Aldol condensation reactions of acetone and formaldehyde over vanadium phosphate catalysts: Comments on the acid-base properties

The acid base properties of vanadium phosphate catalysts are investigated using the aldol condensation of acetone and the reactions of 2-methylbut-3-yn-2-ol (MBOH). Three well characterised samples of VOHPO4.0.5H(2)O were prepared using the reaction of V2O5 and H3PO4 with aqueous hydrochloric acid or isobutanol as reducing agents, or from the reaction of VOPO4.2H(2)O with isobutanol. (VO)(2)P2O7, prepared by heating VOHPO4.0.5H(2)O in He (8 h, 750degreesC), before and following partial oxidation in air or butane/air, and alpha(I)-VOPO4 were also investigated. The reaction of MBOH was used to probe the nature of the acid-base properties of the vanadium phosphates. The V4+ phases (VOHPO4.0.5H(2)O and (VO)(2)P2O7) exhibited only acidic active sites, whereas the V5+ phases (alpha(I)-VOPO4 and oxidised (VO)(2)P2O7) exhibited some basic sites in addition to the acid sites. For the aldol condensation reactions of acetone, the V4+ phases were found to be selective for the formation of isophorone from acetone alone and methyl vinyl ketone from the reaction of acetone and formaldehyde. In contrast, vanadium phosphate catalysts containing V5+ phases are not selective to these products and only form hydrocarbons (typically isobutane and isobutene). For all these reactions, the catalyst activity is short lived and the deactivation that is observed is due to the surface becoming fouled by the adsorption of products of polymerisation of the reaction products. However, the catalyst reactivity can be restored by a simple oxidation treatment. The nature of active sites in n-butane oxidation to maleic anhydride is also discussed and it is concluded that basic sites are required in addition to acidic surface sites for the selective formation of maleic anhydride. For the reaction of MBOH, the data are found to give a linear relationship for a Cremer-Constable plot and this is discussed in terms of the enthalpy of adsorption of MBOH.