Niobium-Enhanced Kinetics of Tantalum Phosphate in Catalytic Glucose Dehydration to 5-Hydroxymethylfurfural

The application of water-tolerant bifunctional solid acids with high kinetic performance in converting glucose to 5-hydroxymethylfurfural (HMF) presents a number of challenges. In this study, the effect of doping a hierarchically porous tantalum phosphate monolith with transition metal ions (Nb5+, V5+, Zr4+, Ni2+, and Zn2+) was explored in delivering superior glucose dehydration kinetics and stability. Doping with Nb5+ (x%Nb-TaP) resulted in the best catalytic performance with enhanced tantalum phosphate stability. The incorporation of Nb5+ ions inhibited tantalum phosphate crystallization, increased the density of Lewis acid and Brønsted acid sites and average mesopore size, with consequently enhanced kinetics, enabling the reaction kinetics of fructose to approach a steady state. Application of a 25% mol/mol Nb (25%Nb-TaP) activated at 600 °C to convert 1.0 wt.% glucose in a water/methyl isobutyl ketone (MIBK) biphasic system delivered an HMF yield and selectivity of 72.6% and 95.6%, respectively. Moreover, an HMF productivity of 0.11 mol·h−1·kg-solution−1 was achieved by treating a 15.0 wt.% glucose feed at 170 °C in a water/MIBK biphasic system at a catalyst loading of 10.0 wt.%. The 25%Nb-TaP catalyst largely retained its initial activity after three recycles in the water/MIBK biphasic system, generating an HMF yield of 54.1% and selectivity of 87.0%. The results of this study demonstrate the significant potential of Nb-TaP for industrial-scale HMF production.