Stannate-catalysed glucose–fructose isomerisation in alcohols

Abstract

Isomerisation of glucose to fructose is a crucial step in the valorisation of biomass-derived carbohydrates to renewable chemicals, polymers, and fuels. Glucose isomerisation is base-catalysed but superior catalytic activity can be obtained with Lewis acidic materials such as Sn-beta zeolite, where partially hydrolysed framework Sn-sites, Sn–(OH)_x, are considered the catalytically active sites. Alkali-metal stannate salts (M_xSnO₃·3H₂O) contain a similar type of Sn–(OH)_x sites in the form of [Sn(OH)₆]²⁻ complex anions. In this work, we report the use of M₂SnO₃·3H₂O (M = Na, K) as effective catalysts for glucose–fructose isomerisation in alcohols. Isotope-labelling experiments and kinetic studies demonstrated that the isomerisation with Na₂SnO₃·3H₂O proceeded in methanol by base-catalysis via a proton-transfer mechanism with a low activation energy (63.5 ± 7.0 kJ mol⁻¹). This allowed the stannate to facilitate a higher fructose yield and a higher fructose selectivity than Sn-beta within a shorter reaction time at a lower reaction temperature. Furthermore, in isopropanol and higher alcohols where the stannates are insoluble the reaction proceeded heterogeneously, thus suggesting the catalytic system to be a green and viable alternative to other soluble catalyst systems.