Performance, Structure, and Mechanism of ReO_x–Pd/CeO₂ Catalyst for Simultaneous Removal of Vicinal OH Groups with H₂

Abstract

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.

Valorization of Biomass‐derived Small Oxygenates: Kinetics, Mechanisms and Site Requirements of H2‐involved Hydrogenation and Deoxygenation Pathways over Heterogeneous Catalysts

“…The optimized ReO x −Pd/CeO 2 catalyst, however, was much less efficient when using 3‐pentanol, instead of H 2 , as the reductant, suggesting that ReO x could not be reduced to its most active state by a secondary alcohol . In a later study, they found that the addition of noble metals (Ru, Rh, Ir, and Pt), but not base metals like Ni and Cu, to ReO x /CeO 2 actually all improved the conversion to similar extents as Pd (within a factor of 2) with excellent DODH selectivity, and that the loading of Pd hardly affected the catalytic performance . By comparing the conversion rates of glycerol and a couple of other polyols over ReO x −Au/CeO 2 (with ∼3 nm Au particles) and ReO x −Pd/CeO 2 , Nakagawa et al.…”

“…Although heterogeneous DODH reactions were previously conducted in ether, hydrocarbon solvents or at solvent‐free conditions, there has not been sufficient attention to the solvent effects, except for only one recent study using ReO x −Pd/CeO 2 catalyst and H 2 as reductant, which basically showed that there was not a great difference in activity and selectivity in neat conditions or with ether, alkane, and alcohol solvents, except for water . The very low activity in water could either be due to the fact that some of the Re oxides (e. g., Re 2 O 7 and ReO 3 ) are soluble in water, leading to rapid leaching, or that water irreversibly binds to the metal center preventing the formation of diolate.…”

“…The ReO x −Pd/CeO 2 catalyst does not seem to require reduction pretreatment prior to DODH reactions, since Ota et al. showed that reducing the catalyst in the reactor even at 423 K remarkably decreased the catalytic activity, which continued to decrease with reduction severity . It was argued that this decrease in catalytic activity stemmed from the reduction of Ce IV to Ce III , since if otherwise, Re reduction would have been beneficial.…”

Vanadium‐Catalyzed Deoxydehydration of Glycerol Without an External Reductant

“…Hydrogen has also been explored as a reductant in DODH reactions. The heterogeneous catalyst ReO x ‐Pd/CeO 2 has been shown to convert glycerol to 1‐propanol by sequential DODH and hydrogenation . In a subsequent study, hydrogenation was curtailed by replacing Pd with Au.…”

Synthesis of 2‐Butanol by Selective Hydrogenolysis of 1,4‐Anhydroerythritol over Molybdenum Oxide‐Modified Rhodium‐Supported Silica

“…[10] It was reported previously that the hydrogenolysis of erythritol over Ir-ReO x /SiO 2 gave 2-butanol as a byproduct, [14] however, the maximum yield of 2-butanol was 17 %. Erythritol, which can be produced easily by the fermentation of sugars, is regarded as a promising C 4 intermediate from biomass, [15][16][17][18][19][20] and the transformation of erythritol [21][22][23][24][25][26][27][28][29][30][31][32] and its derivatives, which include 1,4-anhydroerythritol (1,4-AHERY), to various C 4 chemicals such as butanediols, [14] tetrahydrofuran, [32,33] 2,5-dihydrofuran, [34] and 3-hydroxytetrahydrofuran (3-HTHF) [35] has been investigated. Therefore, the development of effective heterogeneous catalysts for the transformation of erythritol and its derivatives to 2-butanol is desirable.…”