Experimental study combined with density functional theory and molecular dynamics simulation on the mechanism of glucose alcoholysis reaction

The reaction characteristics and mechanism of carbohydrates is important in exploring and developing suitable technology for biofuels and biochemical production from lignocellulosic biomass. In this study, experiments combined with density functional theory (DFT) calculation and molecular dynamics (MD) simulation were used to investigate the glucose alcoholysis reaction mechanism catalyzed by extremely low concentrated sulfuric acid. The effects of temperature and tetrahydrofuran (THF) on the distribution of the product in the alcoholysis process of high-concentration glucose were investigated. DFT calculation results showed that the addition of THF in ethanol can decrease the energy barrier of glucose degradation. Meanwhile, the MD simulation indicated that the catalyst tends to localize more around the glucose molecular, which was beneficial to the glucose degradation in ethanol. Combining the experimental and theoretical results, plausible glucose alcoholysis reaction pathways were proposed in this work. It was found that when temperature was lower than 190°C, the three main pathways were glucose conversion into ethyl glucoside (EDGP); glucose conversion into 1,6-anhydro-β-d-glucopyranose (LG), then further to levoglucosone (LGO), furans, or 1,4:3,6-dianhydro-α-d-glucopyranose (DGP); and glucose conversion to fructose. However, at higher temperatures (190 and 220°C), the main reaction pathways were, glucose conversion into LG, LGO, and ethyl levulinate (EL); glucose conversion into fructose, 5-hydroxymethylfurfural (HMF), 5-ethoxymethylfurfural (EMF), and EL; and glucose conversion into humins. DFT, MD simulation, and experimental results showed that the addition of THF is beneficial for the EDGP formation from high concentration glucose catalyzed by Brønsted acid.