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

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, N-2 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/H-2 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/H-2 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 H-2 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 H-2 to form N-formamide. Subsequently, N-formamide reacts with activated H-2 to yield N-methylamine.