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

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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.

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