SO2 Capture by Guanidinium-Based Ionic Liquids: A Theoretical

ARTICLE pubs.acs.org/JPCB

SO2 Capture by Guanidinium-Based Ionic Liquids: A Theoretical Study Guangren Yu and Xiaochun Chen* College of Chemical Engineering, Beijing University of Chemical Technology, 100029 Beijing, China ABSTRACT: Ionic liquids (ILs) show good performances in SO2 separation science, e.g., SO2 capture from high-temperature flue gas or separation from gas mixtures. In this work, the mechanism of capturing SO2 by three guanidinium-based ILs, 1,1,3,3-tetramethylguanidinium lactate ([tmgHH][L]), 1,1,3,3tetramethylguanidinium bis(trifluoromethylsulfonyl)imide ([tmgHH][Tf2N]), and 1,1,3,3-tetramethylguanidinium tetrafluoroborate ([tmgHH][BF4]), is investigated by using molecular dynamic simulation and ab initio calculation. The results of condensed phase molecular dynamic simulation for the mixtures of SO2 and these three ILs indicate the similar SO2 organization and interaction among them; SO2 may organize around [tmgHH]þ while it favorably organizes around the anions through Lewis acidbase interaction. Gas phase ab initio calculations show that [tmgHH][L] chemically interacts with SO2 while [tmgHH][Tf2N] and [tmgHH][BF4] do not, which is supported by the earlier FT-IR and 1H NMR data and is also consistent with the experimental result of a much higher absorption capability of [tmgHH][L] for SO2 than the latter two. The anion plays a key role in the chemical interaction between [tmgHH][L] and SO2, the S atom is bonded to the N atom on NH2 of [tmgHH]þ, and some products with aminosulfate or aminosulfinic acid fragment may be formed. This work shows that IL structures should be carefully tailored for their final application in SO2 capture.

’ INTRODUCTION Room-temperature ionic liquids, simply known as ionic liquids (ILs), are defined as a class of organic salts with melting points below 100 C.1 Owing to some unique physical and chemical properties, ILs have been receiving an upsurge of interest as neoteric solvents in the last few decades and being studied in a variety of fields such as separation, organic synthesis, catalysis, electrochemistry, fuel and solar cells, lubricants, and functional materials, etc.112 ILs entirely consist of organic cation and inorganic/organic anion. They generally present undetectable vapor pressure; e.g., it is ∼1010 Pa at room temperature for 1-butyl-3-methylimidazolium hexafluorophosphate13 ([C4mim][PF6], as to the abbreviation of the complex of several molecules or ions in this work, the dot sign denotes moleculeion or moleculemolecule interaction, and it is removed for interionic interaction, e.g., SO2 3 [tmgHH][L], a moleculeion interaction between SO2 and [tmgHH]þ, and an interionic interaction between [tmgHH]þ and [L]). This means they are almost nonvolatile. Unlike traditional organic solvents, ILs cannot lead to fugitive emissions; hence, there is neither volatilization loss nor environmental pollution.6 Thus, ILs are quite suitable in gas separation, e.g., sulfur dioxide (SO2) capture from flue gas or separation from gas mixtures, where one desired gas is selectively absorbed into ILs and is then regenerated by a simple swing of pressure or temperature. In such IL-based processes, there is no concurrent loss of solvents, some purification processes for objective gas are simplified, and the energy consumption and cost are reduced. Some other desirable properties of ILs include wide liquidus r 2011 American Chemical Society

range (from below ambient to well over 300400 C), high thermal stability (cannot decompose at < ∼300 C), good solvent ability (dissolving both organic and inorganic compounds), and nonflammability.112 The applications of ILs in some gas processes are also favored when some harsh operation conditions are encountered (e.g., high temperature in the case of flue gas). Capture or separation of SO2 is one important gas-treatment process in industry, such as flue gas desulfurization (FGD) and SO2 purification from gas mixtures.1419 A variety of technologies, such as water scrubbing, metal ion solution, catalytic oxidation, activated carbon adsorption, wet lime or limestone scrubbing, double alkali process, ammonia scrubbing, NH3 gas injection, and organic solvent absorption, are used to capture SO2,1417 but they are always with some shortcomings. For example, in the most popular wet lime or limestone scrubbing process, a large amount of water is required, the resultant wastewater needs to be treated, and a large volume of low-valuable solids are produced. Recently, using ILs to capture SO2 is receiving increasing attention.2030 The physical absorption capabilities of some imidazolium-based and pyridinium-based ILs for SO2 were investigated (some results are summarized in Table 1). These ILs show remarkable absorptions. It is 0.51.5 mol of SO2/mol of IL in an atmosphere of pure SO2, near room temperature and ambient pressure. When gas mixtures are treated, however, the absorption quantity is significantly reduced, e.g., it is only 0.005 Received: August 9, 2010 Revised: February 17, 2011 Published: March 16, 2011 3466

dx.doi.org/10.1021/jp107517t | J. Phys. Chem. B 2011, 115, 3466–3477

The Journal of Physical Chemistry B

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Table 1. Absorption Capability of ILs for SO2a initial gas mixture ionic liquid

component

SO2 mole fraction

absorption capability (mol of SO2/mol of IL) 20 C

25 C

40 C

50 C

60 C

pressure (bar)

ref

[C1mim][MeSO4]

