Insight into Capture of Greenhouse Gas(CO2)based on Guanidinium Ionic Liquids

He-xiu Liu,Rui-lin Man,Bai-shu Zheng,Zhao-xu Wang,Ping-gui Yi

a.School of Chemistry and Chemical Engineering,Central South University,Changsha 410083,China

b.Key Laboratory of Theoretical Chemistry and Molecular Simulation of Ministry of Education,Hunan University of Science and Technology,Xiangtan 411201,China

(Dated:Received on September 24,2013;Accepted on February 10,2014)

Insight into Capture of Greenhouse Gas(CO2)based on Guanidinium Ionic Liquids

He-xiu Liua,b,Rui-lin Mana∗,Bai-shu Zhengb,Zhao-xu Wangb,Ping-gui Yib

a.School of Chemistry and Chemical Engineering,Central South University,Changsha 410083,China

b.Key Laboratory of Theoretical Chemistry and Molecular Simulation of Ministry of Education,Hunan University of Science and Technology,Xiangtan 411201,China

(Dated:Received on September 24,2013;Accepted on February 10,2014)

Quantum mechanics and molecular dynamics are used to simulate guanidinium ionic liquids. Results show that the stronger interaction exists between guanidine cation and chlorine anion with interaction energy about 109.216 kcal/mol.There are two types of spatial distribution for the title system:middle and top.Middle mode is a more stable conformation according to energy and geometric distribution.It is also verif i ed by radial distribution function.The continuous increase of carbon dioxide(CO2)does not af f ect the structure of ionic liquids, but CO2molecules are always captured by the cavity of ionic liquids.

Ionic liquids,Quantum chemical calculation,Molecular dynamics simulation, Interaction energy,Radial distribution

I.INTRODUCTION

Ionic liquid is a new type of liquid compound with low melting point,low vapor pressure,high solubility, and high heat stability[1-4].Green ionic liquid is the ionic liquid made up of recycled cations and anions, which has less volatile,recycled characteristics and is in line with the green chemistry.Over the past twenty years,synthesis of green medium and functional materials has received extensive attention from the domestic and international academic and industrial f i elds.The“programmable”feature of ionic liquids makes it possible to adjust its physical and chemical properties by changing the type and size of cations or anions and to design the ionic liquid with special function according to specif i c applications and needs.Great progress has been made in the application and research of ionic liquids so far,which has been widely used as a solvent, reaction medium,catalyst and functional material in the synthesis and catalysis,extraction and separation, electrochemistry,biological chemistry and other f i elds [5].

In recent years,the climate warming caused by CO2has become one of the focuses on environmental issues in the world.It is urgent to solve the problem of CO2greenhouse ef f ect.But the CO2is a safe and abundant carbon resource from a point of recycling.The gaseous CO2can be used as a fertilizer and sterilization,etc. Supercritical CO2can also be used for food,pharmaceutical and other industries.Solid CO2can be used for artif i cial rainfall,concrete production and environmental protection,etc.Therefore,there is important scientif i c signif i cance in f i xing and recovering CO2industrially.Existing CO2f i xation technologies such as biological method,physical method and chemical method have some limitations that carbon resource waste and organic solvent volatilization can lead to the problems of environment pollution,the equipment corrosion and post-processing complexity.Because of the features of low vapor pressure and strong dissolving ability of the ionic liquid,to f i x CO2using ionic liquids has attracted great attentions.There are the following advantages in applying ionic liquid to f i x CO2.The CO2recycling use takes the place of the direct abandon of traditional method.Nature of the ionic liquid is stable,non-volatile and recycled.It is shown that ionic liquids have good abilities to absorb and dissolve CO2and show ef f ective catalytic or sub-catalytic performances in the CO2conversion reaction under certain conditions[6-10].

