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Review

Carbon-Based Heterogeneous Catalysis for Biomass Conversion to Levulinic Acid: A Special Focus on the Catalyst

by
Laura G. Covinich
1,*,
Nicolás M. Clauser
1,2 and
María C. Area
1
1
Programa de Celulosa y Papel (PROCYP), Instituto de Materiales de Misiones, Universidad Nacional de Misiones (UNaM), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Posadas cp 3300, Misiones, Argentina
2
Facultad de Ingeniería, Universidad Nacional de Misiones (UNaM), Oberá cp 3360, Misiones, Argentina
*
Author to whom correspondence should be addressed.
Processes 2025, 13(8), 2582; https://doi.org/10.3390/pr13082582
Submission received: 7 July 2025 / Revised: 4 August 2025 / Accepted: 8 August 2025 / Published: 15 August 2025
(This article belongs to the Special Issue Processes in 2025)
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Abstract

The conversion of cellulosic biomass into renewable chemicals can serve as a sustainable resource for levulinic acid (LA) production. LA yield is significantly influenced by reaction temperature, reaction time, substrate concentration, active sites, catalyst amount, catalyst porosity, and durability. Beyond the features of the catalyst, such as acidity, porosity, functional groups, and catalytic efficiency, the contact between the solid acid catalyst and the solid substrate is of vital importance. Solid-based catalysts show remarkable catalytic activity for cellulose-derived LA production, thanks to the incorporation of functional groups. For a solid carbon-based catalyst to be effective, a synergistic interaction between the binding domain (functional groups capable of anchoring cellulose to the catalyst surface, such as chloride groups, COOH, or OH) and the hydrolysis domain (due to their ability to cleave glycosidic bonds, such as in SO3H) is essential. As a relatively new market niche, carbon-based catalyst supports are projected to reach a market value of nearly USD 125 million by 2030. This review aims to highlight the advantages and limitations of carbon-based materials compared to conventional catalysts (including metal oxides or supported noble metals, among others) in features like catalytic activity, thermal stability, and cost, examine recent advancements in catalyst development, and identify key challenges and future research directions to enable more efficient, sustainable, and scalable processes for LA production. The novelty of this review lies in its focus on carbon-based catalysts for LA production, emphasizing their physical and chemical characteristics.

1. Introduction

The conversion of cellulosic biomass into renewable chemicals can serve as a sustainable resource for the production of platform chemicals [1,2,3]. For the production of levulinic acid (LA), the use of fructose as a raw material is preferable since glucose is less reactive than fructose because of its stable glucopyranose form [4]. However, the production of glucose from cellulose is preferred over fructose as a raw material due to its greater abundance and lower cost [5], since cellulose is a naturally occurring polymer [3]. The hydrolysis and depolymerization of cellulose into glucose represent an essential first step [6]. Cellulose degradation is restricted due to its crystallinity, the degree of polymerization, the availability of chain ends, and the fraction of accessible bonds [7]. Subsequently, glucose is further catalytically converted into intermediates, chemicals, and fuels [6].
Figure 1 shows the production mechanism of LA from cellulose [8,9,10,11,12,13].
The sequence in Figure 1 depicts the integration of glucose isomerization to fructose and its subsequent dehydration–rehydration, which requires the presence of both Lewis [14] and Brönsted [15] acid sites. Recent reviews [16,17] explain the biosynthesis of LA from lignocellulosic biomass.
To initiate the hydrolysis of cellulose, acid catalysts are required [18,19]. Typically, H2SO4, HNO3, HClO4, H3PO4, HCl, HF, and FA are used as homogeneous acid catalysts [20]. In contrast, the use of solid acid catalysts for cellulose conversion that can be recovered and reused is a highly appealing alternative [21]. Solid acid catalysts are environmentally friendly in terms of safety, waste generation, corrosiveness, separation, and recovery [22]. Unlike homogeneous catalysts, solid heterogeneous acid catalysts can be easily separated from the reaction mixture through centrifugation or filtration, which facilitates their recovery, reduces equipment corrosion, and simplifies handling [23]. Some of them are even magnetic, which further enhances the separation process [24] from the reaction medium. Furthermore, these solid catalysts can be easily applied to a continuous-flow fixed-bed reactor [6]. The parameters of interest when studying heterogeneous catalysts that determine their catalytic activity and selectivity toward the desired intermediates and products are detailed in Ref. [25]; namely, the active species, the active sites, the pore dimensions, and the surface area.
The conversion of cellulose through heterogeneous catalysis is a solid-to-solid reaction. The rate of this reaction is primarily limited by mass transfer and surface reactions [26] as cellulose is insoluble in water and most common solvents [3], due to its supramolecular structure [27]. Then, the accessibility and reactivity of cellulose are determined by the hydrogen bond-breaking activation steps of OH groups [28], the breaking of β-O-4 glycosidic bonds, achieved by attacking the oxygen atom of the linkage [29] (in turn, related to the pka of the catalyst acid [30,31]), and by its interaction with the reaction media [27] (e.g., swelling, which usually precedes dissolution [32]).
Carbon-based heterogeneous catalysts have recently attracted considerable attention due to their low cost, ease of preparation, high thermal stability, elevated acid density, and versatile functionalization potential [33] (see references [34,35] for other applications of carbon-based materials). To be effective in various reactions, these catalysts typically require surface functionalization. For cellulose hydrolysis, the presence of SO3H groups is necessary, in addition to COOH and OH groups [36]. Cellulose hydrolysis is an essential initial step for many chemical reactions, but it can only occur after cellulose is adsorbed onto the surface of a solid acid. In contrast, hydrolysis products are more hydrophilic and are easily desorbed [37].
This review aims to highlight the advantages and limitations of carbon-based materials compared to conventional catalysts, including a discussion of the practical applications of carbon-based catalysts in real biomass systems, examine recent advancements in catalyst development, and identify key challenges and future research directions to enable more efficient, sustainable, and scalable processes for LA production. The novelty of this review lies in its focus on carbon-based catalysts for LA production, emphasizing their physical and chemical characteristics. The literature reviewed to date does not summarize the production of LA from the perspective of carbon-based catalysts but instead focuses on the reaction conditions of the catalytic system.

