The Unexpected Role of Aldehydes and Ketones in the Standard Preparation Method for Vanadium Phosphate Catalysts

VO(H2PO4)2 is formed as the exclusive product from the reaction of aldehydes or ketones with V2O5 and H3PO4 whether aqueous (85%) or crystalline (100%) orthophosphoric acid is used. This exclusive product formation has been observed with a broad range of aldehydes and ketones (C4–C10). This finding casts doubt on the current commercial preparation of vanadium phosphate catalysts used for the oxidation of n-butane to maleic anhydride. This catalyst is derived from a crystalline precursor VOHPO4·0.5H2O, formed from the reaction of V2O5 and H3PO4 with an alcohol. The alcohol acts as a reducing agent forming an aldehyde or ketone. These aldehydes and ketones, once formed, will lead to the formation of VO(H2PO4)2 as an impurity, at levels which will be difficult to detect but which are known to affect catalytic performance adversely. The use of isobutanol containing a small quantity of butanone for the reaction of V2O5 and H3PO4 (100%) confirmed that low levels of VO(H2PO4)2 can be formed together with VOHPO4·0.5H2O.