New insights into the effects of Mn and Li on the mechanistic pathway for CO hydrogenation on Rh-Mn-Li/SiO2 catalysts

Abstract

It has been reported widely that Rh-Mn-Li/SiO2 catalysts can exhibit high selectivity to C2+ oxygenates during CO hydrogenation, with promoters of Mn and Li playing an important role in this behavior. In this study, a series catalysts of Rh-Mn-Li/SiO2 with different amounts of Mn and Li were prepared, and the new insights into the effects of Mn and Li on the mechanistic pathway for C2+ oxygenates synthesis from syngas were investigated. The XPS analysis showed that Rh existed mainly as metallic Rh after reduction, however partially positively charged Rhδ+ atoms appeared on the surface of Mn-containing catalyst due to the interaction of Rh-Mn. The results of H2-TPR indicated that both of Li and Mn can inhibit the reduction of Rh2O3. With the increase in the ratio of Mn/Rh, the most effective interaction, associated to the presence of two reduction centers of Rh2O3, was obtained when the ratio of Mn/Rh=1. In situ-FTIR was used to probe the effects of Mn and Li on CO absorption and hydrogenation. With regards to the CO adsorption, the doping of Mn can enhance the CO adsorption ability of Rh and weaken the CO-Rh bond strength very effectively when the amount of Mn reached 1.5wt.%. Improved capacity of CO adsorption is conducive to increase of CO conversion, while the weakening of CO-Rh bond is beneficial to the CO insertion reaction, thus contributing to the generation of C2+ oxygenates. Moreover, the low amount of Li (≤0.075wt.%) can also enhance the CO adsorption, resulting in the improvement of reactivity. On the other hand, Mn and Li promoted dissociation of H2, which is favorable to an increase in the rate of hydrogenation. But an opposite effect appeared at high content of Mn (>1.5wt.%). As the optimized results, when Rh, Mn, Li content was 1.5wt.%, 1.5wt.% and 0.075wt.%, the catalyst for CO hydrogenation of C2+ oxygenates achieved the best performance of C2+ oxygenates synthesis.