Mechanistic Study of Hydrogen-Driven Deoxydehydration over Ceria-Supported Rhenium Catalyst Promoted by Au Nanoparticles

Abstract

The structure and activity of ReOx-Au/CeO2 catalysts for deoxydehydration (DODH) of polyols to alkenes with H2 were investigated in detail. Based on X-ray diffraction (XRD) and transmission electron microscopy (TEM), the sizes of Au particles are in a similar scale to CeO2 support particles and the number of Au particles is much smaller than that of CeO2 support particles. Nevertheless, the catalytic activity and temperature-programmed reduction (TPR) data of the physical mixture of Re/CeO2 and Au/CeO2 indicate that Re species on all the CeO2 particles can be reduced with H2 and can work as a catalytic center. The H2 activation ability of the catalyst with larger (∼12 nm) Au particles (impAu; “imp” means impregnation for loading) is lower than that with smaller (∼3 nm) Au particles (dpAu; “dp” means deposition–precipitation for loading), and the DODH reaction rate over ReOx-impAu/CeO2 is limited by the H2 activation rate. The CeO2-supported dpAu particles have also higher activity in C═C hydrogenation, C═C migration, and cis/trans isomerization of diols. The C═C hydrogenation and C═C migration are side reactions in DODH, and in the DODH reaction of polyols, such as glycerol and erythritol, ReOx-impAu/CeO2 shows higher yield of DODH products than ReOx-dpAu/CeO2. In contrast, in the case of diols, the selectivity decrease by these side reactions is small, and ReOx-dpAu/CeO2 is a better catalyst than ReOx-impAu/CeO2 because of the higher activity. In addition, the cis/trans isomerization activity of ReOx-dpAu/CeO2 enables the DODH of trans-1,2-cyclohexanediol, which is usually unreactive in DODH, into cyclohexene via isomerization to cis-1,2-cyclohexanediol and the subsequent DODH.