How Topological Differences between Two Oxide Surfaces Determine Selectivity—The Case of the Dehydra-Decyclization of Tetrahydrofuran

Abstract

Production of butadiene from biomass-based tetrahydrofuran (THF) is explored as an alternative to the existing petroleum-based processes. Metal oxide catalysts have been shown to exhibit varying product selectivities when reacted with THF. Among those oxides, ZrO2 showed the highest selectivity for butadiene. In contrast, Al2O3 showed the highest selectivity for the competing retro-Prins products, C3H6 and HCHO. The reasons behind the varying selectivity across oxides are unclear. In this work, we employ periodic density functional theory and mean-field microkinetic modeling to investigate the mechanism of the reaction of THF to butadiene and retro-Prins products on t-ZrO2 (101) (dry and hydrous) and on γ-Al2O3 (110). Our simulations reproduce the experimental selectivity trends. High selectivity for butadiene is promoted by the presence of neighboring Lewis acid metal sites that facilitate E1cB hydroxyl elimination from a 3-butenoxide intermediate; on hydrous Al2O3; where such neighboring Lewis acid centers are not available, the butenoxide undergoes E2 elimination and retro-Prins products ensue. The THF ring opening is rate-determining on ZrO2, whereas the γ-proton elimination that yields the 3-butenoxide intermediate is rate controlling on hydroxylated Al2O3. We conclude that the local topology around the active site greatly influences the mechanism and selectivity.