Preparation of vanadium phosphate catalysts from VOPO4•2H2O:: effect Of VOPO4•2H2O preparation on catalyst performance

The preparation of VOPO(4)(.)2H(2)O is described and discussed. Three samples of the dihydrate are prepared with different ageing times following the initial reflux of V2O5 with H3PO4 in water for 24h. The materials were characterised using a combination of powder XRD, BET surface area measurement, laser Raman spectroscopy and scanning electron microscopy. A sample of VOPO(4)(.)2H(2)O was isolated by immediate filtration of the reaction mixture and comprised flat oval crystallites with a broad size distribution between ca. 2 and 20 mum in diameter. Materials isolated following ageing of the initial reaction mixture (20degreesC, 24h) comprise square platelets again with a very broad size distribution. Using pyrophosphoric acid as the phosphorus source in place of phosphoric acid also affected the morphology of the VOPO(4)(.)2H(2)O. The dihydrates were reacted with isobutanol to form VOHPO(4)(.)0.5H(2)O and these were transformed to (VO)(2)P2O7 by reaction with 1.7% n-butane in air at 400degreesC. The most active catalyst was derived from VOPO(4)(.)2H(2)O prepared from ageing a reaction mixture following the removal of the first crop of crystals. The study shows that the method of preparation of VOPO(4)(.)2H(2)O and, in particular, its morphology is of importance in the preparation of vanadium phosphate catalysts using the two stage method based on the reaction of the dihydrate with an alcohol to form the hemihydrate precursor. (C) 2004 Elsevier B.V. All rights reserved.