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

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)(2)P2O7 and delta-VOPO4. Alkali-doped (VO)(2)P2O7 was prepared by heating alkali-doped VOHPO4.0.5H(2)O in He(8 h, 750degreesC), 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)(2)P2O7 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 greater than or equal to90%) from the aldol condensation of acetone, and the Cs-doped and undoped (VO)(2)P2O7 exhibited similar intrinsic activities (mol(isophorone) m(-2) h(-1)). In contrast, alkali-doping of the V5+ vanadium phosphate gamma-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 delta-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.