Quantum Chemistry Study On Adsorption Of CO₂ In Ionic Liquid

On the basis of the theory of quantum chemistry, the absorption of two imidazolium-based ionic liquids and four phosphonium-based ionic liquids for CO₂ were caculated using Material Studio software Dmol3 module. The bond length, bond angle, hydrogen bonding, electric charge, molecular orbital, microscopic parameters such as Fukui function of the cation and anion of ionic liquids, ion-pairs and CO₂-ionic liquids were analysied in detail. The main conclusions are as follows: 1. Hydrogen bonding between cation and anion in 1-butyl-3-methylimidazolium tetrafluoroborate（[Bmim][BF₄]）, 1-butyl-3-methyl-imidazolium bis（trifluoromethylsulfonyl）-imide（[Bmim][TFSI]）, trimethyl（alkyl） phosphonium tetrafluoroborate([P₁₁₁₂][BF₄]), [P₁₁₁₄][BF₄]) and trimethyl（alkyl） phosphonium bis（trifluoromethylsulfonyl）-imide([P₁₁₁₂][TFSI], [P₁₁₁₄][TFSI]) are mainly interactions force. There were 5 hydrogen bongdings in both imidazium ionic liquids between cation and anion. Then 6⁷ hydrogen bondings were formed respectively between F atoms on the anion and H atoms on the cation for [P₁₁₁₂][BF₄] and [P₁₁₁₄] [BF₄], while 5 6 hydrogen bongdings were formed between O and N atoms on the anion and H atoms on the cation for [P₁₁₁₂][TFSI] and [P₁₁₁₄][TFSI].2. The positon of nucleophilic absorption site is C5 position of the imidazolium ring which is due to N = C bond causing electron deficiency. And the positon of nucleophilic absorption site is P2 position of the phosphonium ionic liquids. The position of electrophilic absorption sites occurs on the anion, which is confirm with the results of HOMO orbital concentration distribution.3. In this work, the stable configurations of several ionic liquids absorption CO₂ were obtained. In the stable configuration, the bond angle of CO₂ is only slightly change. The bond angle ∠O31-C32-O33 of is changed from 180.0 ° to 176.9 ° for [Bmim][BF₄]·CO₂, from 180.0 ° to 177.9° for [Bmim][TFSI]·CO₂, from 180.0 ° to 176.8° for [P₁₁₁₂][BF₄]·CO₂ and from 180.0 ° to 177.6° for [P₁₁₁₂] [TFSI]·CO₂. Thus the bond angles are changed a small margin, indicating the interaction of these types of ionic liquids and CO₂ is physical absorption process.4. The anion and cation have influence on molecular binding energy and the order of binding energy is Ebinding（[Bmim] [TFSI]） > Ebinding([P₁₁₁₄][TFSI]) > Ebinding([P₁₁₁₂][TFSI]) > Ebinding（[Bmim][BF₄]） > Ebinding([P₁₁₁₄][BF₄]) > Ebinding([P₁₁₁₂][BF₄]). The binding energy of [TFSI]-) is higher than that of [BF₄]- and the binding energy of imidazium ionic liquids is higher than that of phosphonium ionic liquids hasing the same anion. Moreover, with the increasing of carbon chain with phosphonium ionic liquids, the binding energy increases. The absorption equilibrium data shows that CO₂ uptake of imidazolium ionic liquid is higher than phosphonium ionic liquid with the same anionic, and CO₂ adsorption capacity of [Bmim] [TFSI] is the 1.37 wt % under the 313 k and 0.1 Mpa.5. The charge transfer of ionic liquids affects the viscosity and the viscosity increases with the charge-transfer resulting longer CO₂ adsorption equilibrium time. The density of charge transfer of phosphonium ionic liquid is less than imidazium ionic liquids with the same anion, which leads to the viscosity of phosphonium ionic liquids decreasing. And experiments have confirmed that CO₂ adsorption equilibrium time of phosphonium ionic liquid is shorter than that of imidazolium ionic liquid.The charge transfer of [TFSI]- is less than that of [BF₄]- with the same cation, leading to ionic liquids with [TFSI]- has lower viscosity.6. The results of molecular orbitals analysis showed that the HOMO orbital of phosphonium ionic liquids is mainly occupied by anions and the orbitals between CO₂ molecules and anion（[BF₄]- and [TFSI]-） are almost no overlap. Therefore, CO₂ molecules is easy to absorb on the anion. When the anion is [BF₄]-, the LUMO orbital of [P₁₁₁₂][BF₄]·CO₂ is mainly composed of absorption CO₂ in the C and O atoms, while the LUMO orbit of [P₁₁₁₄][BF₄]·CO₂ is mainly composed of cation. After ionic liquid with [TFSI]- absorbtion CO₂, the LUMO orbital is mainly dominated by cation and the absorption CO₂.