Preparation of vanadium phosphate catalyst precursors using a high pressure method

The preparation of vanadium phosphate catalysts is known to be an important factor in determining their performance for the oxidation of butane to maleic anhydride. In this paper the preparation of catalyst precursors from V2O5 and H3PO4 using both aqueous hydrochloric acid and isobutanol as reducing agents are evaluated. In particular, the preparation of materials at higher temperatures using an autoclave method is described and compared with the materials prepared using the typical reflux methodology. The materials were characterised using a combination of powder XRD, BET surface area measurement, laser Raman spectroscopy and scanning electron microscopy. With the reflux method (1 bar pressure) both methods lead to the formation of VOHPO4·0.5H2O. The aqueous hydrochloric acid method leads to the formation of VO(H2PO4)2 as a minor impurity that is readily removed by water extraction. The high temperature autoclave route gives significant differences. The aqueous hydrochloric acid route surprisingly does not give V4+ phases but gives a mixture of hydrated VOPO4·xH2O phases. In contrast the isobutanol method at higher temperature and pressure gave VOHPO4·0.5H2O as expected. However, for both methods the use of the higher temperature and higher pressure led to lower surface area materials being formed and consequently this will limit the usefulness of this methodology with these reducing agents.