Cheng Liu,Huiling Chen,Qi Chen,Jinhong Bi,Jimmy C. Yu,Ling Wu
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.
Liu Cheng,Shi Ying-Zhang,Chen Qi,Ye Bing-Hua,Bi Jin-Hong,Yu Jimmy C.,Wu Ling
Defect engineering in metal organic frameworks (MOFs) has captured significant attention in the field of photocatalysis. A series of UiO-66(Ce) (UiO = University of Oslo) MOFs with different contents of missing-linker defects have been developed for the photocatalytic selective oxidation of benzylamine (BA) and thioanisole (TA) under visible light. The introduction of missing-linker defects promotes the formation of unsaturated Ce sites with a high Ce 3+ content. It also generates a high concentration of oxygen vacancies. In situ Fourier transform infrared spectroscopy (FTIR) results revealed that BA and TA molecules were activated on coordinatively unsaturated Ce sites via the H–N···Ce and the C–S···Ce interactions, respectively. Simulated in situ electron paramagnetic resonance (EPR) data indicate that O 2 activation and reduction occur at coordinatively unsaturated Ce 3+ sites to form ·O 2 – . This is accelerated by the Ce 3+ /Ce 4+ redox cycle associated with the photogenerated electrons. The corresponding photogenerated holes are involved in the deprotonation of the activated BA and TA. The most active sample exhibits 98.4% and 95.5% conversion rates for BA and TA oxidation. Mechanisms for the molecular activation are proposed at the molecular level. Graphical abstract
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