Preparation and Characterization of Lamellar Vanadyl Alkylphosphates as Catalyst Precursors for the Selective Oxidation of Butane

Vanadyl alkylphosphates consisting of V^4+ and P^5+ were synthesized by the reaction of a solid mixture of V2O5 and V4O9 (2:3 in mol) with P2O5 in primary aliphatic (from C1 to C8), secondary aliphatic (C3 and C4), and alicyclic (C5) alcohols, and were characterized by means of elemental analysis, thermogravimetric analysis, Xray diffraction, IR spectroscopy, and magnetic susceptibility. The chemical formulae of the products were determined to be VO(RO)x(HO)1-xPO3·(ROH)y (x = 0.50.7, y = 0.3-0.5 for primary aliphatic alcohols; x = 0.1-0.2, y = 0.4-0.9 for secondary aliphatic and alicyclic alcohols; and R = organic group). X-ray diffraction revealed that the compounds had a lamellar structure, where the organic groups existed as double layers with a tilt of about 56° against the basal plane. Magnetism and IR spectroscopy demonstrated that the construction of the basal planes in the vanadyl alkylphosphates was similar to that of VOHPO4·0.5H2O, which consists of V^4+ dimers linked with PO4, except for vanadyl sec-butylphosphate, consisting of isolated V^4+ monomers. Calcination of vanadyl methylphosphate and vanadyl cyclopentylphosphate brought about crystallites of (VO)2P2O7. These (VO)2P2O7 exhibited high selectivities to maleic anhydride (65-68%) for the selective oxidation of butane under a high concentration of butane (5.0%).