An Overview of Solid Acid Catalysts in Lignocellulose Biorefineries

Abstract

The continuous depletion of fossil fuels demands their replacement with renewable energy sources for the production of fuels, chemicals, and materials. Lignocellulosic biomass can serve as a sustainable raw material for the manufacturing of various industrial products, such as fine chemicals, biofuels, polysaccharides, and biofuel precursors. Though numerous homogeneous catalysts are available for converting lignocellulosic biomass into fermentable sugars and biofuels, they require harsh environmental conditions, and their recovery is often difficult. Heterogeneous solid acid catalysts are efficient for biomass conversion, are environmentally benign, and can replace homogeneous catalysts in biorefineries to make them green. Zeolites, metal oxides, heteropoly acids, mesoporous silica nanoparticles, and carbon solid acid catalysts are some of the heterogeneous catalysts employed in lignocellulose biorefineries. This comprehensive review covers the different solid acids that can be used in biomass refineries, the factors influencing their catalytic activity, and the progress made towards their application in lignin depolymerization and the production of fermentable sugars, biofuels, and platform chemicals.

1. Introduction

Lignocellulose, composed of lignin, hemicellulose, and cellulose, is the most abundant renewable organic material on Earth. Worldwide annual biomass production is estimated at around 200 billion metric tons 3 [1]. This includes agricultural residues (e.g., straw, husk), forestry residues (e.g., sawdust, wood chips), and dedicated energy crops (e.g., miscanthus, switchgrass). Biorefineries aim to convert the lignocellulosic biomass into fuels, chemicals, and materials that can substitute traditional petroleum-based products. Integrated biorefineries help to minimize landfilling and the open burning of agricultural and forestry wastes. This, in turn, helps to reduce environmental pollution and greenhouse gas emissions. Cultivating dedicated energy crops for lignocellulosic feedstock enhances carbon sequestration in soils, contributing to climate change mitigation. Thus, biorefineries can significantly contribute to achieving the Sustainable Development Goal (SDG) 7 (Affordable and clean energy), SDG 12 (Responsible consumption and production), SDG 13 (Climate action), and SDG 15 (Life on land).

However, the complex structural composition of lignocellulose necessitates effective pretreatment and conversion strategies to achieve efficient biomass conversion and utilization. Various catalytic processes are required to break down the complex lignocellulose and transform the resulting intermediates into value-added products. Typically, in a biorefinery, processes such as lignocellulose pretreatment, the depolymerization of lignin, the pyrolysis and gasification of biomass, the hydrolysis of carbohydrates, hydration, dehydration, esterification, transesterification, and the upgradation of bio-oil rely heavily on the use of catalysts. Enzymatic catalysts (e.g., cellulases, hemicellulases), homogeneous catalysts (e.g., H₂SO₄, H₃PO₄, NaOH) and heterogeneous catalysts (e.g., solid acid, solid base, redox, metallic, bifunctional) are used in the above mentioned processes. Among these, solid acid catalysts are well suited to handle the complex and diverse chemical composition of the biomass sources and play a crucial role in the lignocellulose conversion process, offering distinct advantages, such as recyclability, ease of separation, and reduced environmental footprint, compared to traditional liquid acids. Furthermore, solid acid catalysts offer higher product yield and fewer byproducts. Solid acid catalysts enable efficient conversion, minimize waste generation during processing, and reduce the need for harsh chemicals, aligning with green chemistry principles. Moreover, using solid acid catalysts helps to reduce the carbon footprint of biorefinery operations through energy-efficient processing and offers a longer catalyst lifetime compared to conventional acids. For these reasons, solid acid catalysts are considered to be more sustainable. This review discusses the different solid acid catalysts used in biorefineries and their role in lignocellulose pretreatment, lignin depolymerization, carbohydrate hydrolysis, hydration, dehydration, esterification, and transesterification.

2. Solid Acid Catalysts

Solid acid catalysts (SACs) are heterogeneous catalysts that exhibit acidic properties through Brønsted acid sites (donate protons) and/or Lewis acid sites (accept electron pairs). These solid acids catalyze the reactions for the conversion of lignocellulosic biomass into second-generation biofuels, such as bioethanol, biodiesel, and bio-oil. SACs improve the efficiency in cellulose and hemicellulose hydrolysis, as well as the subsequent dehydration and isomerization steps. They are found to be effective in the production of platform chemicals, such as levulinic acid, furfural, and 5-hydroxymethylfurfural (HMF), which serve as building blocks for bioplastics, solvents, and a range of value-added products. SACs are advantageous because they can be easily recovered from the reaction mixture and recycled efficiently. The efficiency of the catalysts depends on various factors, including the reaction conditions (such as the reaction temperature, pressure, and reaction time) and the properties of the catalysts (such as the acid site density, strength, and type of functional groups), affecting the selectivity and yield. A variety of SACs are employed for this catalytic conversion, and some of them are classified in Figure 1 below.

2.1. Zeolite

Zeolites are crystalline microporous aluminosilicates and exhibit diverse crystal structures with large open pores. The International Zeolite Association has recognized 267 types of zeolites with differences in terms of shape, size, pore size, and rings, including 40 naturally occurring zeolites [2]. They have high thermal and chemical stability, making them suitable for reactions under harsh conditions. Shape selectivity, tunable acidity, and highly porous structures are the major advantages of zeolites [3]. The acidity and reactive properties of zeolites are strongly influenced by their crystalline structure, pore structure, and Si/Al ratio [4]. The Si/Al ratio directly impacts the Brønsted acidity of the zeolites and their physical and chemical properties. A higher Si/Al ratio increases their hydrophobicity and thermal stability. Y-zeolites typically have an Si/Al ratio of ~1 to 5 and exhibit strong acidity due to their greater number of Brønsted acid sites. Zeolites, such as HZSM-5, typically have intermediate Si/Al ratios in the range of 10 to 50. They combine strong acidity with higher thermal and hydrothermal stability. Meanwhile, in zeolites with a high Si/Al ratio (>100), the number of acid sites decreases, lowering overall acidity. These zeolites are more hydrophobic and resistant to deactivation in water-rich environments, making them suitable for biomass conversion reactions. The acidity is enhanced when Al³⁺ is substituted for Si³⁺ in a silanol structure. Therefore, adjusting this ratio allows for the fine-tuning of catalytic activity in acid-catalyzed conversions commonly used in biorefineries.

Figure 2 shows a representative mechanism for polysaccharide hydrolysis using zeolites. Initially, the water molecules adsorb on the acid sites in the zeolite through hydrogen bonding. The water-soluble oligomeric substrate enters the internal pores of the zeolite. After hydrolysis, the products diffuse out through the pores.

2.2. Metal Oxides

Metal oxides have emerged as one of the highly effective SACs in a wide range of acid catalytic processes in biorefineries due to their tunable Brønsted and Lewis acidity, thermal stability, and structural versatility. Metal oxides exhibit Lewis and Brønsted acidity depending on their surface structure and chemical composition. The strength of acidity is influenced by electronegativity, the oxidation state, the surface structure, and defect chemistry [5,6,7]. Lewis acid sites originate from coordinatively unsaturated metal cations, such as Al³⁺ in Al₂O₃, which accept electron pairs from bases like NH₃ or pyridine, forming coordinate bonds. The strength of these sites increases with lower coordination numbers, commonly found at edges or defect sites. Mixed oxides, such as SiO₂–Al₂O₃, enhance Lewis acidity due to the inductive effects of the silica network, further polarizing Al³⁺. Transition metal oxides, including TiO₂, ZrO₂, and SnO₂, exhibit Lewis acidity via their exposed Ti⁴⁺, Zr⁴⁺, and Sn⁴⁺ cations, respectively, with their acid strength tunable through synthesis and calcination [8]. Brønsted acidity, on the other hand, arises from surface hydroxyl groups (M–OH), especially prominent in hydrated metal oxides, where moisture leads to protonated surface oxygen atoms. Brønsted acidity in metal oxides is fundamentally associated with the presence of surface hydroxyl groups (M–OH). These surface-bound hydroxyl groups possess a proton (H⁺) that can be donated to an adsorbed molecule. Strong Brønsted sites are observed in supported metal oxides, such as WO₃/Al₂O₃ [9] and MoO₃/Al₂O₃ [10], where interaction with the support enhances the proton-donating ability. In WO₃, the protonation of terminal W=O groups forms active Brønsted sites. Crystalline aluminosilicates, such as zeolites, display robust Brønsted acidity through bridging hydroxyls (Si–OH–Al), with strength modulated by the Si/Al ratio and framework structure. Comparative trends show that CrO₃ (Cr⁶⁺) is more acidic than Cr₂O₃ (Cr³⁺) due to a higher oxidation state. Smaller ionic radii and higher charge densities (e.g., Ti⁴⁺ > Ti³⁺) correlate with stronger Lewis acidity. Furthermore, modification strategies, such as the sulfation of ZrO₂, introduce SO₄²⁻ groups, generating superacid sites by strongly polarizing Zr⁴⁺, and thus dramatically enhancing both the Lewis and Brønsted acid strength.

Various metal oxides have been studied for sugar conversions, such as layered transitional metal oxides, nanosheet aggregates, and mesoporous and amorphous metal oxides, as well as sulfated metal oxides and mixed metal oxides [11]. The thermal stability and structural integrity of metal oxides, such as zirconia (ZrO₂) [12], alumina (Al₂O₃), and tungsten trioxide (WO₃) [13] make them suitable for biomass conversion processes. To enhance the acidity and catalytic performance of metal oxides, they can be modified or functionalized (e.g., by adding sulfate or phosphate groups) and transformed into superacid catalysts [14]. Synthesis and structure are key to tailoring acidity, porosity, and thermal stability for specific applications in biorefineries. Metal oxides, such as ZrO₂, WO₃, and Al₂O₃ possess distinct crystal structures, typically based on octahedral or tetrahedral coordination of metal cations with oxygen atoms. These crystalline frameworks can exhibit polymorphism, where different crystalline phases (e.g., γ-Al₂O₃ vs. α-Al₂O₃) possess different acidic properties. The well-defined mesoporous pore structure of the catalysts enhances the reactants’ access to acid sites and improves catalytic activity.

The synthesis of metal oxides is a key step in determining their structural properties, acid site distribution, and catalytic performance. Common synthesis routes employed to tailor these properties include precipitation and co-precipitation, sol-gel, and hydrothermal and solvothermal methods. Similarly, the post-synthesis steps, such as surface functionalization and calcination, also play a critical role in fine-tuning the properties of metal oxide catalysts.

2.3. Heteropolyacids (HPAs)

Heteropolyacids (HPAs) are made of hydrogen, oxygen, Group IV transition metals (primarily molybdenum and tungsten), and a p-block element (typically phosphorus, silicon, or arsenic) [15]. They are used in liquid form for various organic reactions, such as hydration, oxidation, polymerization, and transesterification. They are used in heterogeneous conditions for etherification and esterification reactions. High stability, high proton mobility, strong Brønsted acidity (near the superacid region), additional Lewis acid sites, and fewer side reactions make them environmentally benign catalysts. HPAs can contain Brønsted and/or Lewis acid sites. Their type and number affect polysaccharide conversion [15].

HPAs are becoming very important in the field of biomass conversion. HPAs have flexible molecular structures that can be adjusted for different reactions, and they work well in eco-friendly and efficient processes. HPAs can perform both acid-based and redox (electron transfer) reactions, making them useful for many steps in converting biomass into valuable chemicals. New techniques, such as chemical looping and the use of green solvents, widen the application of HPAs and make HPAs strong candidates for sustainable biorefinery systems. HPAs, particularly vanadium-substituted Keggin-type polyoxometalates (POMs), have drawn significant attention as multifunctional solid acid catalysts in biomass valorization owing to their dual Brønsted acidity and redox capabilities. Recent studies have emphasized their efficiency and versatility in selective oxidation and hydrolysis reactions under mild conditions [16]. Keggin-type polyoxymetalate, H₅PV₂Mo₁₀O₄₀ (HPA-2), exhibited isomer-specific activity and additive effects in the aerobic oxidation of sugar-derived intermediates to formic acid. The addition of alcohols such as methanol suppresses CO and CO₂ formation while preserving high formic acid yields under relatively low temperatures (80 °C) and pressure (5 bar O₂) [17]. Complementing this, He et al. [18] developed a chemical looping strategy using H₈PV₅Mo₇O₄₀ (HPA-5) as both the catalyst and oxygen carrier, which significantly enhanced formic acid selectivity (up to 95.4%) by temporally separating substrate oxidation and catalyst regeneration steps. This method demonstrated applicability beyond glucose, extending to xylan, cellulose, and raw biomass, highlighting its potential for scalable biorefinery integration [18]. Raabe et al. [17] provided a comprehensive review of HPA-5’s structural and catalytic versatility, emphasizing its role in green chemical transformations, such as the esterification, oxidative delignification, dehydration, and fractionation of lignocellulosic biomass. Incorporating vanadium into the Keggin structure enhances redox activity while preserving the catalyst’s structural integrity, allowing HPA-5 to outperform many conventional catalysts [17]. Rafiee and Eavani [19] provided insights into the heterogenization of HPAs and reviewed in detail the use of HPAs in organic synthesis, i.e., oxidation, reduction, and multicomponent reactions. The conversion of HPAs to heterogeneous catalysts will help to utilize them as efficiently as possible in biorefinery processes with easy catalyst recovery and reuse, which is critical for economic viability in industrial applications [19].

