(597g) Alcohol Solvent Effects in Glucose Isomerization in MOF Uio-66

Solvents are often used to aid in the catalytic upgrading of biomass to chemicals and have been shown in various systems to help improve rates and selectivities. Metal-organic frameworks (MOFs) such as the modulated UiO-66 provide larger void spaces than zeolites that allow for the adsorption of reactant as well as solvent molecules that can facilitate the reaction by stabilizing the transition states. In this work, we focus on the transformation of glucose to fructose over modulated UiO-66 in different alcohol solvents – methanol and 1-propanol. First principles density functional theory calculations that utilize a combination of local explicit solvent molecules and an implicit solvation model were carried out to examine the effects of alcohol solvents on the energetics of the elementary steps involved in the mechanisms for glucose isomerization and etherification (side reaction) over the defective UiO-66.

Our theoretical calculations show that while different alcohol solvents have little effect on the barriers for isomerization, there are significant changes in the barriers for etherification. We show that the higher hydrogen bonding acidity (correlating with the propensity of donating hydrogen for hydrogen bond formation) of methanol solvent compared to propanol solvent stabilizes the more basic methoxy species in the etherification transition state thus favoring glucose etherification in methanol, in agreement with the experiments. Additionally, we performed catalytic experiments in mixtures of methanol and 1-propanol solvents with varying compositions that show that methanol-rich solvation environments favor glucose etherification to alkyl glucosides whereas solvent mixtures with higher 1-propanol content significantly favor glucose isomerization to fructose.

Our combined computational and experimental studies identify the role of alcohol solvents and the effect of their acidities on reaction rates and selectivities towards fructose or alkyl glucosides. This presents opportunities for further improving fructose selectivities by tailoring the solvation environments in MOFs based on solvent acidity.