2. Reaction Conditions and General Considerations

High cellulose conversion rates and LA yields are typically achieved at elevated temperatures, with low biomass concentrations and extended reaction times [38] (see Table 1 for the reaction conditions). The catalytic conversion of cellulose increases from 6.33% to 20.32% when the temperature rises from 140 °C to 170 °C (1 g of cellulose and 0.3 g of sulfonated carbon in 10 mL of water over 6 h) [39]. The reaction rate increases with temperature (since atoms then donate or receive electrons more easily [40]), which boosts the production of LA [41,42]. Additionally, the time needed to achieve the maximum product yield decreases considerably with increasing temperature [43]. The increased conversion of LA likely relates to the enhanced solubility of cellulose at elevated temperatures, i.e., approximately 190 °C for 24 h [44]. In one study [45], the authors found that the reaction time for glucose conversion to LA decreased from 210 min to 150 min when the reaction temperature increased from 160 °C to 200 °C in 50 mL of distilled water. At 180 °C for 180 min, the highest LA yield (64.4 mol%) was achieved using 0.75 g of an 8% Cr/HZSM-5 catalyst because, at very high temperatures, side reactions are also accelerated [29], favoring the formation of humins from glucose [46,47]. Temperatures exceeding 200 °C generally led to the formation of humins, whereas lower temperatures facilitated the production of LA. Moreover, humins were more easily formed from HMF than from glucose [43], due to aldol addition and the condensation reactions of HMF [48,49] (humins are not produced from LA [50,51]).
Another disadvantage of working at high temperatures is that LA becomes unstable above 230 °C, where it undergoes dehydration to form unsaturated lactones [52].
Although FA is theoretically produced in equimolar amounts with LA, several studies have reported significantly lower concentrations of FA [53], suggesting that it decomposes into CO2, H2, CO, and H2O under acidic and heated conditions [54] at temperatures exceeding 180 °C [55]. Conversely, there are reactions in which the ratio of LA to FA is low, due to glucose reactions that do not correspond to HMF production, but rather to alternative pathways to produce furfural from hexoses via a pentose unit, with FA as a by-product [56,57].
Water is the most commonly used solvent for producing LA [52,58]. At temperatures higher than 160 °C [59], water is also a by-product of the reaction [60]. The use of water as a solvent is attractive from both an economic and environmental perspective because it is a cheap, non-toxic, and non-flammable solvent [61]. Properties such as the partition coefficient, boiling point, thermal stability, and recyclability are also important [25].
However, the choice of a solvent that can solubilize cellulose (such as GVL [62], 2-sec-butylphenol [63], ionic liquids [64], dioxane [65], tetrahydrofuran (THF) [66], and butanol [67]) increases the possibilities of cellulose–catalyst interaction, so that maximizing LA production [68] is sometimes preferred. The use of biphasic systems, typically consisting of polar aprotic solvents and water, stabilizes the acidic proton and reduces the activation free energy of the catalytic process, resulting in accelerated reaction rates [69,70]. For example, under identical reaction conditions (160 °C, 120 min), the use of GVL as a solvent instead of water enhanced the yield of LA from glucose by a factor of five when catalyzed by CH3-SBA-15-SO3H [71]. The polar phase of biphasic systems dissolves and converts the substrate, while the less polar phase extracts the products [72]. As a consequence, LA is extracted into an organic phase [9].
The recommendation has been made to use biphasic systems to prevent the subsequent reaction of products into humins [9] since their formation is favored at high water contents [73]. Therefore, controlling the water content is a strategy for managing the formation of humins [60,74]. With a water ratio greater than 0.5 concerning an organic solvent, the hydration of HMF to LA is beneficial, due to its water-like properties [75].
Table 1. Yields of LA with various solid acid catalysts.
Table 1. Yields of LA with various solid acid catalysts.
Raw MaterialReaction ConditionsYieldRef.
Cellulose2 g of cellulose, 2 g of zirconium dioxide, 100 mL of water, 180 °C, 3 h53.90 mol%[48]
Cellulose2 wt.% of cellulose, 0.2 g of Amberlyst 70, 3 mL of solvent (90 wt.% GVL/10 wt.% water), 180 °C, 16 h69.00%[76]
Cellulose2 wt.% of cellulose, 0.2 g of C–SO3H, 3 mL of solvent (90 wt.% GVL/10 wt.% water), 180 °C,
16 h		56.00%[76]
Cellulose0.05 g cellulose, 0.1 g of C–SO3H, 5 mL of water, 180 °C, 12 h51.50%[77]
Cellulose1 g of cellulose, 1.5 g of Al-doped mesoporous niobium phosphate, 15 mL of water, at 150 °C, 3 h, total acid density: 1.09 mmol H+/g52.90%[78]
Cellulose0.5 g of cellulose, 0.4 g of H-ZSM-5 (Al/Si = 1/25), 10 mL of water, 180 °C, 24 h27.00%[79]
Cellobiose100 mg of cellobiose, 200 mg of CP-SO3H, 2 mL of water, 180 °C, 5 h, total acid density: 1.69 mmol H+/g40.00%[68]
Glucose30 g of glucose, 0.5 g of graphene oxide, 200 mL of water, 200 °C, 2 h, total acid density: 2.70 mmol H+/g78%[28]
Glucose30 g of glucose, 0.5 g of sulfonated graphene (GO–SO3H), 200 mL of water, 200 °C, 2 h, total acid density: 2.70 mmol H+/g78.00%[28]
Glucose1 g of glucose, 1 g of 10%Fe/HY zeolite, in 50 mL of water, 180 °C, 180 min, total acid density: 2.68 mmol H+/g62%[40]
Glucose1 g of glucose, 1 g of 10% Fe/HY, 50 mL of water, 180 min, total acid density: 2.30 mmol H+/g62.00%[40]
GlucoseCatalyst/glucose ratio: 1:1, CrCl3/HY zeolite (2:1), water 180 °C, 160 min, total acid density: 2.94 mmol H+/g44.00%[53]
Glucose1.5 mL of glucose, 0.3 g of CH3-SBA-15-SO3H, 13.5 mL of GVL/water, 180 °C, 150 min60.00%[71]
Glucose0.05 g glucose, 0.1 g of C–SO3H, 5 mL of water, 180 °C, 12 h61.05%[77]
Glucose100 mg of glucose, 0.05 g of Fe-NbP, 10 mL of water, 180 °C, 3 h, total acid density: 3.59 mmol H+/g, Brønsted/Lewis ratio: 1:264.20%[80]
Glucose11.9 g of glucose, 18.1 g of Amberlyst 70, 85 mL of water, 180 °C, 120 min55.00%[81]
Fructose1 g of fructose, 1 g of LZY zeolite, 140 °C, 15 h43.20%[82]
Fructose9 wt.%, 0.28 mmol of fructose, Fructose: Amberlyst 15 (1:1), 0.5 mL of water, 120 °C, 24 h55.00 mol%[83]
Mixture of pentosesPentoses: 1.56 g/100 g biomass, Y zeolite treated with 0.25 M NaOH, 190 °C, 180 min4.71 g/100 g of biomass[47]
Xylose0.9 g of xylose and 0.5 g of catalyst, Y zeolite treated with 0.25 M NaOH, 170 °C, 3 h, N2 at
15 bar, total acid density: 1.409 mmol H+/g		30.40%[84]

