Mechanism study on the formation of furfural during zinc chloride-catalyzed pyrolysis of xylose

Abstract

Furfural (FF) is a promising platform compound that can be selectively produced by the fast pyrolysis of xylan-based biomass in the presence of zinc chloride (ZnCl2). Whereas, the catalytic effect of ZnCl2 and the FF formation mechanism remain uncovered. The present work aims to give a deep insight into the formation mechanism of FF under the catalysis of ZnCl2 by combining quantum chemistry calculations and fast pyrolysis experiments. Density functional theory (DFT) calculations and fast pyrolysis experiments were conducted with xylose as the model compound. ZnCl2 affects the pyrolysis reactions through the binding of Zn atom and O atom of xylose initially, with manifest electron exchange between Zn and O. The activation free energies of the pyrolysis reactions can be decreased when ZnCl2 stays at the reaction center of the transition states. Both ring-opening and 1,2-dehydration can be promoted by ZnCl2, whereas ring-contraction is hindered by ZnCl2 because ZnCl2 is away from the reaction center of the ring-contraction. Due to the distinct influence of ZnCl2 on different reactions, the rate-determining steps in some of the possible FF formation pathways are changed. Hence, the competitiveness of these pathways is altered to a great degree, which should be responsible for the selective production of FF. Xylose tends to generate FF through the acyclic d-xylose decomposition channel due to the catalysis of ZnCl2. The most favorable FF generation pathway involves the successive cyclization, 1,2-dehydration between 2-OH and 1-H, electrocyclic dehydration, and 1,2-dehydration between 4-OH and 5-H following the formation of a d-xylulose intermediate.