Catalytic Biomass Conversion into Fuels and Materials: Sustainable Technologies and Applications

Abstract

The production of fuels and materials from residual biomass through sustainable processes is one of the most challenging goals for science and technology. Such development is highly ambitious because of the complex composition of the feedstock (lipidic and lignocellulosic). To succeed, biomass conversion technologies must be able to compete economically with technologies based on fossil carbon. The use of specific and more available catalysts combined with improved reaction conditions can significantly reduce overall industrial costs and maximize efficiency. The synthesis and application of optimized catalytic systems are essential to modulate their activity, ensuring at the same time a high resistance to deactivation. For this reason, the study of multifunctional systems is gaining increasing interest alongside new industrial technologies. Here, we review significant recent advances in sustainable catalytic biomass conversion using emerging heterogeneous catalysts.

1. Introduction

Biomass has emerged as a cornerstone of strategies for carbon neutrality and circular bioeconomy. As a renewable source of fixed carbon, it can be potentially converted into fuels and added-value materials while recycling atmospheric CO₂, making it a promising alternative to fossil resources. Biomass is currently the most widely used renewable energy resource worldwide, accounting for 10.4% of the world’s total primary energy supply or 77.4% of global renewable energy supply [1,2]. Its use as an energy source and a renewable carbon feedstock for chemicals supports the importance in sustainable development and drives extensive research into its efficient conversion technologies. Benefits can also be achieved through industrial and social growth, especially at the local level, with new investments and job opportunities, allowing emphasis on depressed areas. However, scaling up biomass treatments is inherently challenging for cost-competitive and more efficient applications due to the complexity and often recalcitrance of their structure [3]. Especially the lignocellulosic type that is a heterogeneous composite of cellulose, hemicellulose, lignin and minerals with minor components, showing strong interlinking bonds and high oxygen content, requires thermal, chemical or biological pretreatments often with low yield, to obtain a complex mixture of oxygenated monomers more easily feasible [4] for the subsequent specific chemical pathways towards value-added commodities.

These issues highlight that emerging catalytic strategies are crucial in biomass treatments to compete economically with largely adopted petroleum technologies, improving the atom efficiency, conversion rate, reaction conditions and overall process costs [5]. A wide variety of catalysts are nowadays under investigation, ranging from homogeneous or heterogeneous ones, including acid–base bifunctional species, innovative nanostructured oxides, carbon-based, and engineered zeolite composites, each tailored to address specific challenges posed by biomass feedstocks and their naturally present poisoning impurities. For example, in the field of heterogeneous catalysis, bi- and tri-metallic systems are advancing due to the ease of modifying their electronic and chemical properties to achieve better selectivity and durability [6], or the use of porous, high-surface-area metal–organic frameworks (MOFs) with their tunable structures and coordinatively unsaturated metal sites, which demonstrate both catalytic and support properties [7]. Additionally, single-atom catalysts (SACs), characterized by isolated metal atoms stabilized on supports, are gaining interest as a way to maximize atomic efficiency and facilitate challenging biomass conversions (e.g., lignin cleavage, CO₂ hydrogenation with biomass derivatives), owing to their uniform active sites and strong metal–support interactions [8].

In conjunction with catalyst innovations, using more eco-friendly reaction media as deep eutectic solvents (DES) [9], along with studying reactor aspects, such as whether to use batch or flow systems for catalyst separation, recycling co-feeds or by-products, and product purification, can help improve performance. This approach represents the overall development of technology aimed at maximizing large-scale utilization [10].

Therefore, to achieve sustainable use of biomass, an integrated biorefinery approach is needed, including chemical and engineering aspects, overcoming major hurdles such as reducing process energy requirements and achieving economic targets [11]. The integration of processes is not without challenges: different procedures have different optimum conditions and feed requirements, so spatial or temporal separation with intermediate storage may be needed. To smooth process integration, metal catalysts will be required (using non-critical, abundant elements), capable of handling feedstock variability (such as different kinds of biomass, moisture content, impurity levels) and robust enough over an extended period [12].

The present review emphasizes recent advancements mainly in sustainable catalytic technologies and applications, with attention to life-cycle and feasibility studies. For these reasons, biomass types and properties have been very briefly described. The heterogeneous catalytic innovative processes will also be briefly reviewed, both in the biomass pretreatment phase and in their conversion.

2. Biomass Feedstocks and Their Catalytic Pretreatments

2.1. Lignocellulosic Biomass (Agricultural Residues, Forestry Waste)

Lignocellulosic biomass, including agricultural residues (e.g., corn stover, wheat straw) and forestry wastes (wood chips, sawdust), is composed primarily of cellulose, hemicellulose, and lignin, with minor ash and extractives. For example, corn stover typically contains about 30% cellulose, 18% hemicellulose, and 35% lignin (dry basis). Forestry-derived biomass (woody feedstocks) often has higher lignin and lower ash content than herbaceous residues like straw. The compositional differences influence their conversion: agricultural residues tend to produce more char and less liquid in pyrolysis relative to wood, due to their higher ash and alkaline mineral content, which catalyzes char formation. The recalcitrance of lignocellulose, rooted in a crystalline cellulose fibril network embedded in lignin/hemicellulose, limits direct biocatalytic or catalytic utilization. Thus, pretreatment is essential to disrupt the structure of lignocellulosic biomass. Pretreatment strategies for lignocellulosic materials include physical (milling, extrusion), physico-chemical (steam explosion, liquid hot water), and chemical methods (dilute acid/base, Ionic Liquids, DES) (Figure 1).

The removal or redistribution of lignin and hemicellulose, accompanied by an increase in cellulose, may represent an important goal to increase the surface area accessible to catalysts or enzymes. For instance, liquid hot water (LHW) pretreatment at high pressure can autohydrolyze hemicellulose and partially delignify biomass, enhancing subsequent enzymatic hydrolysis [13]. Corn stover pretreated by catalytic LHW (180 °C, 0.5% H₂SO₄) yielded ~91% of theoretical glucose in enzymatic hydrolysis, compared to ~53% without acid.

Overall, agricultural and forestry residues can be effectively valorized when pretreatment is tailored to their composition (e.g., accounting for high ash in straws or high lignin in woods) and coupled with robust catalysts tolerant to residual impurities. Also, catalysts used in biomass pretreatment structurally provide Brønsted or Lewis acid sites to facilitate initial biomass depolymerization. Sulfated zirconia and many sulfonated polymers can leach sulfate or organic sulfonate groups when exposed to hot water, losing their activity after the first use. In pretreatment contexts, this leaching is exacerbated by the long duration (hours) of treatments in the presence of water. Thus, as a challenge, it is worth designing solid acids that are truly water-tolerant. Niobium-based catalysts (niobic acid, Nb₂O₅, Nb-containing molecular sieves) have gained attention for this purpose. In particular, niobium oxides have solid acid properties but are more stable in water and less prone to leaching (niobic acid is a superacid solid that retains significant Brønsted acidity even in water) [14]. These kinds of catalysts have been tested for cellulose hydrolysis, showing moderate activity but high stability. A breakthrough was reported using a porous Ru/Nb₂O₅ catalyst to hydro-deoxygenate lignin at only ~250 °C. This catalyst achieved near quantitative cleavage of aromatic C–O bonds and removed essentially all oxygen from lignin fragments, yielding a total of 35.5 wt% liquid products with ~71 wt% of those products being mononuclear aromatic hydrocarbons. Niobium-based oxides, in fact, exhibit Lewis acidic Nb(V) sites, structurally coordinated by oxygen atoms, capable of electron pair acceptance. These Lewis sites activate ether bonds (e.g., β-O-4 linkages) in lignin, enabling selective bond cleavage and depolymerization. Another Nb system, like mesoporous Ru/NbOPO₄ catalyst, was developed to tackle not just C–O but also the even stronger C–C bonds in lignin [15]. This catalyst, under a slightly higher temperature (310 °C) and in the presence of H₂, successfully broke both interunit C–O and C–C linkages in lignin, producing 124–153% of monocyclic hydrocarbons in terms of ratio of molar yields with respect to the Nitrobenzene oxide (NBO) method, showing an arenes selectivity in the range 62–68%. Other water-tolerant acids include tungsten oxides and zirconium phosphates [16]. Zr phosphate and mixed Nb–W oxides have been used for catalytic pretreatment as they have acid sites and a framework that withstands hydrothermal conditions. The easily tunable acid properties of Zr phosphate and its recoverability permitted the use in the co-catalyzed hydrolysis of lignocellulosic (Pennisetum Sinese Roxb) for the enhanced extraction of high crystalline (79% within the crystal type of cellulose I) cellulosic nanocarbons under 65 °C, and a 5 h reaction time, resulting in a yield of 50% [17]. Another example regards the layered Nb-W oxides that, when combined with Ru/C and phosphoric acid, were successfully employed for the hydrodeoxygenation (HDO) of cellulose to Hexane in aqueous media, with a conversion higher than 90% and almost 80% of selectivity; they show beneficial effects by suppressing the formation of isosorbide, caused by the steric restrictions of sorbitol dehydration within the interlayers [18].

2.2. Aquatic Biomass

Aquatic biomass (microalgae and macroalgae/seaweed) offers a markedly different feedstock profile from land biomass. Microalgae are rich in lipids, proteins, and carbohydrates, and they do not contain lignin. Macroalgae contain carbohydrates (such as alginate or agar), minerals from seawater and a smaller amount of lipids. Algae can be cultivated in open or raceway ponds on non-arable land using wastewater. Based on the strains, they can grow rapidly and can fix high amounts of CO₂, making them attractive third-generation biomass resources. Importantly, the high water content of algae favors wet conversion processes like hydrothermal liquefaction (HTL) over dry thermochemical routes. From the catalytic HTL of wet microalgae, biocrude oil is obtained, by-passing the energy-intensive drying step. Macroalgae have also been explored for direct conversion to fuels and chemicals. They contain high carbohydrate content (e.g., glucans, mannans) that can be hydrolyzed into fermentable sugars or dehydrated into platform chemicals. It has been demonstrated that red seaweed (Gracilaria verrucosa) can be converted, via HTL, into levulinic acid (LA) (30% yield) by using a Purolite ionic resin catalyst at 200 °C [19]. This is notable given seaweeds’ high mineral content and unique polysaccharides, showing that, with appropriate catalysts such as a sulfonic acid resin and reaction conditions, even marine biomass can be a viable feedstock for platform organic acids. Overall, algal biomass expands the feedstock versatility for biorefineries [20,21,22]. Catalytic strategies leverage algae’s high lipid content for biodiesel or hydrothermal oil, and their carbohydrate fractions for chemical production, all while utilizing mild “wet” pretreatments (or none at all) due to algae’s lack of lignin. Process integration (e.g., nutrient recycling from algal residues) further enhances the sustainability of algal bioconversion.

2.3. Waste Biomass (Municipal Solid Wastes, Industrial Byproducts)

There are two main categories of waste biomass, which can be classified as woody and non-woody. The former, already discussed, exhibits low ash and moisture content, but high calorific value. The latter, abundant, readily available and inexpensive, includes animal waste, urban and industrial solid waste, which are characterized by low bulk density, high ash and moisture content and low calorific value. For these reasons, non-woody biomass is not widely used for energy production and often needs harsh pretreatments to create suitable feedstock for gasification systems.

Generally, the pretreatment includes one or a combined process for size reduction, drying and densification. After pretreatments, different catalytic processes can be used. For example, Shang et al. report the co-gasification of sewage sludge with wheat straw in a fixed-bed reactor using a corn stalk char-supported nickel–iron–lanthanum as a catalyst (Ni-Fe-La/CSC) to enhance hydrogen output [23] (an H₂ yield of 11.96 mol/kg feed at 600 °C is reached, versus 3.80 mol/kg without a catalyst). The threefold increase in H₂ was attributed to the catalyst’s ability to crack tar and promote water–gas shift reactions. Similarly, municipal solid waste (MSW) can be processed via catalytic pyrolysis or gasification. For instance, using inexpensive mineral catalysts (like calcined dolomite or iron-rich sludge ash) has been shown to reduce tar formation and increase syngas H₂ content in MSW gasification [24]. These examples highlight that even heterogeneous waste can yield clean fuel gases when coupled with catalysts engineered to handle impurities (e.g., using waste-derived char as both feed and catalyst support is a clever approach to impart resilience and low cost).

