Visible-Light-Induced Catalytic Transfer Hydrogenation of Aromatic Aldehydes by Palladium Immobilized on Amine-Functionalized Iron-Based Metal–Organic Frameworks

Visible-light-induced selective transfer hydrogenation of aromatic aldehyde to the corresponding alcohol was achieved by using Pd nanocatalyst supported on amine-functionalized iron-based metal–organic frameworks [Pd/MIL-101(Fe)-NH2] with triethylamine (TEA) as an electron donor and HCOOH as a proton source. The Pd/MIL-101(Fe)-NH2, obtained by an in situ photodeposition method, showed homogeneously and highly dispersed Pd nanoparticles (NPs) with a uniform size throughout the MIL-101(Fe)-NH2 support due to an effective stabilization role of amine groups on the backbone linkage of MIL-101(Fe)-NH2. The resulting Pd/MIL-101(Fe)-NH2 exhibited excellent catalytic performance toward a visible-light-induced transfer hydrogenation of benzaldehyde by producing a benzyl alcohol yield of 77% with a full benzaldehyde conversion in the presence of TEA-HCOOH. In addition to benzaldehyde, biomass-based renewable platform molecules such as furfural and 5-hydroxymethylfurfural (HMF) were successively converted into the corresponding alcohols with the yields of 29% for furfuryl alcohol and 27% for 2,5-dihydroxymethylfuran (DHMF), respectively, which are the highest yields reported so far by visible-light-induced transfer hydrogenation method. Our experimental investigation reveals that a preliminary photoirradiation promotes in situ photodeposition of Pd salt [MIL-101(Fe)-NH3]+·1/2[PdCl4]2– to form Pd catalyst Pd/MIL-101(Fe)-NH2 in the presence of TEA-HCOOH, and a further photoirradiation successively triggers Pd/MIL-101(Fe)-NH2-promoted transfer hydrogenation of aldehyde, again, with the help of TEA-HCOOH. Our theoretical research based on density functional theory (DFT) further confirms a dual function of amine group in the Pd/MIL-101(Fe)-NH2 for Pd NPs stabilization as well as for enhancement of the electron density of the Pd center upon light adsorption. The photocatalytic system of Pd nanocatalyst and TEA-HCOOH thus demonstrates an environmentally friendly and efficient strategy for aldehyde hydrogenation by using renewable solar energy as a driving force.