Functionalized UiO-66(Ce) for photocatalytic organic transformation: the role of active sites modulated by ligand functionalization

Abstract

Ligand functionalization has been considered as an efficient strategy to modulate the properties of metal–organic frameworks (MOFs). In this work, a series of functionalized UiO-66(Ce)-X (X = H, CH₃, Br, NO₂) have been successfully synthesized for the photocatalytic oxidation of benzylamine under visible light. The UiO-66(Ce)-CH₃ exhibited the highest conversion among the samples. The quantity and the acid strength of coordinatively unsaturated Ce sites as Lewis acid sites are modulated by ligand functionalization, both following the order of UiO-66(Ce)-CH₃ > UiO-66(Ce)-H > UiO-66(Ce)-Br > UiO-66(Ce)-NO₂, which is closely related to photocatalytic performance. Based on in situ FTIR results, coordinatively unsaturated Ce sites can facilitate the activation of benzylamine molecules via the surface –Ce⋯N– coordination species, and the activated degree of the N–H bonds is evaluated and compared via the force constant. In the photocatalytic process, photogenerated holes achieve the deprotonation of activated benzylamine molecules to form benzylamine cations, and photogenerated electrons captured by oxygen vacancies simultaneously reduce activated O₂ to form ˙O₂⁻, effectively achieving this selective photooxidation. Finally, a synergetic photocatalytic mechanism associated with the coordination activation was proposed to explain this reaction process.