Citrate-Based Ionic Liquids for CO2 Capture—A Computational Approach Toward Environmentally Benign Ionic Liquid Anions

Abstract

This work aims to explore and characterize citrate as an anion of nontoxic and biocompatible origin, which is a crucial step to developing a sustainable CO2 capture process through ionic liquids (ILs). Citrate ILs have recently been synthesized and utilized as solvents and catalysts for various synthetic purposes in the industry. These are found to be easily recycled, nonpolluting, less corrosive, and easy to synthesize. In this work, citrate–CO2 and citrate–bmim (1-butyl-3-methylimidazolium) ion pair (IP)–CO2 interactions have been theoretically explored via carboxylation reactions and various electronic structure calculations. The results indicate favorable citrate–CO2 interactions in the gas as well as the aqueous phase resulting in monocarboxylates, dicarboxylates, and tricarboxylates of citrates owing to the availability of its three carboxylate O atoms. Even as citrate is paired with bmim, it shows the possibility of multiple site CO2 absorptions. This system should thus serve as a pathway for enhanced CO2 capture and better desorption by reducing the formation of carbene–CO2 complex (reduced basicity of the anion and enhanced steric hindrance of the cation). The study reveals that in the IP, at least one of the citrate O atoms can form a covalent carboxylate (chemisorption) with CO2 while other available O sites may weakly bind CO2 (physisorption).