SO2

1

0.950

1.01

20

[C2mim][MeSO4]

SO2

1

0.960

1.01

20

[C4mim][HSO4]

SO2, O2

0.67

0.200

1.01

20

[C4mim][Cl]

SO2, O2

0.67

1.690

1.01

20

[C4mim][PF6]

SO2, O2

0.67

0.330

1.01

20

[C4mim][MeSO3]

SO2, O2

0.67

0.780

1.01

20

[C4mim][MeSO4]

SO2

1

0.980

1.01

20

[C4mim][OAc]

SO2, O2 SO2, O2

0.67 0.67

0.710 1.350

1.01 1.01

20 20

[C4mim][Tf2N] [C4mim][BF4]

SO2

1

1.330

SO2, N2

0.1

0.007

SO2

1

1.500

SO2, N2

0.1

0.005

0.709 0.842

SO2, O2

0.67

[C6mim][Tf2N]

SO2

1

0.916

SO2 SO2

1 1

1.237

[C6mim][MeSO4] [C6mpy][Tf2N]

SO2

1

[tmgHH][BF4]

SO2

1

1.270

SO2, N2

0.10

0.064

[tmgHH][Tf2N]

0.460

1.092 0.681

1

1.180

0.10

0.061

[tmgHH][L]

SO2, N2

0.08

0.978

1 1

1.700

P([tmgHH][A])

SO2 SO2

P([tmgHH][A]-co-MBA)

SO2

1

[tmgPO2][BF4]

1.800

SO2

1

1.600

SO2, N2

0.10

0.080

SO2

1

1.618

SO2, N2

0.10

0.151

SO2

1

2.012

SO2, N2

0.10

0.200

0.361

0.761

1.049 0.923 1.358

1.0

21

1.0

21

1.0

21

1.0

21

1.01 0.330

0.490 1.120

SO2

[tmgHPO][BF4]

0.290

0.580

SO2, N2

[tmgBu2][Tf2N]

0.391

0.401 0.775

b 1.003 1.01

23 20

1.10

22

1.0

21

1.0

21

1.0

21

1.0

21

1.0

27

1.150

1.2 1.01

1.350

1.01 0.529 0.560 0.789

20 22

27 29 30

1.0

21

1.0

21

1.0

26

1.0

26

1.0

26

1.0

26

a The results are from other work; see the respective literature sources in this table. b The pressures are 0.994, 0.977, and 1.04 bar for 25, 40, and 60 C, respectively.

mol of SO2/mol of [C4mim][BF4] in an atmosphere of 10 mol % SO2 in N2 (this is close to the real SO2 content in flue gas) at 20 C and 1 bar. Besides liquid absorption, the permeability of SO2 in some imidazolium-based ILs supported by a polyethersulfone microfiltration membrane was measured, where a comparable absorption capability was found.24,25 Compared with earlier imidazolium-based and pyridiniumbased ILs, the synthetic routes are more direct and simple for some guanidinium-based ILs with high yield and purity. This is favorable for the commercial production of ILs in a large scale. Some 1,1,3,3-tetramethylguanidinium-based ILs such as [tmgHH][BF4], [tmgHH][Tf2N], [tmgBu2][Tf2N], [tmgHPO][BF4], and [tmgPO2][BF4] (where BF4 is tetrafluoroborate, Tf2N is bis(trifluoromethylsulfonyl)amide, Bu is butyl, and PO is 2-hydroxy-1-propyl) were investigated for capturing SO2.21,26 Similar absorption capabilities to those of imidazolium-based ILs were obtained; e.g., it is 1.27 mol of SO2/mol of [tmgHH][BF4] in an atmosphere of pure SO2, while it is only 0.064 mol of SO2/mol

of [tmgHH][BF4] in an atmosphere of 10 mol % SO2 in N2 at 20 C and 1 bar. FT-Raman and 1H NMR spectra indicated the nature of physical absorption for these guanidinium-based ILs. Some other 1,1,3,3-tetramethylguanidinium-based ILs,2730 e.g., [tmgHH][L] and [tmgHH][A] (where L is lactate and A is acrylate), however, show significantly improved absorption capabilities for SO2, e.g., 1 mol of [tmgHH][L] can almost absorb 1 mol of SO2 in an atmosphere of 8 mol % SO2 in N2 at room temperature and ambient pressure. FT-IR and 1H NMR spectra also indicated the nature of chemical absorption for [tmgHH][L] and [tmgHH][A]. Therefore, some different mechanisms of capturing SO2 exist for these 1,1,3,3-tetramethylguanidinium-based ILs but remain to be understood. The interactions between organic amines and SO2 have been thoroughly understood, where the R3N 3 SO2 complex is formed through Lewis acidbase interaction between the N atom of amines and the S atom of SO2 (R is H or alkyl, see Figure 1 for complex structure and Table 2 for specific structure 3467

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parameters).3134 Due to the electron-withdrawing effect from the carbocation unit in [tmgHH]þ, it is obvious that the N atom on NH2 of [tmgHH]þ is less nucleophilic than the organic amine N atom, as indicated in our previous electronic characterization of the [tmgHH][L] ion pair.3537 Thus, a weaker interaction between the S atom and the N atom of [tmgHH]þ is expected; however, spectrum data indicated that a bond between the S atom and N atom is formed instead of nonbonding Lewis acidbase interaction.27,28 What is the underlying mechanism? Following our previous ab initio calculation studies on [tmgHH][L],3537 in this work, the interaction mechanisms between SO2 and [tmgHH][L], [tmgHH][Tf2N], and [tmgHH][BF4]

Figure 1. Amine 3 SO2 complex.