2.4. Clays

Clays are naturally occurring layered silicate materials with a unique structure, a high surface area, and tunable acidity. They are composed of alternating layers of tetrahedral silicon-oxygen sheets and octahedral aluminum-oxygen sheets. Due to their natural abundance and desirable physicochemical properties, such as their ion-exchange capacity and thermal stability, they are considered attractive candidates for biomass conversion processes. Both natural clays (e.g., kaolinite, montmorillonite, and bentonite) and their modified forms (e.g., acid-activated clays and pillared clays) are recognized for their superior performance in biomass conversion processes.

2.5. Carbon-Based SAC

Carbon-based SACs have recently gained attention in biorefineries. Like other SACs, carbon-based SACs are sustainable and exhibit high thermal stability and tunable surface properties. Due to their low cost, chemical and mechanical stability, and high efficiency, carbon-based SACs are suitable for cellulose/lignocellulose conversion.

Surface groups on the carbon catalysts play a significant role in their function. Functional groups like –COOH and phenolic –OH enhance the substrate affinity of carbon solid acids [20]. The synthesis of functionalized carbon solid acids is illustrated in Figure 3. The synthesis of solid carbon acids with strong acid sites usually involves a two-step process where the carbon sources, e.g., glucose, cellulose, and lignin, are partially carbonized and subsequently sulfonated with acid at lower temperatures. Alternatively, it can be synthesized directly by a single-pot method using a suitable carbon precursor [21]. Carbon solid acid catalysts with higher adsorption capacity have demonstrated better catalytic activity [22].

2.6. Graphene/Graphene Oxide

Graphite oxidized by potent oxidizing agents like sulfuric acid shows numerous oxygen functionalities, such as –OH, –COOH, and epoxide, in their structure. These functionalities convert graphite into hydrophilic graphene oxide (GO), enhancing the interaction with cellulose. GO can be reduced to graphene by numerous thermal, chemical, and electrochemical reduction methods. This reduced graphene oxide has residual –OH and –COOH functionalities. Graphene oxide has been used as a structure-directing agent while preparing carbon-based solid acid using cellulose [23]. The addition of GO led to the formation of a lamellar structure and the increased hydrophilicity of the catalyst. It caused better catalyst dispersion in the aqueous system and improved interaction with the substrate, resulting in nearly 94% glucose selectivity [23].

2.7. Mesoporous Silica Nanoparticles (MSNs)

Mesoporous silica nanoparticles (MSNs) are porous, honeycomb-like structures with a high surface area and tunable pores. Particles of smaller size can pass through the channels and interact with the functional groups attached to the channels. MSNs exhibit a high surface area and tunable pore sizes. Silanol groups on pure SiO₂ facilitate functionalization with versatile moieties to tailor the surface properties of SiO₂ [24]. Functionalization with sulfonic acid or phosphoric acid groups enhances the hydrophilicity and polarity of these materials [25,26,27]. Thus, MSNs and functionalized MSNs can catalyze hydrolysis, esterification, deoxygenation, and dehydration reactions in biorefineries. The introduction of sulfonic groups can be achieved through post-synthetic grafting methods. The thiol groups are first attached to the silica surface and subsequently oxidized to sulfonic groups using oxidizing agents like hydrogen peroxide.

Much research has been done using mesoporous functionalized silica in the hydrolysis. Pure silica has hydrophobic characteristics, but Brønsted and Lewis acid sites like Al make them hydrophilic [28]. Unlike liquid mineral acids, MSNs are easily separable and can be reused several times, and they are thermally stable to withstand harsh reaction conditions required for biomass conversion reactions.

The most common types of MSNs are Santa Barbara Amorphous-15 (SBA-15) and Mobil Composition of Matter No-4y81 (MCM-41). MCM-41 is made by using the surfactant cetyl trimethyl ammonium bromide (CTAB) and dissolved silica precursor, while SBA-15 is prepared using an amphiphilic block copolymer (Pluronic 123) and silica source, such as tetraethylorthosilicate (TEOS). The silica precursor reacts with the micellar rod template formed by the surfactant, resulting in MSN particles. The template is washed off, resulting in a highly stable/ordered structure with mesopores (Figure 4) [29]. Though MSN displays a high surface area, it requires surface functionalization to improve its catalytic activity. Cellulase-immobilized MSNs are also used for glucose conversion from cellulose hydrolysis.

2.8. Metal-Organic Frameworks (MOFs)

MOFs are made of metal clusters and ligands that can serve as active sites. They have a highly nanoporous structure, a large pore volume, tunable cavities, a high specific surface area, and mainly copious Brønsted and Lewis acid sites. Also known as porous coordination polymers, they can form porous structures made of inorganic metal ions (joints) with organic ligands (linkers). Nodes from metal ions bind with arms of linkers, resulting in a one-, two-, or three-dimensional cage-like structure. Joints consist of transition and post-transition metals and lanthanides, while linkers are made of anions (e.g., heterocyclic compounds, sulfonates, or carboxylates) [30]. The presence of organic ligands makes MOFs heat sensitive, and, therefore, their application is limited to low-temperature applications. MOFs can be combined with graphene, porous carbon, enzymes [31], or metal oxides [32] and used in various applications. Carbon/metal oxide MOF has good stability under high temperatures [33]. MOFs can be tailored to applications like catalysis and separation by carefully selecting metal ions and ligands.

2.9. Polymeric Catalysts

Polymeric catalysts are ion-exchange resins in which the polymeric chains contain the sulfonic acid groups. For example, Amberlyst-15 is a polystyrene-based sulfonic acid resin, and Nafion^® is a perfluorinated sulfonic acid resin. Polymeric catalysts are hydrophobic, attracting fatty acid or alcohol tails that enhance catalytic activity for esterification reactions. The length of polymer chains influences their hydrophobicity, and such catalysts are generally used for non-aqueous reactions. For cellulose hydrolysis, the catalysts must be hydrophilic to interact with the substrate (cellulose, β-1, 4 glucan). The catalytic activity is determined by the nature of the reaction medium, owing to which polymeric catalysts exhibit good activity in a non-aqueous medium. Generally, such catalysts are not soluble in water; therefore, they are either used in combination with organic solvents or in modified form for aqueous reactions. For example, Villandiera and Corma [34] employed Amberlyst 15 DRY in ionic liquid media for cellulose hydrolysis and the glycosidation of the monomers obtained in a single pot reaction. Jin et al. [35] modified commercial Amberlyst 15 by chloromethylation and reported the enhanced hydrolysis of cellulose in aqueous medium. The improved performance (67.2% glucose yield) of the catalyst was attributed to the synergistic effect between the sulfonic group and the chlorine in the synthesized catalyst.

2.10. Magnetic Solid Acid Catalyst

Extensive research has been carried out for synthesizing magnetic silica-coated nanoparticle solid acids. Most of the works used thiol group formation for –SO3H functionalization rather than sulfonation using sulfuric/sulfonic acids. Takagaki et al. [36] prepared CoFe₂O₄ -embedded silica nanoparticles. Lai et al. [37] claimed to have synthesized the first ordered mesoporous magnetic solid acid by combining magnetic nanoparticles (MNP) and mesoporous SBA-15. They fabricated magnetic –SO₃H functionalized SBA-15 by co-condensing tetraethoxysilane and mercaptopropyl trimethoxysilane. Mercapto groups were oxidized by 30% H₂O₂ with Fe₃O₄ particles and triblock copolymers to give Fe₃O₄- SBA- SO₃H. Good catalytic activity with 96, 50, and 26% total reducing sugar (TRS) yield for cellobiose, amorphous cellulose, and microcrystalline cellulose, respectively, was reported. They also extended the catalyst for lignocellulose, such as corncob, and obtained a 45% TRS yield [37]. Xiong et al. [38] prepared MNPs by immobilizing sulfonic acid groups on Fe₃O₄@SiO₂. They added chlorosulfonic acid to Fe₃O₄@SiO₂ and mixed it at room temperature for 30 min (Figure 5). The nanoparticles were sulfonated by reacting –OH groups of Fe₃O₄@SiO₂ and sulfonic acid chlorides. The resulting MNPs were used for cellulose hydrolysis in [BMIM]Cl at 130 °C for 3 h with a TRS yield of 73.2%. The magnetic property of the catalyst was retained and achieved nearly 69.4% TRS yield even in the 6th cycle.

Carbon-encapsulated magnetic catalysts have been prepared by various methods, such as spraying, catalytic chemical vapor deposition, ion beam co-sputtering, magnetron, high-temperature annealing of carbon-based materials and metal precursor mixtures, organometallic compound pyrolysis, and methane catalytic decomposition. However, these methods typically require sophisticated hardware and are energy-intensive. Xuan et al. [39] followed a simple one-step process to synthesize carbon-coated Fe₃O₄ particles. They used glucose for FeCl₃ reduction and adsorption on Fe₃O₄ nuclei to reduce interfacial tension and as a carbon shell precursor. As a result, 100 to 200 nm nanoparticles with an average carbon shell thickness of 10 nm were synthesized. Increasing glucose content produced impure and irregular Fe₃O₄ carbon particles. Therefore, the synthesis of thick shell-coated Fe₃O₄ remains a challenge.

Zhang et al. [40] followed the same procedure and sulfonated carbon-coated Fe₃O₄ for synthesizing Fe₃O₄@C-SO₃H nanoparticles. They used the catalyst for hydrolyzing ball-milled cellulose and obtained 48.6% cellulose conversion. They also exhibited good stability both in terms of catalytic and magnetic properties. Therefore, such an environmentally friendly process can be studied further for biomass conversions. Hu et al. [41] used electronegative –Cl groups to enhance the catalytic surface property for binding with cellulose. The enzyme-hydrolyzed corn stover lignin and chloromethyl polystyrene were used for synthesizing Cl-doped Fe₃O₄@C-SO₃H. This catalyst was applied for the hydrolysis of microcrystalline cellulose and rice-straw-derived cellulose in [BMIM]Cl, achieving a reducing sugar yield of 78.5% and 73.2%, respectively. Though they displayed good stability, many steps were involved, including FeCl₃ impregnation and evaporation, co-carbonization with chloromethyl polystyrene, and sulfonation with H₂SO₄. Therefore, alternate methods with minimal process steps should be investigated.

