We report here about an investigation of the key catalyst features and reaction parameters in glycerol oxidehydration into acrylic acid over W-V-Nb mixed oxides with the hexagonal tungsten bronze structure. The incorporation of Nb allowed to considerably improve the catalytic behaviour not only because of the higher yield to acrylic acid and acrolein compared to W-V bronzes, but also to increase of more than order of magnitude the oxygenated productivity rate, because of the greater concentration of glycerol used in the inlet feed and the lower contact time needed. Experiments were carried out by reacting acrolein, and by oxidation of methanol, as a model molecule for the determination of surface acid and redox properties. These experiments demonstrated that with the W-V-Nb catalyst the optimal ratio between the two consecutive steps of acid-catalysed glycerol dehydration and acrolein oxidation, and between the two parallel reactions of acrolein oxidation and transformation into by-products (ketals and oligomers), was achieved in the presence of defined glycerol-to-oxygen inlet ratio. Indeed, oxygen played the fundamental role of accelerating the oxidation of the intermediately formed acrolein into acrylic acid, by allowing a greater concentration of the oxidizing V 5+ sites.

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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 Ti 4+ 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 W 5+ 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 acidredox 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 Ti 4+ 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.