Density functional theory study of acid-catalyzed conversion of glucose to hydrochar precursors under hydrothermal conditions

Hydrothermal carbonization is a thermochemical technology to convert sludge into hydrochar through the use of Lewis and Brønsted acids. However, the mechanisms by which Lewis and Brønsted acids catalyze the conversion of small-molecule carbohydrates into hydrochar remain unclear. In this study, we used glucose as a small-molecule modulo, and Lewis (CuCl2) and Brønsted (NH4+) acids as catalysts to investigate the reaction mechanisms in the sludge hydrothermal process using density functional theory. Glucose was isomerized to fructose catalyzed by CuCl2 and Cl−, followed by three consecutive dehydrations of fructose to produce 5-hydroxymethylfurfural catalyzed by NH4+. 5-Hydroxymethylfurfural proceeded through multiple NH4+-catalyzed hydration and dehydration reactions to produce small organic molecules, polymerization of which generated hydrochar precursor polymers. The addition of CuCl2 and NH4+ reduced reaction energy barriers during this process. The NH4+-catalyzed polymerization reaction energy barrier between two 5-hydroxymethylfurfural was decreased by 28.0 kcal/mol, whereas that between 5-hydroxymethylfurfural and 1,2,4-benzenetriol was decreased by only 0.72 kcal/mol. Compared to the noncatalytic process, the intermolecular hydrogen transfer in the NH4+-catalyzed conversion of 5- hydroxymethylfurfural to levulinic acid was the best catalytic effect throughout the catalytic process. In this study, using quantum chemical theory, we investigated the conversion mechanism of the CuCl2-and NH4+-catalyzed hydrothermal formation of hydrochar precursor polymers from glucose at the atomic level, thus contributing to the design of catalysts for sludge hydrothermal carbonization and providing theoretical support for this technology.