Transfer hydrogenation of 5-Hydroxymethylfurfural to 2,5-dihydroxymethylfuran by synergistic catalysis with a mixture of ZIF-8 and UiO-66-NH2

The metal-organic framework materials ZIF-8 and UiO-66-NH2, synthesized by hydrothermal methods, were characterized in detail and combined to catalyze the transfer hydrogenation of 5-hydroxymethylfurfural (5-HMF) to 2,5-di(hydroxymethyl)furan (2,5-DHMF) using isopropanol as the hydrogen source. The ZIF-8/UiO-66-NH2 catalyst mixture exhibited abundant Lewis acid-base sites and a hierarchical multiporous structure with a large specific surface area (564.0 m2/g). The conversion of 5-HMF reached 98.2 %, and the selectivity and yield of 2,5-DHMF were 96.2 % and 94.5 %, respectively. The recycling performance of ZIF-8/UiO-66-NH2 was also investigated, and the conversion of 5-HMF and selectivity of 2,5-DHMF significantly decreased to 78.4 % and 79.1 % in the second run, respectively. The loss of −NH2 groups was found to be the main reason for the decrease in activity. However, the catalytic activity of the recovered catalyst can be almost completely restored by the addition of a small amount of fresh UiO-66-NH2 and ZIF-8, and the conversion of 5-HMF and selectivity of 2,5-DHMF were increased to 89.4 % and 95.5 %, respectively. In the proposed reaction mechanism, Lewis acid-base sites play important roles and exhibit a synergistic catalytic effect in the selective conversion of 5-HMF to 2,5-DHMF.