3. Catalyst Design and Characterization

The yield of LA is significantly influenced by factors such as reaction temperature, reaction time, substrate concentration, the availability of active sites, the amount and porosity of the catalyst, and its durability [48,85]. Beyond the features of the catalyst, the contact between the solid acid catalyst and the solid substrate is of vital importance [81]. To enhance mass transfer between the solid acid catalyst and the substrate, sodium chloride (NaCl) can be added to the reaction medium, which improves the yield of LA. The addition of NaCl disrupts the hydrogen bonds in cellulose, leading to increased solubility under hydrothermal conditions [86].
The activity of solid catalysts is correlated with their functionalization, surface area, and porous structure [87]. They must be water-resistant [88], since the hydrolysis activity of a solid acid catalyst decreases with increasing amounts of water due to the hydration of the acid sites [89]. Both the final yield and the product distributions during the reaction depend on the catalyst’s characteristics [53]. A small amount of by-products, such as fructose, suggests that the isomerization process is highly efficient and selective [90]. That is to say, fructose dehydration occurs rapidly as the catalyst provides adequate acidity for its transformation [91]. Additionally, any fructose derived from glucose is likely converted into HMF [50]. Figure 2a presents the main features of the catalyst used for producing LA, and Figure 2b shows the main characterization techniques for solid catalysts.
Acid strength precisely indicates the amount of energy needed to detach a proton from a solid acid. This energy, known as deprotonation energy (DPE), is an intrinsic characteristic of the acid itself and does not depend on the probe used to measure it [94]. For the NH3-TPD profiles, the higher the desorption temperature, the stronger the acid strength of the catalyst [95]. In the case of the sulfonated carbon catalyst, its temperature falls within the range of 500 °C to 600 °C, corresponding to an acid strength of 6.28 mmol/g [96].
In the case of the functionalized catalyst with Lewis acid, like Fe/HY catalysts for the production of LA [40], the strength of the acid sites increased when Fe was impregnated into the HY-zeolite (the impregnation of Fe induced new acid sites). According to infrared spectroscopy analyses of adsorbed pyridine, Brønsted acid sites are observed at a band around 1545 cm−1, while the band at 1450 cm−1 is associated with Lewis acid sites. In the case of FTIR vibration bands, the identification of Brønsted acid sites is at 1400 cm−1, which corresponds to O=S=O stretching in SO3H, and the bands at 1035 cm−1 correspond to SO3 stretching [97].

