Synergistic and autocatalytic proton shuttle with a bifunctional solid acid in low-toxic molten salt hydrate for efficient levulinic acid production from cellulose

Abstract

The efficient conversion of cellulose into levulinic acid (LA) is a challenge due to the recalcitrant crystalline structure of cellulose and the facile formation of humin during the conversion process. Herein, we report a highly efficient and green catalytic system, which is composed of LiCl·3H₂O (molten salt hydrate, MSH) and a bifunctional NbP/HZSM-5 catalyst, achieving a LA yield of 85.8% from microcrystalline cellulose (MCC) under mild conditions. Mechanistic investigation indicates that MSH not only helps disrupt the cellulose crystallinity but also serves as a co-catalyst to promote catalytic activity of NbP/HZSM-5. Their synergistic effect markedly lowers the activation energy of the 5-hydroxymethyl furfural (HMF)-to-LA reaction. Moreover, MSH can activate the produced LA and formic acid (FA) as an autocatalytic proton shuttle to form a self-accelerating cycle, which drastically reduces the dependence on external acid inputs. In addition, the catalytic system demonstrates excellent recyclability, still maintaining an 82.1% LA yield after four cycles. This research not only offers an efficient route for LA production from cellulose but also exemplifies a green chemistry-advancing strategy by integrating benign solvents, recyclable catalysts, and atom-efficient autocatalysis which align with the core principles of green chemistry.