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

Abstract

Increase of loading amount of Ir-ReOx/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-ReOx/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-ReOx/SiO2 catalyst. High 1,3-propanediol selectivity (ca. 70%) was obtained over 20 wt%-Ir Ir-ReOx/SiO2 at 20% conversion level of glycerol. This high selectivity was almost independent of glycerol concentrations, while 4 wt%-Ir Ir-ReOx/SiO2 catalyst showed lower selectivity when glycerol concentration was lower. The 20 wt%-Ir Ir-ReOx/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 ReOx 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-ReOx/SiO2 catalyst. This 20 wt%-Ir Ir-ReOx/SiO2 catalyst can be also applied to various substrates with OCCH2OH structure for the selective cleavage of the CO bond.