Synergistic catalysis of acid–base bifunctional ionic liquids for pentanal self‐condensation reaction

Abstract

AbstractBACKGROUNDPentanal self‐condensation to 2‐propyl‐2‐heptenal is an important process of carbon chain extension for industrial manufacture of 2‐propylheptanol. This reaction in industry employs an aqueous caustic alkali catalyst, causing several problems of equipment corrosion, treatment of alkali wastewater, etc. Herein, several acid–base bifunctional ionic liquids were synthesized for catalyzing pentanal self‐condensation.RESULTS[PEmim]Cl‐0.5Zn(CH3COO)2 exhibited a better catalytic performance, and the yield and selectively of 2‐propyl‐2‐heptenal could reach 82.0% and 86.6%, respectively. The recovered [PEmim]Cl‐0.5Zn(CH3COO)2 could be reused three times after being regenerated without a significant loss in its catalytic performance. The structures and charge properties of [PEmim]Cl‐Zn(CH3COO)2 and a complex of [PEmim]Cl‐Zn(CH3COO)2 and pentanal were simulated by density functional theory calculation. The results show that the action of acid and base sites of [PEmim]Cl‐Zn(CH3COO)2 can effectively activate pentanal by enhancing the electrophilicity or nucleophilicity of different pentanal molecules.CONCLUSIONThe acid–base synergetic effect of [PEmim]Cl‐Zn(CH3COO)2 could account for its high catalytic performance. An acid–base synergetic catalysis mechanism of [PEmim]Cl‐Zn(CH3COO)2 in pentanal self‐condensation was established. © 2019 Society of Chemical Industry