The mechanism of glucose conversion to 5-hydroxymethylfurfural catalyzed by metal chlorides in ionic liquid: A theoretical study

The complete catalytic cycle of the reaction of glucose conversion to 5-hydroxymethylfurfural (HMF) by metal chlorides (MCl3) in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) ionic liquid has been studied using density functional theory (DFT) calculations. Insights into the most preferred mechanistic pathways were gained for both isomerization of glucopyranose to fructofuranose as well as subsequent dehydrations of fructofuranose to the final product HMF, which were considered as two main reactions in the whole process. The first part of the mechanism was predicted to proceed slowly and thermodynamically less favored. A five-membered-ring chelate complex of the metal atom with glucopyranose was assumed as a key intermediate. The second part consists of sequential releases of three water molecules from fructofuranose. The removal of the first water appears to be rate controlling, whereas further loss of the second and third water were highly exothermic. A variety of transition metal cations in the same oxidation states (WCl3, MoCl3, and FeCl3) were screened and parallel DFT studies were carried out to determine their reactivities in the catalytic reaction. It turns out that the metal centers exerted significant influences on the stabilities of the intermediates as well as the energy barriers associated with each elementary reaction step. The overall free energy barriers at 353 K indicated that the reaction activities of the entire processes over different MCl3 active sites decrease in the order of WCl3 > MoCl3 > CrCl3 > FeCl3, in which WCl3 may be the most promising catalyst at low temperatures.