Theoretical Study On The Mechanism For Fixation Of CO₂ Catalyzed By Quaternary Ammonium-based Ionic Liquids

Carbon dioxide（CO₂）is the major greenhouse gases.,However,CO₂ is abundant,inexpensive,and non-toxic C1 resource.The effective utilization of CO₂ would not only reduce the environmental pollution but also alleviate the energy crisis caused by the depletion of fossil fuels.Therefore,it is very meaningful to study the conversion and elimination of CO₂.However,the kinetic inertness of CO₂ is very high.As a result,it is difficult to activate the CO₂ in benign condition.The synthesis of propylene carbonate（PC）from CO₂ and propylene oxide（PO）has been attracted much attention due to the atomic economy and high efficiency.Numerous catalysts have been developed for this reaction.However,there are some common disadvantages,such as,the harsh reaction conditions,requirement of co-catalyst or organic solvents.The ionic liquids（ILs）stand out from other catalysts with the advantages of high efficiency,simple component,and water stability.One of the most outstanding advantages of ionic liquids is the tunable structure.Moreover,additional solvent is not necessary since ionic liquids are the excellent solvents.It is necessary to elucidate the reaction mechanism,which is the precondition to develop the high-efficiency catalysts.The mechanism of CO₂ and PO has been reported in some literatures.However,there are two common problems.One is that the predicted catalytic activity by theoretical method is inconsistent with the experimental results;the other is that the solvent effect aroused by the ionic liquid is not sufficiently considered.In this dissertation,Quantum Mechanics（QM）and Molecular Dynamics（MD）were used to study the the mechanism of coupling reaction of CO₂ and PO catalyzed by a series of quaternary ammonium-based ionic liquids.Our goals are to improve the accuracy of the theoretical prediction,to consider the solvent effect of ionic liquids by a new model,and to establish the structure-activity relationship for ionic liquids.The details are as follows:1.The mechanism of coupling reactions of CO₂ with PO catalyzed by a series of hydroxyl-functionalized quaternary ammonium ionic liquids（N,N,N-triethyl-2-hydroxyethanaminium bromide（NEt₃（HE）Br）,N,N-Diethyl-2-hydroxy-N-（2-hydroxyethyl）ethanaminium bromide（NEt₂（HE）₂Br）,N-Ethyl-2-hydroxy-N,N-bis（2-hydroxyethyl）ethanaminiumbromide（NEt（HE）₃Br）,tetrakis（2-hydroxyethyl）ammonium bromide（N（HE）₄Br）,the number of hydroxyl groups in the cation gradually increases from 1 to 4）is investigated by combination of Density Functional Theory（DFT）and MD methods.When the number of hydroxyl groups varies from 1 to 3,the catalytic activity increases steadily.The catalytic activity of NEt（HE）₃Br is the highest.However,the corresponding PO conversion decreases when N（HE）₄Br is employed as the catalyst,indicating that the catalytic activity is not strictly proportional to the number of hydroxyl-functionalized groups.The barrier heights of the rate-determining step are calculated at the level of M06/6-311+G（2d,2p）（PCM）//B3PW91/6-31G（d,p）.When only one ionic liquid is considered,the barrier heights of rate-determining steps are NEt₂（HE）₂Br（22.86 kcal/mol）>NEt₃（HE）Br（21.11 kcal/mol）>N（HE）₄Br（19.88 kcal/mol）≈NEt（HE）₃Br（19.75 kcal/mol）,which is not consistent with the experimental catalytic activity of NEt（HE）₃Br（92%）>NEt₂（HE）₂Br（88%）>NEt₃（HE）Br（81%）>N（HE）₄Br（77%）.It is attributed that the contribution from other hydroxyl groups is not included.Other hydroxyl groups are not involved in the core catalytic region due to steric hindrance.In order to include more hydroxyl groups in the core catalytic region,the Double-IL model is firstly proposed by us,that is,two ionic liquids as catalysts.The barrier heights of rate-determining step decrease in the order of N（HE）₄Br（19.22 kcal/mol）>NEt₃（HE）Br（18.32 kcal/mol）>NEt₂（HE）₂Br（16.02 kcal/mol）>NEt（HE）₃Br（13.60 kcal/mol）on the basis of the Double-IL model,which is consistent with the experimental sequence.There are many possible reaction routes in the Double-IL model.The most favorable situation is that one ionic liquid acts as an electrophile to