Synthesis of butyl salicylate by phase-transfer catalysis with dual-site phase-transfer catalyst and ionic liquid in tri-liquid system

The synthesis of butyl salicylate using dual-site phase-transfer catalyst 1,4-bis(trihexylammoniomethyl)benzene dibromide (BTHAMBB) and ionic liquid trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethyl pentyl)phoshinate (THTDPBP) under ultrasound irradiation in tri-liquid system was investigated. BTHAMBB was synthesized from p-xylylene dibromide reacting with trihexylamine and used to form the third-liquid phase, which contained highly concentrated catalyst for conducting intrinsic reaction. No extra addition of inorganic salts was needed using BTHAMBB to prepare third-liquid phase. The product yield in 4 h of reaction was 0% without catalysts. With BTHAMBB under ultrasonic irradiation (28 kHz/300 W) the reaction rate can give 69.7% of product yield in 4 h of reaction at 70 °C and 250 rpm, and the product yield was further enhanced to 95.4% by incorporating with THTDPBP. In addition, the third-liquid phase was recovered to be reused for two times and showed insignificant loss in catalytic activity. The overall reaction was successfully described by pseudo-first-order kinetic model. This work developed an efficient method to synthesize esters by third-liquid phase-transfer catalysis.