Green synthesis of carbon solid acid catalysts using methane sulfonic acid and its application in the conversion of cellulose to platform chemicals

Levulinic acid (LA) is an important platform chemical and used for the production of various biofuels and bio-based chemicals. Formic acid (FA) is a major product of biomass conversion and sustainable biomass-derived energy source for hydrogen production. Due to the growing demand for the use of renewable sources, cellulose-containing plant wastes/lignocelluloses can act as substrates for LA and FA production. Among various cellulose conversion strategies, solid acid appears to be a promising alternative for mineral acids recently. Hitherto only harsh conditions and strong mineral acids have been used for carbon solid acid synthesis. Therefore, recyclable non-corrosive catalysts with suitable thermal and chemical stability for cellulose hydrolysis are in great demand. Methane sulfonic acid (MSA) is a green acid since it is non-oxidizing, readily biodegradable, and less toxic than mineral acids. In this work, we have explored the use of this milder acid-based carbon solid acid catalyst synthesized from inedible mahua (Madhuca longifolia) oil cake through one-step hydrothermal synthesis and examined its efficiency for LA and FA production from cellulose. The catalyst was characterized using Raman spectroscopy, FTIR, BET, XPS, TGA, and XRD analysis. A drastic change in catalyst surface area from < 1 to 319 m²/g and acid site density (0–5.77 mmol/g) was achieved by increasing synthesis temperature above 240 °C which reflected in increased cellulose conversion to LA and FA. This indicated a need for significant change in synthesis temperature while using milder acids than the conventionally used temperature of 180 °C for strong acids. The increase in synthesis time to 24 h resulted in a catalyst with a good surface area of 367 m²/g. The use of high acid concentration for catalyst synthesis (1:4, oil cake: acid ratio) destroyed porous structures leading to reduced surface area (277 m²/g) and pore volume but increased the amount of catalyst acid sites. Interestingly, the lowering of LA and FA yield for this catalyst signified the importance of surface area and acid site concentration in determining the catalyst efficiency. The catalyst achieved efficient conversion of cellulose with an LA and FA yield of 38 and 65%, respectively.