Theoretical Investigation On The Co-catalytic Mechanism Of Ionic Liquids/ZnBr₂ For The Fixation Of CO₂

The fixation and utilization of carbon dioxide（CO₂） have received increasing attention in light of the growing problems of the greenhouse effect and depletion of the fossil fuels. Emission of CO₂ into the atmosphere has already resulted in global warming and climate change, while CO₂ also serves as a cheap, nontoxic, and abundant C1 building block. In the past two decades, various pathways of CO₂ utilization have been explored. Among them, the synthesis of five-membered cyclic carbonates via the coupling reaction of CO₂ with epoxides is one of the most promising methodologies.However, the thermodynamic stability and kinetic inertness of CO₂ are roadblocks that are in the way of utilizing it as a raw material. Exploring efficient catalysts is a promising method to overcome this obstacle. Although some catalysts have been explored for the cycloaddition of CO₂ to epoxides, such as, alkali metal salts, metal oxides, transition-metal complexes, and others, they are still suffer from the low activity, harsh reaction conditions, the need of organic co-solvents, or a combination of these. Recently, ILs have attracted considerable attentions because of their unique properties especially due to their environmental benign and feasibility for design. The involvement of the Lewis acids, such as, ZnBr₂, would dramatically enhance the catalytic activity. Moreover, it also offers the advantages of recyclability and reusability. In this thesis, the mechanisms of coupling reaction for CO₂ with epoxide（PO） catalyzed by the binary catalysts “ILs/ZnBr₂” are investigated by the density functional theory（DFT） method. The main contents are as follows:1. To explore the reason for the high activity of the cycloaddition reaction of PO（propylene oxide） with CO₂ catalyzed by choline chloride（CH）/ZnBr₂ co-catalyst, the mechanism has been constructed using a DFT method. The combination of CH and ZnBr₂ will generate three new stable complexes, i.e., [Ch]₂[ZnBr₂Cl₂], Ch⁺ZnBr₂Cl^-, and Ch⁺ZnBrCl₂^-. The latter two are derived from the dissociation of [Ch]₂[ZnBr₂Cl₂]. The detailed mechanism of a coupling reaction catalyzed by the more stable complex Ch⁺ZnBrCl₂^- is explored. It has been elucidated that the attack from the Zn complex and the Br- anion is the major factor in promoting the cleavage of the C-O bond of PO. Finally, the performance of [Ch]₂[ZnBr₂Cl₂] is also investigated, providing less activity, indicating that it should dissociate to gain better catalytic effect. It is hoped that our work will provide valuable clues for further theoretical and experimental studies for the synthesis of more powerful catalysts.2. Numerous binary catalysts of IL/Lewis acid have been developed for the coupling reaction of carbon dioxide and epoxides to form cyclic carbonates with high catalytic activity under benign environment. However, the mechanism is still obscure for most of catalysts. The catalytic mechanism of the binary catalysts hexaalkylguanidinium bromide/ZnBr₂ is elucidated by theoretical method in this work to obtain the reason of their high catalytic activity. Owing to the complicated of the binary catalysts, there are lots of possible attack forms. Finally, it is confirmed that the electrophilic attack from the Zn complex and the nucleophilic attack from the Br- anion are the essential factors to promote the ring opening of PO. Following the most favorable route, the catalytic activity of different binary catalysts, including the ILs/ZnBr₂ and Bu₄NBr/Zn（salphen）, is compared. Moreover, the influence of the bulk of hexaalkylguanidinium salt on the catalytic activity is studied. The catalytic activity is enhanced with the increased bulk of the hexaalkylguanidinium salt. It is expected that our theoretical study would provide valuable clues to further refine the binary catalysts.