Highly active niobium-loaded montmorillonite catalysts for the production of 5-hydroxymethylfurfural from glucose†
Bio-based platform chemical 5-hydroxymethylfurfural (5-HMF) can be converted into large-scale, high value-added compounds generally obtained from petroleum. Hence, developing an efficient catalytic system to transform glucose (a cheap and abundant branch of biomass) into 5-HMF has drawn significant attention. Earlier reports have concentrated on a two-step conversion of glucose to 5-HMF involving glucose isomerization into fructose and subsequent dehydration. In this work, efficient heterogeneous niobium-loaded montmorillonite (Nb-MMT) catalysts containing Lewis and Brønsted acid sites have been synthesized by a readily cation-exchanged synthetic method and were used to convert glucose and showed high selectivity and conversion. An optimum 5-HMF yield of 70.52% with a 99% conversion of glucose was achieved in a biphasic solvent of methyl isobutyl ketone (MIBK)/water at 170 °C for 3 h. The catalytic system also exhibited an excellent activity in the conversion of disaccharides and polysaccharides to 5-HMF with satisfactory yields. In addition, Nb-MMT was effectively reused four times without significant loss of activity. Nb-MMT also featured good catalytic activity and selectivity towards carbohydrate conversion in water. Furthermore, the reaction mechanism for the transformation of glucose to 5-HMF over Nb-MMT was investigated by density functional theory (DFT) calculations, and the results showed that niobium oxyhydroxide species of Nb-MMT played an essential role in glucose conversion, and the catalytic mechanism was a synergetic proton transfer process via hydrogen bonding. This study paves the way for improvements in Nb-containing solid acid catalysts for conversion of carbohydrates.