Crude glycerol, a byproduct of biodiesel production and used cooking oils reforming, is a prominent industrial waste undergoing catalytic valorization in the production of propylene glycol, an important building block, or other molecules such as glycerol carbonate and its derivatives. A good conversion of glycerol (80%) and high selectivity (100% under controlled conditions) have been achieved by using γ-ZrP as a catalyst [25]. Interestingly, the easy separation of the catalyst and its full recovery, the recovery of NH₃ released during the reaction and the continuous separation of carbonate have been discussed. The aspect related to the conversion of glycerol carbonate that, in the presence of a heterogeneous catalyst, can be efficiently converted into its derivatives through the functionalization of the –OH moiety, with high yield and high selectivity, is also attractive. In particular, glycerol carbonate, previously obtained by glycerolysis of urea, can be converted under mild conditions with very high yield and selectivity into epichlorohydrin. This product has extensive industrial applications, via a multi-step reaction: a direct thio-chlorination of glycerol carbonate under a nitrogen flow at 42 °C, followed by desulfurization at room temperature at 0.1 MPa, and then final distillation [26,27].

It has been reported that basic oxide-supported catalysts, such as Cu–Zn/MgO–La₂O₃ give a 100% glycerol conversion, with a 93% yield of 1,2-PDO at 210 °C and 4.5 MPa H₂ [28]. The basic oxide (MgO-La₂O₃) used as support provided necessary alkalinity for C–O bond cleavage, while Cu–Zn facilitated hydrogenation; La₂O₃ improved catalyst reducibility and stability, yielding nearly complete glycerol conversion to glycols. Overall, the waste biomass conversion is an important route towards the development of “circular economy” [29]. Tuning catalyst properties (e.g., adding basic or acidic functions or using coke-tolerant supports) for the treatment of specific waste feed enables high yields while addressing waste disposal.

3. Heterogeneous Catalysts and Their Use in Biomass Treatments

The principles of sustainable chemistry have now replaced technologies that involve the application of so-called stoichiometric reactions, such as those catalyzed by mineral acids/bases (H₂SO₄, H₃PO₄, etc.), Lewis acids (AlCl₃, ZnCl₂, etc.), and inorganic bases (NaOH, KOH, etc.), especially due to their toxicity and difficulty in recovery and reutilization, drawbacks generally avoided using heterogeneous catalysts, which show also less reactor and plant corrosion problems, and environmentally safe disposals [30].

Photocatalytic reactions, mainly oxidation and reduction, can occur under mild conditions, using either homogeneous or heterogeneous photocatalysts. The former are costly, and sometimes the separation from the reaction mixture can be difficult, whereas heterogeneous catalysts offer many opportunities for industrial application, particularly if they are active and durable. Typically, they are semiconductors such as specific metal oxides or metal complexes that can generate electron–hole pairs when exposed to photon radiation with energy exceeding the band gap energy of the photocatalyst. The electron–hole pairs are involved in redox reactions with organic substrates or gas molecules (Figure 2) [31]. In biomass conversion, they have been investigated for oxidizing polyols to organic acids and even fragments of lignin, aiming to produce value-added chemicals or fuels using sunlight as the energy source [32].

The most used photocatalyst is TiO₂, which typically exists in anatase, brookite or rutile phases (Figure 3) [33], with anatase preferred for higher photocatalytic efficiency, as reported by Absalan et al. that compared the activity of different crystal phase ratio catalysts in the batch reactor methyl orange degradation at room temperature, radiating 1 mL of MeO solution (0.06 M) at 365 nm up to 3 h.

Mechanistically, TiO₂ absorbs photons to generate electron–hole pairs, then the holes oxidize water, forming hydroxyl radicals (•OH) that oxidize biomass substrates into aldehydes, ketones, or carboxylic acids [34]. Metal or non-metal doping elements can introduce structural modifications, enhancing visible-light absorption and photocatalytic activity by altering electronic band structures. For example, dopants such as Cr³⁺, Fe³⁺ or V⁵⁺ [35,36,37] can introduce mid-gap states or narrow the band gap to capture visible photons, while nonmetals like N, C, or S can, similarly, modify the electron structure or reduce electron–hole recombination [38].

Maleic acid was selectively converted to formic acid under visible light using a doped TiO₂ photocatalyst [39].

Heterogeneous catalysts are widely employed in biomass valorization due to their thermal stability and ease of separation from products. Transition metal catalysts like Ni, Co, Fe, and Cu offer cost-effective alternatives to precious metals, and they can drive key reactions including HDO, hydrogenolysis, and reforming. This review categorizes the most commonly used catalysts for biomass conversion into solid acid and basic types, examining their properties and applications in the recent literature, both in feedstock pretreatment and in the conversion of biomass-derived molecules.

3.1. Solid Acid Catalysts

Solid acid catalysts can catalyze the conversion of biomass through different processes such as hydrolysis, dehydration, and esterification. A wide variety of solid acid catalysts have been explored, containing acidic clays, modified zeolites, hetero-poly-acids (often silica-supported), sulfonated carbons, ion-exchange resins, sulfated or phosphate metal oxides and acidic metal oxides.

Designing a solid acid catalyst for biomass conversion requires the examination of several critical factors: (1) acid strength (sufficient to activate stable bonds like glycosidic linkages), (2) density of acid sites, (3) surface area/pore structure, which govern accessibility, and (4) stability under harsh hydrothermal conditions, as needed for long reaction times in hot, aqueous media (as for cellulose hydrolysis) in which catalysts must resist deactivation by leaching or structural collapse [40] or acid sites deactivation by water. Although they are easily recoverable, they generally exhibit low activity; for example, large amounts of solid catalysts are required to obtain high cellulose conversion because of the difficulty in cleaving the β-1,4-glycosidic bonds, so even an excess of solid acid (sometimes a greater mass of catalyst than cellulose) should be used.

Moreover, reactions typically run under hydrothermal conditions (water medium at 150–250 °C), so catalyst durability is very important. In the field of metal oxides, according to what was reviewed by Zhang [41], bare acidic zirconia (ZrO₂) achieves a notable LA yield (53.9 mol%) from cellulose decomposition at 180 °C, whereas sulfated zirconia enhanced catalytic outcomes towards 5-hydroxymethylfurfural (5-HMF) from fructose dehydration yielding approximately 65 mol%, attributed to its strong Brønsted and Lewis acidity. Also, metal (IV) phosphates, such as Cerium (IV) phosphates, have been used. It has been reported that Ce (IV) Phosphate can convert fructose into 5-HMF, with 52% yield and 95% selectivity [42]. Moreover, mixed oxides based on CeO₂ are also able to perform the selective oxidation of 5-HMF to 2,5-furandicarboxylic acid with 99% of both yield and selectivity, in water, using oxygen as the oxidant [43]. Shen et al. proposed the use of Al-induced orthorhombic SnO₂ in the lactic acid production from sugar-derived dihydroxyacetone, demonstrating that the acid site content is strictly connected with the orthorhombic tin oxide amount, exhibiting an initial increase and subsequent decrease in catalyst activity as the Sn proportion rises, because the presence of Sn²⁺ shows a lower acidity compared to Sn⁴⁺ [44].

Metal oxides or modified metal oxides were also applied in the lipid conversion, like tungstate zirconia optimized with 20 wt% tungsten loading that provided very high conversions (>93%) in the transesterification processes of vegetable oils to fatty acid methyl esters (FAMES), attributed to the optimal dispersion of WOₓ domains generating robust acid sites [45]. Mixed oxides based on ceria loaded with calcium oxide or alumina, either separately or in combination, have also been used for the simultaneous transesterification of lipids and esterification of free fatty acids (FFA) from bio-oils for the effective production of fatty acid methyl esters [46].

Catalysts based on sulfonated carbon are considered eco-friendly, efficient, non-toxic, and low-cost species derived from the carbonization and sulfonation of biomass-based precursors [47]. Structurally, these catalysts show aromatic carbon frameworks with covalently bonded sulfonic acid groups (-SO₃H), providing strong Brønsted acidity [48]. Their mesoporous architecture allows cellulose penetration, facilitating the protonation of glycosidic bonds and subsequent hydrolysis. These carbon-based acid catalysts offer a simultaneously high surface area, tunable surface chemistry (hydrophobic/hydrophilic balance) and excellent hydrothermal stability, making them a promising class of green solid acid catalysts. The design of such catalysts can draw inspiration from enzymes: in one study, a ‘cellulase-mimetic’ (CP-SO₃H) solid acid with carefully positioned acid sites and hydrophobic regions was proposed to enhance cellulose binding and hydrolysis efficiency [49].

Carbon solid acid catalysts prepared by sulfuric acid sulfonation have also been widely used in furfural production. Li et al. [48] used sulfonated hierarchical sucrose-based activated carbon (SSAC) and N-doped SSAC (SSUAC) to obtain furfural from xylan and wheat straw with high yield and catalyst recyclability. In another work, Ausavasukhy et al. [50] applied a sulfonated catalyst derived from palm kernel shells in the etherification of glycerol with tertiary butanol.

In addition, sulfonated carbon catalysts can be modified with metal oxide in order to obtain different surface properties. One example is reported by Liu et al. in which a ZrO₂-modified sulfonated charcoal-based catalyst was prepared and used for the conversion of xylose, glucose, and fructose into the corresponding furans with very high yield, demonstrating that the amount of ZrO₂ can regulate the content and molar ratio of Lewis and Brønsted acid sites [51]. Also, sulfonated mesoporous silica and sulfonated zirconia materials possess a considerable amount of accessible -SO₃H acid groups that can be applied to both lignocellulosic and oleaginous biomass [52]. They exhibit large and uniform mesopores that require considerable effort to incorporate organic and inorganic components for easy modification or functionalization. This represents a good way to modify the hydrophilic/hydrophobic network in order to increase the preferential solubilization and subsequently improve the selectivity. For example, grafting aluminate species under alkaline conditions increased the hydrophilicity of mesostructured pore surfaces, leading to markedly increased hydration reactions at elevated temperatures and a simultaneous improvement in the catalyst surface area [53]. Instead, the incorporation of organics like alkoxysilanes [RSi(OR′)₃] or chlorosilanes [RSiCl₃] is exploited to improve the organic wettability, enlarging at the same time the catalyst pore dimension [54]. One example was reported by Hegde et al. [55], where the sulfonic acid functionalization of mesoporous silica catalysts of different morphology (spherical S–SOH and cubic C–SOH) was applied in the esterification of free oleic acid with methanol to obtain methyl oleate. They demonstrate that using a mole ratio alcohol/acid equal to 40:1, at 60 °C and after 8 h of reaction, both species were quite active but the cube-shaped sulfonated mesoporous silica exhibited higher activity (92% with respect to 78% for spherical), with the difference in catalytic activity majorly due to the shape effect and not to the size (100–150 nm in size for the spherical whereas 300–400 nm for the cubic one); cubic C–SOH displayed higher total acidity and surface area as compared to the spherical S–SOH [55].

3.2. Zeolites

Zeolites are crystalline alumino-silicates characterized by microporous structures formed by SiO₄ and AlO₄ tetrahedra arrangement. The acidic strength derives from proton-exchange sites associated with aluminum frameworks (Figure 4) [56].

The presence of hierarchical meso-porosity increases activity and product selectivity. Zeolites are quite stable materials at a high temperature and in LHW; they are less effective for cellulose hydrolysis than amorphous sulfonated materials, probably because in aqueous media, the effective acidity of the structural protons is reduced [57] and their microporous structure (<1 nm) fails to interact with the bulky polysaccharide substrates.

