Ruthenium isomorphic substitution into manganese oxide octahedral molecular sieve OMS-2: Comparative physic-chemical and catalytic studies of Ru versus abundant metal cationic dopants
Ferran Sabaté, J.L. Jordá, María J. Sabater
Abstract
A fraction of structural Mn cations have been replaced by different cationic dopants (i.e. earth abundant cationic dopants and heavier, Ru3+) in the cryptomelane oxide K-OMS2. Then, a comparative study has been addressed in order to understand the relationship between physic-chemical properties and catalytic activity in this series of doped oxides. The most striking effect of doping was the weakening of the Mn-O bond so that oxygen atoms can be more easily removed from the surface layer, facilitating the production of oxygen vacancy defects (OVDs), which are key for certain catalytic applications. This weakening of the MnO bond was more pronounced for the case of ruthenium doped cryptomelane [Ru]-K-OMS2 oxide, which was the most active catalyst (TON = 145) during the oxidation reaction of benzyl alcohol to benzaldehyde. This feature has been correlated with the reducibility according to TPR-H2 data and a more weakened MnO bond force constant value, kMn-O (N·m-1), which varied from 301.7 to 291.8 N·m-1 for undoped K–OMS2 and [Ru(2%)]-K-OMS2 respectively. All the doping processes enhanced the alcohol conversion (ca. 2–50%) giving a high selectivity (>99%) towards the aldehyde.