15-SO3H, SBA-15-SO3H-R, R/SBA-15-SO3H (R = Me, Et or Ph)	Co-condensation, co-condensation, post-synthesis	Palmitic acid in soybean oil (esterification)	Methanol-to-palmitic, acid molar ratio = 20 : 1; 85 °C; 2 h; catalyst, loading 10 wt%	Conv. (%) = 88, conv. (%) = 84, conv. (%) = 70	No data about, deactivation, behaviour	112
Propyl-SO3H-KIT-6 (5.2 nm), propyl-SO3H-KIT-6 (6.2 nm), propyl-SO3H-KIT-6 (7.0 nm)	Post-synthesis	Palmitic acid (esterification)	Methanol-to-palmitic, acid molar ratio = 30 : 1; 60 °C; 6 h; catalyst, loading 0.4 wt%	Conv. (%) = 14, conv. (%) = 28, conv. (%) = 39	Stable under the, mild operating, conditions, employed	113
Arene-SO3H-SBA-15	Co-condensation		Methanol-to-oil, acid molar ratio = 30 : 1; 160 °C; 8 h; catalyst, loading 8 wt%	Yield (%) = 82		94
Propyl-SO3H-SBA-15 arene-SO3H-SBA-15 arene-SOH-SBA-15 (capped)	Co-condensation	Crude palm oil	Methanol-to-oil, acid molar ratio = 20 : 1; 140 °C; 2 h; catalyst, loading 6 wt%	Yield (%) = ∼72, yield (%) = ∼78, yield (%) = ∼90		73
SiO2-propyl-SO3H	Co-condensation	Acetic acid (esterification)	Methanol-to-oil, acid molar ratio = 1 : 1; 110 °C; catalyst loading 25 mg	Conv. (%) = 86	The activity decreased by 5% after 5 cycles	26
SBA15-propyl-SO3H SBA-15-phenyl-SOH	Co-condensation	Interesterification of extra virgin olive oil with ethyl acetate	Methanol-to-oil, acid molar ratio = 20 : 1; 130 °C; 6 h; catalyst loading 13 wt%	Conv. (%) = 6, conv. (%) = 20	—	30
Propyl-sulfonic SBA-15	Co-condensation	Ethyl hexanoate (transesterification)	Methanol-to-oil, acid molar ratio = 4 : 1; 60 °C; 4 h; catalyst loading 200 mg	Conv. (%) = 63	No significant decrease after 4 cycles	28

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5.2. Pre- and in situ water removal

Apart from alkyl modification of the catalyst, several methods are employed to salvage catalyst deactivation in biodiesel production in the presence of water. These include introduction of molecular sieves into the reacting media to scavenge water, feedstock and catalyst pretreatment, and reduction of the temperature profile of the system by initially promoting transesterification at a high temperature and the subsequent temperature reduction to minimize hydrolysis of FAME.¹¹¹ The first method entails drying of the catalyst at 80 °C and the feedstock at 100 °C and 70 mbar for 12 h prior to the reaction¹¹¹ or drying by adding a dehydrating agent such as molecular sieves.^132,133 Li et al.¹³² investigated this pretreatment strategy towards methanolysis of waste oil over a lipase catalyst. They reported that the pretreated waste oil was effectively converted to biodiesel with a higher yield of methyl ester than untreated waste oil. The yield from the treated waste oil is comparable with the yield from refined oil. Alternatively, the second method involves in situ water removal from the system by addition of dehydrating molecular sieve such as zeolite LTA using a catalyst/zeolite mass ratio of 10.^111,133 Prior to the reaction, the molecular sieve must degassed at about 110 mbar and 180 °C for about 12 h in an oven.¹¹¹ Molecular sieves are appropriate for dehydrating organic solvents. In the reaction media, both water and the products are strongly adsorbed on the surface of the molecular sieve, but the pores are only permeable by water molecules.¹³³ This facilitates in the effective removal of water molecule from the reaction media. Melero et al.¹¹¹ performed a comparative study of two of these strategies. They compared pretreatment of feedstock and catalyst with in situ water removal from the reaction media over arene-SO₃H-SBA-15. Fig. 7 shows the comparative yield of FAME as a function of time for the strategies. The pretreatment method gave a better FAME yield compared to the untreated catalyst and feedstock (about 5% difference). The most effective strategy is in situ water removal. This is mainly because the in situ water removal strategy scavenged the in situ generated water from the FFA esterification process. Therefore, it is tenable to say that water formation is more paramount to the catalytic process than the initial water content of the feedstock and the catalyst because of its preferential interaction with the active sites due to proximity.

