Role of Alcohol Solvents in Glucose Isomerization and Etherification in Modulated UiO-66

Abstract

Solvent choice significantly affects the rates of glucose isomerization and its selectivity to fructose. Glucose isomerization was examined over modulated UiO-66(Zr) in methanol and 1-propanol solvents. Reactions in 1-propanol favor glucose isomerization to fructose, whereas reactions in methanol favor glucose etherification side reactions to glucosides, which reduces fructose selectivity. The fructose that forms subsequently reacts with both propanol and methanol solvents to form fructosides. Density functional theory calculations examine the key reactions and the important role of solvation in affecting fructose selectivity. The higher hydrogen-bonding acidity provided by the methanol solvation environment, as compared to 1-propanol, stabilizes transition states involving the alkoxide attack on glucose in the etherification pathway. Methanol molecules form a more intimate hydrogen-bonding network thus stabilizing anionic intermediates during reaction. As such, methanol, which is more acidic than propanol, enhances alkoxide-mediated etherification relative to Lewis-acid-catalyzed isomerization. Experiments carried out in mixtures of these alcohols show that methanol-rich environments favor glucose etherification (over isomerization), in contrast to 1-propanol-rich environments favor isomerization. Notably, the calculated free energy differences between isomerization and etherification (+1.0 kcal/mol in methanol and –3.9 kcal/mol in 1-propanol) are consistent with those from kinetic experiments (0.3 kcal/mol in methanol and −2.2 kcal/mol in propanol).