Aromatic heterocycle-grafted NH2-MIL-125(Ti) via conjugated linker with enhanced photocatalytic activity for selective oxidation of alcohols under visible light

A novel and facile post-grafting strategy, via a Schiff base chemical reaction between aldehyde and -NH2 groups within NH2-MIL-125(Ti) has been developed to construct aromatic heterocycle-grafted metal organic frameworks (MOFs) photocatalysts. The improved conjugated system in MOFs showed lower band gaps and their excellent catalytic activities were manifested by selective oxidation of alcohols to corresponding aldehydes under visible light irradiation with high conversion and selectivity. Density functional theory (DFT) was applied for simulating the band gaps and electron orbits of heterocycle-grafted MOFs. The post-synthetic aromatic heterocycles in the MOFs network did not influence the original framework, but effectively expanded its pi-delocalized system and promoted the separation and transfer of photo-excited charge carriers. As a result, the aromatic heterocycle-grafted MOFs exhibit significantly enhanced visible-light photo-catalytic activity for selective oxidation of alcohols over pristine NH2-MIL-125(Ti). Especially, the NH2-MIL-125(Ti) grafted by 2-quinolinecarboxaldehyde and 3-thiophenecarboxaldehyde showed better performance (more than 85% conversion of benzyl alcohol) due to the excellent photon adsorption and charge carriers transfer of aromatic heterocycles grafted on MOFs. This general grafting strategy can be utilized to graft large numbers of aromatic rings with different molecular structures onto NH2-MIL-125(Ti).