To increase the tolerance of zeolite in LHW, modification of the zeolite external surface using organosilane has been performed [58]. In particular, HY zeolite was modified with 3-Aminopropyltriethoxysilane (APTES-HY) with a Si/Al ratio of 2.134. This modification enhanced the stability of zeolite against LHW treatment for 72 h [58]. The substitution of a part of Si⁴⁺ with Sn⁴⁺ in the BETA zeolite led to the synthesis of a catalyst (Sn-BETA) active in the isomerization of glucose to fructose in aqueous media, and resistant to a wide range of pH (including pH< 2) and temperature [59]. As already reported by Davies et al. [60], the isomerization process occurs via an intramolecular hydride shift mechanism analogous to enzymatic isomerization (the Bilik mechanism), where glucose converts to an open-chain form and a hydride shift from C2 to C1 yields fructose. Zeolites also offer good properties as support, i.e., they offer the possibility to optimally disperse the active metal centers while simultaneously increasing the recoverability of the catalyst. An example was reported by Samart et al., where the synergistic effect of the bimetallic system Co-Cu loaded on HY zeolite allowed the oxidative fractionation of lignin [61]. The high meso-porosity of zeolites also allows for obtaining a high incorporation of sulphonic acid, improving some zeolite properties as recently reported [62] for the fructose dehydration into 5-hydroxymethyl furfural under microwave irradiation and in a biphasic medium.

3.3. Solid Alkaline Catalysts

Solid basic catalysts include mainly alkaline earth oxides/hydroxides/carbonates, basic mixed metal oxides and hydrotalcites. These catalysts are generally used in transesterification processes (activation of alcohol) and hydrothermal reactions. Moreover, they are employed in biomass pretreatments and liquefaction, where they promote the decomposition of macromolecules into smaller molecules, enhancing the conversion rate and consequently improving the yield [63]. Additionally, they can be used in deoxidation, desulfurization, and denitrification processes. Their basicity originates from surface lattice oxygen anions (O²⁻) or vacancies. Structurally, these catalysts exhibit interconnected metal-oxide lattices capable of abstracting α-protons from diverse substrates like acetone or biomass-derived aldehydes, forming reactive enolate intermediates essential for aldol condensations or isomerization. For example, MgO–ZrO₂ mixed oxides possess robust basic sites due to Mg–O–Zr bridging bonds [64]. Some solid bases have achieved near-homogeneous activity. The Na₂O·nSiO₂ catalyst exhibited comparable performance to NaOH in the transesterification of soybean oil, yielding biodiesel efficiently under mild conditions [65].

3.4. Supported Metal Catalysts

Supported metal catalysts involve transition metal, often as nanoparticles dispersed on high-surface-area supports such as activated carbon, silica, or metal oxides. Supports may have multiple positive effects, such as a better dispersion of active species, increasing recoverability and durability. Structural integrity is maintained through metal–support interactions, optimizing catalytic activity. For instance, in sequential biomass hydrogenation/hydrogenolysis reactions, the metal nanoparticles dispersed on a support can dissociate molecular hydrogen into reactive atomic hydrogen, as happens with Pd/C or Ru/Nb₂O₅, which exhibit enhanced hydrogen activation due to electronic interactions and the surface dispersion of metal nanoparticles. Noble metals are highly effective hydrogenation catalysts and are often used in biomass conversion processes. However, their high cost and limited availability lead to the proposal of less expensive alternative catalytic systems with similar performance.

Nickel Raney (a skeletal Ni catalyst) can be combined with a solid acid to upgrade bio-oil phenolic compounds. In this context, Lercher and coworkers showed nearly 100% yield of hydrocarbons from phenolic bio-oil fractions by using Ni Raney for hydrogenation and Nafion-silica solid acid for dehydration in tandem [66]. This dual catalyst system operates in aqueous phase (bio-oil contains 30% water) and under relatively mild conditions, showing the importance of water-tolerant catalysts for real biomass feeds. The Ni component hydrogenates aromatic rings and cleaves C-O bonds, while the solid acid takes care of the hydrolysis and dehydrogenation steps, achieving the complete deoxygenation to hydrocarbons. Of interest are also modified nickel catalysts, and metal carbides/nitrides are explored. For example, the Nickel–Tin alloy Raney^® supported on aluminum hydroxide, yielding a Ni₃Sn₂ alloy, is used in the synthesis of γ-valerolactone (GVL) from biomass-derived sugars [67].

Other supported metal catalysts, such as copper and iron supported on montmorillonite, are used in the direct 5-HMF production from cellulose under the DMSO/H₂O two-phase system as a solvent [68], or metal nanoparticles supported on mussel shell for catalytic biodiesel production [69].

MOFs have emerged as intriguing catalyst supports (and even as catalysts themselves) due to their highly tunable structures. MOFs consist of metal nodes and organic connectors, offering tailorable pore sizes and the possibility to incorporate catalytic sites either at the metal nodes or via functionalized linkers (Figure 5).

Several studies have shown MOF-based materials to be effective in biomass conversion. One example is the use of a bimetallic Ni/Ce-MOF catalyst for the selective transfer HDO of vanillin (VAN) (a lignin-derived aromatic aldehyde) to 2-methoxy-4-methylphenol (MMP) and guaiacol (GUA) (deoxygenated products) under 2 MPa N₂ pressure at 240 °C in 15 mL ethanol [70]. Ce doping MOF has the role of a promoter by supplying oxygen vacancy sites, facilitating the removal of –CHO and –OCH₃ groups from VAN, effectively aiding the Ni active sites (Scheme 1).

This Ni/Ce-MOF catalyst exemplifies how MOF supports can stabilize multiple active components and promote unusual reaction pathways (e.g., transfer hydrogenation using internal hydrogen sources).

Another approach is to use MOFs as precursors for deriving nanostructured carbon or oxide supports. In addition, when MOFs are pyrolyzed, they can yield high-surface-area carbons doped with metal or heteroatoms. For instance, MOF-derived Ni₂P embedded in a porous carbon was found to be an effective catalyst for HDO reactions, outperforming its non-MOF-derived counterpart due to better dispersion and a carbon matrix that mitigated sintering [71].

Despite their promise, MOFs face challenges in stability (many of them decompose above 300 °C or in the presence of water). The trend is to identify more robust MOFs (e.g., Zr-based MOFs or carbide-MOFs) that can withstand biomass reaction conditions, as well as exploring MOF composites (like MOF@silica) to improve durability.

Carbon-based supports, generally derived from biomass itself (activated carbon, graphitic carbon, biochar, carbon nanotubes, etc.), are widely used in biomass conversion because of their chemical inertness, high surface area, and tunable surface functionality. Carbons often provide a neutral or hydrophobic surface, which can be advantageous when used in aqueous-phase processing (reducing strong interactions with water) [72]. Carbon support also resists poisoning by sulfur or chlorine better than many oxides, an important consideration given that biomass feeds can contain such elements. Metals deposited on carbon tend to have different selectivity than oxides, partly due to the lack of strong metal-support acid sites [73].

In pyrolysis oil upgrading, carbon-supported catalysts (like Ru/C or Ni/C) have shown good performance in hydrogenation and less tendency for undesired polymerization reactions, as carbon does not catalyze acid-driven polymerizations that acidic oxides might. For example, N-doped carbon supports can anchor metal single atoms or impart basic sites that catalyze the aldol condensation of biomass-derived aldehydes. A caveat is that carbon support can gasify or combust in very oxidative or high-temperature environments, so they are best suited to liquid-phase or inert atmosphere processes.

Beyond the choice of support, nano-structuring of catalysts is a pivotal aspect of advanced design. Tuning the size, shape, and distribution of active phases can significantly influence performance. Nanoparticle size and very small metal clusters may favor deoxygenation pathways, unlike larger particles, due to quantum size effects or differing facet exposures. Researchers have also designed hollow and core–shell nanoparticles as support for biomass conversion. Hollow structures (e.g., hollow metal oxides or phosphides) can offer interior voids that improve mass transfer and resist sintering by confining the metal [74]. Similarly, core–shell architectures have been used for acids on the shell and metals in the core (or vice versa) to control the intimacy of bifunctional sites. For example, a recent report described a nanometer-scale encapsulation of acid sites around metal nanoparticles, allowing reagents to encounter metal sequentially. In another work [75], a highly resistant core–shell catalyst was produced for the furfural oxidation with the oxidant active manganese oxide (+4) that surrounds the cerium oxide support. This level of precision reduces side reactions and has led to increased selectivity in the upgrading of polyols and sugar derivatives [76].

3.5. Functionalized Porous Catalysts

Functionalized porous catalysts are hybrid materials with specific morphological properties and often mesoporous channels (~2–10 nm) that incorporate active moieties such as sulfonic acid groups or metal complexes. Catalytic mechanisms generally involve the adsorption of substrates onto functional groups, protonation, and subsequent intramolecular transformations, as occur in the fructose dehydration to HMF through consecutive protonation, intramolecular cyclization, and water elimination. A multi-functional porous catalyst has also been designed. A compelling example involves the preparation of a mesoporous t-SiO₂@B@A catalyst that shows bifunctional activity with Brønsted basic (─C═N─) and acidic (−COOH) sites in the production of HMF from glucose in water [77]. Sidhpuria et al. prepared silica nanoparticles coated with an acidic imidazolium-based ionic liquid, creating a supported ionic liquid nanoparticle (SILnPs) of ~300–600 nm. These materials showed improved activity in fructose dehydration compared to conventional solid acids like zeolites or resins, achieving > 99% fructose conversion with 63% HMF yield [78]. MOF (metal–organic frameworks) composed of metal ions and organic ligands, displaying diverse crystal structures, regular pore channels, and rich porosity, can be used as a feasible support in catalysis. Perez-Ramirez incorporated polyoxometalates (POMs) into MOFs to enhance biomass catalytic valorization [79] while Wang et al. modulated the strength and amounts of Lewis acid sites using different mono- and di-topic ligands (2-sulfoterephthalate monosodium salt (STPA), 3-sulfobenzoate sodium salt (mSBA), 4-sulfobenzoic acid monopotassium salt (pSBA), and 4-(chlorosulfonyl)benzoic acid (pCl-SBA)) in the glucose isomerization and conversion into added value products [80].

3.6. Single-Atom Catalysts (SACs)

SACs consist of isolated metal atoms (often stabilized on a support by coordination to heteroatoms like N or O) that act as uniform active sites. By eliminating metal–metal coordination, SACs can dramatically change reaction pathways and improve metal utilization efficiency, reducing the cost and increasing the stability as compared to nanoparticle catalysts (Figure 6) [81].

A breakthrough example is a single-atom Ni catalyst on N-doped carbon for the transfer hydrogenation of furfural [82], where each Ni atom is bound to four nitrogen atoms (Ni–N₄ sites) in a carbon matrix. With such a catalytic system, a TOF of 832 h⁻¹ for converting furfural to furfuryl alcohol at 130 °C, with 97% selectivity, was observed. Remarkably, that activity was about nine times higher than that of a conventional Ni nanoparticle catalyst under the same conditions. DFT studies indicated that the Ni–N₄ configuration altered the electronic density of Ni, optimizing its binding to the furan ring and facilitating hydrogen transfer. The single-atom Ni was also noted to be robust, maintaining its activity over repeated batch runs without aggregation into particles (the N-doped carbon effectively traps the Ni in isolated sites). Other papers report the use of SACs for the same reactions, including Pd₁ on a MoC support [83] and Ru₁ on carbon for ammonia synthesis from biomass-derived nitrates [84], both with selectivity higher than 90%. The main challenges for SACs are ensuring the single atoms remain isolated under reaction conditions (avoiding sintering) and scaling up their synthesis to gram or kilogram quantities. The atomic layer deposition and MOF-derived SAC preparation are currently applied and improved to solve these problems. If scalable, SACs could significantly decrease the use of costly metals (where each atom is active) and create new selective pathways in biomass conversion that traditional catalysts cannot achieve.

