Effectively enhancing conversion of cellulose to HMF by combining in-situ carbonic acid from CO2 and metal oxides

Abstract

Efficient valorization of cellulose is a sustainable route yet a challenging topic to solve the crisis of fossil resources. Although heterogeneous catalysts such as metal oxides have various advantages, the low surface contact area between metal oxides and cellulose is unfavourable for the hydrolysis of cellulose and thus restricts the application of metal oxides in direct conversion of cellulose to 5-hydroxymethylfurfural (HMF). This work presented the utilization of in-situ carbonic acid from CO2 as a green acid to enhance the conversion of cellulose to HMF in the presence of ZrO2 and TiO2. Combination of in-situ carbonic acid, ZrO2 and TiO2 exhibited a distinctive catalysis on the hydrolysis of cellulose and monosaccharide, isomerization of glucose and monosaccharide conversion in tetrahydrofuran (THF)/H2O/NaCl system. Especially, in-situ carbonic acid played a significant role in cellulose degradation and accelerated transforming insoluble polysaccharide into soluble oligosaccharide or monosaccharide, and thus promoted the cellulose conversion catalyzed by metal oxides. A high yield of HMF from cellulose (48.4%) was achieved by physically mixing commercial TiO2, ZrO2, and CO2. Due to the significant effects on cellulose degradation and HMF selectivity improvement, in-situ carbonic acid offers a green, low-cost and environmentally friendly co-catalyst for highly effective converting cellulose into HMF.