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

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 (ZnCl 2 ). Whereas, the catalytic effect of ZnCl 2 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 ZnCl 2 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. ZnCl 2 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 ZnCl 2 stays at the reaction center of the transition states. Both ring-opening and 1,2-dehydration can be promoted by ZnCl 2 , whereas ring-contraction is hindered by ZnCl 2 because ZnCl 2 is away from the reaction center of the ring-contraction. Due to the distinct influence of ZnCl 2 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 ZnCl 2 . 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.