Table 2. Structure Parameter and Interaction Energy for Amine 3 SO2 Complexa 1

NH3 3 SO2

MA 3 SO2

DMA 3 SO2

TMA 3 SO2 62.8

ΔE, kJ 3 mol

49.0

58.2

60.7

r (Å)

2.63

2.45

2.40

2.36

β, deg

85

85

85

85

R, deg

0

60

60

60

a

The results are from refs 31 and 34; MA, methylamine; DMA, dimethylamine; TMA, trimethylamine.

Figure 2. The ILs studied in this work.

are investigated by using molecular dynamic simulation and ab initio calculation (see Figure 2 for their molecular illustrations). This work is necessary for understanding the mechanism of SO2 capture by ILs, rationally designing the functional ILs, and promoting their industrial applications.

’ COMPUTATIONAL METHODS Molecular Dynamic Simulation. The force field paramters in the AMBER framework for SO2, [tmgHH]þ, [L], [Tf2N], and [BF4] are from refs 38, 37, 37, 39, and 40, respectively. The simulation is performed with the MDynaMix simulation package.41 There are three simulated systems, i.e., the mixtures of SO2 and [tmgHH][L] (128 SO2, 256 [tmgHH]þ, 256 [L]), SO2 and [tmgHH][Tf2N] (128 SO2, 256 [tmgHH]þ, 256 [Tf2N]), and SO2 and [tmgHH][BF4] (128 SO2, 256 [tmgHH]þ, 256 [BF4]). The simulation is started in the NVE ensemble and then equilibrated and sampled in the NPT ensemble for 2 ns (1 000 000 time steps) and 1 ns, respectively. Some other simulation details can be found in our previous work.37,4244 Ab Initio Calculation. The calculation is performed with the Gaussian 03 program.45 The density functional theory (DFT) method of the well-established Becke’s three-parameter functional with the nonlocal correlation of Lee, Yang, and Parr (B3LYP)46,47 is employed with the 6-31G* basis set to perform the potential energy surface (PES) scan, optimize the structure of the stable point, and obtain the data of vibration frequency and zero point energy (ZPE) correction. The applicability of this calculation method in studying ILs has been widely demonstrated in our previous work3537,42,43 and other works.4854 The initial structures of stable points are obtained from PES scan, and the final structures are obtained by performing optimization. When the structure of transition states (TS) cannot be obtained through direct optimization (the key word in Gaussian is opt=TS), the method of quadratic synchronous transit (the key word in Gaussian is opt=QST2 or opt=QST3) is used to obtain the TS. The method of intrinsic reaction following (the key word in Gaussian is opt=IRC) is used to check if one TS is the desired one. TS is indicated by only one virtual frequency, where the scaling factor of 0.96 is used for the vibrational frequency correction. Some single point energies are improved at the higher level of MP2/6-311þG(2df,p).55,56

Figure 3. Organization of cation and anion in three mixtures of SO2 and [tmgHH][L], SO2 and [tmgHH][Tf2N], and SO2 and [tmgHH][BF4]: (a) mass center RDFs between cation and anion (the data in pure [tmgHH][L] are from ref 37); (b) RDFs between the N atom on NH2 of [tmgHH]þ and the C atom on COO of [L] and the N atom of [Tf2N] and the B atom of [BF4]. The coordination number of the first shell is also labeled in italic. 3468



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Figure 4. Organization of SO2 around ion in three mixtures of SO2 and [tmgHH][L], SO2 and [tmgHH][Tf2N], and SO2 and [tmgHH][BF4]: (a) RDFs between the S atom of SO2 and the O atom on COO of [L] and the O atom of [Tf2N] and the F atom of [BF4]; (b) RDFs between the S atom of SO2 and the N atom on NH2 of [tmgHH]þ. The coordination number of the first shell is also labeled in italic.

Figure 5. Structures of the isolated monomers, length in Å and angle in deg.

Figure 6. Reaction path and PES for SO2 and [tmgHH]þ in the absence of anion at the MP2/6-311þG(2df,p)//B3LYP/6-31G* level, length in Å and angle in deg.

Table 3. Energy of the Stationary Points on the Reaction PES between SO2 and [tmgHH]þ in the Absence of Anions (in kJ 3 mol1)a B3LYP/6-31G* stationary point

a

without ZPE

MP2/6-311þG(2df,p)//B3LYP/6-31G* with ZPE

without ZPE

with ZPE

R1

40.1 (0.0)

36.8 (0.0)

36.7 (0.0)

33.4 (0.0)

TS1 P1

164.7 (204.9) 62.2 (102.3)

157.0 (193.8) 65.7 (102.4)

185.7 (222.4) 70.8 (107.5)

178.0 (211.4) 74.2 (107.6)

Relative to the sum of the energy of SO2 and that of [tmgHH]þ, and the values in parentheses are relative to the energy of R1.