Compared with imidazoles and phosphonium salt, guanidinium ionic liquid has more signif i cant thermal and chemical stability,better catalytic activity and stronger biological activity etc.[11-13].In addition, the charge dispersion degree is high in guanidine salt cation.Moreover,the dif f erent substituent on the three nitrogen atoms and the counter anions can be directionally designed and chosen in order to make the ionic liquid own some excellent physical or chemical properties.For example,Wang et al.studied the force f i eld of the TMGL ionic liquid and the solubility of SO2and CO2from molecular danymics simulation[14].Zhang et al.investigated the microscopic structure,interactions,and properties of pure ionic liquid[ppg][BF4]and mixtures of[ppg][BF4]and CO2by molecular dynamics(MD)simulations and ab initio calculations [15].Zhang et al.reported the solubilities of CO2in 1-butyl-3-methylimidazolium hexaf l uorophosphate and 1,1,3,3-tetramethylguanidium lactate at elevated pressures[16].Therefore,it is important to investigate the interaction between guanidinium ionic liquids and CO2gas molecules,study the inf l uence of the structure of ionic liquid on the absorption of such gas,and understand further the mechanism that guanidinium ionic liquids absorb CO2gas by quantum chemistry calculation and MD simulation from molecular level,which can provide theoretical basis for designing and developing new high absorptive guanidinium ionic liquids.To the best of our knowledge,there are no reported calculations involving the guanidine cation,chlorine anion and CO2systems.In the present work,we report results of the quantum mechanics and molecular dynamics on the guanidine cation and the chlorine anion systems,and guanidine cation and the chlorine anion-CO2systems and seek to develop an understanding of the capture ability of the guanidine cation and the chlorine anion ionic liquid toward the CO2gas.

II.FUNDAMENTAL THEORIES AND COMPUTATIONS DETAILS

The initial structure may be top,side and middle type(Fig.1)for the simple guanidinium interacting with anion because there are a variety of modes of interaction for guanidinium ion pair.The second order perturbation theory(MP2)and density functional method (X3LYP)with 6-311++G(d,p)and aug-cc-PVDZ basis set are adopted to optimize monomers and dimers for possible initial conf i guration.Two stable conf i gurations are got:top and middle type.The main anionic attack method can be initially determined as middle type through the analysis of electrostatic potential of monomer and the highest occupied orbital.The binding energy of the compound can be calculated according to the equation:

here Emono,1,Emono,2,and Edimerrepresent the energy of cation,anion,and the dimer respectively.The basis set superposition error(BSSE)approach was incorporated into the calculations via the counterpoise(CP) method proposed by Boys and Bernardi[17].All calculations in this work were performed using the Gaussian 03 package[18].

FIG.1 The anions attacking on three positions of the guanidinium cation and the HOMO(16)mapped with electronic potential from total SCF density of guanidinium.

FIG.2 Methanetriamine positive ion(METM+)and Cl-in dif f erent geometries(middle,top).

In molecular dynamics simulation,we adopted Gromacs 4.5.4 package[19,20],combined with OPLSAA/L atomic force f i eld developed by Lopes and others [21].The parameters of cationic and anionic force f i eld come from the program.The force f i eld parameters of CO2are from Ref.[22].128 cations and 128 anions are added in the initial structures randomly,followed that 0,8,18,36,and 64 CO2molecules are joined respectively.The energy minimization of 500 ps and the steepest descent method are used in the simulation.When the maximum force converges in 100 kJ·mol·nm,the NPT system simulation is adopted.Berendsen algorithm is applied in the temperature control and pressure control[23].The Newtonian motion equations are solved by leapfrog(leap-frog)algorithm in which time step is set to 2 fs.The linear constraint solver [24]are used in the restrictive algorithm of bond in which the truncation radius is 1.4 nm[25].The particle mesh Ewad algorithm is applied in long-range electrostatic interaction[26,27].The temperature and pressure are 460 K and 1.0 bar respectively.When NPT reaches the convergence criteria,the 5 ns MD simulation is carried out f i nally.The temperature and pressure are controlled by the Nose-Hoover algorithm[28] and Parrinello-Rahman algorithm[29].

