Active site modulation in UiO-66(Ce) MOFs by Al3+ doping for boosting photocatalysis

• Al 3+ doping induces crystal structure distortion to form coordinately unsaturated metal (Al, Ce) sites. • Coordinately unsaturated Al sites as stronger Lewis acid sites promote the activation of benzylamine. • The activation degree of N–H bonds in benzylamine was evaluated. • A synergistic photocatalytic mechanism associated with molecular activation was discussed at the molecular level. A series of Al 3+ -doped UiO-66(Ce) MOFs (U(Ce x Al 1−x )) was developed to reveal the roles of metal doping on active site formation and substrate activation in photocatalytic selective transformation of amines to imines. Al 3+ doping induced crystal structure distortion to form coordinately unsaturated metal (Al, Ce) sites as Lewis acid sites to chemisorb and activate benzylamine (BA). In situ FTIR revealed that the Al site with stronger acid strength facilitated BA activation. The activation degree of N–H bonds in BA was evaluated by changes of calculated force constant. The intermediate products were confirmed through time-dependent in situ FTIR. The Al 3+ -doped sample U(Ce 0.90 Al 0.10 ), the optimal catalyst, showed a significantly increased BA conversion (97.6 %) than that (47.5 %) of undoped sample, which was attributed to enough active sites and the optimal charge mobility. Finally, the present study proposes a synergistic photocatalytic mechanism associated with molecular activation to demonstrate selective oxidation pathways at the molecular level.