3. Characteristics of Solid Acids

The catalytic performance of solid acid catalysts depends on several characteristics, including the surface area, pore size distribution, density and strength of acid sites, thermal and mechanical stability, etc. Acid sites are a major governing factor for the catalytic activity of solid acids. Both the type of acid sites and their strength affect the conversion process. Weak acid sites do not undergo leaching through ion exchange and exhibit hydrothermal stability [42]. On the other hand, degradation side reactions are prompted by the leaching of strong acid sites. Thus, weak acid sites are preferred, owing to their superior stability. The leaching of strong acid sites from sulfonated resins (Amberlyst 70/30) has been reported in reactions at high temperatures. One of the most powerful methods for the estimation of Brønsted acid densities is temperature-programmed desorption (TPD) [43,44]. The acidic strength is directly proportional to the temperature the probe gas molecules require to get released [45]. Ammonia is the most commonly used probe gas in TPD to estimate the acidic strength. However, for the determination of acidic strength, NH₃ has several limitations, and the most important limitation is that ammonia is not specific to the Brønsted acid sites [43]. To overcome this limitation, TPD with alkyl amines, such as isopropylamine, is preferred, since alkyl amines exclusively bind to Brønsted acid sites, providing more accurate measurements. The interaction of amines is stronger compared to ammonia, as they form a stable bond with Brønsted acid sites. Alcohols, such as isopropyl alcohol (IPA), can also be used as probe molecules in TPD. Alcohol adsorbs on the surface and undergoes desorption or surface reactions, such as dehydration or dehydrogenation, upon heating, providing mechanistic insights into surface interactions. Each probe molecule interacts differently with the materials’ acid sites, offering unique perspectives on acidity [43,46]. In addition to TPD techniques, infrared (IR) spectroscopy with pyridine adsorption serves as a complementary method. Pyridine forms pyridinium ions, specifically with Brønsted acid sites. On the other hand, when pyridine binds with Lewis acid sites, it forms coordination complexes. The interaction of pyridine with Brønsted and Lewis acid sites can be studied using Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy to reveal specific absorption peaks for each type of site [47]. This method provides both qualitative and quantitative data on acid site characteristics. Together, these techniques enable comprehensive characterization of the acid sites in SAC materials [47]. Hydrogen-deuterium exchange (HDX) is another effective method for estimating acid sites in solid acid catalyst (SAC) materials. This approach involves the replacement of hydrogen atoms within the catalysts’ acid sites with deuterium atoms, resulting in the formation of deuterated materials. The exchange process is monitored using techniques such as mass spectrometry, which detects the presence of deuterium within the material. By analyzing the rate and extent of H-D exchange, it is possible to determine the quantity and strength of the acid sites with precision [48]. Reactive gas chromatography using tert-butylamine has been found to be highly sensitive to low acid site concentration and has been found to quantify Brønsted acid site densities more accurately than traditional NH₃-TPD and FT-IR techniques [49]. Accordingly, acidic strength has been graded as weak (100–200 °C), medium (200–300 °C), strong (300–400 °C), and very strong (400–500 °C) depending on the release temperature. Sulfonated silica exhibits weak activity but more stability, whereas zeolites are active and stable [50]. Therefore, along with the strength of the acid site, the hydrothermal stability of the acid site is also a critical factor in determining the efficiency of the catalyst. For weak acid sites, carbon has been preferred due to hydrophilic groups such as phenolic OH and –COOH. These groups are active in carbohydrate hydrolysis. Even silica and alumina, having weakly acidic hydroxyl groups, can induce reactions by forced interaction with glycosidic oxygen groups. Gazit et al. [51] studied silica with weakly acidic surface silanols and grafted β–glu strands. The β–glu strands were hydrolyzed by the surface silanols at a pH of 4. The silica surface creates a robust hydrogen bonding environment for β–glu strands. They reported that the role of silanols is analogous to the precisely positioned carboxylic groups used by Capon [52] to give a similar effect on glycoside hydrolysis. The Lewis acidic properties of silica can be altered by careful Al incorporation to yield silica with different Si/Al ratios, providing varied acidic properties. Similarly, Brønsted acidity can be modified by –SO₃H group addition. Though a strong acid site is a principal factor for determining the efficiency of solid acid catalysts, excess acid sites will cause the formation of unwanted products like furfural and humins. Shen et al. [53] observed an increase in humin formation when the catalyst-to-substrate ratio was increased above 5. The –SO₃ sites promoted the transformation of 5-HMF to both levulinic acid and humin. Vu et al. [54] used 50% γ-valerolactone with ionic liquid for curbing humin formation during cellulose hydrolysis and obtained a high total reducing sugar yield of 94.5%. Despite these drawbacks, researchers have preferred strong acid sites for efficient hydrolysis in many works. Tan and Lee [55] used the strong acid sites of Dowex^TM DR-G8 to treat macroalgae cellulosic residue and subsequent enzymatic conversion to yield glucose. Glucose yield reached 80.6% after enzymatic hydrolysis for solid acid-pretreated macroalgae, while conventional acid and alkali pretreatment yielded 80% and 64.9%, respectively.

Pore size distribution is another important characteristic that determines catalytic activity. Most strong acid sites are typically found in the internal pores of the catalysts rather than on their external surface [56]. For example, zeolites have strong acid sites in their internal pores. However, these internal active acid sites are not accessible to cellulose due to pore diffusional resistance. Particularly, when water is used as a green solvent for hydrolysis, cellulose hydrolysis takes place only on the interface of the catalyst and substrate, as the insoluble cellulose does not diffuse into pores where the active sites are located. Therefore, a larger pore diameter and external area are favored for the hydrolysis of biomass [57]. For example, in ZSM-5, only 62.6% of the Brønsted acid sites are easily accessible, as its pore size is a mere 5.6 A° [58]. Thus, the overall catalytic activity of zeolites depends on their pore size. They can only effectively bind to the substrates that fit into their pores [59]. Jow et al. [60] obtained a 56% levulinic acid yield from fructose using LZY zeolite for 15 h at 140 °C. They explained the high yield based on the molecular tailoring of LZY zeolite. Fructose enters through the 0.75 nm pores of LZY and gets converted to HMF by acidic sites in pores. HMF, having a molecular size of 0.82 nm, gets trapped inside the pores unless it gets converted into levulinic acid, resulting in the high yield and selectivity of levulinic acid. These zeolites are unsuitable for treating cellulose as they have 2–20 nm diameters. Ishida et al. [61] studied the effect of four different zeolites, mordenite, faujasite, ZSM-5 type (Si/Al = 50), and ZSM-5 type (Si/Al = 150), for cellulose hydrolysis in both water and ionic liquid, [BMIM]Cl. Mordenite and faujasite, with pore sizes of around 0.7 to 0.8 nm, were superior to the other two, with pore sizes of 0.5 nm. Cai et al. [62] employed zeolites of different pore sizes and the same acidity to understand the influence of pore size on cellulose hydrolytic activity in [BMIM]Cl. They found that the cellulose hydrolyzing efficiency increased with an increased catalyst pore size. The pore diameter varied as follows: HY-0.74 × 0.74 nm > HBeta, HZSM-5-0.66 × 0.67 nm, 0.53 × 0.56 nm > and SAPO-34-0.38 × 0.38 nm. HY, having the largest pore size, had the highest conversion (84.3%); HBeta and HZSM-5, with medium-sized pores, had moderate conversion (77 and 80.9%); and the lowest pore size catalyst, SAPO⁻³, had the poorest conversion (2.90%). In systems with water as the solvent, the reason for their dependence on pore size was the restriction posed by the pores for interaction with acid sites. In ionic liquid, cellulose is in the solution in a dissolved state and experiences relatively lower diffusional resistance. However, Cai et al. [62] explained the effect of pore size based on the release of H+ from the reaction of ionic liquid cation with the Bronsted acid sites of HY. Y zeolite is commonly used in petroleum refineries as a cracking catalyst. Ultra-stable Y zeolite (USY zeolite) is obtained by processing Y zeolite through steaming and yields a hydrothermally stable catalyst. The Al extraction from the framework (dealumination) decreases the acidic sites of the catalyst. Zeolites with at least two different porosity levels with meso/micro or macro pores are called hierarchical zeolites. These additional larger pores can overcome the mass transfer limitations arising from smaller pores. Zhou et al. [57] prepared H-USY-meso zeolite using oxalic acid dealumination of H-USY zeolite and studied the effect on cellulose conversion. Even though H-USY-meso zeolite many fewer lower acid sites than the parent H-USY zeolite, it had a better external surface area and higher conversion. Parent zeolite and H-USY-meso zeolite achieved a 5.8% and 55.7% TRS yield for hemicellulose hydrolysis, respectively. It was attributed to the macro- and mesopore formation in USY zeolite, which increased the internal acid site accessibility and reactant/product mass transfer, avoiding byproduct formation. The low number of acidic sites in these H-USY-meso zeolites will not allow cellulose hydrolysis. Zhou et al. [57] sulfonated H-USY-meso zeolite (H-USY-meso-SO₃H) for its application to hemicellulose and cellulose hydrolysis. TRS yield reached 78 and 43.7% for hemicellulose and microcrystalline cellulose, respectively. For hemicellulose, TRS yield increased from an initial 5.8% for USY zeolite to 55.7% for H-USY-meso and, finally, 78% for H-USY-meso-SO₃H. It signifies the equal importance of the catalyst pore size and acidic sites in catalytic activity. Therefore, cellulose must be either dissolved in a suitable solvent or the zeolite pore size must be carefully selected to ensure proper contact between the acid sites and cellulose. Nevertheless, the product of cellulose hydrolysis, glucose monomers, can effectively pass through the pores, diffuse through the inner channels, and be converted into 5-HMF or levulinic acid in suitable conditions [63].

Silica nanoparticles (MSNs), such as SBA-15, MCM-48, MCM-50, and FSM-16, are widely studied due to their high surface area (typically > 600 m²/g) and tunable mesopore diameters, ranging from 2 to 50 nm. Under conventional synthesis conditions, MCM-41 and MCM-48 typically exhibit pore sizes between 2 and 4 nm, while SBA-15 has pore diameters ranging from 5 to 10 nm. Degirmenci et al. [64] used zirconia-modified sulfonated SBA-15 for efficiently converting cellobiose to glucose. The sulfation of zirconia patches results in strong acidity on the surface of the SBA-15. IR spectroscopy revealed that, in the sulfated zirconia-modified SBA-15, the Brønsted acidity arises from interactions between sulfate groups attached to zirconia and the silanol groups [64]. The reaction temperature also influences the pore structure of nanoparticles, with damage observed at 130 °C and an ordered structure at 150 °C. To prevent aggregation during synthesis, diluted surfactant concentrations or additives, such as triethanolamine, pluronic polymer, PEG, and L-lysine, are commonly used. Pore size can be tailored through the surfactant chain length. Longer chains yield larger pores, and shorter chains yield smaller pores. Swelling agents, such as DMHA and TMB, can also create pore-expanded MSNs [65]. Lanzafame et al. [66] developed a bifunctional catalyst (possessing both Brønsted and Lewis acidic sites) by modifying SBA-15 with sulfated zirconia for cellulose hydrolysis. They achieved an enhanced surface area and acid functionality without compromising the mesoporous structure. After cellulose depolymerization, interaction within the internal pores leads to glucose formation and its subsequent dehydration to 5-HMF. It was stated that even though catalytic activity was not affected notably by internal pores, they play an essential role in product selectivity. Glucose can penetrate the inner channels, preventing secondary polymerization and exhibiting a shape-selective effect. MSNs have tunable mesopores (2–50 nm), offering high accessibility and broader applications, whereas zeolites, with micropores (<2 nm), specialize in catalysis. A study explored the tunability of MSNs in the sub-micrometer range using NaOH as a base catalyst. Two synthetic approaches were compared: one using a homogenous aqueous/ethanol solution and another using a two-phase system where ethanol was replaced with ethyl acetate. The two-phase system produced a broader pore width distribution, mainly in the 2–5 nm range, which was highly dependent on the reaction temperature [67]. Conventionally prepared MSN using CTAB as a surfactant has a small pore size of 2 nm, which restricts large substrates like cellulose. Therefore, even though MSN has a high internal surface area, the small pore size did not allow the substrate to diffuse into the mesopores. Large-pore-sized MSN can be obtained through two methods: (1) surfactants with a high molecular weight as templates or (2) hydrophobic swelling agents as additives. PEO-b-PMMA, a high-molecular-weight surfactant, was used by Fan et al. [68] to synthesize MSN with an ultra-large pore size of ~37 nm. A pore size of ~25 nm was achieved using swelling agent 1,3,5 trimethylbenzene (TMB) for MSN synthesis [69]. For small-pore-sized catalysts, cellulose should be dissolved in suitable solvents and simultaneously treated by catalysts. Peng et al. [70] first dissolved cellulose in ionic liquid, followed by hydrolysis, isomerization, and dehydration using bi-functionalized MSN. They used acid (SO₃H) and base (NH₂)-bi-functionalized MSN for cellulose conversion to HMF. Moreover, wormhole mesoporous silica nanoparticles (WMSNs), with disordered pore channels, serve as an alternative to traditional honeycomb-structured MSNs. They are synthesized using triethanolamine as a cosolvent, resulting in narrower pore size distributions. In contrast, NaOH-based MSN synthesis results in honeycomb-like pore structures [71]. This can also be achieved by double-chained or specific swelling agents [72].

The synthesis of mesoporous silica nanoparticles (MSNs) has recently gone beyond template-directed synthesis and swelling agents. Atomic layer deposition (ALD) allows for the precise control of pore architecture and surface functionality. ALD enables the conformal coating of internal pore walls with metal oxides, such as TiO₂, WO₃, or Nb₂O₅, at an atomic level. Thus, fine control over acid site density and pore size without collapsing the mesostructure is achieved. Ke et al. [73] demonstrated that TiO₂ films could be deposited on SBA-15 at the submonolayer level using ALD. The synthesized catalysts retained mesoporosity while gradually reducing the pore size from ~8 nm to ~6–7 nm as film thickness increased. The uniformity of these coatings ensured that diffusion pathways remained open, and hence the resulting catalysts were suitable for size-sensitive reactions [73]. In another work using ALD, niobia was deposited on SBA-15, yielding highly ordered and hydrothermally stable solid acids. Although the pore diameter decreased with increasing ALD cycles, the mesoporous framework was preserved. These catalysts exhibited enhanced activity in reactions such as alcohol dehydration due to increased acid site dispersion and accessibility [74]. Majumder et al. [75] investigated polymer–nanoparticle interfaces by coating silica nanoparticles with metal oxides, such as TiO₂ and CaCO₃. Their findings showed that surface chemistry directly influenced polymer infiltration, suggesting ALD’s broader relevance for modulating interfacial interactions in catalytic and materials applications [75].