’ RESULTS AND DISCUSSION (a). Molecular Dynamic Simulation. CationAnion Organization. The sitesite radial distribution function (RDF) is a good

parameter for indicating the intermolecular organization or interaction. The mass center RDFs between cation and anion

for these three simulated systems are shown in Figure 3a, where the result for a pure [tmgHH][L] system is also given. As indicated in Figure 3a, similar mass center RDFs are obtained for these systems; there are six to seven ions in the first shell; both the peak position and the range of the first shell are similar. The peak for [tmgHH][Tf2N] is lower than that for [tmgHH][L] 3469



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Figure 7. Structures of the ion pairs (length in Å and angle in deg; for clarity, some H atoms on [tmgHH]þ are not presented).

Figure 8. Structures of the dimers of SO2 3 [tmgHH]þ, SO2 3 [L], SO2 3 [Tf2N], and SO2 3 [BF4] (length in Å and angle in deg).

and [tmgHH][BF4], and it may be ascribed to the weaker cationanion interaction for the former, as demonstrated in the following parts. The addition of SO2 into [tmgHH][L] only slightly lowers the peak height, and this means that the addition of SO2 has a little effect on the organization of [tmgHH]þ and [L], which is also found for the mixture of CO2 and some ILs.57,58 Figure 3b gives the distribution of anions around the NH2 of [tmgHH]þ. It is shown that there are about two anions around NH2 for all three systems; there is a weaker organization of [Tf2N] around NH2. SO2 Organization around Ion. The RDFs between SO2 and ion are shown in Figure 4. As indicated in Figure 4a, for all three of these simulated systems, SO2 organizes around the anions through the Lewis acidbase interaction between the S atom of SO2 and the O atom on COO of [L], and the O atom of [Tf2N], and the F atom of [BF4], while there is a weaker organization of SO2 around the cations as indicated in Figure 4b, where there is a little peak at ∼4 Å; this result is similar to that obtained by Wang et al.59 As shown in Figure 4, although there are some differences for the organization of SO2 among these three simulated systems, the differences are little; this implies a similar physical solubility of SO2 in these three ILs, although the pure physical solubility of SO2 in [tmgHH][L] has never been measured experimentally. In fact, the physical absorption capabilities for SO2 by [tmgHH][Tf2N] and [tmgHH][BF4] are similar, as shown in Table 1. (b). Ab Initio Calculation. ILs entirely consist of cation and anion through nonbonding interaction, mainly including Coulombic electrostatic, hydrogen bonding, and van der Waals interactions. As shown in Figure 3b, coupled with our previous results of the gas phase ion pair from ab initio calculation and the condensed phase from molecular dynamics simulation, there are one to two anions around the NH2 of [tmgHH]þ through hydrogen bonding interaction.3537 Thus, in this work, three kinds of reaction paths between SO2 and the ILs are studied in three corresponding conditions, i.e., in the absence of anions (only [tmgHH]þ exists), in the presence of one anion, and in the presence of two anions, respectively.

Table 4. Interaction Energy between SO2 and Ion at the B3LYP/6-31G* Level (in kJ 3 mol1)a interaction energy SO2 3 ion

with ZPE þ

SO2 3 [tmgHH] SO2 3 [L]

SO2 3 [Tf2N] a

SO2 3 [BF4]

without ZPE

36.8

40.1

103.0

108.2

37.2

39.5

70.0

73.1

The interaction energy is defined as the difference between the energy of the complex of SO2 3 ion and the sum of the energies of the isolated SO2 and the ion.

Monomer Structure. The structures of [tmgHH]þ, [L], [Tf2N], [BF4], and SO2 are shown in Figure 5, where some selected geometric parameters are labeled. In [tmgHH]þ, the main framework of NCNN presents a complete plane, and terminal NH2 has a 21 rotation away from the plane; three CN bonds with the same length of 1.35 Å indicate a characteristic of the carbocation; both of the NH bonds are 1.01 Å. In [L], the main frameworks of OCCO and OH are approximately on a plane; the characteristic parameters of COO are 1.25, 1.27, and 129.8. In [Tf2N], almost the same sizes are obtained for two CF3 and OSO units, and the four units are staggered. In [BF4], a symmetry character of the Td point group is found. The geometric parameters of SO2 (C2V) are in good agreement with the experimental values;60 the SO bond is 1.464 Å (experimental, 1.431 Å), and the OSO angle is 119.1 (experimental, 119.3). Addition of SO2 to NH2 of [tmgHH]þ in the Absence of Anion Starting Configuration. A number of experimental and calculation studies indicated that organic amines interact with SO2 through Lewis acidbase interaction between the S atom and the sp3 N atom and amine 3 SO2 complexes are formed (Figure 1 and Table 2).3134 However, all the attempts to form the [tmgHH]þ 3 SO2 complex through SN Lewis acidbase 3470



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Figure 9. Structures of the trimers of SO2 and ion pair (length in Å and angle in deg; for clarity, some H atoms on [tmgHH]þ are not presented).

