Understanding the CO2 valorization to propylene carbonate catalyzed by 1-butyl-3-methylimidazolium amino acid ionic liquids

Abstract

Nowadays, CO2 emission impacts on our life standards, revealing itself as one of the major environmental challenges of present century. One main strategy is to develop new technologies within integrated Carbon Capture and Utilization (CCU), to obtain valuable chemical products. The design of new systems able to efficiently retain CO2 and convert it at mild operating conditions is an important challenge. Ionic liquids (ILs) were demonstrated as multitask absorbents-catalysts in processes involving CO2 capture and cycloaddition reaction to obtain cyclic carbonates. In this work, the use of amino acid based ionic liquids (aa-ILs) is proposed to promote a sustainable catalytic process for CO2 valorization using propylene oxide (PO) to produce propylene carbonate (PC). Three aa-ILs were proposed in the study: 1-butyl-3-methylimidazolium glycinate ([bmim][GLY]), 1-butyl-3-methylimidazolium methioninate ([bmim][MET]), and 1-butyl-3-methylimidazolium prolinate ([bmim][PRO]). Computational calculations by DFT method reveals that the reaction firstly occurs through the PO ring opening by aa-IL catalyst, followed by CO2 nucleophilic addition, rather than direct CO2 activation. The carbonate formation using aa-IL catalysts was experimentally demonstrated by ATR-FTIR experiments at mild conditions (4 bar of initial pressure and 80–120 °C), obtaining PC yields ordered as [bmim][MET] > [bmim][GLY] > [bmim][PRO], same trend than their CO2 chemical absorption capacity. Further stirred tank reactor experiments were carried out to scale-up the process, confirming the suitability of aa-ILs as homogeneous catalyst for CO2 conversion to cyclic carbonates, with extremely low consumption (0.5% m/m).