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Critical Role of CUS in the Au/MOF-808(Zr) Catalyst for Reaction of CO2 with Amine/H2 via N-Methylation and N-Formylation

Jincheng Mu,Jianfang Liu,Zhenzhen Ran,Muhammad Arif,Min Gao,Chen Wang,Shengfu Ji

MOF-808(Zr) with the coordinated unsaturated (CUS) Zr4+ centers was synthesized by the thermal reflux method, and a series of Au/MOF-808(Zr) catalysts with different Au loading percentages have been prepared by the impregnation–reduction method. The related X-ray diffraction, Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, N2 adsorption–desorption, Inductively coupled plasma-mass spectrometry, and NH3 temperature-programmed desorption characterizations have been undertaken to understand the structure of catalysts. The catalytic activity in CO2 with aniline/H2 to N-methylation and N-formylation has been studied in a batch reactor. Results show that the MOF-808(Zr) structures in catalysts are of high quality and damage-free. The Au nanoparticles (NPs) are highly dispersed on the surface of the MOF-808(Zr) cage with a mean size of 2–20 nm. Also, the catalysts could catalyze the conversion of CO2 with aniline/H2 to N-methylation and N-formylation products with a high efficiency. With a 3.0 wt % Au/MOF-808(Zr) catalyst, the conversion of aniline is 18.4%, and the selectivity of N-methylaniline is 80.9% and that of N,N-dimethylaniline is 19.1%. The preliminary reaction mechanism and the critical role of CUS were also studied. It is determined that H2 is adsorbed and activated on Au NPs. CO2 is adsorbed on the CUS Zr4+ acidic sites and O2– basic sites. The primary and secondary amines are adsorbed on the CUS Zr4+ acidic sites, where they react with CO2 and H2 to form N-formamide. Subsequently, N-formamide reacts with activated H2 to yield N-methylamine.