Figure 10. Relaxed PES scan along one reaction coordinate in the trimers of SO2 3 [tmgHH][L], [tmgHH][L] 3 SO2, SO2 3 [tmgHH][Tf2N], SO2 3 [tmgHH][BF4], and [tmgHH][BF4] 3 SO2; see Figure 9 for atom numbering.

interaction result in a new structure, R1 (see Figure 6, subscript 1 denotes it is on the first kind of reaction path). In R1, instead of Lewis acidbase interaction between the S atom and N atom, a hydrogen bond is formed between O24 of SO2 and H5 of NH2, where N2H5 is 1.02 Å, H5 3 3 3 O24 is 1.99 Å, and — N2H5 3 3 3 O24 is 166.1. This structure characteristic is attributed to the strong ability of proton donation of NH2, where two H atoms on NH2 are partially protonic with a natural atomic orbital charge of 0.47.35 Reaction Path. The reaction path is shown in Figure 6. This path can be described as follows: first, SO2 rotates around O24 and is located above the plane of NH2; second, proton H5 is partially transferred from N2 to O24, and a distorted four-center transition state of TS1 is formed; and finally, driven by the deprotonation of NH2 (departure of H5) and the bond formations of O24H5 and N2S23, the product of P1 with a structure of aminosulfinic acid is formed. This is a concerted result of proton transfer and electrophilic attack of the S atom on the N atom.

Table 5. Intermolecular Interaction Energy and Relative Energy for the Trimers of SO2 and Ion Pair at the B3LYP/ 6-31G* Level (in kJ 3 mol1)a relative energy trimer

without ZPE with ZPE without ZPE with ZPE 0b

445.9

440.7

58.2

52.2b

504.1

492.9

0c

0c

385.7

377.9

0d

0d

425.8

417.7

38.8d

36.0d

464.6

453.7

0b

SO2 3 [tmgHH][L]

b

[tmgHH][L] 3 SO2 SO2 3 [tmgHH][Tf2N] SO2 3 [tmgHH][BF4] a

interaction energy

[tmgHH][BF4] 3 SO2

The interaction energy is defined as the difference between the energy of the trimer of SO2 3 [cation][anion] or [cation][anion] 3 SO2 and the sum of the energies of the isolated SO2, cation, and anion. b The energy relative to that of SO2 3 [tmgHH][L]. c The energy relative to that of SO2 3 [tmgHH][Tf2N]. d The energy relative to that of SO2 3 [tmgHH][BF4]. 3471



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Energy. The detailed energies of the stationary points are listed in Table 3. As shown in Table 3 and Figure 6, the energy barrier of 211.4 kJ 3 mol1 is so high that a spontaneous reaction is not possible; in addition, R1 is more thermodynamically stable than P1. The data in Table 3 show that the energy barriers obtained at the MP2/6-311þG(2df,p)//B3LYP/6-31G* level are ∼20 kJ 3 mol1 higher than that obtained at the B3LYP/631G* level, which is consistent with the recognition that density functional methods underestimate the energy barrier;61 ZPE Table 6. CationAnion Interaction Energy (kJ 3 mol1) and Proton Affinity of Anion (kJ 3 mol1) for [tmgHH][L], [tmgHH][Tf2N], and [tmgHH][BF4] cationanion interaction energy

ionic liquids

proton affinity of anion

B3LYP/

MP2/6-31G*//

B3LYP/ MP2/6-31G*//

6-31G*

B3LYP/6-31G*

6-31G*

B3LYP/6-31G* 1441.4

435.1

448.9

1452.2

[tmgHH][Tf2N] 342.3

362.6

1296.8

1277.8

[tmgHH][BF4]a 418.4

421.2

1279.9

1258.5

[tmgHH][L]

a

A molecular complex of BF3 3 HF is formed after the addition of a proton to [BF4].

Figure 11. Distribution of negative charges for [L], [Tf2N], and [BF4]. Charges are obtained from restraint electrostatic potential fits. The charges for all the F atoms in [BF4] are the same; the charges for all the F atoms in [Tf2N] are the same; the charges for all the O atoms in [Tf2N] are the same.

