Oxidation-state-controlled deoxydehydration: Rhenium vs. molybdenum and tungsten in the conversion of biomass-derived mucic acid to adipate esters

This study systematically compared the catalytic performance of ReO_x-, MoO_x-, and WO_x-incorporated Ru on activated carbon (0.8Ru2Re/AC-P, 0.8Ru2Mo/AC-P, and 0.8Ru2W/AC-P, respectively) for the one-step conversion of mucic acid (MA) to hexanedioic acid esters (HADIE/HADME). The respective HADIE/HADME yields were 88.2 %, 48.9 %, and 14.3 %. In 0.8Ru2Re/AC-P and 0.8Ru2Mo/AC-P, atomically dispersed Re and Mo species were deposited on Ru⁰ nanoclusters and activated carbon, whereas in 0.8Ru2W/AC-P, both Ru and W existed predominantly as nanoclusters. Ru⁰ act as active sites for hydrogen atom generation via dehydrogenation of isopropyl alcohol which spills over to neighboring single-atom Re⁷⁺ sites, generating Re⁵⁺ sites that facilitated adsorption and activation of vicinal diols in MA, yielding trans,trans-muconic acid (TTMCA) that was subsequently hydrogenated on Ru⁰ to produce HADIE. In contrast, both 0.8Ru2Mo/AC-P and 0.8Ru2W/AC-P utilize both of hydrogen atoms derived from methanol and external H₂ to reduce Mo⁶⁺/W⁶⁺ to catalytically active Mo⁴⁺/W⁴⁺ sites. Methanol-derived hydrogen atoms preferentially hydrogenate C[dbnd]C bonds of TTMCA, whereas H₂-derived hydrogen atoms facilitate the reduction of terminal hydroxyl groups to form HADME. Theoretical calculations reveal the first deoxydehydration (DODH) step constitutes the rate-determining step, with the ReO_x-based system exhibiting the lowest activation energy barrier, while the WO_x-based system presents the highest. Charge density difference analysis further indicates that charge depletion at the C2 of the MA-derived intermediate on Re⁵⁺ sites facilitates C–O bond cleavage, promoting DODH more effectively than on Mo⁴⁺/ W⁴⁺ sites. Isotope labeling studies corroborate the mechanistic distinction in hydrogen sources and their roles across the different catalytic systems.