Mechanistic Study on Deoxydehydration and Hydrogenation of Methyl Glycosides to Dideoxy Sugars over a ReO_x–Pd/CeO₂ Catalyst

Abstract

We found that nonprotected methyl glycosides with cis-vicinal OH groups could be converted to the corresponding methyl dideoxy glycosides by deoxydehydration and consecutive hydrogenation (DODH + HG) over a ReO x –Pd/CeO2 catalyst with gaseous H2. In the study, the reactivity of the methyl glycosides in DODH was clearly lower than that of simple cyclic vicinal diols, such as cis-1,2-cyclohexanediol and cis-1,2-cyclopentanediol, and the reactivity of the methyl glycosides was also different. Herein, we investigated the reactivity difference based on kinetic studies and density-functional theory (DFT) calculations. The kinetic studies suggest that the reactivity difference between the methyl glycosides and the simple diols is derived from the OH group of methyl glycosides except the cis-vicinal diols, and that the reactivity difference among the methyl glycosides will be associated with the configuration of the substituents adjacent to the cis-vicinal diols, while the reaction mechanism of DODH is suggested to be basically similar judging from almost the same reaction orders with respect to the substrate concentration and H2 pressure in all substrates. The adsorption and transition states of methyl α -l- rhamnopyranoside and methyl α-l-fucopyranoside, which have a large reactivity difference (methyl α-l-rhamnopyranoside≫ methyl α-l-fucopyranoside), were estimated by DFT calculations with ReO x /CeO2 as the active site of the ReO x –Pd/CeO2 catalyst, showing that the main difference is the activation energy in DODH of these substrates (65 kJ mol–1 for methyl α-l-rhamnopyranoside and 77 kJ mol–1 for methyl α-l-fucopyranoside), which was also supported by the results of Arrhenius plots (63 and 73 kJ mol–1 for methyl α-l-rhamnopyranoside and methyl α-l-fucopyranoside, respectively). The activation energy was influenced by the torsional angle of the substituents adjacent to the cis-vicinal OH groups, which is derived from the interaction of the OH group adjacent to the cis-vicinal OH groups and the surface hydroxy groups on CeO2.

Deoxydehydration of Biomass‐Derived Polyols Over Silver‐Modified Ceria‐Supported Rhenium Catalyst with Molecular Hydrogen

“…Then, we carried out density functional theory (DFT) calculation of the reactions of butenediols, erythritol, and 1,2-butanediol over the surface of reduced ReO x /CeO 2 , which is the main active site for DODH reaction (Table S8 and Figure S2; calculation details and the results are described in the Supporting Information). We have already conducted the DFT calculation of the DODH reaction of methyl glycosides over the surface of reduced ReO x /CeO 2 catalyst, [43] and we used the same calculation model. The results show that the adsorption strength of substrate as the diolate is in the order of erythritol at 1,2-position � 1,2-butanediol � 3-butene-1,2-diol (energy change of adsorption E A � À 20 kJ mol À 1 ) > erythritol at 2,3-position (E A = + 0 kJ mol À 1 ) > cis-2-butene-1,4diol (E A = + 27 kJ mol À 1 ).…”

“…Our group reported heterogeneous CeO 2 ‐supported Re catalysts using H 2 as a reductant. We first developed ReO x ‐Pd/CeO 2 catalysts for hydrodeoxygenation of diols via DODH and subsequent hydrogenation [40–43] . The CeO 2 support leads to the formation of highly dispersed ReO x species with high valence state, and the Pd activates H 2 molecule to reduce such Re species.…”

“…We first developed ReO x -Pd/ CeO 2 catalysts for hydrodeoxygenation of diols via DODH and subsequent hydrogenation. [40][41][42][43] The CeO 2 support leads to the formation of highly dispersed ReO x species with high valence state, and the Pd activates H 2 molecule to reduce such Re species. Pd also has high activity in C=C hydrogenation, resulting in the subsequent hydrogenation of the olefins produced by DODH.…”

Rhenium – A Tuneable Player in Tailored Hydrogenation Catalysis

“…The ceria support, in turn, avoids further reduction of Re 4+ . The deoxydehydration reaction mechanism catalyzed by Re 4+ is suggested to happen in three steps: species formed on the CeO 2 support was proposed to have three elementary reaction steps: two‐electron reduction of high‐valent Re, coordination of vicinal diol as diolate, and reduction of diolate with reduced Re to alkene [70,185,195–198] . Estimated adsorption and transition states for structural isomers of substrate methyl glycosides were obtained by DFT with ReO x /CeO 2 as the active site, pointing out that the difference in reactivity between isomers is due to the angle of torsion caused by interaction of the third −OH (not cis‐vicinal) group with surface −OH groups on ceria [70] …”

“…Due to the selectivity of rhenium‐based catalysts towards the hydrogenolysis of C−O bonds rather than C−C bonds, bimetallic catalysts of general composition M−ReO x (M=noble metal) have been studied for the hydrogenolysis of bio‐derived polyols such as glycerol [66] . Silica‐supported Rh−ReO x , Ru−ReO x , Pt−ReO x , Ir−ReO x and Pd−ReO x were evaluated in this reaction by Amada et al., [67] where Rh−ReO x was the most active, yet Ir−ReO x had the highest selectivity to the desired product 1,3‐propanediol ‐ interestingly, Pd−ReO x /SiO 2 showed no activity in the hydrogenolysis of glycerol, [67] whereas Pd−ReO x /CeO 2 is reported as active in the hydrodeoxygenation of other polyols, [68,69] indicating an influence of the CeO 2 support [70] . Rh−ReO x was among the first bimetallic catalyst containing ReO x studied by the research group of Tomishige et al.…”