Study On The Mechanism Of Fixed CO₂ Catalyzed By The Functionalized Ionic Liquids

Energy and environment are two historical problems all over the world. Besides the famous green house gas, carbon dioxide (CO2) is a cheap, abundance, and non-toxic C1 resource. In this work, the fixation reaction mechanisms of CO2 catalyzed by a series of ionic liquids are studied by the density functional theory (DFT) method. The main contents are as follows:1. Utilization of CO2 to synthesize useful organic compounds has attracted much attention from both the academia and industry. Experimentally, four hydroxyl-functionalized ionic liquids have been successfully applied to catalyze the fixation of CO2. However, their detailed mechanisms are still ambiguous. Herein we performed a theoretical study on the mechanism of the fixation of CO2 catalyzed by 2-hydroxyl-ethyl-triethylammonium bromide by using the DFT method, and total five reaction pathways were identified including two-step (ring-opening and ring-closing) and three-step mechanisms (ring-opening, CO2 insertion and ring-closing). Furthermore, the similarities and differences of two different catalysts,2-hydroxyl-ethyl-triethylammonium bromide and 2-hydroxyl-ethyl-tributylammonium bromide, were discussed in detail. The results show that increasing the chain length of substituted group will decrease the catalytic activity. In addition, the catalytic performance of different cations was also studied. The result demonstrates that imidazole structure in the cation is a good choice to improve the catalytic ability. The hydroxyl group and the active H atom are the main factors to impove the catalytic performance.2. To gain an insight into the high catalytic activity of N"-(2-aminoethyl)-N,N,N’,N’-tetramethylguanidine bromide, the mechainism of cycloaddition reaction of CO2 and the epoxide catalyzed by a series of guanidine-based ionic liquid was studied by means of the DFT method. Besides the hydroxyl-and carboxyl-groups, the amino-group is introduced as a functional group, which is considered to show the excellent catalytic activity. Except for the amino-functionalized ionic liquid, others own the three-step reaction mechanism (ring-opening, CO2 insertion and ring-closing). The results show that the mechanism of amino-functionalized guanidinium-based ionic liquid is divided into two steps:(i) two ionic liquids molecules can react with one CO2 molecule to form the carbamic acid, and (ii) the coupling reaction of CO2 is catalyzed by the carbamic acid. The following reaction mechanism is the same with that catalyzed by the hydroxyl-and carboxyl-functionalized ionic liquids. Besides the cation, the effect of different anions is investigated. The result reveals that the catalytic performace of BF4-is better than that of PF6-. Introducing more acidic functional groups or choosing anions with stronger ability of nucleophilic attack can improve the catalytic activity.3. Three functionalized phosphonium-based ionic liquids with the carboxyl, hydroxyl, and amino groups were firstly employed as catalysts for the synthesis of cyclic carbonate through the coupling reaction of CO2 and propylene oxide with the catalytic performance of carboxyl-group> hydroxyl-group> amino-group. However, when these functional groups were introduced into the guanidinium-based ionic liquid, the different catalytic order was found. So the catalytic activity of functionalized ionic liquid is dependent on not only the functional group but also the nature of cation. What is the best combination for the functional group and cation structure? Will the other functional group be better choice with phosphonium-based ionic liquid? To elucidate these questions, the mechanism of coupling reaction of CO2 catalyzed by the functionalized phosphonium-based ionic liquid is explored by the DFT approach. The catalytic activity of different catalysts is compared. The results indicate that the combination of amino functional group with guanidinium-based ionic liquid is a good choice to refine the catalytic performance, carboxyl group is better to couple with guanidinium-based and phosphonium-based ionic liquids, and hydroxyl group should be introduced into the imidazolium-based ionic liquid. In addition, a new phosphonium-based ionic liquid was also designed by introducing sulfonyl hydroxide group and its catalytic activity was theoretically evaluated.