Catalytic cooperation between MoO 3 and Sb 2O 4 in N-ethyl formamide dehydration II. Nature of active sites and role of spillover oxygen

Abstract

The present work reports experiments designed to further investigate the role of MoO 3 and Sb 2O 4 in the cooperative effect they exhibit in N-ethyl formamide dehydration. Temperature-programmed desorption of NH 3 showed that Bronsted acid sites are responsible for activity. TPD of CO 2 gave no indication of basic sites. The Brønsted sites are sensitive to reduction and reoxidation. Thermoreoxidation of previously reduced molybdenum oxide and oxidation of carbon deposited on MoO 3 are more rapid when α-Sb 2O 4 is admixed. The interpretation of the phenomenon is that α-Sb 2O 4 dissociates molecular O 2 to an oxygen spillover species which can maintain MoO 3 at a high degree of oxidation and, if necessary, reoxidize it or remove deposited carbon. Brønsted sites necessitate a high degree of oxidation and their number is consequently promoted by spillover oxygen. Mechanisms are proposed for the dissociation of oxygen, the reoxidation of pre-reduced MoO 3, and the formation of Brønsted acid sites. The importance of the reported results for selective oxidation or multicomponent catalysts is discussed.