Aldol condensation reactions of acetone over alkali-modified vanadium phosphate catalysts

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) over alkali-doped vanadium phosphate catalysts. Two well defined vanadium phosphates were investigated, namely (VO)₂P₂O₇ and δ-VOPO₄. Alkali-doped (VO)₂P₂O₇ was prepared by heating alkali-doped VOHPO₄·0.5H₂O in He (8 h, 750 °C), and Na⁺-, K⁺- and Cs⁺-doped samples were investigated. The reaction of MBOH was used to probe the acid–base nature of the surface sites. The undoped V⁴⁺ (VO)₂P₂O₇ exhibited only acidic active sites, whereas when doped with 10% Na⁺, K⁺ or Cs⁺ the active sites became predominantly basic in nature. Both the doped and undoped samples were selective for the formation of isophorone (selectivity 90%) from the aldol condensation of acetone, and the Cs-doped and undoped (VO)₂P₂O₇ exhibited similar intrinsic activities (mol(isophorone) m⁻² h⁻¹). In contrast, alkali-doping of the V⁵⁺ vanadium phosphate γ-VOPO₄ did not induce the formation of a significant number of surface basic sites and the active sites for MBOH decomposition remained predominantly acidic in nature. Both the doped and undoped δ-VOPO₄ were non-selective in the aldol condensation of acetone, and during the short-lived reaction, only hydrocarbons (typically isobutane and isobutene) were observed. The results are discussed with respect to the nature of active sites for the selective oxidation of n-butane to maleic anhydride.