A Pathway for Sugar Production from Agricultural Waste Catalyzed by Sulfonated Magnetic Carbon Microspheres

Lignocellulose is an important renewable biomass resource. However, at present, there is a lack of efficient and environmentally friendly catalytic systems that can selectively convert lignocellulose components into high-value sugars, and the value realization of agricultural waste (such as straw) remains challenging. Carbon-based solid acids are used in the valorization of biomass due to their simple preparation and excellent catalytic performance. In this study, the magnetic carbon microspheres catalyst was prepared using concentrated sulfuric acid and hydroxyethyl sulfonic acid as sulfonating agents. Two sulfonation catalysts were applied to the hydrolysis of typical agricultural waste (rice straw). The performance of catalyst conversion to reducing sugar was compared, and the glucose yield was lower than 30%. The sulfonation catalyst of hydroxyethyl sulfonic acid obtained a higher yield of pentose (76.67%) than that of concentrated sulfuric acid (74.25%) in 110 min. The optimal reaction conditions were found: substrate was 0.04 g straw, catalyst was 0.04 g, H₂O/γ-valerolactone ratio was 8:2 in the solvent, and the reaction time was 110 min at 140 °C. Under these conditions, the sulfonation properties of hydroxyethyl sulfonic acid as a green sulfonating agent are similar to those of concentrated sulfuric acid. Its excellent catalytic performance is attributed to the medium B/L acid density ratio on the catalyst surface. In addition, the prepared catalyst can be effectively separated from the reaction residue in the catalytic system. This work provides a green catalytic system for the high-value utilization of agricultural waste from renewable carbon sources.