A Sustainable Phosphonoacetic Acid–Based Sulfonated Acidic Ionic Liquid Catalyst for Efficient Conversion of Alkaline and Dealkaline Lignin into Phenolics

Abstract

The valorization of lignin into high-value phenolic compounds represents a pivotal step toward sustainable biorefineries, yet its recalcitrant structure poses significant challenges. Accordingly, we hypothesized that a hybrid inorganic and organic acid framework with controlled proton activity could provide tunable acidity. Herein, we report a sustainable, phosphonoacetic acid-based, novel sulfonated Brønsted acidic ionic liquid (IL) [C3SO3HMIM]-HCMP, designed as a dual-acidic catalytic system with a Hammett acidity of 2.09. The catalyst combines strong proton-donating ability with hydrogen bonding, promoting high lignin conversion. Under optimized conditions (120 °C, 1 h, 30 mL H2O/CH3OH (1:5 v/v) solvent system, and 1.5 mmol catalyst loading in a 100 mL batch reactor), the catalyst achieved remarkable lignin conversion of 44–48% for both alkaline (AL) and dealkaline lignin (DAL), yielding 36–40 wt % bio-oil. GC-MS analysis of volatile bio-oil fraction, based on normalized peak area percentages, revealed vanillin as the dominating product (66% for AL, 54% for DAL), followed by vanillic acid (10.7% AL, 6.8% DAL), apocynin (9.3% for AL, 5.4% DAL), and other phenol derivatives (1.4% for AL to 19.54% for DAL), indicating effective cleavages of both β–O–4 and Cα–Cβ linkages. These values represent the relative abundance of compounds detected in GC-MS total ion chromatograms. Comprehensive characterization of lignin before and after reaction via FTIR, 2D HSQC, NMR, XRD, TGA, CHNS, and SEM confirmed successful lignin depolymerization and phenolic compound formation. Crucially, the IL catalyst retained its activity over four consecutive cycles without performance loss. This study demonstrates that rationally designed hybrid acidic ILs offer a green, energy-efficient, and noncorrosive platform for selective lignin valorization, advancing sustainable aromatic chemical production within a circular bioeconomy.