3.1. Active Sites on the Catalyst

Solid-based catalysts show remarkable catalytic activity for LA production from cellulose, thanks to the incorporation of functional groups [98]. These can be grouped into Lewis (e.g., AlCl3, CrCl3, SnCl4, zeolites, Zirconia, γ-Al2O3 [58,99,100,101]) and Brønsted (e.g., HCl, H2SO4, HNO3, lactic acid, formic acid, acetic acid, Amberlys, Heteropolyacids [25,102,103,104,105]) acid types. Lewis acid sites are considered to be strong acid sites, while Brønsted acid sites are classified as medium acid sites [28,106].
The acid sites on the solid surfaces are highly polarized groups “acting as H+ donor (Brønsted acid), or coordinatively unsaturated cationic sites, which leave M+ exposed to interact with guest molecules as an acceptor of an electron-pair (Lewis acid)” [107]. In addition, it is necessary to optimize their strength and Lewis/Brønsted ratio in cascade reactions to maximize the production of the desired product [108]. The selectivity of LA increases as the ratio of Lewis to Brønsted acids decreases [58]. For example, the yield of LA rose from 21% to 51% when glucose was reacted in water at 180 °C for 4 h; this was achieved by lowering the Lewis/Brønsted ratio from 0.3 to 0.06 [109]. For the production of HMF from glucose, a yield of 42% was obtained at 195 °C for 30 min, using H-ZSM-5 as the catalyst, at a Lewis/Brønsted ratio of 0.25 [5].
The formation of humin species during the reaction is higher at higher Lewis/Brønsted ratios [58,80]. High Lewis acidity can lead to rapid, undesired side reactions of fructose after glucose isomerization [110] due to cross-oligomerization between glucose and HMF [45]. Lewis acid sites may be deactivated in water [111] by coordinating with them [112], leading to the formation of Brønsted acidic protons [92,113,114].
For a solid carbon-based catalyst to be considered efficient, it is essential to verify a synergistic effect between the binding domain and the hydrolysis domain [20,115]. Therefore, the presence of functional groups capable of anchoring cellulose to the catalyst surface is crucial [116], including chloride groups [68], COOH [36], OH [28], and sulfonated groups that convert cellulose into glucose [117], due to their ability to cleave glycosidic bonds [20]. SO3H is not an adsorption site [36]. Therefore, this synergistic effect may explain the excellent catalytic activities observed in carbonaceous solid acid catalysts, which also exhibit low activation energy. The contents of -SO3H, -OH, and -COOH groups on carbon-based materials typically range from 0.1 to 4.9 mmol/g, 0 to 0.9 mmol/g, and 0 to 7.8 mmol/g, respectively [118].
Carbon-based catalysts attack both the chain ends and the random interchain components of cellulose [37], which is an advantage.

