Fundamental Insights into Deactivation by Leaching during Rhenium-Catalyzed Deoxydehydration

Deoxydehydration, a transformation that selectively converts diols into olefins in a single catalytic step, has great potential for biomass upgrading because it removes oxygen, and the obtained double bond is useful for further functionalization. Recent efforts to produce heterogenized versions of known soluble oxido-rhenium catalysts have produced mixed results, with stability being a recurring issue. In the present work, the reasons for catalyst deactivation were systematically investigated by comparisons within a series of materials distinguished by the support. Catalysts were prepared by incipient wetness impregnation of ammonium perrhenate onto TiO2 and ZrO2 at 1, 2, or 4 wt % rhenium and SiO2, Fe2O3, and Al2O3 at 4 wt % rhenium, with subsequent calcination. All catalysts were active in the conversion of 1,2-decanediol to 1-decene at a temperature of 150 degrees C, in toluene as the solvent at autogenous pressure and with triphenylphosphine as the reductant. The rhenium-normalized 1-decene formation rates for TiO2- and SiO2-supported catalysts were comparable with that of a homogeneous methyltrioxorhenium catalyst with selectivities between 80 and 90%, whereas the other catalysts were significantly less active. High activity was correlated with rapid deactivation. It was found that rhenium leaching is the primary cause of deactivation and is induced via association of the catalyst with the diol to give a soluble glycolate complex. Homogeneous catalysis contributions were significant. In addition to choosing a suitable (i.e., reducible or Lewis acidic) support, leaching can be minimized through an appropriately low rhenium loading that depends on the support, selecting a solvent that is less than ideal for the substrate, and by driving the reaction to 100% substrate conversion, which results in redeposition of rhenium.