On the nature and structure of new MoVTeO and MoVTeNbO crystalline phases
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On the nature and structure of new MoVTeO and MoVTeNbO crystalline phases E. García-González1, J. M. López Nieto2, P. Botella2, B. Solsona2 and J.M. González-Calbet1 1 Dpto. Q. Inorgánica; Facultad de Químicas, Universidad Complutense, 28040-Madrid, Spain; 2 Inst. Tecnología Química, UPV-CSIC, Avenida de los Naranjos s/n, 46022-Valencia, Spain. ABSTRACT New TeMoV and TeMo(V,Nb) bronze type phases have been obtained during the heat-treatment (600º C in N2) of the corresponding precursor mixture. Both mixed oxides seem to be structurally related to one of the crystalline phases proposed in Mo-V-Te-Nb-O catalysts. Chemical and microstructural characterization revealed the formation of an orthorhombically distorted cell deriving from a bronze type HTB structure, which does not behave as a bronze type structure in the chemical sense. Microstructural details responsible for the new unit cell are discussed in terms of tellurium location inside the hexagonal tunnels and based on the structural model proposed. INTRODUCTION Acrylonitrile and acrylic acid are two important intermediates for the preparation of fibers, synthetic rubbers, synthetic resins, etc. (1). Both are actually produced by gas phase ammoxidation and oxidation of propylene, respectively. The trend in the next years will be to replace it by propane, a less expensive raw material. MoVTeNbO multicomponent catalysts have been proposed as serious alternatives to convencional catalysts used in the oxidation and ammoxidation of propene (2) and the oxdehydrogenation of ethane (3). These catalysts present at least two different phases, the socalled M1 and M2 (4,5). However, their catalytic performances are still under discussion (6). The catalyst preparation method, the optimal chemical composition as well as the nature of the active and selective sites is still under discussion and research (2-6). However, the main of the papers and patents published in the last years suggest the presence of at least two crystalline phases, although their structures are not well resolved. Recently, the synthesis of new MoVTeO and MoVTeNbO mixed oxides with stoichiometry TexMOz (x = 0.33-0.4; M= Mo/V or Mo/V/Nb) (6) has been reported, which present XRD patterns similar to that observed for Sb4Mo10O31. These mixed metal oxides are one of the crystalline phases detected in MoVTeNb based catalysts. In this way, they are active phases in the oxidation of propene to acrolein (MoVTeO phase) and acrylic acid (MoVTeNb phase), respectively, but are inactive in the partial oxidation of propane (6). So, the incorporation of Nb in the crystalline phase has an important influence on the catalytic activity and selectivity to partial oxygenated products.In this paper, we have performed a structural investigation on these new phases and the microstructural characterization carried out can be used as the first step in elucidating their role in the effectiveness of catalysts. EXPERIMENTAL DETAILS Samples were prepared by evaporation of aqueous slurry comprising ammonium heptamolybdate, tellu
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