3.2. Sulfonation of Carbon-Based Catalyst

A wide variety of carbon-based materials can serve as catalyst supports [119]. Their properties notably influence the performance of the catalyst [120]. The most important are activated carbons [121], nanotubes and nanofibers [122], and graphene [123]. They all have one thing in common: for the conversion of cellulose, the presence of sulfonic groups is necessary [124]. Carbon-based catalysts offer high sustainability, abundant raw material sources, excellent chemical stability, and adaptable surface modifications to enhance their catalytic activity [125].
Regarding surface modification, as said, sulfonated materials are the best candidates for the hydrolysis of cellulose to glucose [88]. The sulfonation process consists of the covalent attachment of SO3H groups to the carbon framework through the substitution of hydrogen [126]. These sulfonated carbon-based materials are typically characterized by an increase in surface acidic strength and hydrophilicity [127]. The sulfonation procedure results in the reordering of the catalyst structure [28]. There are some studies [33] where sulfonation with H2SO4 decreases the surface area and pore structure of the catalyst [128]; the pores also become smaller [129] because a reduction in total pore volume can be attributed to partial oxidation, condensation, carbonization, and partial pore structure destruction during sulfonation [130]. Although sulfonation introduces sulfonic acid groups and oxygen atoms [131], it also results in acid-catalyzed dehydration reactions, causing slight decreases in the O/C and H/C ratios of the solid material [132].
The zeta potential of carbonaceous materials increases with a higher sulfonation degree and surface charge, which contributes to electrostatic attraction forces [133]. The sulfonation temperature, reaction time, and sulfonating agent load determine the degree of sulfonation achieved [134]. The sulfonation temperature significantly affects acid density [88] and the specific surface area of the solid catalyst, with higher temperatures yielding greater total acid density [124]. As the sulfonation temperature increases, the hydrophilicity of the solid material also increases [135]. That is, sulfonation increases the water dispersibility of materials [131]. The former provides better access to the reactants in solution to the sulfonic groups, which gives rise to the high catalytic performance of the solid catalyst [104].
A higher acid content for the solid catalyst is obtained with a longer sulfonation time [128]. Using the same catalyst, the acid amount increased from 0.7 to 1.1 mmol H+/g for sulfonation times of 6 h and 15 h, respectively [128]. In the sulfonation of ordered mesoporous carbon materials, the density of SO3H groups increased from 2.02 to 2.33 mmol SO3H/g throughout 2 to 6 h [136].
The sulfonation degree could be controlled with different sulfonating agents [137]. The most commonly used sulfonating agents are sulfuric acid [132], 3-mercaptopropyltrimethoxysilane [138], thiourea [68], ammonium sulfate [139], and 4-benzene-diazonium sulfonate [118]. However, the most widely used reagent is sulfuric acid, although separating the excess material is a difficult task. While fuming sulfuric acid exhibits higher catalytic activity, concentrated sulfuric acid is preferred due to handling issues. The use of 4-benzene-diazonium sulfonate is recommended as an alternative to sulfuric acid, due to the possibility of working at lower sulfonation temperatures [118]. Table 2 shows the various sulfonation strategies.
Table 2. Carbon-based catalyst sulfonation strategies.
Table 2. Carbon-based catalyst sulfonation strategies.
Sulfonating AgentSulfonation ConditionsTotal Acid DensityRef.
H2SO41 g of carbonized lignin, 10 mL of H2SO4, 200 °C, 5 h1.34 mmol H+/g[26]
H2SO41 g of char, 10 mL of H2SO4, 150 °C, 14 h, N20.02 S/C ratio[33]
H2SO4150 °C, 24 h, 60 mmol H2SO4/g carbon0.90 mmol H+/g[37]
H2SO4fast pyrolysis biochar (7 g) and 150 mL of H2SO4, 80 °C, 10 h, N2-[96]
H2SO42.6 g of sucrose, 0.25 g of H2SO4 and 4.2 g of distilled water, 160 °C, 15 h0.90 mmol H+/g[128]
H2SO41 g of alkaline lignin, 10 mL of H2SO4, 150 °C, 10 h, N26.21 mmol H+/g[134]
H2SO420 g sample of D-glucose powder, 200 cm3 of concentrated H2SO4 (> 96%), 150 °C, 15 h, N20.75 mmol/g a[140]
H2SO450 mg of reduced graphene oxide, 20 mL of H2SO4, 150 °C, 15 h-[141]
H2SO412.5 g of biochar, 12 mL of H2SO4, 100 °C, 15 h0.65 mmol H+/g[142]
H2SO41 g of ordered mesoporous carbon, 15 mL of H2SO4, 200 °C, N2 (30 mL/min), 16 h1.82 mmol H+/g[143]
H2SO41 g of polyethylene pellets, 35 g of H2SO4, 120 °C, 3 h85% b[144]
H2SO41:10 carbon: H2SO4, 100 °C, 5 h1.15 mmol H+/g[145]
H2SO41 g of carbon, 20 mL of H2SO4, 150 °C, 13 h, N20.64 mmol H+/g[146]
H2SO4300 mg of lignin-based carbon, 10 mL
H2SO4, 150 °C, 8 h		1.58 mmol H+/g[147]
H2SO420 mL of H2SO4/g of glucose-based carbon, 150 °C, 15 h Ar5.43 mmol H+/g[148]
H2SO40.2 g of carbon, 10 mL of H2SO4, 80 °C, 8 h under reflux1.12 mmol H+/g[149]
H2SO41 g of lignin-based carbon, 20 mL of H2SO4, 180 °C, 12 h, N21.46 mmol H+/g[150]
H2SO4carbonized cellulose, 15 wt.% of H2SO4, 80 °C, 10 h0.92 mmol/g a[151]
H2SO4cup-stack carbon nanotubes, plasma- 100 mL of 1 M H2SO4, 30 min1.60 wt.% c[152]
4-benzene
diazonium
sulfonate		Fe3O4-reduced graphene oxide, 50 °C,
1.5 h		1.06 mmol/g a[153]
4-benzene-diazonium sulfonate100 mL of water, 12 g of 4-benzene-diazonium sulfonate, 2 g of activated carbon, 30 min1.01 mmol H+/g[154]
Fe(SO4)3/H2SO41 g of carbon material, 10 mL of the sulfonating mixture, 190 °C, 10 h1.15 wt.% of sulfur[155]
Sulfanilic acid373 mg of graphene, 789 g of sulfanilic acid, 25 °C, 24 h1.75 mmol/g a[156]
5-sulfosalicylic acid dihydrate32 g of cellulose, 50 mL of aqueous solution of 20 g of 5-sulfosalicylic acid dihydrate; stainless-steel autoclave for 4 h at 180 °C, with stirring3.54 mmol H+/g[157]
Na2SO30.4 ratio of Na2SO3 to lignin in weight, 90 °C, 5 h1.55 mmol/g a[133]
SO3/H2SO40.3 g of mesoporous carbon, 5 mL 50 wt% SO3/H2SO4, Teflon-lined autoclave at 60 °C, 48 h1.30 mmol H+/g[158]
Ferrous sulfide1 g of carbonized lignin, 25 g/L of ferrous sulfide solution, 105 °C, 10 h, N21.79 wt.% c[159]
Chlorosulfonic acid1 g of graphene oxide, 0.5 g of chlorosulfonic acid, 50 mL of
Chloroform, 1 h, 70 °C, 4 h ultrasound		2.70 mmol H+/g[28]
Note: a mmol SO3H/g, b crosslink percentage was used to indicate the sulfonation degree, and c elemental S content.