4. Catalytic Technologies for Biomass Conversion

As introduced in the previous paragraphs, the catalysts used in biomass conversion often require multifunctional structures, combining acid/base or redox properties with metallic functionalities for cascade transformations. As an example that clearly illustrates how multifunctionalities are often necessary, it is worth mentioning nanostructured Ruthenium dispersed on Niobium-based supports, showing bifunctional structures and providing hydrogenation-active Ru sites alongside Lewis acid Nb(V) sites used in the selective hydrogenolysis of lignin-derived aromatic ethers by cooperative metal–acid interactions [85]. Also, it should be considered that biomass is often fermented to organic acids (lactic acid, succinic acid) or ethanol, which should be used as they are or upgraded to fuel by using heterogeneous catalysis.

A classic case regards GVL, obtained directly from sugars or through fermentation via LA and subsequent hydrogenation (using Ru/C). GVL is a stable liquid reputed as a platform molecule because it can be upgraded to a C₄ molecule (butene), or methanol and CO₂. The conversion into butene in water over NbAlS-1 zeolite with a quantitative yield is a standout demonstration of going from a biomass-derived lactone to drop-in fuel alkenes using a solid catalyst [86]. Butene can be oligomerized to jet fuel range alkenes or used in alkylate gasoline. Another cascade process is the conversion of surplus produced ethanol into butadiene (Lebedev process) but revisited with modern catalysts (Zn/ZrO₂, Ag/ZrO₂, silica-supported metals, etc.) in which the multi-step routes (ethanol → acetaldehyde → aldol → crotyl alcohol → butadiene) require carefully balanced catalytic sites [87]. Catalysts, in any case, must be optimized based on the technology with which they are applied.

This paragraph reviews the latest advancements in catalytic technologies applied in biomass conversion.

4.1. Thermochemical Processes

4.1.1. Catalytic Pyrolysis and Cracking

Catalytic fast pyrolysis (CFP) uses solid catalysts (often acidic zeolites or metal oxides) either in situ (mixed with biomass) or ex situ (vapor-phase upgrading) to crack heavy vapors and deoxygenate bio-oil into lighter hydrocarbons. Zeolite catalysts like H-ZSM-5 are the most applied in CFP because their shape-selective micropores and strong Brønsted acidity can oligomerize and aromatize biomass-derived oxygenates into benzene, toluene, xylene (BTX used as fuel) and other aromatics. Different studies are present in the literature in which metal loading on these zeolites can improve the catalytic process. Wang et al. demonstrate that in the microalgae CFP, the incorporation of Fe into H-ZSM-5 can significantly reduce the production of acids, aldehydes, ketones, furans and nitrogen-containing compounds, increasing the selectivity towards the aromatic hydrocarbon content, through a series of reactions as dehydration, cracking, decarboxylation, decarbonylation, and oligomerization reactions, followed by cyclization and aromatization reactions [88]. A yield of 42.5% was obtained with 8% of iron loading, highlighting that the increased strong acid sites were crucial, although its specific surface area, micropore volume and average pore diameter decreased. Yuan et al. reported the benefits of ZSM-5 modification with La and P on the catalyst surface properties [89]. In particular, they found that some crucial catalyst surface properties change with different La-to-P ratios. In fact, the specific Brunauer–Emmett–Teller (BET) surface area tended to increase and then decrease with increasing P loading, while the average pore size decreased first and then increased, because the low P loading promoted the shrinkage of zeolite pore channels as well as the increase in zeolite channel curvature. At the same time, an increase in phosphate groups promotes the generation of large amounts of non-acidic silicon hydroxyl groups and terminal hydroxyl groups of Al and subsequent Lewis acid sites. Working on the Cassava residue pyrolysis, they obtain a total aromatic hydrocarbon yield up to 83,5% at 650 °C, which increases over 90% using acid feedstock pretreatment. As reported by Zhong et al., working on the same feedstock, V-modified ZSM-5 shows a decrease in BET surface area and pore volume, while the related pore diameter increased, probably due to the partial blockage of mesopores caused by V; simultaneously, the acidity of the catalysts increased with the V loading, especially strong Bronsted sites. The 3V/ZSM-5 presented the highest acidity and B/L molar ratios, which significantly favored the formation of aromatics (monocyclic and polycyclic aromatics), and their synergistic regulation can change the reaction selectivity towards monocyclic or polycyclic aromatics [90].

Catalytic hydropyrolysis (CHP) represents a promising method to produce renewable fuels from lignocellulosic biomass in which hydrogen is co-fed into the reactor and oxygen can be eliminated as water. This approach has shown increased yields of deoxygenated liquids but requires careful catalyst selection (e.g., Ni-based on acidic supports) to manage the higher H₂ uptake and water formation. Shafaghat et al. selectively converted sawdust into bio-jet fuel (BJF) components via an ex situ CHP process (Figure 7), using a structure-modified Beta zeolite in combination with a hydroconversion catalyst of Mo/TiO₂ in an integrated catalyst setup, obtaining a very high total carbon recovery [91]. Otherwise, a Ni/Mo bimetallic system on ZSM-5 was used to hydrogenate unstable pyrolysis intermediates, leading to improved liquid stability and different product slates [92].

Despite these advances, catalyst deactivation in this process remains a significant issue. The use of catalysts in pyrolysis reduces bio-oil production (since more carbon is converted into gas or coke), thereby improving its quality (energy density). At the same time, it involves the coke deposition on acid sites, which represents the most important drawback in CFP, since heavy oligomers or some char precursors can be formed during the reaction that tend to clog the pores, modifying the surface properties of the catalyst itself. Furthermore, it is known that the vapors from biomass pyrolysis also contain high amounts of phenols and polyoxygenated products that can polymerize and form coke right on the catalyst surface, requiring frequent regeneration. Base minerals (alkali metals) can also poison the acid sites of the catalysts through neutralization or dealumination of zeolites. To address these challenges, researchers are investigating systems that can offer greater coke tolerance or that produce less coke, such as hierarchical and nanostructured catalysts that provide larger mesopores for vapor diffusion.

Fluid catalytic cracking (FCC) represents the most widely used industrial process to convert heavy fractions contained in crude oil (such as polycyclic aromatic hydrocarbons) directly into easy-to-combust hydrocarbons or for a hot syngas cleaning unit in gasification plants [93]. It generally involves a first step in which, using a bed reactor, the catalyst is contacted with the feedstock, producing cracked products and spent coked catalyst. The latter is separated from the cracked products, stripped of residual oil by steam, and then regenerated by burning the coke from the deactivated catalyst in a regenerator that occurs at 650–760 °C and a pressure of around 2 atm. The hot catalyst is then recycled to the riser reactor for additional cracking. Iron is a cheap and frequently used catalyst. The main advantage of this technology is the high efficiency in the oxygen removal from biomass-derived molecules in the form of CO, CO₂ or H₂O and recently, scientists have been focusing on maximizing hydrogen production. Yue et al. reported a combination of fast pyrolysis and tar cracking in a novel dual micro-bed reactor, studying through a kinetic model the effect of temperature zone on the product selectivity [94]. Boccia and co-workers used two waste-derived materials (red mud and sewage sludge) having a high content of iron and compared the efficiency of an alumina-supported iron catalyst with an only-iron one, demonstrating that although pure iron produces a higher hydrogen amount with respect to the supported species, the presence of alkali and alkaline-earth metals in the catalyst increases its lifetime [95].

4.1.2. Gasification and Fischer–Tropsch Process

Gasification is the process that converts biomass into synthesis gas (syngas) containing CO, H₂, CO₂, CH₄, and minor contaminants, by its partial oxidation at high temperatures (800–1000 °C) [96]. Subsequently, the syngas can be selectively converted into liquid fuels via the Fischer–Tropsch (FT) reaction or fermented by microbes [97]. (Scheme 2)

Catalysis plays multiple roles in the overall process, both inside or downstream of the gasifier to reform by-produced tars (heavy polyaromatic compounds condensing from gasifier effluent) to useful gas products, often required to avoid pipeline blockages and catalyst deactivation, thus adjusting the gas composition [98], but also in the FT reactor to convert the CO/H₂ mixture into hydrocarbons [99]. Although biochar itself reveals a high surface area and meso-porous structure that can be used in the catalytic tar over-oxidation, for its regular channel size and tunable surface chemistry, it appears particularly suitable to be decorated with one or more active metals. For instance, nickel-based catalysts resulted in very effective biomass gasification, showing a good compromise between activity, stability, and costs. Lisak et al. [100] discussed the catalytic activity of nickel supported over char, commercial alumina and a spent fluid cracking solid in the steam reforming of biomass pyrolysis gas, investigating the trend of methane production and catalyst stability under different conditions and gas-phase composition. Similarly, the Zhao group [101] compares the catalytic activity of bio-char and char impregnated with Zn or Ni, demonstrating that the catalyst’s activity is a function of the catalyst surface acidity, which is strictly correlated to the properties of the metal deposited on the support. The greatest flaw of these catalysts lies in the premature deactivation of the metal center (nickel), especially by sulfur poisoning and coking, requiring periodic regeneration [102]. Alternatively, inexpensive alkali and alkaline earth metal-based catalysts (either inherent in biomass ash or added as salts) have attracted more attention, because in some cases the activation energy of the reaction is lower and therefore also the temperature for char gasification, promoting tar cracking. In such a system, the catalyst presents less activity, but it is more durable as the premature deactivation of the metal center does not occur [103]. Depending on the alkali metal used, the reaction selectivity can be tuned. For instance, potassium tends to favor low molecular weight compounds and gaseous species, magnesium promotes dehydration reactions, and calcium and magnesium oxides allow volatiles to upgrade by deoxygenation and deacidification [104].

The Fisher–Tropsch reaction is commonly used on the cleaned-up syngas, particularly rich in H₂ and CO (after particulates, sulfur, chlorine, and phosphorus removal) [105]. It catalytically polymerizes CO and H₂ over metal species (commonly Co or Fe compounds) to long-chain hydrocarbons, which can be further hydrocracked into diesel, jet fuel, and naphtha-range products. Cobalt-based catalysts (e.g., Co on alumina, sometimes promoted with Re or Ru) are generally used for biomass-to-liquid fuel when the H₂/CO ratio is high, producing long-chain paraffins with low oxygenates, as reported by Prieto et al. that obtain CO conversion higher that 20% under mild conditions as T = 473 K, P = 20 bar, H₂/CO = 2, WHSV = 5.1–11.0 h^–1 [106]. However, cobalt is very sensitive to contaminants: even a few ppm of sulfur or chlorine compounds in syngas can irreversibly poison Co active sites by chemisorption [107]. Phosphorus compounds (originating from working on certain herbaceous biomass) can also deposit on catalysts and cause deactivation. A recent study on Co/Al₂O₃ catalysts demonstrates that up to ~800 ppm of phosphorus in syngas did not significantly diminish CO conversion but did alter the hydrocarbon distribution at temperatures below 260 °C, as amorphous carbon and waxes can be formed, shifting selectivity towards light alkanes. As reported by Khodakov [108], in some cases, support selection can mitigate such effects. For instance, phosphorus poisons were less severe on inert SiO₂-supported Co than on acidic Al₂O₃, because the latter can bind phosphate, reducing the interaction with Co sites. To improve selectivity and catalyst robustness, promoters can be added, obtaining multi-metallic formulations. Liu investigated the use of small amounts of Mn and Na on cobalt deposited over different supports, revealing the modification of Co crystallite dispersion [109].

Recently, SACs (e.g., isolated Fe or Co atoms on supports) have also been used at a high temperature [110].

To increase energy efficiency, interesting results have been achieved coupling gasification with downstream FT in modular systems, using intensified reactors or co-producing power [111], or using the hybrid biorefineries, where part of the biomass is gasified to provide hydrogen (via water–gas shift) or heat for the FT process, thereby improving overall carbon efficiency.

