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 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.