A Computational Exploration of H2S and CO2 Capture by Ionic Liquids Based on α-Amino Acid Anion and N7,N9-Dimethyladeninium Cation.

Hydrogen sulfide (H2S) and carbon dioxide (CO2) adsorption on a series of the aliphatic amino acid ionic liquids (AAILs) composed of N7,N9- dimethyladeninium cation with amino acid anions (AA = Gly, Ala, Val, Leu, and Ile) as the functionalized ILs with dual groups of amine have been investigated. On the basis of the obtained data, the possible sites of H2S adsorption are twice those of CO2 on the ionic liquids, and also the average adsorption energy of H2S (ΔE = -51.5 kJ mol-1) in the most stable region of adsorption is twice greater than that of CO2 (ΔE = -25.5 kJ mol-1). An increase in the length of the side chain of the amino acids increases the interaction energy of the H2S and CO2 capture (on the amine group of the [AA]- anions). Quantum theory of atoms in molecules analysis confirmed the noncovalent nature of the N···C bond formed between CO2 and N atom in both of the amine groups and S-H···O and S-H···N bond critical points of H2S on [dMA][AA]. Natural bond orbital analysis indicates that charge transfer in H2S adsorption is more important than CO2 capture. Finally, a correlation between the adsorption energy and the sum of stability energies (∑E(2)) in the most stable region has been obtained and discussed.