The role of lattice oxygen in alcohol oxidation by octahedral molecular sieves: A kinetic, mechanistic and structure-activity relation study

The oxidation of benzyl alcohol by molecular O2 in the liquid phase using a heterogeneous octahedral molecular sieve (OMS) catalyst was studied. Typical batch reactor kinetic data were obtained and fitted to the classical Langmuir-Hinshelwood-Hougen-Watson (LHHW) model as well as the Mars-van Krevelen (MvK) model of heterogeneously catalyzed reactions. A two-step Mars-van Krevelen model involving an exchange between the gas phase and lattice oxygen was found to give a better fit. This was further corroborated by an oxygen isotope labelling study. The changes in the 16O and 18O content of the product water confirm this observation. Kinetic isotope effects were also evaluated in mechanistic studies. The occurrence of this mechanism in the liquid phase explains the activity and high selectivity of the OMS-2 catalyst for this selective oxidation reaction. ^ Further, the manganese oxide catalysts were synthesized in different media and modified by exchanging the tunnel cation by H+, using acid treatment or exchanging with NH4+ followed by thermolysis. Various alkyl alcohol oxidations were performed using these catalysts to ascertain the influence of synthesis method on their activity. A correlation was made between lattice oxygen instability and activity of the catalysts, which indicates involvement of the lattice oxygen in the mechanism. The exchange of the tunnel cation with the smaller H+ ions leads to weakening of the Mn-O bond, as verified by temperature programmed desorption (TPD) results. Only the chemisorbed oxygen on the surface (O −) and the lattice oxygen in the layers close to the surface were involved in the oxygen transfer during the reaction. Other properties like the types and densities of acid and basic sites, and the redox nature of the catalyst in terms of the average oxidation state of Mn were also correlated to the activity. The whole structure-activity relation study served as an exercise in intelligent catalyst design.