correction has an effect of ∼10 kJ 3 mol1. Therefore, it is almost impossible for the addition of SO2 to NH2 when no anions exist. Addition of SO2 to NH2 of [tmgHH]þ in the Presence of One Anion Ion Pair. The ion pair structures are shown in Figure 7, where one interionic hydrogen bond is formed for each ion pair. However, from the respective parameters of hydrogen bond, it can be pointed out that hydrogen bond interaction weakens in the following order: [tmgHH][L] > [tmgHH][BF4] > [tmgHH][Tf2N]. Interaction between SO2 and Ion. The structure of the dimers of SO2 and ion are shown in Figure 8, and the energies are listed in Table 4. The values of interaction energy in Table 4 indicate that the interaction between SO2 and ion weakens in the following order: SO2 3 [L] > SO2 3 [BF4] > SO2 3 [Tf2N] > SO2 3 [tmgHH]þ. Therefore, the interaction of both SO2-cation and SO2-anion are considered in constructing the trimer of ion pair and SO2. Trimer of Ion Pair and SO2. The structures of trimers are shown in Figure 9, and their energies are listed in Table 5. As shown in Figure 9, the addition of SO2 does not have a very remarkable effect on the original ion pair structures; SO2 interacts with the ion pairs through the Lewis acidbase interaction between the S atom of SO2 and the N atom on NH2 of [tmgHH]þ (e.g., SO2 3 [tmgHH][L], SO2 3 [tmgHH][BF4]), or between the S atom and prominent Lewis base site on the anion (e.g., [tmgHH][L] 3 SO2, [tmgHH][BF4] 3 SO2). In the case of [tmgHH][Tf2N], only one trimer, SO2 3 [tmgHH][Tf2N], is obtained, where the trimer is stabilized by two hydrogen bonds and one SO Lewis acidbase interaction. In SO2 3 [tmgHH][L] and SO2 3 [tmgHH][BF4], the structural characteristic of the SO2 3 [tmgHH] fragment, with a weaker SN interaction, is similar to the amine 3 SO2 complex (Table 2, Figure 1). The energies in Table 5 show that SO2 prefers to interact with anion (e.g., the energy of [tmgHH][L] 3 SO2 is 52.2 kJ 3 mol1 lower than that of SO2 3 [tmgHH][L]), although SO2 3 [tmgHH][L] and SO2 3 [tmgHH][BF4] may exist in the liquid phase, as indicated in Figure 4b. Reaction Path. The results of relaxed PES scan along one reaction coordinate in the trimers of SO2 3 [tmgHH][L],

Figure 12. 3D PES scan along reaction coordinates S34N2 and N2H5 in SO2 3 [tmgHH][L] and [tmgHH][L] 3 SO2, with the domains of stable points labeled. 3472



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Figure 13. For trimer SO2 3 [tmgHH][L], the reaction path and PES for SO2 and [tmgHH]þ in the presence of one [L] at the MP2/6-311þG(2df, p)//B3LYP/6-31G* level (length in Å and angle in deg; for clarity, some H atoms are not presented).

Figure 14. For trimer [tmgHH][L] 3 SO2, the reaction path and PES for SO2 and [tmgHH]þ in the presence of one [L] at the MP2/6-311þG(2df, p)//B3LYP/6-31G* level (length in Å and angle in deg; for clarity, some H atoms are not presented).

Table 7. Energy of the Stationary Points on the Reaction PES between SO2 and [tmgHH]þ in the Presence of One [L] with the Starting Point of SO2 3 [tmgHH][L] (in kJ 3 mol1)a B3LYP/6-31G* stationary point

a

without ZPE

MP2/6-311þG(2df,p)//B3LYP/6-31G* with ZPE

without ZPE

with ZPE

R2

445.9 (0.0)

440.7 (0.0)

441.5 (0.0)

436.3 (0.0)

TS12

444.2 (1.7)

448.8 (8.1)

441.1 (0.5)

445.7 (9.3)

IN2

457.5 (11.6)

453.2 (12.6)

449.5 (7.9)

445.2 (8.8)

TS22

442.4 (3.5)

438.4 (2.3)

434.4 (7.2)

430.4 (5.9)

P2

481.0 (35.1)

470.7 (30.1)

466.3 (24.7)

456.0 (19.7)

Relative to the sum of the energies of SO2, [tmgHH]þ, and [L], and the values in parentheses are relative to the energy of R2.

[tmgHH][L] 3 SO2, SO2 3 [tmgHH][Tf2N], SO2 3 [tmgHH][BF4], and [tmgHH][BF4] 3 SO2 are shown in Figure 10. As shown in Figure 10, with shortening S 3 3 3 N and enlarging N 3 3 3 H, the energies monotonously and sharply increase for SO2 3 [tmgHH][Tf2N], SO2 3 [tmgHH][BF4], and [tmgHH][BF4] 3 SO2, while the energy troughs and peaks alternately emerge for SO2 3 [tmgHH][L] and [tmgHH][L] 3 SO2, where trough implies product or intermediate and peak implies transition state. Therefore, [tmgHH][Tf2N] and [tmgHH][BF4] cannot chemically interact spontaneously with SO2, as was demonstrated by FT-Raman and 1H NMR spectra,21,26 while [tmgHH][L] can. Their different interactions are understood from the weaker anioncation interaction and proton affinity of the anion of [tmgHH][Tf2N] and [tmgHH][BF4] than [tmgHH][L] (Table 6), along with the more scattered negative charges of [Tf2N] and [BF4] (Figure 11). Two 3D PES scans are performed for SO2 3 [tmgHH][L] and [tmgHH][L] 3 SO2,

