Keggin-type SiW12 encapsulated in MIL-101(Cr) as efficient heterogeneous photocatalysts for nitrogen fixation reaction

The incorporation of polyoxometalates (POMs) in metal-organic frameworks (MOFs) with host-guest structure have proven to be effective strategy to rational design of heterogeneous catalysis. In this study, the Keggin-type POM@MIL-101(Cr) composite catalysts (PMo12, PW12 and SiW12) are synthesized for nitrogen fixation reaction without sacrificial agents at room temperature in the first time. The SiW12 molecules are encapsulated in smaller cavities of MIL-101(Cr) by solvothermal method and in larger cavities by impregnation method, respectively. Solvothermal synthesized catalyst has a performance of 75.56 μmol·h-1·g-1cat and TOF value of 1.95 h-1, which are about 10 and 88 times than that of Na4SiW12O40. The excellent performance is ascribed to the synergistic effect of SiW12 and MIL-101(Cr). The MIL-101(Cr) adsorbs a large amount of N2 and generates sufficiently photogenerated electrons under sunlight irradiation, and electrons quickly transfer to the SiW12 through hydrogen bonds. Moreover, the agglomeration effect of the homogeneous catalyst SiW12 is weakened due to encapsulation with more exposed active sites. This work provides a feasible route to design and synthesize nanocomposite materials with exceptional performance for photocatalytic nitrogen fixation.