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 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.

Tungsten–niobium oxide bronzes: a bulk and surface structural study

“…However, in the latter cases, it is not possible to incorporate high amounts of dopant within the structure. Dopant contents exceeding the maximum theoretical value for the hexagonal tungsten bronze structure (h-W1-xMxO3; x=0.33 and M= V, Ti) give rise to anatase-type TiO2 nanoparticles 28 or extraframework VOx species. 10 The case of W-Nb-O system is completely different, since niobium is incorporated within the crystal framework throughout all the compositional range, giving rise to a loss of long-range order in the ab plane of the structure.…”

“…An interesting strategy to increase the thermal stability of the hexagonal phase consists in the partial substitution of tungsten species by other transition metals, especially by those elements showing a maximum oxidation state lower than 6+ (for instance, vanadium, [10][11][12][13]19 tantalum 26 or titanium, 27,28 which are stable up to 500-600 ºC). This way, the incorporation of these elements will help to keep the partially reduce nature of the materials, thus increasing the thermal stability of h-WO3 phase.…”

“…However, for applications in catalysis, soft synthesis methods are required. Hydrothermal, [11][12][13][26][27][28][29] reflux 30,31 or slurry 32 methodologies are the most common ones, not only for the synthesis of tungsten-bronze based materials, but also for the preparation of other bronze-type oxides based on molybdenum or vanadium. [33][34][35] Unlike methods based on solid state reactions, these mild synthesis protocols allow to achieve active surface areas large enough to carry out catalytic reactions.…”

A Review and Experimental Revisit of Alternative Catalysts for Selective Oxidation of Methanol to Formaldehyde

“…Tungsten-titanium mixed oxide bronzes were investigated for MeOH transformation by Delgado et al [232] at 200-400 • C with 100 mL/min of 6/13/81 vol.% MeOH/O 2 /N 2 on 0.10 g of catalyst (0.25-0.4 mm). For all the samples, the major product was DME, which is in good agreement with the measured acidity.…”

“…For the Ti containing samples, the redox activity was much lower, with less than 5% selectivity relative to FA in all cases. The redox properties were believed to come from W 5+ , which was present in the Ti-free sample, whereas the Ti containing samples only had W 6+ at the surface [232].…”