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

Abstract

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 VOHPO₄·0.5H₂O were prepared using the reaction of V₂O₅ and H₃PO₄ with aqueous hydrochloric acid or isobutanol as reducing agents, or from the reaction of VOPO₄.2H₂O with isobutanol. (VO)₂P₂O₇, prepared by heating VOHPO₄·0.5H₂O in He (8 h, 750 °C), before and following partial oxidation in air or butane/air, and α_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 (VOHPO₄·0.5H₂O and (VO)₂P₂O₇) exhibited only acidic active sites, whereas the V⁵⁺ phases (α_I-VOPO₄ and oxidised (VO)₂P₂O₇) 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.