Mo–W-containing tetragonal tungsten bronzes through isomorphic substitution of molybdenum by tungsten
Introduction
Tetragonal tungsten bronze (TTB)-type oxides compose a large and extensively studied family of materials with a wide variety of cations. Although the term “tetragonal tungsten bronze” was formerly introduced for the non-stoichiometric compound KxWO3 (x = 0.4–0.6) [1], it was further extended to other compounds with a similar structure and different transition metals as central atom.
Standard methods for preparation of Mo- or W-containing bronzes involve the reaction of the component materials in the solid state at high temperatures [2], or the thermal degradation of polyoxometallates [3], [4], [5], [6]. Conversely, soft chemical methods often allow producing new materials that cannot be synthesized by using solid state reactions at high temperature. The production of highly uniform materials requires precursors containing the appropriate components dispersed at molecular level with the desired stoichiometry. Here, hydrothermal synthesis has been proposed as an interesting alternative to prepare mixed metal oxides [7], [8], [9]. In this synthesis, the use of polyoxometallates as precursors is widely extended, although most of the examples concern applications of Anderson-type oxoanions. Very recently our group has presented the synthesis of Mo-containing TTB derived from the Nb2O5/WO3 system by heat-treatment of amorphous precursors hydrothermally synthesized from aqueous solutions of Keggin-type heteropolyacids together with vanadium and niobium salts [10], [11], [12]. These materials have found application as active and very selective catalysts for the partial oxidation of short-chain olefins when incorporating in the framework an element of the V and VI groups, but especially tellurium [11]. In addition, the tetragonal bronzes display a catalytic behavior similar to that observed for the pseudohexagonal bronze (the so-called M2-phase) although some of them are more active for propene oxidation [10]. However, tuning of the composition of this TTB phase is of interest not only as a candidate for olefin oxidation, but also as a possible component of an active and selective alkane oxidation catalytic system.
In the present work we describe a synthesis method for the preparation of Mo–W–Nb–V–P–Te mixed oxides with TTB structure where molybdenum is partially or fully replaced by tungsten through isomorphic substitution. Structural parameters are tuned in order to achieve homogeneous dispersion of MO6 octahedra (M = Mo,W) in the TTB framework. These materials find application as selective catalysts in the partial oxidation of propene to acrolein and acrylic acid. Improved catalytic performance is explained on the basis of the isolation of Mo-sites and the synergetic effect between Mo- and W-centers.
Section snippets
Synthesis of TTB-bronzes
Materials have been prepared by hydrothermal synthesis from gels containing H3PMo12O40 and/or H3PW12O40 (Aldrich), vanadyl sulfate (Aldrich) niobium oxalate (CBMM) and telluric acid (Aldrich) [12]. The resulting gels, with a concentration of 3.3 mmol of heteropolyacid in 40 ml of H2O and a Mo(W)/Nb/V/P/Te atomic ratio of x(y)/0.17/0.20/0.08/0.04 (where x + y = 1), were introduced in a Teflon-lined, stainless steel autoclave and heated at 175 °C for 48 h. The solid so obtained was filtered off, washed
Structural characterization
After the heat-treatment in N2 these materials show metallic sheen and changing color depending on the composition: from purple for W-free sample (MW0) to dark brown for Mo-free mixed oxide (MW100). In all cases, solids with low surface area (below 10 m2 g−1) were obtained. A summary of the most important physical and chemical characteristics of these materials is presented in Table 1.
Chemical composition of heat-treated materials matches reasonably well with synthesis gel composition, although
Conclusions
Mo–W-containing tetragonal tungsten bronzes (TTB) can be synthesized by hydrothermal treatment of an aqueous solution of phosphomolybdic and phosphotungstic acids, vanadyl sulfate, niobium oxalate and telluric acid and further calcination of the obtained solid at 700 °C in N2.
In the TTB structure, W can substitute Mo in the whole range up to complete replacement without the basic structure being changed. Nevertheless, due to the slightly larger Shannon ionic ratio of W6+, the isomorphic
Acknowledgements
Financial support from DGICYT in Spain through Projects CTQ2006-09358/BQU, is gratefully acknowledged. Authors are also grateful to Manuel Planes Insausti and Jose Luis Moya López, technical supervisors responsible for the Servicio de Microscopía Electrónica (UPV) for the use of their facilities.
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