4.2. Aqueous-Phase Processes: Hydrothermal Liquefaction (HTL)

HTL involves reacting wet biomass (e.g., algae, wet wastes, lignocellulosic slurries) in hot compressed water (typically 250–374 °C, 10–25 MPa) to produce a viscous energy-dense biocrude oil, along with aqueous phase, gaseous products, and solid residues [112]. The presence of water in the HTL process promotes unique reaction pathways: it hydrolyzes biopolymers into smaller fragments and can supply in situ hydrogen via water–gas shift or metal redox reactions. In the HTL process, to increase biocrude yield and improve oil quality (higher H/C ratio, lower O content) (as occurs when municipal primary sludge [113] is used), both homogeneous (e.g., KOH, formate salts) or heterogeneous catalysts (metal oxides, noble metals on supports) [114] can be used. The use of redox catalysts like elemental iron powder can dramatically enhance oil yield by generating in situ hydrogen from water and hydrogenating reactive fragments [115]. In the HTL of oak wood, adding ~10 wt% Fe increased the biocrude yield by ~30% (relative to non-catalytic HTL), whereas Fe₃O₄ gave a 13% increase. The Fe catalyst not only boosted oil yield but also raised the hydrogen-to-carbon ratio of the oil by ~15%, indicating that effective hydrogenation of biomass oxygenates with hydrogen derived from water (through Fe oxidation to Fe₃O₄). After the reaction, the iron was found as magnetite mixed with char. It could be regenerated to Fe(0) by heating in N₂ atmosphere, allowing reuse [116]. Similarly, Bojun Zhao et al. [117] observed that metallic Fe (10 wt%) in the HTL of cornstalks in 50:50 ethanol/water solvent not only increased biocrude yield and quality (higher hydrothermal vaporization–HHV) but also far outperformed Fe₃O₄, underlining the importance of the metal’s redox state. Acid/base properties of catalysts can also significantly influence product distribution. Alkaline conditions (addition of Na₂CO₃ or KOH) often promote deoxygenation and reduce char formation by neutralizing organic acids that would otherwise repolymerize oils [118]. Bifunctional acid–base catalysts have been used to promote condensation (oligomerization) reactions, producing larger fuel precursors from small fragments. Teixeira et al. demonstrated that hydroxyapatite (HAP, Ca₅(PO₄)₃OH), combined with a small amount of base (to provide complementary basic sites), tripled the biocrude oil yield from food waste HTL (from ~ 4% to ~37%), capturing ~49% of the feed’s energy in the oil phase. The HAP catalyst likely provides both acidic and basic functionality, enabling reactions like ketonization and aldol condensation of fatty acids in the feed. Product analysis confirmed that HAP catalyzes in situ conversion of fatty acids into higher molecular weight oils via oligomerization [119]. Such strategies are important for wastes rich in lipids or fatty acids, turning what would be residues into additional oil. Heterogeneous catalysts tested in HTL also include transition metal oxides (ZrO₂, CeO₂, TiO₂), which can show both acid and redox properties [120]. For example, Ce–Zr mixed oxide was reported to promote in situ deoxygenation of water-soluble carbohydrates into oil by transfer hydrogenation, leveraging Ce^3+/4+ redox cycling [121]. Noble metals, such as Ru, Pt, and Pd, dispersed on carbon or alumina, result in highly active catalysts for HTL and hydro-processing in situ, but their cost confines them to small-scale studies.

According to Mazhkoo [122], another example of catalytic HTL using heterogeneous metal-supported catalysts is the liquefaction of grape pomace with Ni–HZSM5 and Ni-ZrO₂-modified steel slag (MSS) in a water–crude glycerol co-solvent system. It was demonstrated how the solvent composition and the catalyst structure may affect the biocrude yield and quality. In particular, it was reported that at 320 °C with 75% crude glycerol, the process achieved a high biocrude yield of 76 wt%, with an HHV of 41 MJ/kg and H/C ratio of 1.81, highlighting the synergy between redox-active Ni species and solvent hydrogen donors. The Ni–HZSM5 catalyst reduced the acid content of the biocrude by 44% and increased the diesel-range fraction to 41%. Meanwhile, the Ni-ZrO₂-MSS catalyst improved overall yield but promoted acid formation. Catalyst characterization (BET, H₂ chemisorption, TGA) revealed changes in surface area and active site distribution after HTL, underlining the importance of stability and regeneration. The challenges in HTL catalysis include catalyst recovery (especially if fine solid catalysts are mixed with feed ending up in char or aqueous phases), stability under hot compressed water which can leach or alter catalysts (e.g., base metals can leach into aqueous phase or form sulfides if biomass contains sulfur), and compatibility with continuous reactor operation. For these reasons, more attention is focused on the reactor development [123]. To avoid solid handling, Johannsen et al. [124] explored continuous-flow HTL reactors with fixed-bed catalysts (Figure 8), where biomass slurries are pumped through the catalyst bed, resulting in clogging or high pressure drops.

Some pilot systems use a mixture of catalyst and biomass, then separate the catalyst from the products by filtration for recycling, which requires the setup of a more complex plant. This highlights that feedstock variability can be an issue: for example, HTL of protein-rich feed (algae, manure) produces nitrogenous compounds (pyrroles, amides) that may deactivate acid catalysts or require downstream removal, while lignin-rich feeds produce phenolic compounds that can polymerize if not catalytically stabilized. In any case, it is necessary to develop low-cost catalysts (e.g., biochar-supported metal systems, or catalysts derived from waste such as red mud or fly ash) and to set up in catalyst regeneration systems in situ. It is worth mentioning that HTL biocrude typically still requires HDO or catalytic hydrotreating to become finished fuel [125].

4.3. Chemical Upgrading

4.3.1. Hydrodeoxygenation (HDO)

HDO is a catalytic hydrogenation process that removes oxygen from bio-oils by forming H₂O (via hydrogenation of carbonyls and subsequent dehydration) or sometimes CO/CO₂ mixture with an opportune ratio (via decarbonylation/decarboxylation). It is analogous to petroleum hydrotreating, except that biomass-derived liquids contain vastly higher oxygen species (15-45 wt% O in pyrolysis oil vs. <1 wt% O in crude oil) and often some nitrogen and sulfur, requiring more H₂ addition and producing more water, which can lead to support instability and catalyst deactivation. The goal of HDO is to upgrade bio-oils such as pyrolysis oils, HTL bio-crudes or fatty acid derivatives into hydrocarbon fuels compatible with the existing refinery infrastructure. Types of catalysts used for HDO processes are sulfided NiMo or CoMo on γ-Al₂O₃, adapted from hydrosulfurization (HDS) catalysts used in refineries. These are effective for removing oxygen but face issues; biomass oils typically should contain small amounts of sulfur to keep the catalyst in sulfide form, which can be converted into oxides (losing the activity) unless a sulfating agent like dimethyl disulfide (DMDS) is continuously supplied. Moreover, sulfided catalysts can introduce sulfur into the product if the over-sulfiding process is practiced [126]. For this reason, in some cases, it tends to use non-sulfur catalysts but those based on noble metals dispersed on supports such as carbon, ceria, alumina, and/or bimetallic systems in which the bimetallic synergistic effects can be exploited when one metal (like Ni or Co) activates hydrogen, while the other one (Fe, Mo) may preferentially bind oxygenates or stabilize the active phase [127]. Dou et al. showed that a Ni-Fe oxide hollow catalyst converted Kraft lignin under mild conditions (250 °C, 6 h) to yield 15.4 wt% of monomeric aromatics [128]. The presence of Fe as a promoter modified Ni behavior, partially inhibiting Ni’s overly strong hydrogenation activity, thus preserving aromatic rings from over-reduction, and also introduced additional acid sites that enhanced the cleavage of C–O bonds. This bimetallic design led to improved product yields and less char formation compared to Ni alone. Likewise, other bimetallic combinations such as Ni–Re and Ni–Mo have recently demonstrated superior HDO performance (e.g., faster phenol conversion and deeper deoxygenation) relative to single-metal catalysts [129].

In other cases, the presence of a metal can act as a poison in a bimetallic system, leading to a decrease in activity and selectivity [130]. The use of Ru nanoparticles supported on hydrophilic mesoporous carbon is interesting, as they show quantitative microalgae oil conversion into alkanes in a one-pot HDO process at low temperatures (140 °C)—also if coking and metal sintering were observed [131]. Zhang et al. reported that Ru supported on ceria (Ru/CeO₂) as single atoms has shown unique selectivity in model-compound studies: atomically dispersed Ru on CeO₂ could selectively hydrogenate aromatic rings in phenolic compounds to cyclohexanol rings (crucial for upgrading lignin-derived phenolics to cycloalkanes for jet fuel) while minimizing excessive ring opening [132]. The strong metal–support interaction and the redox ability of CeO₂ likely facilitate the cleavage of C–O bonds (cleaving phenolic OH) and subsequent hydrogenation in a controlled manner. The SACs achieved nearly complete conversion of phenolic model substrates to cycloalkanes under mild conditions, demonstrating the potential of SACs to improve HDO selectivity.

Also, during an HDO process, the catalyst deactivation is a central point because of large, highly functionalized molecules present in bio-oils (e.g., oligomeric phenolics, sugars, resinols) that can polymerize on catalyst surfaces or coke formation and metal sintering, leading to activity loss. Water produced during HDO can adversely affect acid supports (causing support hydrolysis or phase changes, especially for γ-Al₂O₃, which can transform into boehmite). One approach is to create core–shell or encapsulated structures that protect the active metal. For example, a novel Ni@C catalyst (nickel nanoparticles enveloped in a carbon shell on an alumina support) demonstrated remarkable durability in HDO of m-cresol: it achieved complete m-cresol conversion with 100% selectivity to methylcyclohexane and maintained 100% conversion over six successive catalytic cycles with only minor changes in product selectivity [133]. Strong metal–support interactions can similarly enhance stability. It has also been shown that dispersing Ni on a carbon–ceria composite support, a strong anchoring effect is created, rendering the Ni highly resistant to sintering and coke formation during bio-oil HDO [134]. Two-stage hydrotreating of HTL biocrude was investigated by Haider et al. [135], in which a first stage at lower temperature focused on hydro-demetallation and partial HDO, preventing heavy coke precursors, and a second stage completes the HDO to fully deoxygenate the oil. This approach greatly reduced catalyst fouling and enabled continuous operation in a trickle-bed reactor. Additionally, guard beds packed with materials like Co/MoS₂ or sacrificial adsorbents can remove contaminants (chlorides, metals from feed) before the main HDO catalyst bed.

From a reaction mechanism standpoint, HDO can proceed via direct hydrogenolysis of C–O bonds or via the dehydration of intermediate alcohols. For instance, furfural (obtained from hemicellulose) can be HDO converted into pentane by first hydrogenation to tetrahydrofurfuryl alcohol, followed by dehydration to pentenes and hydrogenation to pentane. The ideal catalyst must facilitate each step while avoiding side reactions like hydrogenation to methane or excessive carbon-carbon bond cleavage (which wastes carbon as light gas). Catalysts like Ru, Ni, and Pd are very active hydrogenating catalysts and can saturate aromatic rings (good for fuel stability), whereas Mo or W carbides/nitrides have shown activity more akin to sulfide catalysts, favoring direct deoxygenation. There is active debate on whether alternative hydrogen sources can be used for HDO to avoid external H₂: e.g., using formic acid or alcohols as hydrogen donors (in transfer hydrogenation) or leveraging the high-pressure water in HTL (water gas shift in situ). As demonstrated by Passos et al. [136], the transfer hydrogenolysis of glycerol to propylene glycol using a Pt–Fe catalyst and 2-propanol as a hydrogen donor eliminates the need for external H₂. Similar concepts might be extended to bio-oil HDO using hydrogen-donor solvents.

