Selective Hydrogenolysis of Glycerol to 1,3-Propanediol over Rhenium-Oxide-Modified Iridium Nanoparticles Coating Rutile Titania Support

The effect of support in Ir-ReOx catalysts for glycerol hydrogenolysis to 1,3-propanediol was investigated. Rutile TiO2 support showed high activity, even higher than previously reported SiO2 support. Anatase TiO2, C, ZrO2, CeO2, Al2O3, and MgO supports showed very low activity of supported Ir-ReOx pairs. Higher Ir-based 1,3-propanediol productivity of Ir-ReOx/rutile catalyst was obtained at the initial stage even with lower Re/Ir ratio (typical Ir loading amount, 4 wt %, nominal ratio of 0.25; actual ratio of 0.24) without addition of H2SO4 than that of Ir-ReOx/SiO2. The 1,3-propanediol productivity over Ir-ReOx catalysts showed dependency on catalyst compositions (metal loading amount), and the relationship between catalyst structure and activity was further established over Ir-ReOx/rutile. Relatively high Ir loading amount in comparison with small surface area (6 wt %, on 6 m2 g–1 rutile TiO2) showed the highest activity (Ir-based activity). From combined characterization results altogether (TPR, TEM, XPS, XAS, CO adsorption, CO FT-IR) with a kinetics study, the Ir metal particles interacted with the partially oxidized ReOx cluster (average valence of Re: +3) almost totally covering the surface of rutile TiO2 particles, and the active site was the Ir-ReOx interface. Small amounts of Ir species (∼20%) were incompletely reduced; however, such IrOx species as well as rutile TiO2 support were not directly involved in glycerol hydrogenolysis. The role of rutile support was regarded as providing a unique environment for stabilization of uniform and small Ir-ReOx particles with very high surface density on rutile TiO2, which increased the number of active sites per Re amount.