3.3. Catalyst Porosity

Porosity is one of the most important properties of solid materials. In addition to active sites, the high selectivity of LA results from an ordered porous structure, which allows reactants to enter the pores quickly and proceed with the reaction [108]. It is recommended to use catalysts with high relative mesoporousness and low relative microporosity [40]. When using solid catalysts and water, cellulose hydrolysis occurs at the interface, due to the low solubility of cellulose in water. Therefore, a large pore diameter and a high surface area favor hydrolysis using solid materials as catalysts [138].
The pore systems are essential for creating high surface areas, which are crucial for increased activity [107], and also determine the selectivity of the products through steric effects [160] and shape selectivity [53]. The molecular dimensions of the reactants, products, or transition states influence the selectivity of the reaction. Reactant selectivity arises when only one type of reactant can diffuse into the pores of the catalyst. Product selectivity occurs when several products can be formed, but only those with molecular dimensions that are compatible with the pore size can diffuse out and be recovered as final products. Transition state selectivity occurs when restricted space within the porous system limits the formation of certain transition states that require more space than is available, thereby preventing the generation of several products [107].
The dimensions of the reactants primarily determine the selectivity of the process [107]. The molecular dimensions of 0.86 nm [161], 0.86 nm [101], and 0.82 nm [53] are suitable for glucose, fructose, and HMF, respectively. For effective reactions with cellulose and glucose materials, the pores of the catalyst must be at least 1 nm in size to allow the substrate to enter [42]. In heterogeneous catalytic systems, the active sites are mainly located within the pores of the catalyst [108]. Therefore, the shape, size, and volume of these pores play a significant role in determining the catalyst’s activity during cellulose hydrolysis [20,138].

3.4. Substrate/Catalyst Ratio

The amount of catalyst significantly influences the substrate’s conversion to LA, necessitating optimization to maximize selectivity for the desired product (see Figure 3) [48]. The optimal substrate-to-catalyst ratio is essential for efficiently using cellulose and maximizing lactic acid yield [52]. Excessive amounts of catalyst can lead to the subsequent formation of humins [162], but for glucose conversion, a low substrate-to-catalyst ratio is necessary [163]. For reactions with cellulose, a substrate-to-catalyst ratio of less than 1 is generally used [88]. In one study, when the substrate-to-catalyst ratio decreased from 0.5 to 0.2, the final yield increased from 40.0% to 58.2% at 150 °C over 6 h [39].
Increasing the substrate concentration while fixing the catalyst dosage could enhance the amount of cellulose available for conversion to LA. However, product feedback inhibition can occur, leading to decreased reactivity or insufficient catalyst dosage for additional cellulose [52].
Starting with fructose at 160 °C with 20 mL of water and 30 mL of 2-butanol, it was found that catalyst ratios exceeding 1:1.2 (substrate to catalyst) promoted the rehydration of HMF into LA because of the increased availability of acidic sites within the system [59]. By increasing the catalyst loading from 0.5 to 10 wt.% (with 2 g of cellulose and 2 wt.% of catalyst in 100 mL of water at 180 °C for 3 h), the yield of LA increased from 30 to 54 mol% [48]. Conversely, when using substrate/catalyst ratios of 9:1 with cellulose as a substrate, yields of less than 6% of LA were obtained (at 190 °C) [55].
The concentration of the substrate also determines the time required for glucose dehydration, increasing along with it [164]. Yields of 13% and 66% of LA were obtained for 15 min and 120 min of reaction time, respectively, using 0.125 g of cellulose, 0.250 g of catalyst, and 10 mL of methanol [96].
Processes 13 02582 g003
Figure 3. Different LA yields (%) from different feedstock:catalyst ratios. Adapted from [28,47,76,78,84,165,166,167].
Figure 3. Different LA yields (%) from different feedstock:catalyst ratios. Adapted from [28,47,76,78,84,165,166,167].
Processes 13 02582 g003

4. Challenges and Perspectives

4.1. Catalyst Deactivation

The recovery and reusability of the solid catalyst for the production of LA are some of the most relevant factors from gan economic point of view [48]. There are cases where the catalyst maintains its catalytic activity through successive runs [70]. However, the catalyst could decrease its catalytic activity [48,68]. Although there are many causes of catalyst deactivation [168], in catalytic systems for producing LA, two chief causes are generally verified [169]: the deposition of insoluble humins on the catalyst surface or the leaching of the active species, which translates into the loss of catalytic activity and/or the selectivity of the catalyst over time [170]. For example, in the case of the conversion of sucrose to LA with Amberlyst-36 as a catalyst at 130 °C, the catalyst surface became covered by the humins generated during the reaction. The activity recorded in successive reactions was 85%, 62%, and 28% after 2, 5, and 10 cycles, respectively. During these cycles, 1 mg of humins was deposited on 150 mg of catalyst, resulting in an activity loss of approximately 3.4% [171]. This reduction in catalytic activity due to humin deposition occurs because it physically obstructs the catalyst, potentially deactivating it by blocking pores or covering active sites.
To remove humins from the used catalyst, two methods are generally used: calcination at high temperatures (around 550 °C) [170,172] and washing the catalyst with organic solvents [85] such as ethanol [173], methanol [174], and acetone [175]. The use of an H2O2 solution is also an alternative to regenerating the used catalyst since it can effectively remove humins from solid acids [176].
In particular, the use of heterogeneous catalysts for cellulose hydrolysis requires the presence of SO3H groups [124]. However, working with this type of material has the effect of leaching away these groups (the active phase) during the reaction. The decomposition temperature of the -SO3H ranged from 280 to 300 °C [177], although the most widely reported temperature is 250 °C [142]. Cellulose hydrolysis at 120 °C for 60 min was 61% with a catalyst bearing –SO3H, COOH, and phenolic OH groups, and it maintained its activity for 5 runs without the loss of the active phase [97]. There are also cases where leaching occurs at lower temperatures than 250 °C. For example, at 170 °C, losses of SO3H density were 31.6% after 1 run [178], but in this case, the leaching of SO3H species was not caused by the cleavage of C-S bonds but by the exfoliation of the catalyst.

