No abstract

Gold nanoparticle cocatalyst and ceria support catalyst enabled the use of H2 as a reductant for Re-catalyzed deoxydehydration (DODH), converting vicinal diols to olefins with higher activity than homogeneous Re catalysts using non-H2 reductants and with comparable selectivity. The suitable particle size (∼10 nm) of Au in ReO x -Au/CeO2 catalyst was especially effective for activating H2 while retaining the CC bond in DODH products. Glycerol was converted to allyl alcohol in 91% yield, and the TON per Re atom reached 300. This system is applicable to other substrates such as erythritol (81% yield of 1,3-butadiene).

The structure and activity of ReO x -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 ReO x -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, ReO x -impAu/CeO2 shows higher yield of DODH products than ReO x -dpAu/CeO2. In contrast, in the case of diols, the selectivity decrease by these side reactions is small, and ReO x -dpAu/CeO2 is a better catalyst than ReO x -impAu/CeO2 because of the higher activity. In addition, the cis/trans isomerization activity of ReO x -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.