and the results are shown in Figure 12, where the domains of the stable points are labeled. For the trimer SO2 3 [tmgHH][L], the reaction path for the addition of SO2 to NH2 of [tmgHH]þ in the presence of one [L1] (subscript 1 denotes the first [L]) is shown in Figure 13. Proton H5 is transferred from N2 of NH2 to O23 of [L1]. In the resultant intermediate IN2 (SO2 3 tmgH 3 HL1, tmgH is 1,1,3,3tetramethylguanidine), the N2 atom becomes more nucleophilic; the distance of S34 3 3 3 N2 is shortened from 2.99 Å in R2 (SO2 3 [tmgHH][L1]) to 2.77 Å in IN2; 2.77 Å is comparable to the distance of S 3 3 3 N in amine 3 SO2 (Table 2); the structure characteristic of the fragment SO2 3 tmgH is similar to that of amine 3 SO2 (Figure 1). From IN2 to P2, the hydrogen bond N2 3 3 3 H5O23 disappears and a new hydrogen bond of O35 3 3 3 H5O23 is formed, and the distance of S34N2 is further shortened to 2.14 Å. In P2, the fragment of tmgHSOO is an aminosulfinate unit; thus, with the presence of one [L], SO2 is bonded on the NH unit of tmgH. 3473



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For the trimer [tmgHH][L] 3 SO2, the reaction path for the addition of SO2 to NH2 of [tmgHH]þ in the presence of one [L1] is shown in Figure 14. The reaction path is similar to that in Figure 13. Proton H5 is transferred from NH2 of [tmgHH]þ to COO of [L1]; the distance between S34 and N2 is continuously shortened and a bond of S34N2 is finally formed in P20 ; from IN20 to P20 , the hydrogen bond interaction between tmgH and HL1 (N2 3 3 3 H5O23) disappears and a new hydrogen bond interaction between SOO and HL1 (O36 3 3 3 H5O23) is formed. The structure of P20 is very similar to that of P2. Energy. The detailed energies of the stationary points on two reaction pathes (in Figures13 and 14) are listed in Tables 7 and 8, respectively. Taking the B3LYP/6-31G* energy without ZPE correction as an illustration, on the reaction path in Figure 13, R2 is transformed into IN2 by passing a very low energy barrier of 0.5 kJ 3 mol1; then, IN2 is transformed into P2 by passing a low energy barrier of 15.1 kJ 3 mol1; the energy of P2 is 24.7 kJ 3 mol1 lower than that of R2; thus, whether in terms of kinetics or thermodynamics, this reaction is facilitated. Comparing the energy characteristic of the reaction path in Figure 14 and that in Figure 13, R20 needs to pass two higher energy barriers to reach the product of P20 , the energy barriers being 40.6 and 31.7 kJ 3 mol1. When ZPE correction is included, the energy barriers are somewhat lowered; e.g., the energies of TS12 both at MP2/6311þG(2df,p)//B3LYP/6-31G* and at B3LYP/6-31G* are even about 89 kJ 3 mol1 lower than that of R2, which was also found in other proton transfer reactions, and is consistent with the nature of the low energy barrier for proton transfer reaction.61,62 Addition of SO2 to NH2 of [tmgHH]þ in the Presence of Two [L] Starting Configuration. When the second [L2] is added to P2 or P20 , a stable point of R3 (HL1 3 OOStmgH 3 [L2]) is

obtained, as shown in Figure 16 where it is stabilized by two hydrogen bonds (O35 3 3 3 H5O23 and O37 3 3 3 H6N2). Reaction Path. The results of 3D PES scan along two reaction coordinates of N2H6 and O23H5 in R3 are shown in Figure 15. As indicated in Figure 15, this is a relatively flat PES; there are many troughs and saddle points as labeled by “P” and “TS”, respectively. Figure 16 gives one reaction path. This reaction path is somewhat similar to that in Figures 13 and 14. First, H6 is transferred from tmgH to [L2]; at the same time, the other proton of H5 is transferred from HL1 to the OOS fragment. After this, the intermediate product of IN3 is formed and stabilized by two hydrogen bonds (N2 3 3 3 H6O37 and O23 3 3 3 H5O35), where a structure of aminosulfinic acid is formed. In TS23, N2 3 3 3 H6O37 is disappearing and a new hydrogen bond of O36 3 3 3 H6O37 is forming. Finally, after a rearrangement by hydrogen bond interaction, a final “ring” product of P3 ([L1] 3 tmgSOOH 3 HL2) is formed through three end-to-end hydrogen bonds. It is worth noting that the added second lactate of [L2] is transformed into the lactic acid of HL2 while the original HL1 is transformed into [L1]. Thus, if we do not differentiate these two lactates or lactic acids, one can say [L] plays a role of catalyst in the chemical capture of SO2 by [tmgHH][L]. Energy. The detailed energies of the stationary points are listed in Table 9. As shown in Table 9, R3 passes two low energy barriers to P3. Their values are 18.7 and 32.7 kJ 3 mol1, respectively, both of which are at the order of magnitude of the molecule collision energy. P3 is lying in a trough with an energy

Table 8. Energy of the Stationary Points on the Reaction PES between SO2 and [tmgHH]þ in the Presence of One [L] with the Starting Point of [tmgHH][L] 3 SO2 at the B3LYP/631G* Level (in kJ 3 mol1)a energy stationary point

without ZPE

with ZPE

R20

506.7 (0)

497.0 (0)

TS120

466.1 (40.6)

462.6 (34.4)

IN20

477.5 (29.2)

471.6 (25.5)

TS220

445.9 (60.9)

439.1 (57.9)

P20

478.6 (28.1)

469.7 (27.3)

Relative to the sum of the energies of SO2, [tmgHH]þ, and [L], and the values in parentheses are relative to the energy of R20 . a

Figure 15. 3D PES scan along reaction coordinates N2H6 and O23H5 in R3, with the domains of some stable points labeled.

