Selective Hydrogenation of Biomass-Based 5-Hydroxymethylfurfural over Catalyst of Palladium Immobilized on Amine-Functionalized Metal–Organic Frameworks

A catalyst of palladium [Pd/MIL-101(Al)-NH2] supported on amine-functionalized Metal–Organic Frameworks (MOFs) allows selective hydrogenation of biomass-based 5-hydroxymethylfurfural (HMF) to 2,5-dihydroxymethyl-tetrahydrofuran (DHMTHF) with 2,5-dihydroxymethylfuran (DHMF) as an observed "intermediate". The Pd/MIL-101(Al)-NH2 was prepared by using a direct anionic exchange approach and subsequent gentle reduction. The presence of free amine moieties in the frameworks of MIL-101(Al)-NH2 is suggested to play a key role on the formation of uniform and well-dispersed palladium nanoparticles on the support. The adsorption experiments reveal that the amine-functionalized MOF supports show preferential adsorption to hydrogenation intermediate DHMF than in the case of reactant HMF owing to an enhanced hydrophilic nature of DHMF as well as improved hydrogen bonding interactions between DHMF and the MOF support, which promotes a further hydrogenation of DHMF to DHMTHF upon the in situ formation of DHMF over Pd/MIL-101(Al)-NH2. Moreover, our results also indicate that the observed high selectivity toward DHMTHF form HMF is closely related to the cooperation between the metallic site and the free amine moiety on the MOF support. Under the optimal conditions, a maximum DHMTHF yield of 96% with a full conversion of HMF is obtained by using Pd/MIL-101(Al)-NH2 (Pd 3.0 wt %) catalyst at a low reaction temperature of 30 °C in aqueous medium. The research thus highlights new perspectives for aluminum-based and amine-functionalized MOF material for biomass transformation.