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

Abstract

The thermodynamics of glycerol hydrogenolysis to produce 1,2-propanediol (1,2-PDO) and 1,3-propanediol (1,3-PDO) over Ru/ZrO 2 , Rh/ZrO 2 , ReO x -Rh/ZrO 2 , and ReO x -Ir/ZrO 2 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/ZrO 2 and Rh/ZrO 2 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 ReO x cluster. In the presence of ReO x -Rh/ZrO 2 , the modified catalyst favors the production of 1,2-PDO, and 1,3-PDO production becomes competitive. However, the ReO x -Ir/ZrO 2 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 ReO x -Ir/ZrO 2 catalysts because of the growth of large Ir-Re clusters, resulting in high selectivity for 1,3-PDO. The decisive role of ReO x present in Re-modified bifunctional catalysts in promoting the activity and 1,3-propanediol selectivity, compared with monometallic clusters, is highlighted.