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Synthesis and Crystal Structure of a New Vanadyl Phosphate [H 0.6 (VO) 3 (PO 4 ) 3 (H 2 O) 3 ]·4H 2 O and Its Conversion to Porous Products

Chemistry of Materials (Impact Factor: 8.54). 07/2001; 13(7):2288-2296. DOI: 10.1021/cm0008821
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    ABSTRACT: Vanadium phosphorous oxide (VPO) is traditionally manufactured from solid vanadium oxides by synthesizing VOHPO4 center dot 0.5H(2)O (the precursor) followed by in situ activation to produce (VO)(2)P2O7 (the active phase). This paper discusses an alternative synthesis method based on sol-gel techniques. Vanadium (V) triisopropoxide oxide was reacted with ortho-phosphoric acid in an aprotic solvent. The products were dried at high pressure in an autoclave with a controlled excess of solvent. This procedure produced a gel of VOPO4 with interlayer entrapped molecules. The surface area of the obtained materials was between 50 and 120 m(2)/g. Alcohol produced by the alkoxide hydrolysis reduced the vanadium during the drying step, thus VOPO4 was converted to the precursor. This procedure yielded non-agglomerated platelets, which were dehydrated and evaluated in a butane-air mixture. Catalysts were significantly more selective than the traditionally prepared materials with similar intrinsic activity. It is suggested that the small crystallite size obtained increased their selectivity towards maleic anhydride.
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    ABSTRACT: Vanadium phosphate catalysts have been extensively studied since the 1960s, when it was discovered that they are effective catalysts for the oxidation of butane to maleic anhydride. They represent the sole example of a commercialised material for the catalytic oxidation of an alkane. To date several hundred papers have been devoted to their study. Yet, virtually all studies have concentrated on two specific phases, namely the vanadyl hemihydrate precursor, VOHPO4·0.5H2O, which is converted into vanadyl pyrophosphate, (VO)2P2O7, which most researchers consider to be the active component of vanadium phosphate catalysts. In this feature article the nature of the active catalyst components is revisited and new approaches being pioneered in the synthesis of vanadium phosphates are described. In particular, detailed comments are made concerning the method of preparation and the role of crystalline and amorphous vanadium phosphate catalysts.
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    ABSTRACT: Hydrothermal reaction of Na2WO4, VOSO4, 2,2'-bpy and H3PO4 has afforded in high yield the compound [V(IV)2V(V)6O14(bpy)8(PO4)2][PW11V(V)O40](bpy).12H2O (1). Compound 1 contains a novel octanuclear mixed valence V(IV,V) cluster, [V(IV)2V(V)6O14(bpy)8(PO4)2]4+, with [PW11V(V)O40]4- as counterion. In the vanadium cluster, four V(V) centers are localized and the remaining two V(IV) and two V(V) ions are disordered over four crystallographically equivalent positions. The isostructural compound [V(IV)2V(V)6O14(bpy)8(PO4)2][PMo11V(V)O40](bpy).3H2O (2) has also been synthesized. Thermodiffractometry experiments indicate that 2 is stable up to 360 degrees C. Redox activities for both the vanadium and molybdenum centers have been observed by solid-state electrochemical measurements performed on mechanically attached microparticles of 2. Magnetic measurements performed on have shown the occurrence of weak ferromagnetic interactions between the V(IV) centres (J = +0.34 cm(-1), H(ex) = -JS1 x S2), and combined with DFT calculations, have allowed to propose a localization of the two V(IV) centers on two of the four equivalent crystallographic sites. Finally high field electron paramagnetic resonance has evidenced the magnetic axial anisotropy of the paramagnetic centers (g(x) = g(y) = 1.975(3); g(z) = 1.939(4)).
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