Unlike cellulose, hemicellulose is more vulnerable to hydrolysis. Xylan is extracted from lignocellulose by either hot water or NaOH treatment, and the precipitated xylan is dissolved in an aqueous medium to react with a solid acid. Charmot et al. [76] used a confining pore radius (3.2 nm diameter) and the high curvature of the mesoporous carbon nanoparticle (MCN) for the conversion of glucan to glucose in buffered solution using weak acidic sites of modified MCN [76]. Cellulose, being resistant to hydrolysis, was converted to β-glucan by mixing crystalline cellulose and a concentrated acid solution. The ensuing β-glucans of different molecular weights were adsorbed on MCN and separated for further hydrolysis in buffer (pH ≥ 2). Nafion^® and Amberlysts are polymer-based catalysts with large pore sizes and strong sulfonic groups. The acidic sites provided by these sulfonic groups are higher than the hydroxyl groups of zeolites [59]. The normal temperature range for hydrolysis reaction is between 120 and 180 °C for solid acid. Brønsted acid sites of Amberlyst 15 get removed, and the catalyst becomes discolored and deactivated at higher temperatures [77]. Among metal-organic frameworks (MOFs), MIL-101 is more frequently used due to its highly porous MOF structure. Their large (3.4 nm) mesoporous cavities allow proper mass transfer through the MOF. Zeolites and MSNs have internal pore structures with acid sites, and therefore, the substrate must be effectively depolymerized or the pore size must be tailored correspondingly. But solid carbon acid catalysts are not open-pore-structured like zeolite. The number of acid sites and external surface areas affects the catalyst efficiency more than the pore size. Generally, carbon-based solid acids are mesoporous/macroporous. Therefore, substrate conversion will not be significantly affected by pore size.

4. Application of Solid Acids in Biorefinery Processes

Renewable energy sources must be explored due to growing demands on fossil fuels. In biorefineries, lignocellulosic biomass is transformed into a range of products such as biofuels, biochemicals, fine chemicals, and energy through various biological, chemical, and thermochemical processes. The wide variety of lignocellulose falls under two major types, namely cellulose-rich and hemicellulose-rich lignocelluloses. Hemicellulose conversion is generally easier than cellulose conversion because of cellulose’s inter- and intra-hydrogen bonding. Hemicellulose contains both C₅ and C₆ sugars, while cellulose is made of repeating units of D-glucose. Most agricultural plants and hardwood hemicelluloses contain xylan, while softwood contains glucomannan. Both hemicellulose and cellulose can be converted to platform chemicals. The U.S. Department of Energy has recognized many potential biobased products as valuable platform chemicals, e.g., levulinic acid, furfural, 5-HMF, ethanol, isoprene, lactic acid, fumaric acid, xylitol, arabinitol, etc. [78]. Platform chemicals can be used as substrates to produce higher value-added products. Other wastes like oilseed cake, algae, and vegetable oil have been used as a substrate for biodiesel production [79]. As mentioned earlier, solid acids can play an influential role in processes such as (i) lignin depolymerization, (ii) carbohydrate hydrolysis, (iii) the conversion (fermentation, hydration, and dehydration) of sugars, (iv) the esterification and transesterification of vegetable oils, etc.

4.1. Lignin Depolymerization

Lignin is an amorphous aromatic polymer found in plant cell walls, making up 18–35% of wood. It is a complex, highly cross-linked macromolecule composed of three main types of phenylpropanoid units: p-coumaryl alcohol (p-hydroxyphenyl unit or H–unit), coniferyl alcohol (guaiacyl unit or G–unit), and sinapyl alcohol (syringyl unit or S–unit). These aromatic alcohols, also called monolignols, are linked together through ether (C–O) and carbon-carbon (C–C) linkages. It is a valuable byproduct of the pulp and paper industry, which produces over 50 million tons annually. However, most lignin is burned as a low-cost fuel in mills for heat generation. Valorizing lignin is important for its industrial, environmental, and economic benefits. Consisting of repeating three phenylpropane monomers, lignin can replace petroleum-derived phenol in phenolic resins. Currently, lignin is primarily used as a low-grade fuel in the pulp and paper industry. Only a small fraction is utilized in binders, dispersants, emulsifiers, carbon fiber, phenolic resins, and biofuels. Recent chemical and thermochemical methods, including hydrolysis, hydrogenolysis, pyrolysis, solvolysis, and oxidation, have been explored for converting lignin into low-molecular-weight phenolic compounds using various catalysts, such as solid acids, metal oxides, and heteropoly acids. Solid acid catalysts have been shown to depolymerize different types of lignin effectively [80,81]. Deepa and Dhepe [81] demonstrated solid acid catalysts could efficiently depolymerize various lignin substrates, offering up to 60% yield at relatively low temperatures (<250 °C) to produce aromatic monomers. The authors evaluated various solid acid catalysts, including zeolites (H-USY, H-MOR, H-BEA, H-ZSM-5), clay, and metal oxides for lignin depolymerization and found that among all the catalysts studied, H-USY zeolites (Si/Al ratio 15) offered a maximum of about 60% monomer recovery. Solid acids such as zeolite are also effective in accelerating lignin depolymerization during thermochemical processes like pyrolysis [82]. Asawaworarit et al. [83] examined lignin depolymerization from bagasse using carbonaceous solid acids (CSA) from glucose, cellulose, and lignin. CSAlignin had the highest phenolic monomer yield and stability over five cycles. The optimal condition achieved a 32.8% phenolic yield, mainly 4-ethylphenol and guaiacol. The findings suggest that CSAs, particularly CSAlignin, are effective for high-value chemical production from lignin, with the catalyst dosage, solvent, reaction temperature, and time being crucial factors. Li et al. [84] reported a new strategy that efficiently cleaves C_ar-C_α and C_β-O bonds in pine lignin using a 2D hybrid solid superacid named n PP@M-[PS][TFMS−], producing guaiacol. The guaiacol yield was 18.2 wt% in aqueous methanol solution, and it was 5.6 wt% higher than that in pure methanol. Adding water promotes methanol decomposition, enhancing the cleavage process. The solid acid-catalyzed phenolation process has been shown to reduce molecular weight and thereby increase phenolic hydroxyl groups in alkali lignin (AL) [85,86]. High-lignin-based phenolic foam (LPF), made by substituting up to 40% phenol with AL or phenolated AL (PAL), exhibited lower water absorption and slag rate and better thermal stability and compressive strength, proving AL’s potential in high-performance phenolic foam production [85].

4.2. Carbohydrate Hydrolysis

Generally, cellulose hydrolysis is carried out at high temperatures where it will be slightly depolymerized and hydrolyzed effectively by solid acid. Therefore, solid acids used in this process must be hydrothermally stable. Furthermore, other features like surface groups, hydrophilicity, pore volume, and surface area play an influential role in hydrolysis. Zeolites, metal oxides, heteropolyacids, graphene oxide, polymeric catalysts, and carbon-based solid acids are most commonly used in cellulose hydrolysis. Lignocellulose contains crystalline cellulose made of reducing sugars that can be converted into various value-added bio-based products. Cellulose, being highly crystalline due to intra- and intermolecular hydrogen bonding, requires additional pretreatment techniques before conversion to reducing sugars. For this purpose, biorefinery generally uses chemicals like acids or bases that are not environmentally benign. Solid acid catalysts can be easily recovered and reused after cellulose treatment and present a viable alternative to conventional catalysts used in biorefineries. Various first-generation feedstocks, such as starch and molasses, have been exploited to produce reducing sugars. However, due to their competition for utilization as food, these feedstocks are not preferred. Second-generation feedstocks made of lignocelluloses have been studied in reducing sugar production using environmentally friendly solid acid catalysts. Dongshen’s group prepared a catalyst composite using SO²⁻ZrO₂ and cationic montmorillonite (Mt) [87]. Clay structures possess numerous unique stacked nano-sheets with a high surface area and good thermal and hydrothermal stability. Though the catalyst had mesoporous structures, only 30% TRS yield was obtained, which can be explained by the low number of –SO₃H functional groups (0.03 mmol/g). Carbon-based catalysts have many advantages over zeolites, including water and thermal stability, good catalytic activity, and reusability. Another vital feature of carbon-based catalysts is their ability to effectively adsorb β-1,4 glycosidic bonds, resulting in hydrolysis to sugars. This adsorption ability is attributed to –COOH, –OH, and –SO₃H groups in the partially carbonized catalyst [88]. Cornstarch hydrolysis using a carbon-based catalyst with a TRS yield of 99.87% has been reported [89]. The catalyst was prepared using one-pot hydrothermal conversion using a glucose-acrylic acid solution and hydroxyethylsulfonic acid. They claimed that acrylic acid addition as a co-monomer resulted in increased carboxylic group content in the carbonaceous catalyst, which enhanced the hydrophilic nature of the catalyst. To improve the reaction of water-insoluble cellulose and solid acid, cellulose can be dissolved in an ionic liquid, and the resulting oligomers can be hydrolyzed by solid acid. Rinaldi and Schüth [90] first reported the hydrolysis of cellulose by Amberlyst-15DRY in ionic liquid. Zeolites exhibited lower activity than Amberlyst-15 in water but higher activity in [BMIM]Cl [61]. A combination of ionic liquid and solid acid catalysts for simultaneous cellulose dissolution and hydrolysis was also reported. Geboers et al. [91] depolymerized cellulose using the hydrolyzing action of water with a trace amount of acids at high temperatures. The resulting soluble glucan was converted to glucose using zeolite and simultaneously hydrogenated by having ruthenium (Ru) present in the bifunctional Ru-zeolite catalyst. Therefore, it can be concluded that zeolites are more suited for soluble glucan and glucose conversion rather than cellulose hydrolysis. A carbon-based catalyst from black liquor obtained by the KOH treatment of rice straw was synthesized and used for the hydrolysis of rice straw [92]. KOH played a dual role as an activation agent for carbon catalyst synthesis and as a lignin-extracting agent for rice straw pretreatment, resulting in a 63.4% TRS yield. Similarly, Vu et al. [54] synthesized catalysts with dual polymeric chains made of acidic polystyrene sulfonic acid for hydrolysis and polyvinyl imidazolium chloride for cellulose solubilization. Alkali-treated cellulose was hydrolyzed to 74% TRS yield using this polymeric catalyst, and no significant loss of activity was observed over repeated runs. Chang et al. [93] explored the synergistic effect of surfactants with ionic liquids for lignocellulose hydrolysis. Si et al. [94] hydrolyzed bamboo biomass using a solid acid catalyst made from crosslinked sulfonated chitosan and obtained a 68% TRS yield. Hydrolysis of crosslinked chitosan using hydrochloric acid was carried out in ionic liquid in the presence of surfactants. Among the various surfactants studied, Tween 80 displayed increased TRS yield from 51 to 68% in ionic liquid while using the solid catalyst. It was reported that the use of surfactants improved the removal of lignin and enhanced cellulose hydrolysis in the ionic liquid. Even third-generation feedstocks like macroalgae have been utilized for reducing sugar production [95]. Jeong et al. [96] reported 51.9% TRS yield from macroalgae using Amberlyst 15 at 140 °C for 120 min. Although good TRS yield was achieved, catalyst recyclability was not studied. Nearly 45.6% TRS yield has been achieved using Purolite CT269DR, a polymeric cation exchange resin, from the hydrolysis of Chlorella sp. microalgae [96].

TRS produced from hydrolysis can be used as a substrate in microbial fermentation. Some main factors to be considered during TRS production are the nature of the catalyst, the reaction temperature, and time. These factors influence the formation of byproducts like phenolic and furan compounds, which inhibit the growth of microorganisms. Recently, a magnetically separable catalyst made using magnetic mesocarbon and chlorosulfonic acid was used for cellulose conversion [97]. Magnetic mesocarbon was prepared through coal tar and ferroferric oxide co-calcination and was treated with aluminum chloride to introduce cellulose binding sites (–Cl). Upon subsequent sulfonation with chlorosulfonic acid, the catalyst yielded 68.6% TRS and showed good activity even after six runs (61% TRS yield). Polycyclic aromatic rings in coal tar carbon enhanced –SO₃H and –Cl functionalization. Similarly, the magnetic catalyst, made from Fe₃O₄ and glucose, used for the hydrolysis of alkali-free-treated sugarcane bagasse resulted in a 73% TRS yield [98]. Thus, various solid acid catalysts have been successfully utilized for high TRS yield from lignocelluloses. Li et al. [99] developed a carbon-based catalyst from sodium lignosulfonate, which was treated with 98% H₂SO₄, followed by H₂O₂ treatment. The catalyst was found to have 0.68 mmol/g SO₃H and 4.78 mmol/g total acid content. When corncob was hydrolyzed with this catalyst at 130 °C for 12 h, it demonstrated high selectivity, yielding up to 84.2% xylose (w/w) with minimal byproducts. Under these conditions, the cellulose retention rate was 82.5%, and the selectivity reached 86.75%. A nanoscale solid acid SO²⁻/Fe₂O₃ was used to hydrolyze wheat straw. With both Lewis and Brønsted acidity, the catalysts selectively hydrolyzed hemicellulose from wheat straw, maintaining cellulose and lignin stability. Under optimal conditions (4.10 h, 141.97 °C, and a 1.95:1 ratio), a 63.5% hemicellulose yield was attained [100].