4.2. Conventional Catalysts vs. Biobased Catalysts

Conventional heterogeneous catalysts involve metal oxides, supported noble metals (such as Pd and Ru), and acid-functionalized materials, like sulfonated resins and zeolites. These last ones and mesoporous silicas are commonly studied, due to their high surface area and tunable acidity. Considering LA conversion, microporous zeolites demonstrated thermal stability and strong acid sites, facilitating the conversion of hexoses to LA [179].
The conversion of LA into value-added products such as GVL has been extensively studied using catalysts like beta zeolite and titania supported with ruthenium (Ru), which exhibit high activity and selectivity, highlighting their potential for industrial applications. However, conventional catalytic processes in the chemical industry often face significant challenges, including high energy demands and reliance on scarce and expensive metals such as Ru [180].
Conversion yield and catalyst reusability are key performance indicators of catalytic processes. In the case of LA production from cellulose, a study reported a yield of approximately 61% using a carbon-based catalyst [181], which remains lower than the yields of 80% or higher that are typically achieved with conventional catalytic systems [182]. Besides this, carbon-based catalysts have demonstrated the potential for reuse for up to five cycles, with a 17% decrease in conversion yield from the 1st and 5th cycle [181]. For biodiesel production, these catalysts have shown even greater durability, maintaining activity across nine reuse cycles [183].
Carbon-based catalysts can overcome several challenges similar to those faced by conventional catalysts. Recent advances in heterogeneous catalysts demonstrate that catalysts derived from biomass can achieve similar LA yields with lower environmental impact. Table 3 shows some of the benefits of carbon-based catalysts and conventional catalysts.

4.3. Technological Challenges of Carbon-Based Catalysts and Integration Opportunities

The current market size of heterogeneous catalysts is estimated to exceed USD 22 billion and is projected to surpass USD 30 billion by 2030, driven by a compound annual growth rate (CAGR) of 4–6% [188,189]. Within this sector, carbon-based catalyst supports represent an emerging niche. One assessment estimates that this segment could reach a market value of approximately USD 125 million by 2030 [190]. Given the growing demand for more sustainable processes, carbon-based catalysts offer an attractive opportunity. As for the global market for biobased products, although precise figures are difficult to determine, some sources estimate its current value at around USD 50 billion [191], with projected CAGRs ranging from 10% to 25% over the coming years [191,192]. Although there is no well-established biobased catalyst available on the market, the market size and trends show a promising future for these emerging materials.
Carbon-based catalysis is a major field of engineering. The use of carbon-based support materials shows several potential advantages compared to other support materials. Their surfaces are inert, and the size distribution and the chemical properties on the surface can be tailored according to the envisaged application [121].
In addition to these promising features, several challenges must be addressed before commercial-scale implementation (see Figure 4).
The synthesis process typically involves multiple steps, increasing both production costs and complexity [181,194]: stability limitations arise from carbon defects and the risk of structural collapse during reuse, due to the disordered phase in the carbon structure [195,196]; acidity control remains a challenge, as tuning the Brønsted–Lewis acid site ratio is necessary to enhance LA yield through a balanced distribution [181]; co-catalyst dependency often requires acidic deep eutectic solvents, adding complexity to both the reaction system and catalyst handling [189,197]; and pore morphology plays a crucial role, as uniform and stable distribution can significantly improve catalytic activity [181].
The integration of carbon-based heterogeneous catalysts into biorefinery processes can support circular economy strategies by enabling the valorization of biomass waste streams. These catalysts also contribute to the concept of process intensification, particularly through one-pot or cascade reaction systems that enhance overall efficiency [198]. The use of carbon-based materials for biomass valorization aligns with sustainability and green chemistry principles, offering reduced toxicity and lower environmental impact compared to conventional metal-based catalysts [181].
To further demonstrate their potential, several value-added products have been explored in conjunction with LA production. These include biomass-derived pellets [199], biogas [200], furfural [201], and carbon-based electrodes [202]. When integrated into LA production using heterogeneous catalysts, these strategies can significantly enhance the overall performance and resource efficiency of the process.

