Theoretical Study On Synthesis Of Cyclic Carbonate Catalyzed By Functionalized Ionic Liquids

Carbon dioxide (CO2) is one of the most abundant greenhouse gases, which has resulted in serious environmental problems. Therefore, how to effectively reduce the content of CO2 in atmosphere has attracted more and more attentions. CO2 is also a cheap, nontoxic, and abundant C1 resources. The synthesis of the chemical products by using CO2 will be not only helpful to solve the greenhouse effect but also solving the depleting of carbon resources. Unfortunately, the utilization of CO2 as chemical feedstock is limited to a few industrial processes, among which the synthesis of five-membered cyclic carbonates via the cycloaddition of CO2 to epoxides is one of the most prospective reactions. Because of the thermal stability of CO2, catalyst is essential for this process. Numerous catalysts have been proposed for the insertion of CO2 into epoxides to generate cyclic carbonates. The catalyzed mechanism of hydroxyl functionalized ionic liquids and carboxyl functionalized ionic liquids were studied in this thesis. It has been testified that they have shown high selectivity and high yield for this reaction. The research work of this paper can not only help to understand the reaction mechanism in detail, but also provide a theoretical basis for the design of new type of functionalized ionic liquid catalysts.1. The mechanism of cycloaddition reaction between carbon dioxide and epoxide, catalyzed by HEMIMC, was investigated using density functional theory method. The equilibrium geometries of all the stationary points, including the reactants, products, minima, and transition states were optimized using the B3LYP method with 6-31G(d,p) basis set. Vibrational frequency calculations were carried out at the same level to determine the nature of stationary points and to derive the zero point energy (ZPE). Starting from the saddle-point geometries and going downhill to two desired minima, the minimum-energy path (MEP) was constructed by intrinsic reaction coordinate (IRC) theory. Energies were corrected at the B3LYP/6-311+G(2d,2p) level. The results showed that in the presence of HEMIMC, the reaction mechanism changed from single-step to multi-path. Seven reaction pathways are classified into two types, one is that two steps, including epoxide ring-opening and ring-closure of cyclic carbonate, and the other is three main steps, including epoxide ring-opening, carbon dioxide insertion, and ring-closure of cyclic carbonate. The rate-determining barrier height of the most favorable path is 17.63 kcal/mol, both ring-opening and ring-closure step have the possibility to be the rate-determining step with the similar energy barrier. The catalytic activity of HEMIMC was studied, and the catalytic mechanism was elucidated. The nucleophilic attack of anion and hydrogen bond are two most important factors to promote the cycloaddition reaction. The nucleophilic attack of the Cl- anion promotes the ring-opening, and the hydrogen bond facilitates the stabilization of the intermediates and transition states, especially the OH functional group in HEMIMC. Finally, the influence of different anions on the catalytic activity was investigated, the reaction barrier is slightly affected by the different type of anions.2. The fixation of carbon dioxide (CO2) with epoxide (PO) catalyzed by a series of carboxylic acidic functionalized ionic liquids (ILs) catalysts is investigated by density functional theory (DFT) method. The equilibrium geometries of all stationary points (reactants, products, intermediates, and transition states) involved in this work were optimized at the B3LYP level with the 6-31G(d,p) level. At the same level, the vibrational frequency was calculated to identify the nature of stationary points and to make ZPE correction. The number of imaginary frequency is employed to identify the minimum or the transition state. Except for the catalysts reported by experiment, the catalytic activity of a new designed catalyst is also explored. These ILs are categorized into four groups according to the different cation structure and number of functional group, and only the most favorable reaction (three-step catalyzed mechanism) route catalyzed by these catalysts were calculated. The effects of different chain length, anion, and cation structure on the catalytic activity are explored. Starting from the transition state, the MEP (minimum-energy path) was constructed by IRC (intrinsic reaction coordinate) theory to confirm the transition state connecting two desired minima. The natural charges were analyzed using NBO method at the same level. Finally, the energies are refined at the B3LYP/6-311+G(2d,2p) level on the basis of the optimized geometries. The results showed that the elongation of alkyl chain length in cation will increase the product yield, while increasing the chain bulk has almost negligible effect on the enhancement of catalytic activity. Utilization of imidazole group as the cation is better than pyridine group. And the cation with two functional groups will have a better catalytic activity than that with one functional group.