4.3. Esterification and Transesterification

Biodiesel can be obtained either by the transesterification of triglycerides in vegetable oil or by the esterification of free fatty acids (FFA) [101]. The transesterification of animal fat or vegetable oil in the presence of a catalyst with low-molecular-weight alcohol, like methanol, yields fatty acid methyl esters (FAME) and glycerol as byproducts [102]. Esterification involves the reaction of free fatty acids and alcohol in the presence of a catalyst to yield biodiesel and water as byproducts. They are renewable, non-toxic, and fully degradable in water within a few weeks. The normal manufacturing of biodiesel utilizes NaOH or KOH as a base catalyst for the transesterification of triglycerides with methanol [103]. Though alkali-catalyzed transesterification is capable of high yields, it is expensive as it requires pure oil feed, as the process is FAA- and water-intolerant. Owing to the soapy byproduct formation and non-recyclable corrosive nature of NaOH, alternate catalysts for esterification are in great demand. Homogeneous acid can utilize low-quality oil for transesterification but is extremely slow. Using sulfuric acid in the esterification of fatty acid leads to difficulties in water removal and the purification of ester. Another method for biodiesel production is supercritical alcohol [104]. Though this process does not require additional catalysts, it is set back by using expensive materials to design equipment to withstand high pressure. Therefore, inexpensive solid acid catalysts with good product selectivity, water tolerance, and thermal stability are required for industrial biodiesel production. Solid acids like zeolites, metal oxide catalysts, carbon-based polysulfonic acid, and ion exchange resins were used in the esterification of dodecanoic acid with different alcohols [105]. Among these solid catalysts, sulfonated zirconia (metal oxide) was suitable for biodiesel production. Unlike hydrolytic reactions for esterification, which involve organic reactions in water, hydrophobic catalysts are preferred. Water covers the catalyst surface, preventing organic molecule adsorption onto the catalyst. Esterification by zeolites was limited as the bulky reactants could not diffuse into the zeolite pores [106]. A higher SiO₂/Al₂O₃ ratio can increase the hydrophobicity of zeolite but will decrease the acidic strength. Therefore, the optimum ratio must be chosen for the esterification reaction using zeolites [107]. Ion exchange resins like Amberlyst and Nafion-NR50 showed good activity initially but were deactivated after a few hours. A catalyst that can catalyze both esterification and transesterification simultaneously was synthesized by Kulkarni et al. [108] for biodiesel production from waste canola oil. HPA 12-tungstophosphoric acid (TPA), a highly acidic acid, was supported on hydrous zirconia, Si, Al, and activated carbon (AC). Canola oil, with 10% free fatty acid, resulted in the highest ester yield of 77% upon treatment with zirconia-supported TPA. This higher activity compared with other catalysts was explained by the Lewis acidity formed from the interaction of the zirconia -OH group and TPA. For silica-supported TPA, weaker TPA and Si surface interaction gave rise to weaker Lewis acid sites. The lower yield of AC-supported TPA was explained by the catalyst’s smaller pore size, which restricts the bulky reactants. In addition to a strong acidic site, good accessibility is also required for the esterification reaction. Andrijanto [109] used sulfonated hyper-crosslinked polystyrene for biodiesel production from oleic acid using methanol. Compared with Amberlyst 15 and 35, the catalyst had fewer acidic sites but exhibited better catalytic activity, as esterification does not require very high acid strength. Degirmenci et al. [64] studied the effect of a carbon catalyst (biochar from wood pyrolysis) sulfonated by concentrated sulfuric acid and fuming sulfuric acid for both the esterification and transesterification of canola oil. Fuming sulfuric acid, being a stronger sulfonating agent, had higher transesterification efficiency. They also studied the importance of surface area on transesterification by activating the biochar with KOH. The importance of the surface area for transesterification was noted when the catalysts with the highest surface area among equally acidic counterparts exhibited the highest efficiency. Carbon-based solid acids prepared from glucose, a glucose and starch mixture, starch, and sucrose have also been applied in esterification reactions [110]. Acidified biochar from sugarcane bagasse was used for algal oil transesterification with a FAME yield of 94.91% [111]. Carbon solid acids prepared by Hara et al. [112] showed better activity than conventional Brønsted acids, such as cation exchange resins and niobic acid, and they were nearly 60% as active as H₂SO₄ for the esterification reaction. The catalyst was used for free fatty acid conversion from low-cost oil feedstock and was recycled without significant activity loss for nearly ten cycles. Similar results were reported by Zong et al. [113], where the sulfonated sugar catalysts used for esterification exhibited better catalytic efficiency than niobic acid and Amberlyst-15. The catalyst showed good stability, yielding ca. 93% of its original activity even after 50 recycles. A carbon catalyst displaying even higher activity than sulfuric acid was synthesized from Jatropha curcas oilcake and concentrated sulfuric acid by Mardhiah et al. [114] for the esterification of Jatropha curcas oil. However, after recycling for four cycles, catalyst efficiency decreased from 99.13 to 81%.

Ordered mesoporous carbon (OMC) made using a rigid template like SBA-15 and a carbon source followed by sulfonation has also been utilized for esterification. Liu and co-workers synthesized OMC using a nano-casting technique using MCM-48 and SBA-15 templates [115]. The sulfonation of OMC using H₃PO₂ resulted in a 73.59% conversion of oleic acid [115]. As OMCs are hydrophobic and challenging to sulfonate, Dong’s group pretreated OMCs with H₂O₂ to introduce hydrophilic moieties, making sulfonation easy [116]. The presence of hydrophilic –OH, –SO₃H, and –COOH groups and a hydrophobic polycyclic aromatic carbon framework resulted in improved biodiesel production owing to the better adsorption of both methanol and aliphatic acid simultaneously. Sulfonated OMC achieved an oleic acid conversion of 88%, and the catalyst exhibited no activity loss even after five recycles. Lee et al. [117] developed a new method for converting VFAs to FAMEs using OMC prepared using various mesoporous carbon materials. CMK-5 exhibited the highest FAME yield of ~98% due to its hollow rod-like structures [117]. A magnetically separable metal oxide nano-solid acid catalyst [SO₄/Fe-Al-TiO₂] resulted in a 95.6% FAME yield using waste cooking oil and displayed good stability over ten cycles [118]. Therefore, new materials with increased porous structure and stability are continuously being explored for biodiesel production.

4.4. Platform Chemicals

Lignocellulosic wastes represent a renewable source for platform chemical production due to their high cellulose and hemicellulose content. Solid acid catalysts can play a significant role in these conversions. While using lignocellulose as a substrate, pentoses (xylan/arabinose) will be released from the hemicellulose fraction. Degradation of these pentoses produces furfural, which can undergo hydrogenation to form furfuryl alcohol. The furfuryl alcohol can be converted to levulinic acid (LA) through acid-catalyzed hydrolysis [119]. The C-6 sugars from cellulose and hemicellulose can also be converted to HMF using a variety of solid acid catalysts.

4.4.1. HMF

A novel bifunctional carbon solid acid catalyst developed from biochar was employed for glucose-to-HMF conversion. The catalyst with sulfonation and Al-Ti metal loading, under optimal reaction conditions, resulted in a 96.1% conversion of glucose with a 74.6% HMF yield. The presence of both Lewis acid sites and Brønsted acid sites resulted in a synergetic effect favoring the two-step (glucose isomerization followed by fructose dehydration) conversion of glucose to HMF. The addition of Ti enhances the stability of the Al [120]. Iron-sulfonated magnetic biochar was developed and employed to produce 5-HMF from white wine industry waste. Under optimal conditions, a 40.9 ± 1.1 mol% 5-HMF yield with a selectivity of 59.8 ± 2.6 mol% was achieved. The biochar itself was obtained through the pyrolysis of pomace and grape stalk wastes from the white wine industry. The magnetic catalysts were recovered and reused for five cycles without significant change in the activity [121]. A list of solid acid catalysts used in HMF conversions is given in

Table 1. HMF production using various solid acid catalysts.

Catalyst	Catalyst Preparation	Substrate	Substrate:Catalyst Ratio	Solvent	Temp (°C)	Time, min	HMF Yield%	Ref.
Carbon-based Al-Ti-loaded bifunctional catalyst	Bio-tar-derived porous carbon from pyrolysis was activated using H2O2 and acetic acid; bifunctionalization using Al-Ti metal loading followed by sulfonation and calcination.	Glucose	1:1	20 wt% NaCl in DMSO: Water (4:1)	140	240	74.6	[120]
Iron-sulfonated magnetic biochar	Biochar obtained from pyrolysis was subjected to sulfonation at 150 °C, and then iron was deposited using the wet impregnation method.	Fructose	0.36:1	GVL:Water (2:1)	130	360	40.9	[121]
Nb2O5·nH2O	Mixing niobic acid with H3PO4, stirring for 52 h, aging for 12 h, washing, drying at 333 K and 383 K, and calcining at 573 K for 3 h.	Fructose	1.2:0.1	Water and 2-butanol	160	50	89	[122]
Silicoalumino-phosphates (SAPO)	Mixing aging gels (pseudoboehmite and phosphoric acid) and cyclohexylamine and silica, followed by stirring, aging, filtration, washing, drying, and calcining at 550 °C.	Fructose	1:0.286	Water and methyl iso-butyl ketone (1:5 v/v)	175	60	78	[123]
Sulfonated polymer polytriphenylamine (SPPTPA-1)	Triphenylamine was polymerized using FeCl3 in dichloroethane to create PPTPA-1; sulfonation of PPTPA-1 with chlorosulfonic acid yielded SPPTPA-1.	Fructose	1:0.2	DMSO	140	20	94.6	[124]
SBA15-PrSO3H	SBA15PrSO3H (2a) is synthesized by combining TEOS, MPTMS, PTES, and CSPTMS.	Fructose	1:0.02	Water/nitromethane	140	30	69.8	[125]
HCSS	Treating corn stalks with water under pressure at 250 °C, centrifuging, and then drying the solid. Then, it is heated with concentrated sulfuric acid at 200 °C, cooled, washed, and dried.	Corn stalks	1:1	[BMIM][Cl]	150	30	44.1	[126]
PorPOPS	Terephthalaldehyde, pyrrole, and FeCl3 are stirred with colloidal silica and then heated in a Teflon autoclave at 180 °C for 48 h. The precipitate is filtered, washed, and dried to get PorPOP. PorPOP is treated with chlorosulfonic acid, washed, and dried.	Fructose	1:0.5	Water	160	120	85	[127]
Phosphoric carbons (PC)	1 g glucose in 56% phosphoric acid is heated at 180 °C for 12 h and then cooled. The water is evaporated and the mixture is calcined at 300 °C for 4 h in nitrogen.	Fructose	1:0.2	DMSO	160	180	93.7	[128]
HTC 24-140	Hydrothermal carbonization of freeze-dried softwood pulp at 200–240 °C, followed by filtration, washing, and drying. The material is then sulfonated with concentrated H2SO4 at 140 °C, filtered, washed until neutral pH, and dried.	Fructose	1:0.05	[BMIM][Cl] and methyl iso-butyl ketone	112	24	98.6	[129]
LDMCS-700	Alkali lignin and KCl are mixed and carbonized in a tube furnace at 700 °C. The carbonized product is washed, dried, and sulfonated with sulfuric acid at 180 °C. The resulting sulfonated product is washed to neutral pH and dried.	Glucose	1:1	Aq. NaCl-THF	160	150	57.8	[130]
Nafion®50 resin (NR50)	Commercially purchased.	Chitosan	1:0.55	Methyl isobutyl ketone and DI water	180	120	32.6	[131]
Cellulose sulfuric acid	Cellulose (5 g) is mixed with CHCl3 and chlorosulfonic acid and stirred at 0 °C for 2 h. The mixture is stirred, filtered, washed with CHCl3, and vacuum-dried at room temperature for 6 h.	Fructose	1:0.28	DMSO	100	45	93.6	[132]

.

