Water-catalyzed conversion of glucose to small molecules during hydrothermal carbonization: a density functional theory study

Abstract

Hydrothermal carbonization (HTC) holds great promise for the conversion of biomass to biochar and proceeds via the decomposition of soluble carbohydrates and carbohydrate hydrolysates (e.g., glucose) into small-molecule intermediates, which are then polymerized to afford hydrochar precursors. Given that HTC involves multiple hydrogen transfer processes, it should be promoted by proton transfer catalysts. Herein, we use density functional theory (DFT) calculations to examine the ability of water to act as such a catalyst, aiming to probe the mechanism of HTC-induced glucose conversion into fructose, levulinic acid (LA), formic acid (FA), 1,2,4-benzenetriol (BTO), and 2,5-dioxo-6-hexanal (DHH) in the presence of explicit water molecules. According to our results, glucose is sequentially converted to fructose, (5-hydroxymethyl)furfural, and small-molecule intermediates (FA, LA, DHH, and BTO), and DHH then isomerizes into four alcohols participating in subsequent polymerization. During this multistage process, water molecules exert catalytic effects by assisting hydrogen transfer in isomerization and dehydration (but not hydration) steps and thus significantly reduce the overall reaction energy barrier. In particular, energy barrier reductions of 48.63 and 33.28 kcal mol⁻¹ were observed for the water-catalyzed DHH isomerization and the ring-opening isomerization of glucose into fructose, respectively. Thus, the present work deepens our understanding of processes occurring during the HTC of biomass and paves the way to the more efficient conversion of this resource into value-added chemicals and fuels.