Utilizing Imogolite Nanotubes as a Tunable Catalytic Material for the Selective Isomerization of Glucose to Fructose

Abstract

The isomerization of glucose to fructose is an important step in the conversion of biomass to valuable fuels and chemicals. A key challenge for the isomerization reaction is achieving high selectivity towards fructose using recyclable and inexpensive catalysts. Imogolite is a single-walled aluminosilicate nanotube characterized by surface areas of 200-400 m2/g and pore widths near 1 nm. In this study, imogolite nanotubes are used as a heterogeneous catalyst for the isomerization of glucose to fructose. Catalytic testing demonstrates the catalytic activity of imogolite for the isomerization of glucose to fructose. Imogolite is a highly tunable structure and can be modified through substitution of Si with Ge or through functionalization of methyl groups to the inner surface. These modifications change the surface properties of the nanotubes and enable tuning of the catalytic performance. Aluminosilicate imogolite is the most active material for the conversion of glucose. Conversion of glucose of 30% and selectivity for fructose of 45% is achieved using aluminosilicate imogolite. Modification of imogolite with germanium or methyl groups decreases the conversion, but increases the selectivity. Generally, the selectivity for fructose decreases as the conversion of glucose increases. Interestingly, the imogolite nanotubes have comparable catalytic selectivity at similar conversion as base catalyzed reactions. Catalyst recycling experiments revealed that organic content accumulates on the nanotubes that results in a minor reduction in conversion while maintaining similar catalytic selectivity. Overall, imogolite nanotubes demonstrate an active and tunable catalytic platform for the isomerization of glucose to fructose.