For HMF production, mostly fructose, glucose, and various other substrates have been employed with different catalysts to investigate the catalyst potential. Acid catalysts like Nb₂O₅·nH₂O and cellulose sulfuric acid have acidic sites in the pores that enable the dehydration of fructose into HMF. The strong Lewis acid and Brønsted sites on these catalysts boost the reaction rate and selectivity towards the production of HMF, with high yields of 89% and 93.6%, respectively, from fructose [122,132]. Zeolite-based catalysts like silicoaluminophosphates (SAPO) have a microporous structure that provides a high surface area and myriad acidic sites that help to catalyze the conversion of sugars to HMF. The unique property of zeolites allows for the effective adsorption and reaction of fructose molecules, yielding up to 78% HMF [123]. Polymer-based catalysts like sulfonated polymer polytriphenylamine (SPPTPA-1) and SBA15-PrSO₃H are sulfonic acid groups functionalized in the polymer, which provide strong acidic sites for catalysis. The polymer backbone offers a flexible and tuneable structure, improving the catalytic performance in organic and aqueous solvents, yielding up to 94.6% and 69.8% of HMF from fructose, respectively [124,125]. Biomass-derived catalysts like HCSS (hydrothermal carbonized sulfuric acid) and HTC 24-140 are derived from lignocellulose biomass materials. These catalysts are synthesized by hydrothermal sulfonation and carbonization, generating a high density of acidic sites. The origin of the biomass makes these catalysts environmentally friendly and sustainable options for HMF production from sugars [126,129]. Heterogeneous catalysts like PorPOPS (porous polymer organic frameworks) and phosphoric carbons (PC) have a porous structure that offers a large surface area and enables mass transfer. The acidic sites in the pores catalyze the conversion of fructose to HMF efficiently with 85% and 93.7% yield, respectively [127,128]. Commercial catalysts like Nafion^®50 Resin (NR50) are also used in HMF conversion [131]. In HMF production, its stability and strong acidic sites under various reaction conditions make it an effective catalyst, though its use may be limited by cost and availability.

The primary role of these catalysts in HMF production is to facilitate the dehydration of sugars such as glucose and fructose. Catalysts activate sugar molecules, causing them to be more reactive towards dehydration. Acidic sites in the catalysts enable the removal of water molecules from sugars, leading to the formation of HMF. Catalysts improve the selectivity towards HMF, diminishing the formation of byproducts and enhancing the overall yield. Effective catalysts sustain their structure and activity over numerous reaction cycles, offering practical and economic benefits for industrial applications. Heterogeneous and biomass-derived catalysts are more environmentally friendly and sustainable, positioning them with green chemistry principles. The choice of catalyst in HMF production from sugars significantly influences the sustainability of the process, efficiency, and yield. Acid catalysts, polymer-based catalysts, zeolite-based catalysts, heterogeneous catalysts, biomass-derived catalysts, and commercial catalysts each have specific advantages that make them appropriate for industrial applications and specific reaction conditions. By optimizing these catalysts and their preparation, researchers can improve the production of HMF from renewable lignocellulosic biomass sources, contributing to the eco-friendly chemical and development of sustainable processes. These findings highlight the importance of catalyst optimization and selection in the field of chemical synthesis, specifically for biomass conversion and other environmentally friendly processes. The future of catalytic processes lies in developing more efficient, sustainable, and cost-effective catalysts to meet the rising demands for renewable resources and green chemistry.

4.4.2. Furfural

Solid acid catalysts are also used for the production of furfural from xylose- and biomass-derived sugars.

Table 2. Furfural production using various solid acid catalyst.

Catalyst	Catalyst Preparation	Substrate	Substrate:Catalyst Ratio	Solvent	Temp (°C)	Time, min	Furfural Yield%	Ref.
Polyaniline carbon-based solid acid catalyst (PAC-S)	Prepared polyaniline precursor was carbonized at 800 °C in nitrogen atmosphere for 2 h and sulfonated using conc. H2SO4 at 180 °C for 3 h to obtain PAC-S.	Xylose	4:1	GVL	170	45	88.2 *	[136]
Polyaniline carbon-based solid acid catalyst (PAC-S)	Prepared polyaniline precursor was carbonized at 800 °C in nitrogen atmosphere for 2 h and sulfonated using conc. H2SO4 at 180 °C for 3 h to obtain PAC-S.	Corn cob, rice husk, corn stalk, rice straw, wheat straw	1:1	GVL:Water (4:1)	180	30	95, 77.4, >40, <20, ~20 *	[136]
Sulfonated carbon derived from lignin, polyvinyl chloride	Synthesized using the calcination-sulfonation method using lignin, KOH, polyvinyl chloride-in mass-ratio of 1:1:1 calcined at 750 °C in N2 atmosphere for 4 h, and sulfonated using p-aminobenzenesulfonic acid.	Xylose	2:1	GVL	160	60	84.3 *	[134]
Sulfonated carbon derived from lignin, polyvinyl chloride	Synthesized using the calcination-sulfonation method using lignin, KOH, polyvinyl chloride-in mass-ratio of 1:1:1 calcined at 750 °C in N2 atmosphere for 4 h, and sulfonated using p-aminobenzenesulfonic acid.	Corn cob, rice husk, corn stalk, rice straw, wheat straw	2:1	GVL:Water (4:1)	160	60	76.4, 66.3, 16.1, 13.5, 5.3 *	[134]
Sulfonated carbon derived from cotton	Using the one-step carbonization-sulfonation method, cotton in sulfuric acid is stirred at different mass ratios and carbonized subsequently at various temperatures.	Xylose	2:1	GVL:Water (5.67:1)	180	90	87.3 *	[133]
Sulfonated carbon derived from cotton	Using the one-step carbonization-sulfonation method, cotton in sulfuric acid is stirred at different mass ratios and carbonized subsequently at various temperatures.	Corn cob	1:1	GVL:Water (4:1)	190	90	63.2 *	[133]
SO42−/TiO2−ZrO2/La3+ Solid acid catalyst	Using the coprecipitation and impregnation method. Ti, La, and Zr salts are used to form TiO2–ZrO2/La3+, impregnated in 1M H2SO4, dried, and calcined at 550 °C for 4 h.	Corn cob	10:1	Water	180	120	6.18 **	[137]
Rice-straw-derived sulfonated SAC	Two-step procedure consists of rice straw carbonization at 300 °C for 2 h in N2 atmosphere, followed by sulfonation using conc. H2SO4, ultrasonication at 150 °C.	Rice straw	10:1	Water	160	300	6.83 **	[135]

* mol of furfural/ mol of xylose (or xylan) unit in biomass × 100, ** g of furfural/ g of dry biomass × 100.

below tabulates a list of solid acid catalysts used in furfural production from sugar/biomass. Carbon solid acids derived from cotton [133], lignin and polyvinyl chloride [134], and rice straw [135] were effective in the conversion of xylose to furfural. Sulfonated carbon solid acid was synthesized by a one-pot method using cotton as a precursor. When used for the conversion of xylose, under optimal reaction conditions, an 87.3% furfural yield was obtained. Meanwhile, the catalyst resulted in a 63.2% furfural yield from corn cob. Though the catalyst activity is reduced upon use, it could be regenerated to restore its activity [133]. In a similar study, a sulfonated carbon catalyst synthesized from lignin and polyvinyl chloride resulted in 84.3%, 76.4%, and 66.3% furfural yields from xylose, corn cob, and rice husk, respectively [134]. Similar furfural yields of 88.2%, 95.0%, and 77.4%, respectively, from xylose, corn cob, and rice husk were reported by Gao et al. [136] while using a nitrogen-doped sulfonated carbon acid catalyst synthesized from polyaniline.

Li et al. [137] developed a solid acid catalyst (SO₂⁻/TiO₂⁻ZrO₂/La³⁺) using the co-precipitation and impregnation method. This catalyst demonstrated high thermal stability, and its strong acid sites facilitated the hydrolysis of polysaccharides and the dehydration of monosaccharides. The maximum furfural yield (6.18 g/100 g) was achieved at 180 °C for 120 min at a corncob/water ratio of 10:100.

4.4.3. Levulinic Acid

Levulinic acid produced by cellulose conversion is an important, versatile intermediary for the production of various chemicals, e.g., γ-valerolactone, δ-aminolevulinic acid, levulinate esters, methyltetrahydrofuran, and diphenolic acid [138,139]. The U.S. Department of Energy recognized LA as one of the top 12 bio-based platform chemicals. LA has found increasing applications in the pharmaceutical and cosmetic industries as a solvent and in pesticide manufacturing. Its applications include fertilizer, plasticizer, fungicide, antifreeze, food, and feed additive. Aminolevulinic acid produced from LA is an effective biodegradable herbicide. Ethyl levulinate production from LA makes it a potential biofuel precursor. Since LA has an LD50 value of 1850 mg/kg, it is also relatively non-toxic [140]. The worldwide reports published on levulinic acid production are illustrated in Figure 6a. The graph (Figure 6b) displays a steady increase in the number of publications related to levulinic acid production. Focus on sustainable growth through the production of bio-based products has led to a rise in LA demand. Global demand for LA elevated from 2606 tons in 2013 to 3820 tons by 2020. By 2020, nearly 84 articles reported the use of lignocellulose for LA production (Figure 6c).

A variety of biomasses, including rice husk [141], paper towel waste [142], food waste [143], waste paper [144,145], and paddy straw [146], have been utilized for LA production. The market growth for levulinic acid is mainly influenced by the demand for LA in the agricultural sector. By 2027, the global market for LA is expected to reach USD 61.2 million. LA is mainly produced from hexose, carbohydrates, and cellulose by the degradation of sugars to 5-HMF followed by rehydration to LA and formic acid (FA) [92]. The reaction proceeds via several steps, i.e., the production of glucose from cellulose hydrolysis, 5-HMF and acetic acid formation through glucose degradation, and the equimolar synthesis of LA and FA through rehydration of 5-HMF. Initial studies for LA synthesis from hexoses involved the conversion of glucose/cellulose/lignocellulose using homogenous acid catalysts like H₂SO₄, HCl, and H₃PO₄ [147,148,149].

Lewis and Bronsted acids greatly influenced the LA yield when the starting material was fructose and glucose, respectively [150]. Beh et al. [150] synthesized solid acid based on amorphous Si alumina and amorphous Si alumina phosphate through flame spray pyrolysis for glucose conversion. Si-alumina solid acid with an Al/(Al + Si) ratio of 0.4 and Si-alumina phosphate with a Si/(P + Si) ratio of 0.25 exhibited the highest LA yield. Kumar et al. [151] entrapped heteropolyacid (polyoxometallate) in gallium micro/nanoparticles to convert glucose to LA. They also applied the catalyst for LA production from rice straw. Acharjee and Lee [152] studied the isomerization of glucose to fructose using various Lewis acid catalysts. Sn-Beta, a zeolite with large pores embedded with tin, displayed the highest activity for glucose isomerization. Dual acid catalysts with Sn-Beta and Amberlyst-15 had the highest LA yield of 45% from glucose. Co-solvent GVL improves cellulose depolymerization and LA production. The polar aprotic property of GVL improves proton reactivity by stabilizing the solvated protons [142]. A proposed reaction pathway for the synthesis of levulinic acid is given in Figure 7.