Fig. 7 Evolution of yield to FAME from crude palm oil over arene-SO₃H-SBA-15 catalyst with the different strategies. Strategy 1: pretreatment of feedstock and catalyst. Strategy 2: in situ water removal from the reaction media. Reaction conditions: 160 °C; 30 : 1 methanol : oil molar ratio; 8 wt% catalyst loading; 2000 rpm stirring rate.¹¹¹

5.3. Process intensification: temperature profile reduction

FAME production does not only proceed through transesterification of triglycerides and esterification of FFAs as discussed above. The presence of water molecules engenders hydrolysis of triglycerides to produce FFAs and subsequent esterification of the FFAs (Fig. 8). The major parameters that influence these reactions are water content and operating temperature.¹¹¹

Fig. 8 Simplified reaction pathway for FAME production from highly acidic feedstock over solid superacid catalyst. Reaction (1), (4), (5) and (6), transesterification routes; (2) and (3) are FFA esterification routes; while (7) is a FAME hydrolysis route.¹¹¹

Temperature profile reduction helps to intensify the processes by minimizing FFA yield and maximizing FAME yield simultaneously. This strategy comprises two steps: the first step promotes transesterification and triglyceride hydrolysis of FFA at high temperature (160–180 °C).^{105,134–140} The second step promotes esterification of FFA, both the ones inherent in the feedstock and the ones produced from the first step, thereby minimizing hydrolysis of FAME at low temperatures (80–100 °C).^111,136,141 Melero et al.¹¹¹ comprehensively investigated the effect of temperature profile reduction at various upper and lower temperature settings (Fig. 9). To reduce the transition period, they used a water–ice bath to step down the lower limit. This strategy remarkably reduced the acid value of the produced diesel (from 20 mg KOH per g to less than 5 mg KOH per g). It was also reported that the FAME yield increases with an increase in the higher limit temperature and a decrease in that of the lower limit. The optimal temperature profile reduction was 160–100 °C, which gave an acid value of 0.4 mg KOH per g and FAME yield of 96%.

Fig. 9 Yield to FAME from crude palm oil over arene-SO₃H-SBA-15 using a decreasing temperature profile. Reaction conditions: 2 h at temperature T₁ followed by 2 h at temperature T₂; 30 : 1 methanol : oil molar ratio; 8 wt% catalyst loading; 2000 rpm stirring rate.¹¹¹

6. Conclusion

Heterogeneous superacid catalysts are more effective in biodiesel production from low-grade/cost feedstock compared to their homogeneous counterparts because of their remarkable activity and reusability. This is made possible by their rational design, which ensures well-defined mesoporosity and an excellent hydrophobicity/hydrophilicity balance. A well designed solid superacid helps to minimize the mass transfer limitation and suppress deactivation by water and glycerol.

Zeolites are among the diverse possible known solid acids are, which are very popular in various industrial processes. However, zeolites being microporous are marred with stearic hindrance due to their micropore size preventing the diffusion of bulky molecules. Therefore, sulfated metal oxides become very popular. They are solid superacids with remarkable catalytic performance in biodiesel production from low-quality feedstock. However, the reusability still needs improvement due to leaching of sulfate groups into the reaction medium particularly in a reaction medium marred with moisture. Therefore, several strategies are employed to enhance the reusability of solid superacids such as functionalized heterogeneous catalysts and sulfonated carbon-based catalysts.

One of the strategies is hydrophobization with alkyl-bridge organosilica moieties for improved hydrothermal stability. The outstanding catalytic performance of hybrid catalysts is mainly due to their inherent Brønsted acidity, enhanced hydrophobic/hydrophilic balance and well-defined mesoporosity, which is traceable to the alkyl-bridged organosilica moieties incorporation. In addition, hybrid catalysts exhibit remarkable reusability because the incorporated moieties suppressed the sulfate groups leaching into the reaction medium.

The second visible strategy involves water removal either pre or in situ. The former involves feedstock and catalyst heat pretreatment, whereas the later involves introduction of molecular sieves to the reacting media as water scavengers. The later proved to be more effective because water formation is more paramount in the catalytic process than the initial water content of the feedstock and the catalyst due to proximity to the active sites, which engenders preferential interaction.

The third strategy is aimed at intensifying the process by reduction of the temperature profile, which aids in the simultaneous minimization of the FFA yield and maximization of the FAME yield. This helps to reduce the acid value towards producing high-quality biodiesel.

The second and the third strategies can be used over all the solid superacids discussed in Section 3 such as functionalized heterogeneous catalysts, sulfonated carbon-based catalysts and alkyl-bridged organosilica moieties functionalized hybrids.

Acknowledgements

We gratefully acknowledge the High Impact Research (HIR) Grant from the University of Malaya for fully funding this study through Project No. “D000011-16001”. We also appreciate the support of Chemical Engineering Department with RP015-2012D Grant.

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