Porous Bimetallic Ti-MOFs for Photocatalytic Oxidation of Amines in Air

A family of microporous titanium-containing metal-organic frameworks (denoted as M₂Ti-CPCDC, M = Mn, Co, Ni) has been synthesized by using a bimetallic [M₂Ti(μ₃-O)(COO)₆] cluster and a tritopic carbazole-based organic ligand H₃CPCDC. M₂Ti-CPCDC are stable and display permanent porosity for N₂ and CO₂ uptake, ranking among the most porous titanium-based metal-organic frameworks. M₂Ti-CPCDC crystals exhibit n-type semiconductor behavior. Further catalytic studies demonstrate that all M₂Ti-CPCDC materials are applicable for triggering photo-oxidative reactions of amines in air. More specifically, amines with electron-donating groups afford the aldehydes as the main products, while amines bearing electron-withdrawing groups give rise to imines as the main product. Among them, Mn₂Ti-CPCDC exhibit the best photocatalytic activity, with conversion of benzylamine up to 99% and selectivity of 99%. Mn₂Ti-CPCDC could be recycled in at least three runs while retaining crystallinity and catalytic activity. The reaction mechanism indicates that photoinduced hole (h⁺), superoxide radical anion (O₂^·-), and singlet oxygen (¹O₂) are the main active species involved in the photo-oxidation process.