III.RESULTS AND DISCUSSION

The geometric conf i gurations of intermolecular interactions between guanidinium cation and chlorine anion are optimized by MP2 with 6-311++G(d,p)basis set and X3LYP with 6-311++G(d,p)and aug-cc-pvdz basis set.The structures of these compounds are shown in Fig.2.The related geometry parameters and the binding energies are listed in Table I.The optimization results show that there are two dif f erent types for the intermolecular interaction between guanidinesalt cation and chlorine anion which are middle and top.Figure 2 shows hydrogen bonds are formed between chlorine anion and two side hydrogen of guanidinium cation in the middle con fi guration,while C-Cl bond is formed between C+ion of guanidine salt cation and chlorine anion for the top con fi guration.According to the structure parameters of compounds and the monomers in Table I,the bond lengths of the complexes all have certain changes compared with monomers.The bond lengths of R(N1-C1)and R(N2-C1)decrease in middle con fi gurations,while increase in the top confi guration compounds.All these bond length changes are caused by the weak interaction between the two monomers.

TABLE I Dimer geometry and interaction energy(Eint)of guanidine ionic liquid.

According to results listed in Table I,the distances of two hydrogen ˚bond in the middle con fi guration are2.018(r1),2.037A(r2)at the X3LYP/6-311++G(d,p) computational level respectively,which indicates that the hydrogen bond formed between the guanidinium cation and chlorine anion is stronger.The distances (r1,r2)of hydrogen bonds in top con fi guration compound are slightly larger than the corresponding˚ datain middle complex,which are 2.674 and 2.971A respectively.Obviously,the middle mode is more stable than the top one for the intermolecular interactions between the guanidinium cation and chlorine anion.For middle con fi guration compounds,this function mode is more advantageous over the molecular space distribution.For the molecular point group of METM+is C2, it can still keep the characteristics of C2 point group after it interacts with chloride ion.When more ion pairs interact,single cation can accommodate three Cl-ions and six hydrogen atoms can all form hydrogen bonds in space.Top model is not conducive to the accumulation of many molecules because its symmetry falls.The guanidinium cation can only hold 1 or 2 Cl-ions.From the perspective of the spatial distribution characteristics of ion pairs in the interaction between guanidinium cationic ions and chlorine anionic ions,the model of the middle is more advantageous over the pattern of the top.In addition,energy variation characteristics of two kinds of patterns are analyzed in this work.It needs 12.5 kcal/mol energy barriers in the process of the middle conf i guration changing into the top structure,which makes the middle mode in the ionic liquid more stable than the top one.At the same time,we can also see middle conf i guration has larger interaction energy under dif f erent methods and basis set from Table I,which explains that the intermolecular interaction is stronger. In short,middle interaction mode is the main spatial distribution pattern in title system.

FIG.3 Conformations of the guanidinium cation and the chlorine anion by capturing CO2molecule.

FIG.4 Radial distribution functions graph between the carbocation and chlorine anion in the dif f erent number of CO2.

In order to further investigate the capacity of guanidinium ionic capturing CO2,density functional method X3LYP/6-311++G(d,p)and molecular dynamics simulations are used to investigate the microstructure characteristics of weak interactions between guanidinium ionic pairs and CO2molecules.The computational results are shown in Fig.3 and Fig.4.Obviously,it is not difficult to f i nd from the Fig.3 that CO2can not only form intermolecular interaction with METM+,but also form intermolecular interaction with the ionic pairs of cation and anion according to the spatial distribution characteristics from A1 to B4.But there isn’t intermolecular interaction between CO2and anion.For example,it can be seen from the electrostatic potential chart of the B2 conf i guration that the negatively charged center of CO2located at the two oxygen atoms, while the electrostatic potential of Cl-is a red area which indicates that the potential is negative.It is consistent with electrostatic distribution of the two oxygen atoms of CO2which hinders the interaction between CO2and Cl-.Similarly,the same conclusions can also be drawn for other conf i guration analysis.It is worth mentioning that the introduction of CO2gas molecules has no larger inf l uence on middle and top conf i gurations of cationic and anionic pairs from the structure of the interaction between guanidinium ion pair and CO2gas molecules.