Table 9. Energy of the Stationary Points on the Reaction PES between SO2 and [tmgHH]þ in the Presence of Two [L] (in kJ 3 mol1)a B3LYP/6-31G* stationary point

a

MP2/6-311þG(2df,p)//B3LYP/6-31G*

without ZPE

with ZPE

without ZPE

with ZPE

R3

593.9 (0.0)

581.9 (0.0)

574.9 (0.0)

562.9 (0.0)

TS13

563.9 (30.0)

564.8 (17.0)

556.2 (18.7)

557.2 (5.7)

IN3

562.9 (31.0)

557.7 (24.1)

557.4 (17.5)

552.2 (10.6)

TS23

542.3 (51.6)

535.1 (46.8)

524.7 (50.2)

517.5 (45.3)

P3

582.1 (11.8)

570.0 (11.9)

559.4 (15.5)

547.2 (15.6)

Relative to the sum of the energies of SO2, [tmgHH]þ, and a double of [L], and the values in parentheses are relative to the energy of R3. 3474



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Figure 16. One reaction path and PES for SO2 and [tmgHH]þ in the presence of two [L] at the MP2/6-311þG(2df,p)//B3LYP/6-31G* level (length in Å and angle in deg; for clarity, some H atoms are not presented).

Figure 17. Chemical absorption of SO2 into [tmgHH][L]. 3475


The Journal of Physical Chemistry B barrier of 34.7 kJ 3 mol1, and is a stable product of SO2 absorption into [tmgHH][L]. The above results indicate that the mechanism for SO2 chemical absorption into [tmgHH][L] is complex, and such as R2, R20 , IN2, IN20 , P2, P20 , R3, IN3, and P3 may be the absorption products where aminosulfate or aminosulfinic acid unit is formed as is in agreement with the spectra data. However, it is safely said that SO2 is bonded to the N atom of [tmgHH]þ with the assistance of [L] and the anion plays a key role. One reaction process is illustrated in Figure 17.

’ CONCLUSIONS To promote the application of ILs in SO2 separation science, the absorption mechanisms of SO2 into [tmgHH][L], [tmgHH][Tf2N], and [tmgHH][BF4] are investigated by using molecular dynamic simulations and ab initio calculations. Molecular dynamic simulation results show that there are little differences for SO2 organization and interaction among these three ILs, which might imply the similar physical solubility. Further, ab initio calculation results indicate that, with the assistance of the anions, [tmgHH][L] can chemically interact with SO2, while [tmgHH][Tf2N] and [tmgHH][BF4] cannot, which is understood by the weaker cationanion interaction, lesser proton affinity of the anion, and the more dispersed negative charges on the anion for [tmgHH][Tf2N] and [tmgHH][BF4]. Ab initio calculation results are consistent with the experimental spectra data. This work shows that, for 1,1,3,3tetramethylgunidinium ILs, the anion plays an important role in chemically absorbing SO2 and the structures of the ILs should be carefully tailored for their final application in SO2 capture. ’ AUTHOR INFORMATION Corresponding Author

*Phone/Fax: þ86-10-6443-3570. E-mail: [email protected].

’ ACKNOWLEDGMENT This work is financially supported by National Natural Science Foundation of China (20806002, 20976005), Beijing Natural Science Foundation (2103051), and PetroChina Innovation Foundation (2010D-5006-0403). The Chemical Grid Project of Beijing University of Chemical Technology is also highly appreciated for generously providing the computer resources. ’ REFERENCES (1) Wilkes, J. S. A Short History of Ionic Liquids—from Molten Salts to Neoteric Solvents. Green Chem. 2002, 4, 73. (2) Rogers, R. D., Seddon, K. R., Eds. Ionic Liquids: Industrial Applications to Green Chemistry; American Chemical Society (Distributed by Oxford University Press): Washington, DC, 2002. (3) Rogers, R. D., Seddon, K. R., Eds. Ionic Liquids as Green Solvents: Progress and Prospects; American Chemical Society (Distributed by Oxford University Press): Washington, DC, 2003. (4) Welton, T. Room-temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. Rev. 1999, 99, 2071. (5) Wasserscheid, P.; Keim, W. Ionic Liquids—New Solutions for Transition Metal Catalysis. Angew. Chem., Int. Ed. 2000, 39, 3772. (6) Brennecke, J. F.; Maginn, E. Ionic Liquids: Innovative Fluids for Chemical Processing. AIChE J. 2001, 47, 2384. (7) AlNashef, I. M.; Leonard, M. L.; Matthews, A.; Weidner, J. W. Superoxide Electrochemistry in an Ionic Liquid. Ind. Eng. Chem. Res. 2002, 41, 4475.

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SO2 Capture by Guanidinium-Based Ionic Liquids: A Theoretical

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