Role of ReOx in Re-modified Rh/ZrO2 and Ir/ZrO2 catalysts in glycerol hydrogenolysis: Insights from first-principles study

The thermodynamics of glycerol hydrogenolysis to produce 1,2-propanediol (1,2-PDO) and 1,3-propanediol (1,3-PDO) over Ru/ZrO2, Rh/ZrO2, ReOx-Rh/ZrO2, and ReOx-Ir/ZrO2 were studied using density functional theory calculations, with a special focus on the mechanism controlling the activity and selectivity of the reactions. It is found that the decomposition of glycerol on Ru/ZrO2 and Rh/ZrO2 proceeds through a dehydration-hydrogenation mechanism. The formation of 1,2-PDO is thermodynamically favored, and the activity of the Ru-based catalyst is higher than that of the Rh-based one. In contrast, a direct hydrogenolysis mechanism is proposed for the Re-modified Rh and Ir catalysts, in which a dissociated H atom on the Rh(Ir) metal surface attacks the C–O bond neighboring the alkoxide species on the ReOx cluster. In the presence of ReOx-Rh/ZrO2, the modified catalyst favors the production of 1,2-PDO, and 1,3-PDO production becomes competitive. However, the ReOx-Ir/ZrO2 catalyst significantly improves 1,3-PDO selectivity. The direct hydrogenolysis pathway, as opposed to the indirect hydrogenolysis mechanism for monometallic catalysts, may be the key to the high 1,3-PDO selectivity on the modified catalysts, where the hydroxylated Re group facilitates the formation of terminal alkoxide species rather than secondary alkoxides. Steric effects are important in preferential terminal alkoxide formation on the ReOx-Ir/ZrO2 catalysts because of the growth of large Ir-Re clusters, resulting in high selectivity for 1,3-PDO.

Graphical AbstractThe decisive role of ReOx present in Re-modified bifunctional catalysts in promoting the activity and 1,3-propanediol selectivity, compared with monometallic clusters, is highlighted.Figure optionsDownload as PowerPoint slide