Selective N-Methylation of N-Methylaniline with CO2 and H2 over TiO2-Supported PdZn Catalyst

A series of Pd-ZnO/TiO2, Pd/TiO2, and Pd/ZnO catalysts were synthesized and investigated for N-methylation of N-methylaniline (MA) to N,N-dimethylaniline (DMA) with CO2 and H-2. A high performance was observed with a Pd-ZnO/TiO2 catalyst, with 99.9% DMA selectivity at 94% MA conversion. By contrast, both Pd/TiO2 and Pd/ZnO were less active and/or selective. The catalytic performance of Pd-ZnO/TiO2 largely depended on reduction temperature and ZnO loading. The rates for MA conversion (rate(MA)) and DMA production (rate(DMA)) increased linearly with the amount of PdZn alloy formed. The reaction was likely to take place via intermediates of N-methylformanilide (MFA) and formate. Formate was produced through the reduction of CO2 with H-2 as confirmed by in situ diffuse reflectance Fourier transform infrared spectroscopy and then added to MA producing MFA, and finally, MFA was subsequently adsorbed and hydrogenated to DMA. All these steps were promoted by the PdZn alloy. The hydrogenation of MFA to DMA was much faster than the N-methylation of MA to MFA; DMA was stable, so the selectivity to DMA was almost 100% over the Pd-ZnO/TiO2 catalyst.