Highly active iridium–rhenium catalyst condensed on silica support for hydrogenolysis of glycerol to 1,3-propanediol

Increase of loading amount of Ir-ReO_x/SiO₂ catalyst up to 20 wt% of Ir enhanced the Ir-based activity in glycerol hydrogenolysis to 1,3-propanediol. The ratio of used precursor amount was set constant (Re/Ir = 1; “nominal” Re/Ir ratio); however, actual Re/Ir amount was lower at higher loading amount because of the loss of Re during calcination. The catalytic performance, reactivity of related substrates and kinetics over optimized Ir-ReO_x/SiO₂ (20 wt%-Ir, Re/Ir = 0.34, actual) was compared with the previously reported catalyst (4 wt%-Ir, Re/Ir = 1, nominal; Re/Ir = 0.83, actual) without H₂SO₄ addition which increases the activity and stability of 4 wt%-Ir Ir-ReO_x/SiO₂ catalyst. High 1,3-propanediol selectivity (ca. 70%) was obtained over 20 wt%-Ir Ir-ReO_x/SiO₂ at 20% conversion level of glycerol. This high selectivity was almost independent of glycerol concentrations, while 4 wt%-Ir Ir-ReO_x/SiO₂ catalyst showed lower selectivity when glycerol concentration was lower. The 20 wt%-Ir Ir-ReO_x/SiO₂ exhibited good reusability under optimized reaction conditions when recovered without exposure to air. Small Ir metal particles (˜3 nm) were observed from both XRD and TEM regardless of high Ir loading amount. The results of XANES and EXAFS suggested high reduction degree of Re species (˜80%); however, further characterization of XRD and CO FT-IR supported the absence of Ir-Re alloy. Combined with TPR and CO adsorption results, we proposed that ReO_x cluster at low average valence (ca. + 1˜+2) was attached to Ir metal. This unique structure decreased the effect of SiO₂ support and increased the number of active sites, accounting for higher activity. The heterolytic dissociation of hydrogen molecule (˜+1 reaction order on H₂ pressure) and strong interaction between glycerol and catalyst surface (˜0 reaction order on glycerol concentration around standard reaction conditions, 67 wt% glycerol solution) were suggested for 20 wt%-Ir Ir-ReO_x/SiO₂ catalyst. This 20 wt%-Ir Ir-ReO_x/SiO₂ catalyst can be also applied to various substrates with –O–C–CH₂OH structure for the selective cleavage of the C–O bond.