Influence of Mineral Acid, Hydrotropic Organic Acid, and ChCl–pTSA Binary Catalyst on Organosolv Fractionation of Pigeon Pea (Cajanus cajan) Stalk
Abstract
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.