Ce–Ni mixed oxides with different Ni/Ce molar ratios, promoted by a rather low amount of Pd (0.2 wt%), were prepared by a simple one-pot precipitation method. The thus-synthesized CeNiXOY and Pd–CeNiXOY catalysts were applied respectively to the direct N-alkylation of amine and alcohol via oxidative coupling, under O2 in the absence of basic additives. The CeNiXOY catalyst could provide a >99% yield of imine in the model alkylation reaction of aniline with benzyl alcohol at 100 °C. The Pd-promoted Pd–CeNiXOY catalyst surprisingly showed an enhanced imine yield (>99%) at a mild temperature of 60 °C. This catalyst exhibited good reusability, and moreover, demonstrated high performance towards imine synthesis from various amines with alcohols. The presence of Pd species and the Ni/Ce molar ratio showed a synergistic impact on the conversion of aniline, as well as on the product selectivity, which was believed to be related to the improvement on the intrinsic activity of the oxidation of benzyl alcohol to benzaldehyde. Various physicochemical techniques, including ICP-MS, XRD, N2 adsorption–desorption, UV-Raman, H2-TPR, TEM, EPMA and XPS, were employed to study mainly the composition, structure and redox properties of the catalysts. The proportion of Ce3+ species and oxygen vacancies, which can be manipulated by the Ce–Ni redox system and the interaction between Ce and Ni, was crucial to the selective activation of alcohols in the presence of reactive amines. The activation of alcohol on the highly reactive Pd0 species was the other crucial factor.
The reverse water gas shift (RWGS) reaction is a critical and essential step in the CO2 hydrogenation of iron-based catalysts to produce high-value chemicals such as olefins and higher alcohols. Modification iron-based catalysts by promoter is a popular way in design efficient catalysts which not only can regulate the iron active species but also alter the CO2 adsorption and hydrogenation performance of the catalyst. Herein, a series of Na and Pd promoted 10Fe/SiO2 were prepared. 1Pd/2Na10Fe/SiO2 catalyst exhibited the highest CO2 conversion (13.2 %), the highest CO selectivity (99.7 %) and good catalyst stability. Combination of various characterization results, such as XPS, HRTEM, Mössbauer spectroscopy, CO/CO2-TPD, H2-TPD and in situ DRIFTS, it was found that the addition of Na promoter altered the reaction pathway from redox and associated pathway to only following an associative pathway, contributing to the high CO selectivity. Besides, when the content of Fe2+ species was similar, the associated pathway would be advantageous to high CO formation rate during RWGS reaction over iron-based catalysts. On the other hand, in the same reaction pathway, the content of Fe2+ species would determine the CO formation rate over iron-based catalysts during RWGS reaction.
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