For cellulose hydrolysis using Amberlyst 70, using 90 wt% GVL as solvent resulted in a high LA yield of 69%, while water resulted in only 20%. GVL solubilizes cellulose and results in oligomers that have better accessibility to solid acid sites. Furthermore, GVL swells Amberlyst 70, leading to better diffusion through pores, resulting in higher catalytic activity. Cellulose remained solid when water alone was used as the solvent, even after 16 h. When GVL replaced 50 wt% solvents, cellulose was completely solubilized. Low glucose and 5-HMF indicated that cellulose hydrolysis to glucose is the rate-limiting step [153]. Zuo et al. [154] synthesized chloromethyl polystyrene resin, containing acid sites (–SO₃H) and binding sites (–Cl) for cellulose conversion. The improved activity of the catalyst was ascribed to the excellent affinity to the substrate rendered by the –Cl sites [154]. Around 60% of the polymer was adsorbed to the cellulose, which indicated the high affinity of the catalyst for cellulose. The water-soluble polymeric catalyst can be recovered by the ultrafiltration of the reaction mixture after reaction completion [155]. Solid acid prepared through the carbonization of lignin (200 °C) and subsequent mixing with ferrous sulfide at 105 °C resulted in a 35.64% LA yield from cellulose [156]. Ding et al. [157] prepared aluminum-modified mesoporous niobium phosphate with an enhanced strong Lewis acid site, resulting in improved LA yield. Simultaneous conversion of the produced LA to GVL was achieved using an Ru/C catalyst after replacing N₂ with H₂ in the reaction medium. The presence of strong Lewis acid sites was enhanced by increasing the aluminum content in the catalyst [157]. Wei et al. [158] synthesized chromium-modified niobium phosphate for LA production from cellulose, since chromium(III) exhibits better glucose-to-fructose isomerization efficiency than Fe. A novel carbon foam-supported aluminotungstic acid (HAlW/CF) catalyst with Brønsted and Lewis acid sites was synthesized and employed for the conversion of cellulose to LA. The synergistic effect between the dual acid sites resulted in an improved cellulose conversion of 89.4%, and the corresponding LA yield was 60.9% [159]. Ya’aini et al. [160] explored the use of hybrid catalysts made from chromium chloride (CrCl₃) and HY zeolite for LA production from empty fruit bunches. HY zeolite (a Brønsted acid) has high catalytic activity and shape selectivity, while CrCl₃ can facilitate the mutarotation and isomerization of glucose to fructose and simultaneous dehydration to 5-HMF. When HY zeolite and CrCl₃ were used separately, only 15% and 20% LA were obtained. HY zeolite, having a 0.75 nm pore size, can trap the substrate and facilitate its rehydration to LA and FA through the strong acid sites, resulting in increased LA yield. In rice straw conversion, Chen et al. [161] used a superacid, SO₄²⁻/ZrO₂–SiO₂–Sm₂O₃. They enhanced the LA yield through the steam explosion and superfine grinding of the substrate (15 µm) using a fluidized-bed-opposed jet mill. Superfine grinding decreased the cellulose crystallinity and increased the accessibility of acid sites. Wang et al. [162] impregnated montmorillonite (MMT) with SnCl₄ and subsequently sulfonated using sulfuric acid to yield Sn-MMT/SO²⁻. This catalyst was used to produce furfural and LA from bagasse through a two-step process. The first step involved the conversion of hemicellulose to furfural, followed by a second step where cellulose was converted to LA at elevated temperatures. This process resulted in an 88% furfural yield and a 62% LA yield in the first and second steps, respectively. Li et al. [99] used biomass-based magnetic ferric oxide solid acid for corn straw conversion to LA. The magnetic particles in the catalyst core aides in the easy separation of the catalyst for reuse. Microbial fuel cell waste obtained after soil remediation contaminated with Cr(VI) was utilized for conversion to LA using Amberlyst 36 at 180 °C [13]. The various solid acid catalysts reported for LA production from cellulose and lignocellulose wastes are indicated in

Table 3. Levulinic acid production from lignocellulosic biomass using solid acid catalysts.

Catalyst	Catalyst Preparation	Substrate and Solvent	Substrate: Catalyst Ratio	Solvent	Temp, °C	Time, min	Levulinic Acid Yield	Ref.
Amberlyst 70	Commercial catalyst.	Corn stover	0.18:0.2	Water	160	120	54 wt%	[153]
Hybrid catalyst chromium chloride (CrCl3) and HY zeolite	The hybrid catalyst was synthesized using the wet impregnation method. Commercial HY zeolite and aqueous CrCl3 (10 wt/v%) were mixed and dried at 120 °C followed by calcination at 400 °C.	Empty fruit bunch (41.1% cellulose)	1:12	Water	145.2	146	15.5 wt%	[161]
		Kenaf (32% cellulose)	1:12	Water	145.2	146	15 wt%
S2O82−/ZrO2–SiO2−Sm2O3 (solid super acid)	The catalyst was synthesized using the precipitation method with ammonia persulfate as the promoter instead of H2SO4.	Steam-exploded rice straw (superfine grinded)	1:2	Water	200	10	22.8 wt%	[163]
Sn-MMT/SO42−	Two wt% montmorillonite (MMT) was mixed in distilled water, followed by the slow addition of SnCl4. The solution was heated at 85 °C for 2 h through microwave irradiation. The resulting Sn-MMT 5 wt% was mixed with 1 mol/L H2SO4 at 30 °C for 6 h to obtain Sn-MMT/SO42−.	Sugarcane bagasse	0.2:0.15	DCM-water	170	144	62 mol%	[162]
Biomass-based magnetic ferric oxide/SO42−	Corn straw was carbonized at 549 °C for 13 h, and the resulting carbon was sulfonated with H2SO4 at 121 °C for 6 h to obtain the biomass-based precursor. Magnetic iron oxide particles were mixed with the precursor (1:2 ratio) in 1 mol/L H2SO4 for 24 h. They were calcined at 500 °C for 3 h to obtain magnetic ferric oxide/SO42−.	Corn straw	2:1	Water	250	67	23.17 wt%	[164]
Amberlyst 36	Commercial catalyst.	Paper towel	1:1	Water	150	60	40 mol%	[145]
Amberlyst 36	Commercial catalyst.	Pennisetum alopecurmoides	1:1	30:70 (wt%) GVL:water	180	60	20 mol%	[13]
Gallium salt of molydophosphoric (Ga@HPMo)	Gallium (Ga) metal was dissolved in molybdophosphoric acid ethanol solution at 50 °C and sonicated for 12 min. The precipitated Ga@HPMo was separated and dried under an N2 atmosphere.	Glucose	1:5	Water	150	600	56 wt%	[151]
Dual acid catalyst (Amberlyst 15 and Sn-Beta)	Zeolite beta 5 wt% was mixed with 13 M HNO3 at 90 °C for 20 h (dealumination). The recovered zeolite was ground with tin(II) acetate for 15 min, followed by calcination at 550 °C for 6 h. The catalyst was mixed with Amberlyst 15 at a 1:10 ratio.	glucose	1:2.2	Water	140	120	45 mol%	[152]
Amberlyst 70	Commercial catalyst.	Cellulose	0.3:1	90:10 (wt%) GVL:water	160	120	69 wt% (based on theoretical LA yield)	[153]
Chloromethyl polystyrene resin	Chloromethyl polystyrene (CP) resin with a -Cl group was partially substituted with a -SO3H group using thiourea as sulfonic acid as a precursor. Thiourea-substituted resin was added to 1N NaOH resin, followed by washing with water and protonation by 2N H2SO4.	Cellulose	1:3	90:10 (wt%) GVL:water	170	600	65 wt%	[154]
Sulfonated hyperbranched poly(arylene oxindole)	A2 monomer isatin and B3 monomer (0.11:0.5) were used for hyperbranched polymer synthesis through polycondensation. The monomers were mixed with methanesulfonic acid (0.05M), and the polymerization was carried out at 35 °C for 48 h. The precipitated powder was dissolved in DCM and reprecipitated to remove impurities, resulting in sulfonated hyperbranched poly(arylene oxindole).	Ball-milled cellulose	1:1	Water	170	180	25 wt%	[155]
Zirconium di oxide	Commercial catalyst.	Cellulose	1:1	Water	180	180	53.9 mol%	[165]
Lignin-based solid acid	Alkali lignin was carbonized at 200 °C for 5 h. The resulting 1 g of carbon was mixed with ferrous sulfide solution (25 g/L) and heated to 105 °C for 10 h. The recovered solid acid was washed with diethyl ether and dried.	Microcrystalline cellulose	1:1	10:1 (wt%) GVL:water	185	120	35.64 wt%	[156]
Aluminum-modified mesoporous niobium phosphate	(NH4)2HPO4 0.01 M was mixed in 20 mL of water with pH adjusted to 2 with H3PO4. Al precursor, prepared by dissolving Al(OH3) in 1.5 M oxalic acid solution, was added. Niobium tartrate, 20 mL, was added to the mixture and poured into CTAB (1 g in 13 mL water). The precipitated niobium phosphorous was aged at 160 °C for 24 h. After drying, the catalyst was calcined at 500 °C for 5 h.	Cellulose	0.5:0.4	Water	180	720	52.9 wt%	[157]
Chromium-modified niobium phosphate (Cr/NbP)	Niobium tartrate solution, 0.5 mol/L, was added to 3.3 g (NH4)2HPO4 and chromium precursor (CrCl3). The mixture was added to CTAB solution with pH adjusted to 2 using H3PO4. The mixture was aged at 60 °C for 24 h followed by calcination at 500 °C for 5 h.	Cellulose	1:0.3	Water	180	180	62.4 mol%	[158]

.

4.4.4. γ-Valerolactone (GVL)

Liu et al. [166] reported 99% conversion of furfural with a GVL selectivity of 93.3% in a single-pot conversion using the Zr-SBA-15 25 catalyst. Liu et al. [167] used a mesoporous silica-based catalyst with the mass ratio of phosphotungstic acid (HPW): ZrOCl₂.8H₂O: SBA-15 as 2:4:15 and attained an 83% GVL conversion from furfural. This work highlighted the balance of Lewis acid sites and Brønsted acid sites using Zr⁴⁺ and HPW in the catalyst preparation. Jinhua Lai et al. [168] achieved a 58.6% GVL yield with ethyl levulinate, indicating graphene oxide supported zirconia (GO/ZrO₂) potential as a catalyst for GVL production, although its activity was comparatively lower than that of the zeolite-based catalysts. Jian He et al. [169] employed Al-Zr-mixed oxide, synthesized using the co-precipitation method, and yielded 83.2% GVL from ethyl levulinate at a higher temperature of 220 °C. Recently, Malu Thayil Jayakumari et al. [170] achieved a 94% GVL yield from levulinic acid at 175 °C using tuned Al sites in Y zeolite, highlighting the importance of active site modification for catalytic performance.

Xiao Yu et al. [171] reported a 67.5% GVL yield from ethyl levulinate, demonstrating the potential of mixed Mn/Cu oxides in GVL production in 2-propanol. Tingting Yang et al. [172] adopted the sol-gel method for the preparation of Ti/Zr porous oxide and reached a 90.1% GVL yield from ethyl levulinate at 180 °C and investigated the effectiveness of porous oxides in this conversion. Chuntao Zhang et al. [173] prepared a ternary catalyst, Cu/ZnO/Al₂O₃, using the co-precipitation method, exhibited an impressive 99% GVL yield from ethyl levulinate at a lower temperature of 140 °C, showcasing the potential of ternary metal oxides for highly efficient GVL production. Tommaso Tabanelli et al. [174] achieved a 64% GVL yield from ethyl levulinate at a higher temperature of 250 °C, demonstrating the catalytic activity of zirconia in this conversion. Shaodan Xu et al. [175] synthesized an Sn-modified silica catalyst and reported an 81% GVL yield from levulinic acid at a lower temperature of 110 °C, indicating the potential of modified silica catalysts for GVL production. The list of solid acid catalysts employed for GVL production is shown in

Table 4. GVL production employing solid acid catalysts.

Catalyst	Catalyst Preparation	Substrate	Substrate:Catalyst Ratio	Solvent	Temp (°C)	Time, min	γ-Valerolactone Yield%	Ref.
Zr-SBA-15_x (x-molar ratio of Si/Zr)	In situ synthesized zirconia-incorporated SBA-15 was calcined at 550 °C for 3 h with different molar ratios of Si/Zr.	Furfural	1.92:1	2-propanol	190	1440	93.3	[166]
Phosphotungstic-acid-supported Zr-SBA-15	Prepared by an impregnation method using SBA-15, ZrOCl2⋅8H2O, calcined at 300 °C for 3 h, and obtained HPW/Zr-SBA-15 (x:y:z). [x:y:z-mass ratio of HPW: ZrOCl2⋅8H2O: SBA-15]	Furfural	1.92:1	2-propanol	190	1440	83	[167]
Graphene-oxide-supported Zirconia catalyst (GO/ZrO2)	GO prepared using the modified Hummer’s method. Further, preparation of ZrO2/GO was carried out using the hydrothermal method using ZrOCl⋅8H2O at 160 °C for 12 h.	Ethyl levulinate	7.2:1	2-propanol	180	180	58.6	[168]
Al-Zr-mixed oxide	ZrOCl2·8H2O (18 mmol) and Al(NO3)3·9H2O (42 mmol) employed using co-precipitation method to obtain Al-Zr-mixed oxides at different calcination temperatures.	Ethyl levulinate	2:1	2-propanol	220	240	83.2	[169]
Al sites tuned Y zeolite	Y zeolite-treated samples treated at different temperatures either in water vapor saturated air or in air atmosphere.	Levulinic acid	1:1	2-propanol	175	720	94	[170]
Mn Cu-mixed oxide	Simple co-precipitation method employed utilizing Cu(NO3)2·3H2O and Mn(NO3)2, calcined at 400 °C in air for 8 h.	Ethyl levulinate	8.64:1	2-propanol	200	180	67.5	[171]
Ti/Zr porous oxides	By using the sol-gel method, titanium isopropoxide and zirconium propoxide as precursors at different molar ratios, and calcined at 500 °C for 2 h.	Ethyl levulinate	2:1	2-propanol	180	360	90.1	[172]
Cu/ZnO/Al2O3	Cu/Zn/Al molar ratio of 6/3/1 utilized to prepare ternary catalyst using the co-precipitation method and calcined at 300 °C for 3 h in 5% H2/Ar mixture.	Ethyl levulinate	0.96:1	2-propanol	140	120	99	[173]
ZrO2	ZrO2 prepared using zirconium(IV)nitrate dihydrate calcined at 500 °C for 12 h.	Ethyl levulinate	-	2-propanol	250	1440	64	[174]
SnO2/SBA-15 (Sn-modified silica catalyst)	Using dimethyldichlorostannane as Sn precursor and SBA-15, a bifunctional catalyst was prepared, calcined at 600 °C for 2 h.	Levulinic acid	2.49:1	2-propanol	110	480	81	[175]

.

