Tungsten-titanium mixed oxide bronzes: Synthesis, characterization and catalytic behavior in methanol transformation

Abstract

Tungsten oxide bronze-based materials show extremely adaptive structural and compositional features that make them suitable for functional properties modulation. Herein we report the preparation of a series of Ti-containing tungsten oxide catalysts presenting a hexagonal tungsten bronze-type structure. The insertion of Ti4+ within the structure (likely in the octahedral framework of the hexagonal tungsten bronze) leads to an increase in the number of strong acid sites, and the disappearance of W5+ surface species found in the undoped tungsten oxide. With the aim of studying the acid-redox properties of the titled catalysts, the catalytic transformation of methanol has been carried out in the presence and the absence of O2 in the feed. Both catalytic activity and the acid-redox properties of these catalysts are highly dependent on catalyst composition and reaction conditions applied (i.e. in the presence or in the absence O2 in the feed). Aerobic experiments show the depletion of the redox functionality (i.e. no formaldehyde detected in the products) when Ti4+ is incorporated in the framework (i.e. 100% selectivity to dimethyl ether). On the other hand, all the catalysts show the loss of the redox function and a decrease in the catalytic activity when anaerobic conditions are used. In the absence of oxygen, the catalysts are still active in the dehydration of methanol to dimethyl ether, i.e. they maintain their acid functionality even when oxygen is not present in the feed. The results are discussed in terms of the available surface active sites present in each case.