Valorization of cellulosic food waste into levulinic acid catalyzed by heterogeneous Brønsted acids: Temperature and solvent effects

Abstract

This study presents the catalytic valorization of vegetable waste into levulinic acid (LA) over a solid Brønsted acid, Amberlyst 36, in aqueous solution with/without polar aprotic solvent (dimethyl sulfoxide, DMSO). With the aid of microwave heating, the cellulosic structure in the vegetable started hydrolysis at 120°C. Increasing temperature to 135–150°C accelerated the yield of LA to 13–17Cmol% in 5min in aqueous solution due to the reduction of mass transfer limitations. At the same time, the generation of insoluble humins became more rapid and significant, leading to lesser amount of total soluble products. In comparison, in the DMSO-water mixture, the early-stage reaction produced up to 17Cmol% of hydroxymethylfurfural (HMF), which was gradually rehydrated to LA with increasing reaction time. This probably illustrated the effects of DMSO, i.e., increased accessibility of glucose to protons for rapid HMF formation and preferential solvation of HMF to dampen further rehydration. The maximum yield of soluble products in DMSO-water mixture at 120–150°C was higher than that in aqueous solution, which was attributed to the enhanced dissolution of cellulose in DMSO as supported by the SEM images. The XRD patterns of the reacted solids demonstrated more intensive peaks at the early stage of the reaction followed by broadening peaks at the later stage, indicating the hydrolysis of amorphous cellulose and formation of insoluble solids on the surface. Therefore, water serves as a green solvent for LA production while DMSO-water mixture is needed for HMF production from vegetable waste.