The ceria-supported rhenium catalyst modified with palladium (ReO x –Pd/CeO2 (Re = 2 wt %, Pd/Re = 0.25)) is still the best catalyst for simultaneous hydrodeoxygenation. Higher Re loading amount decreased the activity. The simultaneous hydrodeoxygenation of cyclic vicinal diols occurs with high cis-stereoselectivity. ReO x –Pd/CeO2 catalysts were characterized by means of XRD, TEM, H2-TPR, XAFS, XPS, Raman, and DFT calculations. The Re species on ReO x –Pd/CeO2 (Re = 2 wt %, Pd/Re = 0.25) catalyst after reduction and after stoichiometric reaction of 1,2-hexanediol to 1-hexene were ReIV and ReVI, and the ReIV species were converted to ReVI through the stoichiometric reaction. The Re species on ReO x –Pd/CeO2 are proposed to be randomly located on the CeO2 surface, and probably only monomeric Re species have catalytic activity for simultaneous hydrodeoxygenation. This model can explain the higher activity of Re = 2 wt % catalyst than those of higher Re loading catalysts. The reaction is proposed to proceed by the tetra/hexavalent redox cycle of the Re center in the catalysis followed by hydrogenation.

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Heterogeneous ReO x -Pd/CeO 2 catalyst showed excellent performance for simultaneous hydrodeoxygenation of vicinal OH groups. High yield (> 99 %), turnover frequency (300 h À1 ), and turnover number (10 000) are achieved in the reaction of 1,4-anhydroerythritol to tetrahydrofuran. This catalyst can be applied to sugar alcohols, and mono-alcohols and diols are obtained in high yields (! 85 %) from substrates with even and odd numbers of OH groups, respectively. The high catalytic performance of ReO x -Pd/CeO 2 can be assigned to rhenium species with + 4 or + 5 valence state, and the formation of this species is promoted by H 2 /Pd and the ceria support.

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Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) were used to investigate the local structure of Pd supported on ZSM-5, which was affected by the acid sites of support and the adsorption of nitrogen oxide. After the thermal treatment of the initially ion-exchanged Pd amine complex, the formation of metal Pd particles was found, whose size was larger than zeolite pore. The subsequent oxidation led to the disruption of Pd particles and the formation of the dispersed PdO, where the degree of dispersion was dependent on the acid amount of ZSM-5. It was also found on Pd/HZSM-5 (Si/Al 2 ) 24) that the formation of metal Pd and dispersed PdO was reversible upon reduction and oxidation treatments. These facts prove the presence of strong interaction between acid sites and PdO. The role of acid sites of zeolite was considered to keep the dispersed state of PdO. The adsorption of NO on highly dispersed PdO induced a significant change in the local structure of Pd at room temperature. At the same time, PdO was reduced to Pd I upon the adsorption of NO. In contrast to Pd/HZSM-5, highly aggregated PdO was found in Pd/NaZSM-5. The deactivation of Pd/HZSM-5 for selective NO-CH 4 -O 2 reaction due to the presence of H 2 O vapor was considered to be caused by sintering of PdO.