Recovery and reuse of ionic liquids and palladium catalyst for Suzuki reactions using organic solvent nanofiltration

Abstract

The separation, post-reaction, of ionic liquids and catalysts from reaction products is an unresolved challenge in the application of ionic liquids to organometallic catalysis. This paper addresses this challenge using organic solvent nanofiltration technology. Suzuki reactions were carried out in a homogeneous solution, comprising 50 ∶ 50 wt% ethyl acetate and ionic liquid. The post reaction mixture was diluted further with ethyl acetate and then separated by nanofiltration into a permeate fraction and a retained (retentate) fraction. The product was recovered in the nanofiltration permeate, while the ionic liquid and palladium catalyst were retained by the membrane and recycled into subsequent consecutive reactions. Thus, the organic solvent nanofiltration was able to separate the Suzuki reaction product from both catalyst and ionic liquid. Three ionic liquids were tested: cocosalkyl pentaethoxi methyl ammonium methosulfate (ECOENG™500), tetrabutylammonium bromide (TBAB) and trihexyl(tetradecyl)phosphonium chloride (CyPhos®101). All the ionic liquids screened showed positive effects on the catalytic stability, significantly reducing the formation of palladium black and providing high reaction yields over consecutive recycles. The best performance was observed for the CyPhos®101 system. Additional investigations employing this ionic liquid showed that the reaction–recycle process can be successfully performed at lower catalyst–substrate ratios, leading to higher catalyst turnover numbers.