Preparation of magnetic Au/MIL-101(CO@SiO2@Fe3O4 catalysts and N-methylation reaction mechanism of CO2 with aniline/H-2

MIL-101(CO@SiO2@Fe3O4 magnetic catalytic supports were synthesized by the in-situ growth method, where a MIL-101(Cr) shell was deposited on the surface of the SiO2@Fe3O4 core. A series of Au/MIL-101(CO@SiO2@Fe3O4 magnetic core-shell catalysts with different Au contents were prepared and Au nanoparticles (Au NPs) were decorated on the magnetic supports by the impregnation-reduction method. The structure of the assynthesized catalysts was characterized by XRD, FTIR, HRTEM, XPS, N2 adsorption-desorption, NH3-TPD, CO2-TPD, and VSM, etc. The N-methylation reaction performance and recyclability performance of the catalysts were evaluated in a high-pressure microreactor. The mechanism of the N-methylation reaction had been deeply studied. The results indicated that the thickness of the MIL-101(Cr) shell was about 120 nm, and the Au NPs (similar to 3 nm) were dispersed on the surface and/or in the pores of MIL-101(Cr). The catalytic performance for Nmethylation of CO2 and aniline/H-2 to N-methylaniline and N,N-dimethylaniline was satisfactory. Moreover, the incredible recyclability performance (external magnet) and applicability of different amine substrates were achieved. Under the optimized conditions, the 2%Au/MIL-101(Cr)@SiO2@Fe3O4 catalyst demonstrated the best performance, the aniline conversion nearly 44.0%, the selectivity of N-methylaniline and N,N-dimethylaniline were 71.8% and 27.6%, respectively. The reaction mechanism investigation indicated that, firstly, H-2 was adsorbed and activated on the Au NPs, and CO2 was adsorbed and activated on the O(2)(- )Lewis base sites of MIL101(Cr) cages. Later, the activated H-atoms reacted with the activated CO2 to generate the formic acid as an intermediate product. Finally, the formic acid reacted with the adsorbed aniline and activated on the Cr3+ Lewis acid sites to produce N-methylaniline and N,N-dimethylaniline. The enhanced selectivity of the reaction might be attributed to the core-shell architecture, high surface area, plentiful active sites, and synergistic interactions between Au NPs and acid-base sites of the MIL-101(Cr) catalyst.