5. Conclusions

The catalytic conversion of lignocellulosic biomass to levulinic acid supports a sustainable, circular bioeconomy. Integrating carbon-based catalysts into biorefineries promotes process intensification, waste valorization, and the co-production of value-added products, enhancing the overall efficiency and sustainability of biomass conversion.
The market for carbon-based catalyst supports is projected to reach USD 125 million by 2030, reflecting growing scientific interest. More broadly, the expanding biobased products sector, valued at nearly USD 50 billion and with 10–25% annual growth, signals strong potential for scaling up carbon-based catalytic systems.
Among the various catalytic approaches, carbon-based heterogeneous catalysts have emerged as a promising alternative to conventional metal-based systems, offering advantages such as high thermal and chemical stability, abundant feedstock availability, and tunable surface properties. These catalysts can be functionalized with acid groups (e.g., SO3H, COOH, and OH) to enhance their activity and selectivity for cellulose hydrolysis and LA production.
The performance of carbon-based catalysts is highly dependent on several factors, including the nature and distribution of active sites, porosity, surface area, substrate/catalyst ratio, and reaction medium. A synergistic interaction between binding and hydrolysis domains is essential for efficient biomass conversion. Choosing the right solvent, along with optimizing reaction conditions, such as temperature and time, is key for maximizing levulinic acid yields while minimizing the formation of by-products like humins.
Despite their promise, carbon-based catalysts face challenges regarding commercial-scale use, such as complex synthesis, structural instability, limited acidity control, the potential leaching of active sites, and lower catalytic yields compared to some conventional catalysts.
Nonetheless, their superior reusability, reduced toxicity, and alignment with green chemistry principles position them as strong candidates for future biorefinery applications.
Currently, the use of conventional catalysts remains the most attractive option, due to their overall performance and commercial availability. Considering the progress made in recent years in the design and development of renewable-based processes and products, including carbon-based catalysts, advances in green conversion processes, new regulations related to the development and commercialization of environmentally friendly products, and investments in the research and development of greener processes, the outlook for carbon-based catalysts is an attractive field in the new era of bioeconomy.
As stated, the most relevant carbon-based catalysts are activated carbons, nanotubes, nanofibers, and graphene; more specific studies are needed to analyze the advantages and disadvantages of using each of them.

Author Contributions

L.G.C.: Conceptualization: Lead; Data curation: Lead; Formal analysis: Lead; Writing—original draft: Lead. N.M.C.: Investigation: Supporting; Writing—original draft: Supporting. M.C.A.: Supervision: Lead; Writing—review & editing: Lead. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors. All data supporting the findings of this study are available within this paper.

Acknowledgments

The authors acknowledge the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and the Universidad Nacional de Misiones (UnaM). This manuscript was reviewed using AI tools solely for minor grammatical corrections and language refinement. The content, analysis, and conclusions are entirely the authors’ own.

Conflicts of Interest

The authors declare no competing financial interests.

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Processes 13 02582 g001
Figure 1. Production mechanism of LA from cellulose.
Figure 1. Production mechanism of LA from cellulose.
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Figure 2. (a) Main features of a solid catalyst for producing LA. (b) Main characterization techniques for solid catalyst characterization, as related to their acid sites (adapted from [45,53,58,91,92,93]).
Figure 2. (a) Main features of a solid catalyst for producing LA. (b) Main characterization techniques for solid catalyst characterization, as related to their acid sites (adapted from [45,53,58,91,92,93]).
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Figure 4. Advantages and limitations of using carbon materials as a catalyst support. Adapted from [193].
Figure 4. Advantages and limitations of using carbon materials as a catalyst support. Adapted from [193].
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Table 3. Key distinctions of biobased and conventional catalysts. Adapted from [181,182,184,185,186,187].
Table 3. Key distinctions of biobased and conventional catalysts. Adapted from [181,182,184,185,186,187].
AspectConventional CatalystsBiobased Catalysts
SourceSynthetic and inorganic materials.Renewable materials (lignin).
SustainabilityEnergy-intensive. Use of rare materials.Biomass-derived, reduced waste.
ReusabilityModerate (in some cases higher than biobased); 14 cycles for biodiesel.Reported values between 5 (for LA) and 9 cycles (for biodiesel).
PerformanceUp to 80% LA yield from cellulose.61% of LA yield from cellulose.
StatusCommercial availability.Under development.
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Covinich, L.G.; Clauser, N.M.; Area, M.C. Carbon-Based Heterogeneous Catalysis for Biomass Conversion to Levulinic Acid: A Special Focus on the Catalyst. Processes 2025, 13, 2582. https://doi.org/10.3390/pr13082582

AMA Style

Covinich LG, Clauser NM, Area MC. Carbon-Based Heterogeneous Catalysis for Biomass Conversion to Levulinic Acid: A Special Focus on the Catalyst. Processes. 2025; 13(8):2582. https://doi.org/10.3390/pr13082582

Chicago/Turabian Style

Covinich, Laura G., Nicolás M. Clauser, and María C. Area. 2025. "Carbon-Based Heterogeneous Catalysis for Biomass Conversion to Levulinic Acid: A Special Focus on the Catalyst" Processes 13, no. 8: 2582. https://doi.org/10.3390/pr13082582

APA Style

Covinich, L. G., Clauser, N. M., & Area, M. C. (2025). Carbon-Based Heterogeneous Catalysis for Biomass Conversion to Levulinic Acid: A Special Focus on the Catalyst. Processes, 13(8), 2582. https://doi.org/10.3390/pr13082582

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