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

The effect of support in Ir-ReO_x 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-ReO_x pairs. Higher Ir-based 1,3-propanediol productivity of Ir-ReO_x/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-ReO_x/SiO₂. The 1,3-propanediol productivity over Ir-ReO_x catalysts showed dependency on catalyst compositions (metal loading amount), and the relationship between catalyst structure and activity was further established over Ir-ReO_x/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 ReO_x cluster (average valence of Re: +3) almost totally covering the surface of rutile TiO₂ particles, and the active site was the Ir-ReO_x interface. Small amounts of Ir species (∼20%) were incompletely reduced; however, such IrO_x 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-ReO_x particles with very high surface density on rutile TiO₂, which increased the number of active sites per Re amount.