4.4.5. Lignin Monomers

A complex and irregular aromatic polymer that is widely present in lignocellulosic biomass, lignin, has a lot of potential as a renewable feedstock for the production of chemicals, fuels, and aromatic compounds with added value (

Table 5. Lignin monomer production using various solid acid catalysts.

Catalyst	Catalyst Preparation	Substrate	Substrate:Catalyst Ratio	Solvent	Pressure Bar	Atmosphere	Temp (°C)	Time, min	Product	Yield%	Ref.
PP@M-[PS][TFMS-]	The sulfonated polypyrrole (PP)@M complex was sulfonated by 1,3-propanesultone (PS), Tri-floromethanesulphonic anion (TFMS-) was attached to the sulfonated PP to generate LASs.	Lignin model (Cβ-O and Car-Cα bonds)	1:0.1	Methanol	10	N2	250	60	Guaiacol	96.6	[84]
Zeolites (HUSY)	Commercial.	Dealkaline lignin	1:1	Water-methanol (1:5 v/v)	7	N2	250	30	Lignin oil	60	[80]
Carbonaceous solid acids	Prepared by hydrothermally carbonizing carbon precursor in water at 220 °C for 3 h. Then, filtered, washed to neutral pH, dried, then treated with concentrated H2SO4 at 150 °C for 15 h.	Organosolv lignin from bagasse	1:0.1	Methyl isobutyl ketone	20	N2	300	60	Lignin oil	32.8	[83]
Ni/Al-SBA-15 (20)	Synthesized using P123, hydrothermally aged at 110 °C, dried, and calcined at 550 °C. Nickel nitrate solution (20 wt.% Ni) was impregnated on Al-SBA-15, dried at 100 °C, and calcined at 550 °C.	Hydrolyzed lignin	1:0.4	Ethanol	10	H2	300	240	Lignin oil	17.83	[176]
Ni-Cu/H-Beta	Ni and Cu precursor solutions were mixed, supports added, and urea introduced. The mixture was stirred, filtered, washed, dried, calcined at 400 °C, and reduced at 550 °C in H2/N2 flow for final activation.	Kraft lignin	1:0.4	Isopropanol	-	N2	350	300	Cycloalkanes	40.39	[177]
Ru-Cu/HY	Prepared by incipient wetness impregnation with metal salt solutions, stirred, dried, calcined at 550 °C, and reduced at 250 °C under hydrogen pressure before use.	Diphenyl ether	1:1	Water	40	H2	250	120	Cyclohexane	81.6	[178]
		Benzyloxy benzene							Cyclohexane	56.1
		Benzofuran							octahydrobenzofuran	85.1
Cu/Mo-ZSM-5	Prepared by dissolving ammonium heptamolybdate and copper sulfate in water, mixing with HZSM-5, and adding NaBH4. The mixture was centrifuged, washed, dried, and calcined at 500 °C.	Kraft lignin	1:0.25	Water-methanol (1:1 v/v)	-	Ar	220	420	Alkyl phenols	20.6	[179]

). However, catalytic depolymerization is severely hampered by its extremely cross-linked and complex structure. Solid acid catalysts have become one of the most promising methods for lignin valorization because of their excellent thermal stability, reusability, environmental friendliness, and capacity to precisely control surface acidity and porosity to target particular bond cleavages in lignin. For effective lignin breakdown, a range of solid acid catalysts with various structures and functions has been studied. The sulfonated polypyrrole-based catalyst, PP@M-[PS][TFMS−], combines sulfonic acid groups and a trifluoro-methanesulfonic anion and is among the most efficient. This catalyst demonstrated exceptional selectivity and catalytic efficacy by achieving a 96.6% guaiacol yield from lignin model compounds with β-O-4 and Car–Cα links in methanol at 250 °C under 10 bar N₂ [84]. Zeolites are common commercial solid acid catalysts, including HUSY, employed for this application. In a water-methanol (1:5 v/v) solvent at 250 °C, their inherent Brønsted acidity and microporous structure enable lignin conversion, producing 60% lignin oil from dealkaline lignin. However, the narrow pore size might make it difficult to reach large lignin fragments, which would reduce the conversion efficiency overall [80]. A more environmentally friendly option is carbonaceous solid acids, which are made by hydrothermally carbonizing and then sulfonating precursors sourced from biomass. These catalysts generated 32.8% lignin oil in methyl isobutyl ketone at 300 °C and 20 bar N₂ when used to organosolv lignin from bagasse. These materials may be less active and durable than metal-doped systems, despite being environmentally friendly and simple to prepare [83]. Ni/Al-SBA-15 and other mesoporous metal-supported catalysts have been used to improve performance. By hydrolyzing lignin in ethanol at 300 °C under H₂; for 240 min, this catalyst produced 17.83% lignin oil, illustrating the complementary roles of mesoporous channels for lignin accessibility and metal sites for hydrogenation [176]. Bimetallic catalysts, such as Ni-Cu/H-Beta and Ru-Cu/HY, enhance catalytic efficacy by facilitating hydrodeoxygenation and extensive hydrogenation processes. Ni-Cu/H-Beta transformed Kraft lignin into 40.39% cycloalkanes at 350 °C in isopropanol under a nitrogen atmosphere [177]. Ru-Cu/HY effectively converted model lignin compounds and softwood lignin into cyclohexane, octahydrobenzofuran, and hydrocarbons, achieving yields of up to 85.1%, utilizing water as a solvent under 40 bar H₂ at 250 °C [178]. Ultimately, Cu/Mo-ZSM-5, a zeolite doped with molybdenum and copper, yielded 20.6% alkyl phenols from Kraft lignin under inert circumstances, underscoring the significance of molybdenum in selective C–O bond cleavage and phenol stabilization [179]. Solid acid catalysts provide a flexible framework for lignin depolymerization, where the catalyst structure, acidity, metal composition, and reaction conditions markedly affect product yield and selectivity. Future developments focus on creating multifunctional and hierarchical catalysts to enhance lignin valorization for sustainable industrial uses.

5. Summary and Outlook

In this comprehensive review, various classes of solid acid catalysts (SACs) and their roles in the catalytic conversion of lignocellulosic biomass into value-added fuels and chemicals were discussed. The catalytic systems examined include zeolites, mesoporous silica nanoparticles (MSNs), metal oxides, carbon-based materials, polymeric catalysts, heteropolyacids (HPAs), metal-organic frameworks (MOFs), and magnetic solid acids. Among these, zeolites stand out due to their crystalline framework, strong Brønsted acidity, selectivity, and exceptional thermal stability [180]. However, their microporous structure limits their accessibility for bulky biomass molecules, reducing their efficiency in raw lignocellulose processing [181]. Strategies such as adjusting the Si/Al ratio or developing hierarchical zeolites can mitigate these limitations by enhancing both acidity and pore accessibility. MSNs, such as SBA-15 and MCM-41, provide improved substrate diffusion through larger pore structures. When functionalized with sulfonic or phosphoric acid groups, MSNs can achieve considerable acidity, making them effective in esterification and hydrolysis reactions [167,182,183,184]. Nonetheless, their thermal and hydrothermal stability requires further enhancement for long-term use in industrial settings. Metal oxides (e.g., ZrO₂, WO₃) offer a versatile platform with both Brønsted and Lewis acid sites. These materials are highly robust and can be modified with sulfate or phosphate groups to increase acidity. However, control over the surface area and dispersion of active sites remains critical for optimizing their performance. Catalyst poisoning, the high cost of noble metals, and intense energy demands are some of the known limitations of metal oxides [185].

Carbon-based SACs derived from biomass offer several advantages, including low cost, tunability, reusability, and environmental friendliness. Carbon-based SACs can be synthesized from underutilized biomasses [186]. These materials exhibit a combination of acidic functional groups and porous structures, allowing the effective hydrolysis and dehydration of sugars. While their reusability and catalytic activity are promising, variability in acid site density and distribution still poses a challenge for standardization and scale-up.

HPAs, although possessing strong Brønsted acidity, are structurally unstable in aqueous systems and tend to leach, which hinders their reuse and applicability in continuous processes. Immobilization techniques on solid supports are promising but add complexity to the synthesis [187,188]. Polymeric catalysts such as Amberlyst and Nafion are attractive for their tunable surface properties and recyclability. However, their low stability under aqueous and high-temperature conditions limits their application in biomass hydrolysis unless chemically modified or used in composite forms. MOFs, with their high surface area and ordered pore structure, present a unique opportunity for selective biomass conversion. Still, they suffer due to poor hydrothermal stability, thermal stability, reproducibility, and scalability, which hinder their industrial viability [189,190,191]. Recent efforts have aimed to hybridize MOFs with more stable materials or use them in tandem with other SACs. Magnetic solid acids combine catalytic function with the ease of recovery via magnetic separation, making them suitable for integration into continuous processes. Nevertheless, their synthetic complexity and less-established long-term durability require further investigation [192,193,194]. In terms of industrial applicability, zeolites and metal oxides are more established due to their availability and proven stability. Carbon-based and mesoporous silica catalysts, while sustainable and efficient, demand advances in reproducibility and resistance to deactivation [195]. Polymeric and magnetic catalysts offer functional advantages, but need tailored modifications for demanding biorefinery environments. While a lot of progress has been made in designing and comparing different solid acid catalysts, future improvements will depend on solving practical problems. These include how to better apply catalysts to real biomass, deal with complex feedstocks, and make the overall process more efficient. Some of these key challenges and promising directions in using solid acid catalysts for converting cellulose and other plant materials are to be addressed to achieve sustainable development goals.

In the last decade, heterogeneous catalysts used in cellulose processing have rapidly expanded from aluminosilicate minerals to magnetic carbon-based solid acids. Several groups have explored the synthesis of biofuels and other value-added chemicals from cellulose using heterogeneous catalysts. Still, deeper insights into the mechanism of action are needed to prevent humin formation upon using different substrates. For enabling biorefinery, lignocellulosic substrates can be initially treated to recover lignin, followed by hydrolysis of the structural carbohydrates. The most important challenges in applying solid acids for cellulose hydrolysis are the chemical and thermal stability of the catalyst, limiting the use of catalysts at high temperatures to produce chemicals like 5-HMF, levulinic acid, and GVL. Though several groups have obtained interesting results with glucose as the main product, more work on direct cellulose conversion to other fine chemicals is needed. Carbon-based solid acids present a viable solution for economic and stable solid acids for cellulose conversion. Heterogeneous catalysts with different properties have been applied for biodiesel and cellulose conversion. Carbon-based catalysts offer good thermal and chemical stability and can be obtained from low-cost lignocellulosic/carbohydrate materials. Also, carbon catalysts with enhanced affinity for hydrophilic substrates have been reported for cellulose hydrolysis. Though many studies have reported disaccharides and glucose conversion to platform chemicals, biomass conversion over heterogeneous catalysts still requires more research. Some groups have synthesized magnetic solid acid catalysts, which can be recovered using an external magnetic field. The appropriate pretreatment of lignocellulose before hydrolysis is preferable to improve catalytic activity and recovery. Recently, applying green solvents like a deep eutectic solvent for lignocellulose treatment has gained momentum. These solvents selectively remove lignin and hemicellulose by disrupting the LCC linkages. The remaining cellulose-enriched substrate can then be converted into a broad range of platform chemicals, 5-HMF, and levulinic and formic acid.

6. Conclusions

This review summarizes the improvements in cellulose processing using heterogeneous catalysts. Over the last decade, various heterogeneous catalysts with different structural and chemical properties have been employed for cellulose treatment. The commercial production of platform chemicals from lignocellulose is still challenged by the efficiency and cost of the catalyst, competing with petroleum-based fuels and products. Though improvements in the separation and recycling of solid acids have been achieved using magnetic solid acids, the minimization of process steps involved in the synthesis is essential for application in biorefinery. Therefore, more focus must be given to economically and environmentally favorable solid acid catalysts, which can be utilized for lignocellulose processing, coupled with other pretreatment technologies.