attack the O atom of PO directly,while the other ionic liquid is utilized to stabilize the Br^-anion via hydrogen bond interaction.Subsequently,the effect of nonbonding interaction on catalytic activity is investigated by NCI（noncovalent interaction）method.The lowest barrier height of rate-determining step is mainly contributed by the strongest C…Br interaction.There are various possibilities for the relative positions of two ionic pairs.In order to verify the reasonable of the Double-IL model,256 pairs of ionic liquids were simulated by MD simulations.In the Double-IL model,not only the interaction between anions and cations in ionic liquid but also the interaction between different ionic liquids are considered.As a result,the accuracy is greatly improved.The Double-IL model is helpful to explore the mechanism catalyzed by ionic liquids with multi-functionalized groups.2.In the above chapter,the sequence of catalytic activity predicted by the Double-IL model is consistent with the experimental sequence.However,the difference of barrier heights is not large enough to distinct them.For example,the deviation between NEt₃（HE）Br（18.32 kcal/mol）and N（HE）₄Br（19.22kcal/mol）is only 0.90 kcal/mol,which is even in the acceptable computational error.The influence of ionic liquids as solvents should be carefully considered.The hydroxyl-functionalized quaternary ammonium ionic liquids are not only the catalysts but also the solvents.Although the solvent effect has been considered by a polarized continuum model（PCM）,it is not suitable for ionic liquids.In this chapter,the influence of the solvent effect induced by ionic liquids is studied by MD and ONIOM methods（B3PW91/6-31G（d,p）:HF/STO-3G）.It is the first time that the ionic liquids are directly added to the catalytic system as solvent leading to the more accurate situation.The sequence of catalytic activity calculated by ONIOM method is consistent with the experimental results,which is the same with the Double-IL model.However,the difference of barrier heights is enlarged in ONIOM calculation,which is more close to the experimental yields.The result calculated by ONIOM method is more reasonable.Furthermore,the critical hydrogen bond interactions and C…Br interaction in transition states are elucidated by AIM（the atoms in molecules）and NCI methods.In NEt（HE）₃Br catalytic reaction,the electrophilic activation is more important than nucleophilic activation.While the latter is more important in other three transition states.3.Protic ionic liquids are another important ionic liquids,but fewer studies have been performed to explore the protic ionic liquids,especially for the task-functionalized protic ionic liquids.The mechanism of coupling reaction of CO₂ and PO catalyzed by N,N-diethyl-2-hydroxyethanaminium bromide（[HMEA]Br）,N-ethyl-2-hydroxy-N-（2-hydroxyethyl）ethanaminiumbromide（[HDEA]Br）,and tris（2-hydroxyethyl）ammonium bromide（[HTEA]Br）is investigated by both the Single-IL and Double-IL models.Moreover,the solvent effect of ionic liquids is considered.At the M06/6-311+G（2d,2p）（PCM）//B3PW91/6-31G（d,p）levels,the sequence of catalytic activity calculated by the Single-IL model is inconsistent with the experimental results,while the calculated result of the Double-IL model is totally consistent with the experimental result.In the most favorable route,one ionic liquid activates the PO and the other one stabilizes the Br^-anion.The difference between two routes is further refined when the solvent effect is considered by ONIOM method（B3PW91/6-31G（d,p）:HF/3-21G）.The AIM and NCI results indicate that the moderate strength of hydrogen bond interaction along with the strongest C…Br interaction finally results in the lowest barrier height for[HTEA]Br.The Single-IL model is not an advisable choice to study the catalytic activity for a series of ionic liquids due to its poor reliability of the predicted catalytic activity.Considering the interaction between ionic liquids is a prerequisite for a reasonable and reliable calculation.Moreover,the ionic liquids should be incorporated into the catalytic system to consider the solvent effect rather than by the existed solvent model.