Effective wheat straw pre-treatment and saccharification using p-toluenesulfonic acid (pTSA)-based deep eutectic solvents and microwave heating

Abstract

Lignocellulose is a promising source of biofuels by enzymatic conversion; however, its valorisation is often restricted due to the lignin present. This study investigated a novel and effective microwave-assisted ternary Deep Eutectic Solvent (DES) system composed of p-toluenesulfonic acid (pTSA), glycerol, and choline chloride (ChCl) to enable lignin recovery and increase polysaccharides bioconversion from wheat straw (WS), an industrial waste stream. Specifically, four DESs with varying pTSA content were evaluated. Higher pTSA also significantly improved delignification yield (up to 88.1 %), although at the expense of sacrificing the cellulose and hemicellulose contents. The DES with pTSA: glycerol: ChCl at 1:2:2 M ratio (DES 3) achieved the best balance, retaining 77.8 % cellulose and 49.4 % xylan while removing 78.3 % lignin after microwave heating at 120 °C for 20 min, and enabling the highest glucose (95.6 %) and xylose (52.0 %) yields after enzymatic hydrolysis. Structural analysis (FTIR and XRD) confirmed cellulose integrity and reduced crystallinity after DES pre-treatment, supporting improved enzymatic digestibility. This DES also recovered 51.7 % of total lignin with minimised lignin condensation and maintained ∼68 % of its initial delignification efficiency and >90 % of its initial saccharification performance after two recycling cycles, demonstrating good solvent reusability. Most important, this study elucidated the role of pTSA in determining DES-microwave interaction through dielectric property measurement and linked it with the microwave heating profiles. DES with higher pTSA exhibited a greater difference in delignification efficiency between microwave and conventional heating, due to more pronounced selective heating mechanisms.