FIG.5 The cumulative particle number of Cl-changing with the distance.

In order to better illustrate the space characteristics of intermolecular interaction between guanidinium ionic pair and CO2gas molecular,the radial distribution functions of the mixed system are analyzed by the molecular dynamics simulation in this work.The inf l uences of CO2on the microscopic structures and dynamics properties of guanidinium ionic liquids is also investigated.Radial distribution function g(r)is the physical quantities of reaction f l uid microstructure characteristics[30,31].As can be seen from the distribution characteristics,Cl-mainly distributes in the largest peak (0.40,4.511)(see Fig.4).The cumulative particle number of Cl-changes with the distance,as shown in Fig.5. About 2.02 Cl-ions appear around the location(r=0.4) which is consistent with the cationic space f i lled with three Cl-ions.With the increase of the distance of cation and anion,Cl-ion number is increased to about 18.41 when the distance arrives at 0.85 nm.The curve fl attens out with the distance from 1.0 nm to 2.0 nm which explains that the emergence probability of anion has a logarithmic growth trend.The maximum location(0.40,4.˚511)of radial distribution function is veryclose to 3.39A,which can also be concluded that middle model in the research system is the main way of spatial distributio˚n.It is worth˚noting that the 0.4 nm is largerabout 0.6A than 3.39A,which is caused by more than one counter charge ion in a large number of ionic liquids, leading to the weakened interaction between cation and anion.Currently,Bern group and Cadena team found that the cavities are closely associated with the capture process of CO2.Moreover,Bern et al.also drew the conclusion that the addition of CO2molecules didn’t in fl uence the imidazoles structure of ionic liquids obviously when they simulated the system of imidazoles ionic liquids and CO2.They think there are some cavities in the ionic liquid,which enable the CO2molecules not to destroy the microscopic structure of the ionic liquid when they enter into the cavities[32,33].It can be seen from Fig.4 that there is no apparent di ff erence in radial distribution function in the ionic liquid system with 8,16,32,and 64 CO2molecules which further indicates that the addition of CO2molecules has no big impact on the microstructure between cation and anion in the guanidinium ionic liquid system.

According to the above results of quantum chemical calculation and molecular dynamics simulation,it can be seen that the interaction distance of METM+and Cl-is 0.40 nm when the cation and the anion are in the condition of 8,16,32,and 64 CO2molecule which is almost consistent with the distance of 0.4 nm when there are no CO2molecules.It further illustrates that there may be some cavities in guanidinium ionic liquid which is similar to imidazoles ionic liquids.These cavities of guanidiniun ionic pairs play a major maintenance role in the process of capturing CO2molecules.

IV.CONCLUSION

The structure characteristics of guanidinium ionic liquids and the mechanism of guanidinium ionic liquids capturing CO2molecular are investigated by quantum chemical calculation and molecular dynamics simulation theoretically.There is stronger interaction between guanidine cation and chlorine anion.Their interaction model is given priority to middle mode.The dissolution of CO2gas molecules in the guanidinium ionic liquid hasn’t obvious inf l uence on the structure of the ionic pair.The conclusions state that the guanidinium ionic liquid is similar to the imidazole ionic liquid.It may have some cavities in which CO2gas molecules mainly exist.This project can provide theoretical basis for designing and developing new high absorptive guanidinium ionic liquids.

V.ACKNOWLEDGMENTS

This work was supported by the Open Project Program of Key Laboratory of Theoretical Chemistry and Molecular Simulation of Ministry of Education, Hunan University of Science and Technology,China (No.E21104),the National Natural Science Foundation of China(No.21201062 and No.21172066),and the International Cooperation Project(No.2013DFG60060).

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∗Author to whom correspondence should be addressed.E-mail：rlman@mail.csu.edu.cn,hxliu12@hnust.edu.cn