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

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).