4.3.2. Transesterification

The production of biodiesel from plant oils, waste cooking oil, or algal oil is accomplished by transesterifying triglycerides with traditionally short-chain alcohol. Homogeneous basic catalysts require downstream neutralization and produce wastewater, and they are sensitive to FFA in feed (FFAs react to form soap, inhibiting the process). To make the process more sustainable, research has focused on heterogeneous catalysts that are recyclable and more tolerant to feedstock impurities. A remarkable report by Liu et al. [137] showed a CaO-based catalyst that could be reused for 26 consecutive cycles without significant loss of activity, maintaining > 90% conversion of oil to biodiesel each cycle. This indicates exceptional resistance to leaching and deactivation. The CaO was likely modified to improve its durability, as pure CaO can suffer surface carbonation or poisoning by moisture or CO₂. For example, combining CaO with certain supports (like CaO–ZnO or CaO on alumina) has yielded bifunctional catalysts that can catalyze both transesterification and esterification (important for high-FFA oils) of waste cooking oil [138]. The one-pot transesterification of lipids and esterification of FFAs have been applied for the production of biodiesel from aquatic biomass, lowering the operational cost. The catalyst 12CaO 7Al₂O₃ 7CeO₂ is active especially in the conversion of bio-oils with high FFA content. [37]. Solid acid catalysts (such as sulfonated carbons, ion-exchange resins, or heteropolyacids on supports) have also been investigated, especially to handle high FFA feeds via esterification, but they generally have slower kinetics for transesterifying triglycerides compared to base catalysts. Nanocatalysts and enzymatic catalysts are two innovative fronts. Nanoparticles of oxides or mixed metal oxides (e.g., Mg–Al hydrotalcite-derived oxides) offer a high surface area and accessible active sites. Some nano-CaO formulations have shown > 90% yield in minutes of reaction and easy recovery by the sedimentation or magnets (if magnetically doped) [139]. Moreover, enzymatic transesterification using lipases (e.g., immobilized Candida antarctica lipase, known as Novozyme 435) can operate at lower temperatures and avoid chemical catalyst removal, but enzyme cost and slower rate are important issues. Interesting approaches include solvent-free enzymatic reactors and cascade reactions where lipases and the base catalyst carry out the esterification of FFAs, and the transesterification, respectively, in the same system [140]. Recently, organic–inorganic hybrid porous coordination polymers (OIHPCPs) have attracted even more interest because of their possibility for the rational design of crystal structures and specific functional properties, due to the possibility of metals, ceramics, and polymer hybridization (mixing) of different components at the atomic, molecular, nanoscale, and mesoscale. Zhang et al. reviewed the application of these hybrid catalysts in biodiesel production, comparing the activity of pristine MOFs, functionalized MOFs, MOF derivatives, unconventional MOFs (UMOFs), and biomass derivative coordination polymer catalysts, focusing also on the reaction conditions [141].

From a sustainability perspective, transesterification is a relatively straightforward step, and it is commercially practiced at a large scale (especially with homogeneous catalysts). The trend is to make it more eco-friendly by either adopting solid catalysts with long lifetimes or using process intensification (e.g., reactive distillation or ultrasound-assisted reactors to increase efficiency so that even dilute base catalysis can be viable with easier separation). In a biorefinery context, biodiesel production can be integrated so that the glycerol byproduct is captured for upgrading via hydrogenolysis or reforming, and any waste heat or alcohol can be recycled. Additionally, transesterification chemistry is now being extended beyond fat: for instance, upgrading bio-oil phenolics by esterification to reduce their polarity, or producing acetate esters from bio-ethanol and bio-acetic acid, etc.

4.3.3. Hydrogenolysis

Hydrogenolysis refers to the cleavage of C-O bonds in polyols or other oxygenated compounds by hydrogen, typically yielding smaller alcohols or diols. Catalysts for glycerol hydrogenolysis are usually Cu-based or Ni-based, often with promoters like chromium, such as Cu-Cr catalysts (though Cr is toxic and being phased out), or in combination with noble metals. Copper is selective for the cleavage of the C-O bond after glycerol has been dehydrated to acetol (hydroxyacetone), whereas nickel is more active but can over-crack glycerol to smaller molecules or propane. Recent studies have achieved very high performance in glycerol hydrogenolysis [28,142]. Beyond glycerol, sorbitol hydrogenolysis (from sugar hydrogenation to sorbitol, then to glycols like ethylene glycol and propylene glycol) is a pathway to produce these commodity chemicals from biomass. Ni–Pt bimetallic catalysts on acidic supports have shown high sorbitol conversion to glycols, where the acid sites help dehydrate sorbitol to shorter intermediates and metal sites hydrogenate and cleave C–C bonds in a controlled fashion [143].

Hydrogenolysis can also be directly applied to lignin or sugar oligomers. In the context of lignin depolymerization, hydrogenolysis (often in the presence of a hydrogen donor solvent like methanol or with H₂ gas) breaks the aryl–ether bonds (β-O-4 linkages) present in the lignin. Catalysts such as Raney Ni, Ni-Mo, or noble metals supported on carbon have been used for lignin hydrogenolysis [144]. The challenge here is to avoid over-hydrogenating the aromatic rings, obtaining phenolic monomers and not cycloalkanes (if the goal is to use them as chemical precursors). A recent paper [145] reports the use of Ru supported on carbon nanospheres, under CO₂ atmosphere, for lignin hydrogenolysis, achieving up to 36 wt% yield of aromatic monomers from corncob, significantly higher than that obtained with conventional Ru/C [145]. CO₂ likely created a mildly acidic environment (by forming carbonic acid in situ) that assisted in cleaving ether bonds without the excessive hydrogenation of rings. In a sustainable biorefinery context, transesterification and hydrogenolysis often play supporting roles: transesterification converts lipid components of biomass (like oils from oilseeds or algae) to biodiesel, while hydrogenolysis can upgrade glycerol or certain streams (sugar alcohols, lignin) into more valuable chemicals, increasing the overall carbon utilization. Integrating these steps requires matching the scale. For example, glycerol from a biodiesel plant could be sent to a chemical plant for continuous hydrogenolysis to propylene glycol, which in turn could be used for biopolymer production.

An example of an integrated process (combination of biological and chemical catalysis) is the Reductive Catalytic Fractionation (RCF) of biomass, where lignocellulose is treated with a solvent, hydrogen, and a hydrogenolysis catalyst in one step, yielding a depolymerized lignin oil (rich in phenolics) and a solid cellulose pulp [146].

The phenolics can then be upgraded (via hydrogenation or etherification) to fuel additives or synthetic building blocks, while the cellulose is enzymatically fermented to ethanol. Such an integrated system highlights the importance of each catalytic technology: thermochemical to break down raw biomass, biochemical to ferment sugars, and chemical upgrading to refine the output, all coordinated in a loop that minimizes waste.

5. Catalytic Technologies for Biomass-Platform Chemical Production

5.1. 5-HMF, Furfural, and LA

5-HMF, furfural, and LA are mentioned as top-tier platform chemicals that can be derived from sugars. HMF is characterized by the presence of two different functional groups. Such moieties and the furan ring can undergo different types of reactions, such as hydrogenation, oxidation, hydrogenolysis of the C-O bond, rearrangement, dissociation and polymerization to produce a variety of derivatives. [34,147,148,149,150,151].

5-HMF and furfural are furan derivatives obtained by the acid-catalyzed dehydration of C₆ and C₅ sugars (obtained from cellulose via chemical or enzymatic hydrolysis), respectively, while LA is a versatile C₅ keto-acid obtained by the rehydration of 5-HMF or the direct acid hydrolysis of carbohydrates (Figure 9).

Catalytic processes aim to maximize the yields of these compounds from lignocellulosic feedstocks or carbohydrate-rich biomass such as energy crops and agricultural residues. In recent years, substantial improvements have been made in furfural yields using solid acid catalysts and biphasic reactors to continuously remove furfural (preventing over-reaction and the formation of humins). Yields above 80% have been reported. For example, a novel carbon-based solid acid (sulfonated lignin-derived carbon with tin sites) converted xylose to furfural with 93% yield, and even direct corncob (hemicellulose) to furfural with 82% yield [152]. Similarly, a cotton-derived solid acid CS−SO₃H achieved 63% furfural yield from corncob [153]. These are remarkable, considering traditional mineral acid processes have ~50–70% yields. Mechanistically, the strong Brønsted acid sites catalyze the dehydration of xylose to furfural, while the hydrophobic carbon surface helps adsorb and then release furfural into the organic phase (e.g., a co-solvent like toluene or MIBK). The addition of NaCl (“salting out”) in biphasic systems has also been found to increase the furfural yields by enhancing the partitioning of furfural into the organic layer, thus protecting it from further degradation. According to Bo Fan et al.’s study, a Sn-biochar-supported catalyst was developed using barley hulls (BH) as a support material and employed for furfural production from corncob biomass in a biphasic cyclopentyl methyl ether–water (2:1 v/v) system with 50 mM ZnCl₂ at 170 °C for 20 min. The process achieved a furfural yield of 80%. The catalyst demonstrated thermal stability over six cycles [154]. Notably, microwave heating has also been applied to furfural production, speeding up reaction rates and sometimes improving selectivity due to rapid internal heating. Overall, furfural production is nearing commercial-like efficiencies using solid catalysts, which can simplify purification and reuse. Catalyst longevity (deactivation by humins deposition) and processing solid biomass directly remain the challenge; advances in flow reactors for biomass slurries and regeneration schemes are addressing these issues.

Production of 5-HMF from glucose is more challenging because glucose is less prone to dehydration than xylose (requiring isomerization to fructose or use of specialized catalysts). The synthesis of 5-HMF from C6-polyols in a biphasic system of organic carbonate and water in the presence of cerium (IV) phosphates as catalysts has been reported. At the end of the reaction, the catalyst can be recovered and reused, while the solid 5-HMF (70%) is isolated (purity >99%) [155]. The positive aspect of the use of the biphasic system is related to the increase in the lifetime of the catalysts compared to only water [33].

Additionally, green solvents or DES have been used as reaction media, achieving more than 60% HMF yield from fructose in a choline chloride-based DES with SnCl₄ as catalyst [156].

Of interest are the results obtained by Wang et al., where SnO_x on carbon catalyst gave 84.1% HMF yield directly from glucose (92% conversion) at 180 °C [157]. Key to this result is the bifunctional nature of the catalyst: Sn (IV) acts as a Lewis acid to isomerize glucose to fructose, which is then dehydrated by residual Brønsted sites on the carbon support. A one-pot conversion of cellulose to HMF was achieved by combining an acid catalyst with a chloride-based IL to dissolve cellulose; yields, however, were moderate (~ 36% HMF) without a dedicated isomerization catalyst [158].

Looking forward to the commercialization, the development of a continuous flow reactor for 5-HMF synthesis is promising. It uses a solid catalyst (Amberlyst 36) that can produce HMF (in solvent) continuously with a yield of 70% starting from fructose [159] and can be recycled and reused several times without losing its catalytic activity.

LA is typically produced by acid hydrolysis of hexoses (5- or 6-step reaction via HMF), and yields are inherently limited by side reactions forming char (humins) and formic acid (which is a stoichiometric co-product). Yields from pure cellulose or glucose usually top out at ~50 mol% (since 1 mol glucose yields 1 mol LA + 1 mol formic acid). In practice, ~30–40% yield (by weight of biomass) is common. Recent work has been aimed at optimizing catalysts and exploring novel feedstocks. For instance, the use of solid acid resins or heterogeneous Lewis acids can facilitate product separation. Park et al. [19] optimized hydrothermal LA production from macroalgae using an ionic resin, achieving 22.6 g/L of LA (30% yield on carbohydrate) with 14.0% formic acid alongside. Considering that seaweed contains salt that can buffer acid, this is an interesting result. For lignocellulosic, one study in 2021 employed Amberlyst-15 in a biphasic reactor and obtained 59% yield of LA from corn stover at 200 °C [160]. Here, the use of an organic solvent to extract HMF as it formed, then sequential hydrolysis to LA, helped push the yield above the typical 50% ceiling. Also, metal–ion catalysts like Er(OTf)₃ have shown exceptional performance—yields above 90% LA from cellulose were reported at 240 °C with Er (III) catalysts, by accelerating both dehydration and rehydration steps [161]. The high cost of rare earths is a drawback, but this mechanistic insight (Lewis acids can catalyze retro-aldol reactions forming LA directly from hexoses, bypassing some humin formation) is inspiring the design of more affordable catalysts.

