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

This study investigates the effects of various pretreatment methods on the physico-chemical properties of activated carbons produced from the biochar of an invasive weed species, Prosopis juliflora. Utilizing a thermochemical conversion technique such as pyrolysis, followed by chemical and physical activation processes, we aimed to assess the functional characteristics of the resulting biochar for potential industrial applications. The pretreatments involved acidic (H₂SO₄, H₃PO₄, and ZnCl₂), basic (KOH and NH₄OH) and oxidizing agents (H₂O₂), and physical methods like air activation. Comprehensive characterization techniques, namely BET analysis, x-ray Photoelectron Spectroscopy (XPS), Ultimate Analysis, and pH_pzc, were utilized to assess the influence of activation conditions. Principal component analysis was employed to elucidate the relationships between different activation methods and the physico-chemical properties of the activated carbons. Results from this study revealed that the choice of activation agent significantly influences the material’s structure and chemistry. The activation process of activated carbon (AC) is significantly influenced by temperature and the agent used. KOH-AC exhibited bulk oxygen content (5.96%) with a high bulk carbon content (91.73%), while Air-AC has a comparatively high bulk oxygen content (40.71%). The XPS data shows variations in carbon content and oxygen functional groups, influenced by the activation agent and temperature used during the process. The ranking of surface oxygen content was Air-AC (39.93%) > H₂O₂-AC (22.77%) > NH₄OH-AC (17.61%). This study highlights the potential of Prosopis juliflora-derived biochar in diverse applications by tailoring its physico-chemical properties through specific activation processes, thereby also contributing to the upcycling of this invasive species.

Lignin is one of the most abundant biopolymers in nature. The major challenge in lignin depolymerization lies in the formation of complex product mixtures that require extensive downstream separation to isolate value-added monomers. Selective depolymerization strategies aim to overcome this limitation by promoting controlled bond cleavage while suppressing undesired secondary reactions such as condensation and repolymerization. In this work, a series of rare-earth-free, perovskite-type mixed metal oxides with general compositions ZnxNi1–xTiO3 and CuyNi1–yTiO3 were synthesized and evaluated as heterogeneous catalysts for the base-catalyzed depolymerization of lignin. Among the investigated materials, CuTiO3 exhibited superior catalytic performance, enabling the formation of vanillin as the dominant monomer with high selectivity. The selected catalyst was further characterized using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) surface area analysis to elucidate its surface chemistry, morphology, and textural properties. The combined effects of key reaction parameters, including temperature, pressure, lignin-to-catalyst ratio, NaOH concentration, and reaction time, were systematically investigated using response surface methodology (RSM). Under the optimized conditions (154°C, 0.3 MPa, lignin-to-catalyst ratio of 24.5:1, 10 mL of 0.5 M NaOH, and 12 h reaction time), a monomer yield of 11.5 ± 0.46% with ~81% GC-selectivity toward vanillin was achieved. These findings demonstrate that perovskite-type titanates composed of earth-abundant elements can serve as robust and reusable catalysts for the selective base-catalyzed valorization of lignin into high-value aromatic compounds, offering a sustainable alternative to rare-earth-based systems.

Lignocellulosic biomass recalcitrance remains a major challenge for the development of integrated biorefineries, necessitating efficient and selective pretreatment strategies. In this study, the influence of three acid catalyst systems, namely sulphuric acid (mineral acid), p-toluenesulphonic acid (pTSA; hydrotropic organic acid), and a ChCl:pTSA (1:1 molar ratio) binary catalyst system, prepared as an acidic deep eutectic solvent (DES) precursor and employed at a working concentration of 150 mM, on the organosolv fractionation of pigeon pea (Cajanus cajan) stalk was systematically investigated under identical operating conditions. All catalyst systems exhibited comparable delignification efficiencies (79.33–81.55%), cellulose retention (88.45–98.9%), and xylan removal (>95.51%), indicating similar biomass deconstruction at equivalent nominal acid equivalents. However, notable differences were observed in lignin recovery. The ChCl:pTSA resulted in the highest lignin recovery (65.9%), followed by pTSA (62.41%) and sulphuric acid (51.82%). Structural characterisation of the cellulose-rich and lignin-rich fractions by SEM, FTIR, XRD, ¹H NMR, and TGA confirmed extensive lignin and hemicellulose removal while preserving the cellulose framework. The results indicate that catalyst formulation influences organosolv fractionation beyond nominal proton loading alone. While all catalyst systems effectively fractionated pigeon pea stalk, pTSA provided a competitive catalyst performance, combining high delignification and lignin recovery without the addition of ChCl, although its cellulose retention was lower than that obtained with sulphuric acid. The ChCl–pTSA system produced the highest numerical lignin recovery, although this was not statistically different from pTSA. The lower apparent molecular weight of the ChCl–pTSA-derived lignin further suggests that catalyst formulation may influence the subsequent reaction and recovery pathways of solubilised lignin, although the specific molecular mechanism remains unresolved.

Lignin is one of the most abundant biopolymers in nature. The major challenge in lignin depolymerization lies in the formation of complex mixtures that require extensive downstream separation. Selective depolymerization strategies aim to overcome this limitation by promoting controlled bond cleavage while suppressing undesired secondary reactions. In this work, a series of rare-earth-free, perovskite-type mixed metal oxides with general compositions Zn_xNi_1–xTiO₃ and Cu_yNi_1–yTiO₃ were synthesized and evaluated as heterogeneous catalysts for the base-catalyzed depolymerization of lignin. Among the investigated materials, CuTiO₃ exhibited superior catalytic performance, enabling the formation of vanillin as the dominant monomer with high selectivity. The selected catalyst was further characterized using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) surface area analysis. The combined effects of key reaction parameters, including temperature, pressure, lignin-to-catalyst ratio, NaOH concentration, and reaction time, were systematically investigated using response surface methodology (RSM). Under the optimized conditions (154 °C, 0.3 MPa, lignin-to-catalyst ratio of 24.5:1, 10 mL of 0.5 M NaOH, and 12 h reaction time), a monomer yield of 11.5 ± 0.46% with ~81% GC-selectivity toward vanillin was achieved. These findings demonstrate that perovskite-type titanates can serve as robust and reusable catalysts.