Surface coordination enhanced visible-light photocatalytic coupling of benzylamine to N-benzylidene benzylamine over the Pd/NH2-MIL-125(Ti) nanosheets

The two-dimensional (2D) Metal Organic Framework (MOF) of NH2-MIL-125(Ti) nanosheets with oxygen vacancies were prepared to construct a multifunctional photocatalyst via surface modification of Pd nanoparticles (Pd/MTNs). The obtained photocatalyst showed the highly efficiency for precisely transformation of benzylamine to N-benzylidene benzylamine under visible light irradiation and 1 atm air pressure at room temperature. With the assistance of theoretical mathematical model, the MTNs loaded 0.5% wt Pd completed the transformation for benzylamine (>99%) precisely to N-Benzylidenebenzylamine (99%). However, the Bulk loaded 0.5% wt Pd exhibits decreased conversion (80.5%) and selectivity (85.9%). AFM, XPS and UV–vis DRS experiments indicated that the open 2D structure of MTNs would contribute to the generation of surface oxygen vacancies (OV) sites and abundant Ti metal sites with underfilling electrons 3d orbit, enhancing the light absorption. In-situ FTIR revealed that these exposed Ti atoms would act as the discriminating sites to specifically adsorb the benzylamine molecules forming chemical coordination bonds -C-N⋯Ti- on interface, polarizing and activating -C-N-bonds in benzylamine. Additionally, series of in-situ EPR and experimental results elucidated that the surface OV sites would act as the functional sites to capture the O2 molecules from air. Furthermore, surface Pd sites would as an active site to accelerate the photo-generated electrons transfer from inside to interface, inducing the activation of adsorbed O2 molecules to O2–. Based on these cooperating function sites in MTNs, a possible catalytic mechanism was discussed in a molecular scale. This work highlights the rational construction of a multifunctional photocatalyst and the synergistic effects of the surface coordination and photocatalysis.