Synthesis of butyl levulinate based on α‐Angelica Lactone in the presence of easily separable heteropoly acid catalysts

A series of choline (Ch)-exchanged heteropoly acids (HOCH2CH2N(CH3)(3))(x)H(6-x)P2W18O62 [abbreviated as Ch(x)H((6-x))P(2)W(18)O(62), x=1-6] was synthesized and used as catalysts for the reaction of -angelica lactone (alpha-AL) with n-butanol to form butyl levulinate (BL). The solubility of Ch(x)H((6-x))P(2)W(18)O(62) in the reaction mixture was temperature dependent: The catalysts were soluble under the reaction conditions and precipitated upon cooling of the reaction mixture. This facilitated recovery of the catalysts from the liquid phase. Importantly, an increase of the Ch content caused a decrease of the catalyst solubility. Catalytic activity of Ch(x)H((6-x))P(2)W(18)O(62) for the reaction with n-butanol appeared to be in good agreement with the concentration of BrOnsted-acidic sites. The results suggest that the reaction proceeded through formation of pseudo-butyl levulinate as intermediate. Ch(2)H(4)P(2)W(18)O(62) exhibited the best balance between catalytic activity and temperature-dependent solubility. The yield of BL reached 79.4% at full conversion of alpha-AL at a moderate temperature of 75 degrees C in an open system. Ch(x)H((6-x))P(2)W(18)O(62) could be successfully reused five times without significant loss of activity.