LA’s value is in its versatility: it can be upgraded to solvents (γ-valerolactone), fuels, or different monomers (Figure 10) [162,163].

The sustainable production of LA at a large scale is being pursued (e.g., GFBiochemicals built a pilot plant using homogeneous catalytic systems)—also if heterogeneous catalysis may simplify purification and reduce waste neutralization in the future [164]. Also, fermentative routes to LA are under research, but not yet as competitive as chemical routes. Importantly, the co-production of formic acid in LA processes is now being seen as a benefit, as formic acid can be used in situ as a hydrogen source for transfer hydrogenation reactions in biorefineries or sold as a chemical itself.

Figure 10. Some LA industrial derivatives [165].

Figure 10. Some LA industrial derivatives [165].

The production of HMF, furfural, and LA has seen notable breakthroughs: furfural yields in the 80–90% range from real biomass, HMF yields ~80% from sugars with robust Sn-based catalysts, and LA yields ~60% from lignocellulose with tuned acid systems. These achievements are largely due to a deepening understanding of reaction pathways (e.g., avoiding condensation side-reactions by swift product removal or using bifunctional catalysts for tandem steps). The structure–activity relations, such as the importance of pore structure in solid acids (to accommodate bulky xylan oligomers for furfural) or the balance of Lewis/Brønsted acidity (for glucose isomerization vs. dehydration to HMF), have been elucidated in recent papers and inform next-generation catalyst design.

5.2. Lignin-Derived Aromatics

Lignin valorization has historically been difficult due to its recalcitrance and the tendency to form char; however, recent research has made significant progress in controlled lignin breakdown with high monomer yields by using tailored catalysts, solvents, and “lignin-first” processing. A standout accomplishment is the work on RCF and related hydrogenolysis processes. In RCF, lignocellulosic biomass is processed in a solvent (often methanol or ethanol) with a hydrogenation catalyst, which dissolves and depolymerizes lignin before significant carbohydrate degradation. This yields a lignin oil rich in monomers while preserving cellulose pulp for other uses. Advances in catalyst development have pushed lignin monomer yields to unprecedented levels. Osorio-Velasco et al. reported a NiMo catalyst (phosphide form) on a SiO₂ support that achieved a 52 wt% yield of phenolic monomers from Kraft lignin in methanol at 400 °C, for 2 h and 100 bar [166]. The monomers were mostly alkylphenols (e.g., propylguaiacol, propylsyringol), indicating the effective hydrogenolysis of β–O–4 linkages while saturating the propanol side chains. Notably, the use of SiO₂ as a support, characterized by an intermediate acidity, proved to be optimal, as overly acidic supports led to more char and light gases (as seen with γ-Al₂O₃ vs. SiO₂), and non-acidic supports afford lower depolymerization extent. The addition of a small amount of phosphorus to NiMo (forming NiMoP) is crucial for its activity and sulfur tolerance. Catalysts like NiMoP/SiO₂ illustrate how high metal dispersion, suitable promoters, and optimal acidity can cleave lignin efficiently into stable monomers (by immediately hydrogenating reactive intermediates that otherwise repolymerize) [166].

When a Ni–Zn–Al catalyst is used, in an ethanol/water medium, a ~33% of phenolics from hardwood lignin is obtained, with in situ hydrogen formation from ethanol reforming assisting cleavage [167]. These reductive processes typically require H₂ pressure (20–50 bar) and temperatures in the range 200–300 °C (or higher for Kraft lignin, which is more recalcitrant), but the pay-off is a liquid oil where a majority of the lignin’s carbon is in low-molecular-weight form (monomers and dimers).

On the oxidative side, the catalytic oxidation of lignin (or lignin-rich streams like black liquor) can yield aromatics such as vanillin, vanillic acid, and syringaldehyde with low yield, since lignin tends to repolymerize if over-oxidized. A microwave-assisted oxidative depolymerization of an enzymatically isolated lignin gave 14% yield of monophenols (mainly vanillin and syringaldehyde) using a CuO/CNT catalyst at 200 °C [168], doubling the yield obtained under conventional heating and suggesting that rapid/homogeneous heating limits condensation reactions (Figure 11).

However, oxidative routes face selectivity issues with excessive oxidation, yielding small acids/CO₂, whereas mild conditions yield large fragments. As such, industry (e.g., Borregaard) has optimized a process from Kraft lignin to vanillin with yields around 4–8%, which is economically viable due to vanillin’s high price, leaving at the same time most unconverted lignin.

Comparatively, the reductive routes (hydrogenolysis/HDO) are now delivering much higher overall yields of useful liquids. The trade-off is that the product is a mixture of phenolics that may require further separation, purification or upgrading (e.g., hydroprocessing to cycloalkanes for jet fuel, or oligomerization to adhesives). Nonetheless, a 50% yield of phenolics from what was once regarded as waste lignin is a huge win for biorefinery economics and carbon efficiency. It implies that we can double the carbon utilization of biomass (using not just cellulose for ethanol, but also lignin for chemicals) [169]. Additionally, these phenolics can be reacted to produce adhesives, resins, or even drop-in petrochemicals after deoxygenation. For instance, phenol itself can be derived from lignin monomers via catalytic deoxygenation (demethylation and propyl chain removal).

Some studies showed that using zeolite catalysts after the initial lignin depolymerization can convert phenolic monomers into benzene, toluene, and BTX with a yield around 20% BTX from lignin, due to losses as coke [170].

The environmental benefit of lignin valorization is significant: it can improve the economics of cellulosic biofuel plants (no need to burn all lignin for heat) and provide renewable aromatics, which are otherwise derived from petroleum (and associated with high CO₂ emissions). Continued research is expected to focus on integrating lignin conversion into biorefinery streams (e.g., RCF integrated with pulping) and on catalyst longevity (handling lignin impurities like sulfur). But the recent successes give a strong indication that lignin will no longer be just a low-value burner fuel, but a source of green aromatics contributing to the sustainable materials supply chain.

5.3. Biopolymers and Composites—Polylactic Acid (PLA) and Polyhydroxyalkanoates (PHA)

Biopolymers such as PLA (Figure 12a) and PHA (Figure 12b) are renewable alternatives to traditional plastics, produced from biomass-derived monomers or directly by microbial action.

PLA is a biodegradable polyester commonly made by fermenting sugars to lactic acid, then catalytically polymerizing lactic acid (via lactide intermediates) into PLA. PHA are bacterial polyesters accumulated intracellularly by various microbes fed on carbon-rich substrates. Recent advances address feedstock flexibility, catalyst improvements for polymerization, and optimization of microbial processes to enhance yield and lower cost [171]. For PLA, the focus has been on finding non-food feedstocks for lactic acid production and improving the catalytic steps of polymerization. Traditionally, PLA uses corn glucose fermented by Lactobacillus to L-lactic acid. However, research has shown that lactic acid can also be produced chemically from cellulose via catalytic pathways. It has been reported that starting from cellulose and using Au/W-ZnO as a heterogeneous catalyst at 245 °C, a yield of 54% of lactic acid is obtained [172]. This catalytic system facilitated sequential steps: partial cellulose hydrolysis and retro-aldol reactions that form C₃ intermediates, which are then rehydrated to lactic acid. Such catalytic routes (especially with base and metal bifunctional catalysts or rare-earth salts) could complement fermentation by utilizing agricultural residues or even waste such as paper sludge to produce lactic acid. On the polymerization side, the ring-opening polymerization (ROP) of lactide to PLA is typically catalyzed by tin (II) octoate.

Further research has been developed to synthesize organic catalysts or enzymatic catalysts to avoid tin residues and to better control PLA’s molecular weight and stereochemistry [173]. For instance, a zinc-based coordination catalyst was reported to produce high-molecular-weight PLA with low racemization, offering a greener alternative to tin (and avoiding the need for perfect enantiopurity in lactide) [174]. Such advances ensure that PLA remains competitive as a drop-in replacement for petroleum-based plastics like PET and PS in certain applications. PLA is already produced on a large scale (NatureWorks facility >150 kT/yr), but expanding it to a broader feedstock base (cellulosics, food waste) is an active area of research. In terms of material properties, new PLA composites with biomass-derived fillers (natural fibers, nanocellulose) are emerging, improving toughness and heat resistance for broader use in packaging and durable goods.

PHA is mainly produced using waste or cheap feedstocks (e.g., waste oils, crude glycerol, agricultural waste hydrolysates) and extremophilic bacteria (organisms that can grow in non-sterile conditions or high salt, eliminating expensive sterile process needs). For example, halophilic bacteria like Halomonas and Salinivibrio were used to treat waste fish oil (with added glycerol) in a fed-batch, reaching cell densities of 69 g/L with 51% of cell mass as PHA copolymer in 78 h [175]. Similarly, Halomonas hydrothermalis, grown on biodiesel-derived crude glycerol, accumulated polyhydroxybutirrate (PHB) up to 75% of its dry cell weight. Yields of this magnitude, 0.75 g PHA per g biomass, are at the high end for any PHA process, indicating highly efficient conversion of substrate to polymer inside the cells [176]. In terms of process yield (polymer per substrate), a recent continuous fermentation feeding waste animal fats attained a 45% yield (probably conversion yield) to PHA at the pilot scale (Figure 13) [177].

Lab-scale experiments have reported even higher yields: one source notes up to 87% yield of PHA from optimized processes [178], though whether this refers to carbon yield or polymer content needs context. Nonetheless, using renewable waste feedstocks (e.g., used cooking oil, which is rich in fatty acids) can achieve high PHA content. From a catalyst perspective, enzymes within the microbes perform the polymerization of PHA, but catalysis also comes into play in downstream recovery (e.g., using chemical or enzymatic digestion to release PHA granules from cells) and in modifying PHA (thermal or chemical post-treatment to improve properties). An exciting development is metabolic engineering to broaden PHA monomer composition—e.g., including 4-hydroxyvalerate or medium-chain monomers to tailor polymer properties (flexibility, melting point). Some extremophiles have been tweaked to co-produce valuable pigments or enzymes while making PHA, improving the overall economics (biorefinery concept).

6. Non-Catalytic Technologies for Biomass Conversion into Carbon Materials for Catalytic Use

Biomass can also be a source of advanced carbon materials, offering a renewable route to structured substances such as graphene, carbon nanotubes, and activated carbon. Its conversion, typically, involves thermal or hydrothermal carbonization to a char, followed by activation or other high-temperature treatment to achieve the desired structure. In the past few years, new pathways to produce high-value carbon materials from waste biomass with low energy input have been discovered. One breakthrough is the Flash Joule Heating (FJH) method to make graphene. It involves rapidly heating carbon-based matter (including biomass, food waste, plastics) to ~3000 K for a fraction of a second, which induces graphitization into few-layer graphene. Remarkably, it can use biomass feedstocks like wood char or even mixed municipal solid waste (MSW). The process is ultrafast and does not require a furnace, as an electric discharge is enough to convert the material instantaneously. The “flash graphene” process could be applied on different materials such as banana peels, sawdust, or plastic bags, converting a ton of waste to graphene for a few hundred dollars in electricity [179]. This method yields graphene flakes in excess of 80% carbon efficiency (most carbon in the input becomes graphene, with minimal emissions). It represents a quantum leap in sustainable material manufacturing: instead of mining graphite or using methane Chemical Vapor Deposition (CVD) (which is energy intensive), one can turn trash to graphene in one step. A recent article further refined the continuous production of biomass flash graphene, highlighting scalability [180]. The graphene produced can be used to reinforce concrete, create conductive composites, or as battery electrodes, all with a dramatically reduced environmental footprint. This is a prime example of how choosing the right high-energy physics approach (Joule heating) and understanding the mechanistic requirement for graphite formation (extreme heating rate and quenching) can enable a new catalytic process (the carbon itself undergoes self-catalyzed reordering).

