One-step synthesis of quaternized polyethyleneimine and its application in transesterification reactions

Abstract

In this study, dimethyl carbonate (DMC) was utilized as a green quaternization agent to transform branched polyethyleneimine (B-PEI) into a highly basic and exceptionally active macromolecular ionic liquid catalyst, D-BPEI, through a one-step quaternization reaction. Notably, DMC could be sourced from the materials and products of transesterification reactions. To qualitatively and quantitatively analyze the structure and properties of the synthesized ionic liquid catalyst, various characterization techniques were employed. Subsequently, the preparation conditions of D-BPEI were optimized, and its catalytic activity and stability were examined. Additionally, the quaternization characteristics of B-PEI were investigated, and the quaternization mechanism between DMC and B-PEI was clarified. The results revealed that the type of the quaternization reagent, solvent polarity, mass ratio of B-PEI to quaternization reagent, reaction temperature, and reaction duration critically influenced B-PEI quaternization. Among various investigated quaternization reagents, DMC demonstrated the highest reactivity owing to the smallest geometric mean radius of the released electrophilic groups. At a synthesis temperature of 100 °C, a reaction time of 6 h, and a B-PEI : DMC mass ratio of 1 : 2, the resulting D-BPEI catalyst demonstrated excellent catalytic performance and achieved the highest quaternization degree of 32.12%. When D-BPEI was used as the catalyst, the ethyl carbonate (EC) conversion was 74.8%, the diethyl carbonate (DEC) selectivity was 92.8%, the DEC yield was 69.4%, and the turnover frequency reached 120.2 h⁻¹. Remarkably, even after six cycles of reuse, D-BPEI maintained high catalytic activity. Furthermore, D-BPEI demonstrated good catalytic performance in various transesterification reactions. Kinetic studies revealed that the activation energy of the transesterification reaction between EC and ethanol (EtOH) catalyzed by D-BPEI was 34.59 kJ mol⁻¹. Based on the characteristics of D-BPEI, a technique for the continuous synthesis of DEC without azeotrope generation was proposed in this study.