Another material derived from biomass is the activated carbon (AC), characterized by a high surface area, high porosity, used for adsorption (water/air purification), for energy storage (supercapacitor electrodes), and as catalyst supports. Biomass like coconut shell, wood, or agricultural residues can be thermochemically used to produce AC [181]. There are two main approaches: physical activation (carbonization, followed by steam/CO₂ activation at ~800 °C) or chemical activation (using activating agents such as KOH, ZnCl₂, H₃PO₄). When KOH is used, AC with ultrahigh surface areas is obtained [182]. For example, KOH activation of corn stover at 700 °C (KOH/carbon ratio ~3) produced carbons with a ~1721–1965 m²/g surface area [183]. Recent studies are exploring template methods combined with activation to create hierarchical porous carbons, for instance, using silica or salt templates to generate mesopores in addition to micropores.

The use of hydrothermal carbonization (HTC) to convert wet biomass or even sewage into carbonaceous hydrochar at ~180–250 °C in water, then activating that hydrochar, avoiding drying steps, is interesting. The hydrochar typically has a lower surface area but is a very oxygenated material (like lignite coal), which, upon activation, can yield good porosity. One study showed that sewage sludge HTC char could be KOH-activated to ~900 m²/g AC with good heavy metal adsorption capacity [184].

Biomass has also been used to produce more specialized carbon forms, such as carbon fibers or graphene oxide, through chemical processes. For example, lignin can be spun into fibers and carbonized to make quite resistant carbon fiber, but not yet as strong as polyacrylonitrile (PAN)-based fibers. Glucose or cellulose have been used to grow graphene oxide quantum dots via hydrothermal methods (citric acid + urea route yields blue luminescent carbon dots). While these are niche, they show the versatility of biomass in carbon material synthesis.

7. Catalyst Stability and Deactivation

Catalyst deactivation remains a grand challenge in biomass conversion processes. Unlike pure fossil feeds, biomass-derived streams contain high oxygen content, moisture, and inorganic impurities that promote rapid catalyst degradation. The formation of coke is a pervasive issue: heavy oxygenates and polyaromatics from biomass form carbonaceous deposits that deactivate the catalytic sites. For example, in microwave-assisted pyrolysis using a biochar-based catalyst, increasing the reaction temperature to 650 °C for 80 min caused the BET surface area to decrease by approximately 50% (108 m²/g to 56 m²/g) due to coke accumulation. The fraction of inert “hard” coke on the catalyst surged from 15.9% to 63.8% under these severe conditions. Notably, crystalline coke deposits were observed on the catalyst, indicating strongly chemisorbed carbon species. Such coke is not easily removed by mild regeneration, leading to permanent loss of activity [185]. Coke deactivation manifests as declining yields and selectivity over time-on-stream. In one case, a Ni–Fe/Al₂O₃ catalyst for plasma tar reforming maintained stable toluene conversion for ~7 h but showed slight decay thereafter due to coke build-up [186] (Figure 14).

Beyond coke, poisoning by inorganic impurities is another major deactivation of catalysts during the biomass conversion. Alkali metals like potassium, sodium, and calcium, abundantly present in lignocellulosic ash, can volatilize during thermochemical conversion and deposit on catalysts. Potassium, in particular, has been identified as a potent poison, as a few hundred ppm of K on a Pt/TiO₂ catalyst can neutralize the strong acid sites, sharply reducing activity for dehydration reactions. At higher loadings (> 800 ppm K), potassium even migrates to metal–support interfaces, poisoning sites for hydrogenation and HDO [187]. Such metal contaminants often cause irreversible deactivation.

As an example, in a 300 h co-processing test of bio-oil in a refinery hydrotreater, the accumulation of inorganic residues (e.g., metals, sulfur) on the NiMo/Al₂O₃ catalyst has been observed, which could not be fully removed by re-sulfiding, resulting in permanent activity loss [188]. This long-term trial highlights that, unlike in a lab-scale trial, at the industrial level, the catalyst must endure hundreds of hours, and even trace contaminants can gradually foul the catalyst beyond recovery.

Catalysts in hydrothermal environments suffer additional stability issues. The high water content of biomass streams (or produced during HDO) induces the dealumination and structural collapse of acid zeolites [189]. For example, HZSM-5 zeolites used in CFP undergo significant performance loss due to steam-induced structural damage.

Mitigation strategies like hydrothermally stable formulations (e.g., phosphorous- or metal-modified zeolites) are being explored to preserve acidity in the presence of water. Similarly, the sintering of metal nanoparticles can occur under high-temperature biomass conversion conditions, especially if regenerative oxidation steps are used to burn off coke. Repeated oxidation-reduction cycles (during burn-off and reactivation) can agglomerate metal sites or collapse the support surface area. Importantly, many deactivation mechanisms are intertwined. Coke formation often accelerates when acid sites are poisoned or when metal sites lose activity, since the incomplete conversion of reactive intermediates yields more char. Conversely, metal poisoning by sulfur or K can promote coke by diminishing hydrogenation activity that would otherwise saturate coke precursors. These synergies complicate the interpretation of deactivation studies and highlight a gap: most studies examine one mechanism in isolation (e.g., coke in pyrolysis, or K poisoning in model compounds).

Integrated approaches are needed to understand how hydrothermal aging, poisoning, and coking jointly impact catalysts under realistic biomass feeds. Despite these challenges, there is progress in deactivation mitigation. Regeneration techniques (oxidative coke burn-off, solvent washing) can partially restore activity. Notably, the K-poisoned Pt/TiO₂ catalyst mentioned above [183] could be fully revived by a simple water leaching, which removed K and restored both acid and HDO active sites. This finding suggests some poisoning is reversible and points to the potential of cyclic operation (reaction−wash−reaction) to extend catalyst lifetimes. Additionally, new catalyst designs (e.g., hierarchical pore structures to reduce coke residence time, or sacrificial alkali traps in reactors) are being developed. Nonetheless, achieving multi-thousand-hour catalyst lifetimes for biomass feeds, comparable to those in petroleum refining, remains an unresolved gap. Future studies must systematically probe the deactivation kinetics under continuous operation and explore robust catalysts or process strategies (e.g., continuous catalyst regeneration units) to overcome the steep deactivation rates observed in current systems [183].

8. Economic Feasibility and Process Intensification

Economic viability is a critical hurdle for catalytic biomass conversions, which currently struggle to compete with fossil fuels on cost. Techno-economic analyses (TEAs) reveal that many bio-conversion pathways have high capital and operating costs at pilot scale, translating to fuel selling prices above fossil benchmarks [190]. Comparative studies have also been performed to evaluate the impacts of the first generation in respect to the third generation bio-fuels, showing that micro-algae are neither competitive yet with traditional oil crops nor with fossil fuel [191]. Key cost drivers include feedstock logistics, extensive downstream upgrading, hydrogen supply, and catalyst replacement due to short lifetimes. Process intensification strategies aim to address these challenges by simplifying process trains, improving carbon efficiency, and leveraging scale. A notable success is the consolidation of reaction steps. Traditional biofuel production often involves multiple discrete steps (e.g., separate dehydration, oligomerization, and hydrogenation reactors to convert bio-ethanol to hydrocarbons). Recent work by Foust et al. demonstrated a single-step route by combining alcohol dehydration and oligomerization into one catalytic reactor (the so-called CADO process) [192]. By using a metal-doped zeolite that can both dehydrate ethanol and couple the resulting olefins, they achieved direct conversion of 40 wt% wet ethanol (aqueous azeotrope) to hydrocarbons in one reactor. This intensification led to remarkable improvements: liquid hydrocarbon yield increased from ~ 36% of the theoretical carbon maximum to over 80%. In parallel, catalyst cost was slashed by an order of magnitude (through improved formulation and longer life), and the process scale was ramped up 300-fold (from lab to pilot), reducing the projected conversion cost per unit ethanol by 12×. This example highlights how integrated catalyst/process development can drastically improve economic metrics. By avoiding intermediate separation and reheating steps, consolidated reactors save energy and capital, contributing to an estimated conversion cost of only ~USD 2.00/GJ (with future potential ~USD 1.44/GJ) for turning wet ethanol into drop-in fuels. Such costs are on par with conventional refining on an energy basis.

Another intensification approach is process integration and heat/mass integration. For instance, coupling exothermic and endothermic reactions can internally balance energy. An illustrative case is the catalytic reactive distillation for biofuel production. A recent design for BJF oligomerization incorporated reaction and product separation in a single column, so the heat from condensing products drives the reaction reflux. This reactive distillation scheme yielded ~20% lower total annualized cost compared to a conventional multi-unit process, while also cutting environmental impact by ~18–50% [193]. The intensified system eliminates the need for a separate distillation train and leverages in situ separation to shift equilibria, enhancing overall conversion. Likewise, integrating vapor-phase upgrading units directly downstream of biomass pyrolysis (to refine pyrolysis vapors before they condense) is being pursued to intensify processing. Such “in-line” upgrading can reduce the need for the costly reheating and re-vaporization of bio-oil, improving energy efficiency. Economically, scale-up itself represents a challenge because biomass plants face diseconomies of scale due to the feedstock transport limit. This has increased interest in modular and distributed processing with smaller reactors deployed near biomass sources to avoid hauling bulky feedstock. Process intensification can drastically swing the economics, but so can external market factors (credits, feedstock costs). One gap in current research is understanding how these techno-economic gains hold under real-world variability—e.g., feedstock composition changes, or catalyst lifespan shorter than assumed in TEA.

9. Conclusions

Catalytic biomass conversion has made remarkable strides in the past few years, transitioning from fundamental research to pilot-scale demonstrations of fuel and chemical production. In this review, critical aspects have been discussed, from catalyst design and reaction mechanisms to process integration, sustainability, and future outlooks. Biomass is a challenging feed, and conventional catalysts often face rapid deactivation or poor selectivity. Advances in catalyst synthesis, such as robust acid resistant to water, metals resistant to poisoning, and novel bifunctional catalysts, have enabled higher yields and selectivity (fuels, olefins, aromatics) while coping with biomass’s high oxygen content. Continued efforts to reduce their deactivation (coke deposition, sintering, poisoning) and developing mitigation strategies (better regeneration protocols, sacrificial trap materials, more resilient catalyst structures) will be crucial for commercial viability. Gaps remain in achieving the longevity seen in petroleum-refining catalysts, pointing to an ongoing need for research in this area.

The conversion of biomass into fuels and materials via catalysis is steadily evolving from a promising concept to a practical reality. Significant challenges remain, such as the scalability and decrease in costs. There are also systemic challenges, such as feedstock logistics and the competition with other biomass uses. Nonetheless, the progress obtained so far has been impressive: several pathways (like CFP with upgrading, ethanol-to-jet, and HTL with catalytic hydrotreatment) have reached a stage where they can produce kg to tons of product for testing in real engines and markets. Each of these successes builds confidence and technical knowledge.

Moreover, the need to develop new technologies in line with societal needs, pushed by the increasing climate emergency, offers an unprecedented opportunity for biomass conversions. As the world seeks sustainable alternatives to petrochemicals, catalytic processes that valorize renewable carbon will be indispensable. The research community should thus continue to develop innovative catalysts and processes, while also actively engaging in pilot projects and policy dialogs. By doing so, they will ensure that the scientific advances translate into tangible sustainability benefits. The vision of a circular, bio-based economy, where waste biomass is efficiently converted into fuels, materials and energy, is nearing realization. Achieving it will require the concerted efforts of chemists, engineers, policymakers, and industry. Through interdisciplinary solution-focused research, the way toward fully sustainable and scalable biomass conversion technologies is being explored.