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Structure and SO2 absorption properties of guanidinium-based dicarboxylic acid ionic liquids Xiaocai Meng, Jianying Wang, Pengtao Xie, Haichao Jiang, Yongqi Hu, and Tao Chang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02962 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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Structure and SO2 absorption properties of guanidinium-based dicarboxylic acid ionic liquids Xiaocai Meng a,b,∥, Jianying Wang b,∥, Pengtao Xie c, Haichao Jiang b, Yongqi Hu *b, Tao Chang *a a College of Material Science and Engineeing, Hebei University of Engineering, Handan 056038, China, E-mail: [email protected]; b College of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China, E-mail: [email protected]; c Chemistry and Materials Science, Handan college, Handan 056038, China.

ABSTRACT: :

New

series

of

functionalized

ionic

liquids

with

1,1,3,3-tetramethylguanidine([TMG]+) cations as well as dicarboxylic acid anions ([-OOC-(CH2-CH2)n-COO-], n=1,3,5)) were synthesized, and their SO2 absorption properties were investigated. A high SO2 absorption capacity of the prepared ILs was achieved. The molar ratios of SO2 to [TMG][succinic acid] [([TMG][SUC]) (n=1)], [TMG][suberic

acid]

[([TMG][SUB])

(n=3)]

and

[TMG][dodecanedioic

acid]

[([TMG][DOD]) (n=5)] were 4.76, 5.96, and 5.96, respectively. The evidence of spectroscopic measurements and quantum chemical calculations confirmed that SO2 could interact with the carboxyl and adjacent methylene on the anion as well as the amino group on the cation. The SO2 absorption capacity of these ionic liquids was strongly influenced by their asymmetry and space resistance. The high symmetry and large steric hindrance could reduce the SO2 absorption capacity of ionic liquids.

1. INTRODUCTION Regulations regarding SO2 emissions have become increasingly stringent. As a result, the attention on capturing SO2 from huge emission sources, such as coal- and gas-fired power plants, has been increased. At present, SO2 from flue gas is mostly removed through flue gas desulfurization (FGD) processes, in which alkaline sorbents were used as SO2 scrubbing agents

[1,2]

. However, these technologies still have disadvantages, such as

generating large amounts of waste water and waste metal salts, which not only caused a 1

ACS Paragon Plus Environment

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serious waste of resources, but also resulted in secondary pollution. Consequently, developing renewable absorbents which could efficiently remove SO2 has become increasingly important. As promising absorbents for SO2 removal, ionic liquids have the properties of high thermal stability, extremely low vapor pressure, and chemical flexibility [3–6]. Over the past decade, the solubility of SO2 in ILs and the underlying absorption mechanism have been hot topics in the scientific community. According to previous results, it was found that the high SO2 absorption capacity in ILs (such as [TMG][L]) is due to physical and chemical absorption [7,8], especially chemical absorption under a low partial pressure of SO2

[9]

. However, strong interaction and low thermal stability lead to incomplete

desorption [10]. Therefore, absorbing SO2 in flue gas requires chemical absorption and low absorption enthalpy. On the basis of the considerable research of SO2 absorption by [TMG][L], dicarboxylic acids were used to replace the lactate acid as anions in [TMG][L] in this study.

A

series

of

tetramethylguanidine

[1,1,3,3-tetramethylguanidinium] [1,1,3,3-tetramethylguanidinium]

dicarboxylic

[succinic [suberic

acid

acid] acid]

ILs

including

([TMG][SUC]),

([TMG][SUB]),

and

[1,1,3,3-tetramethylguanidinium][dodecanedioic acid] ([TMG][DOD]), were synthesized. The absorption behavior and mechanisms of the ILs prepared in this study were also discussed. These new ILs not only retained the chemical absorption and doubled the number of cation, but also increased the integrating sites on anion. The increasing integrating sites could improve the ability of desulfurization and reduce the corresponding enthalpy of SO2 absorption [9].

2. EXPERIMENTAL SECTION 2.1. Materials. 1,1,3,3-tetramethylguanidine, succinic acid, suberic acid and dodecanedioic acid were obtained from Shanghai Aladdin Reagent Co., Ltd., China. Ethanol and anhydrous sodium sulfate were purchased from Tianjin Yongda Chemical Reagent Co., Ltd., China. All Chemicals were used as received. [TMG][SUC], [TMG][SUB] and [TMG][DOD] were synthesized according to literature procedures [7, 8, 11,12] as shown in Scheme 1.

2


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Water content in these ILs measured by Karl Fischer titration was found in the range of 100-300 ppm. 2.2. Absorption of SO2 In Figure 1, a certain amount of IL is placed in a cylindrical glass tube with 12 mm inner-diameter and 120 mm length. The glass tube is immersed in an insulated silicon oil bath which is equipped with a magnetic stirrer, and the temperature of the ILs is controlled by the oil bath. Absorption of pure SO2 gas in the ILs was conducted by passing the gas stream under magnetic stirring at a pressure of 1 bar at 25 °C. The weight change of the sample is monitored by weighing the glass tube on an analytical electronic balance (Mettler AE200, ±0.0001 g) until equilibrium is attained. 3. RESULTS AND DISCUSSION 3.1 Density of dicarboxylic acid-functionalized guanidinium ILs Density is considered to be one of important parameters, which is contributed to understand the interactions between ILs [13, 14]. The density of [TMG][SUC] cannot be tested with liquid densitometer because [TMG][SUC] is in solid state. The fitting curve of the [TMG][SUB] and [TMG][DOD] densities with temperature is shown in Figure 2. The densities of these two ILs had a good linear relationship with temperature, and the density of the ILs decreased with increasing temperature from 293.15 K to 333.15 K. This result was similar to the previous reports of our group [15-17] and C.P. Fredlake’s group [18]. Density (ρ) as a function of temperature is expressed by the following equation [14]. ρ/(g·cm-3)=a + b·T/K,

(1)

Where a and b are adjustable parameters and T is temperature. The fitted parameters a of [TMG][SUB] and [TMG][DOD] are 1.2362 and 1.2046, respectively; the fitted b of [TMG][SUB] and [TMG][DOD] are -6.8622×10-4 and -6.8031×10-4, respectively. The experimental values of lnρ against T were fitted by the least-squares method

[19]

(Figure 3) and empirical equations were derived for the ILs as follows: ln[ρ/(g·cm-3)]=0.2314 -6.72×10-4T/K, ln[ρ/(g·cm-3)]= 0.2063-6.86×10-4T/K . The thermal expansion coefficient of the pure ILs is defined by the following equation.

3


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(2) Where α is the coefficient of thermal expansion and V is the molar volume of ionic liquid. Thus, the values of α obtained from the fitting line are 6.72×10-4 and 6.86×10-4 K-1 for [TMG][SUB] and [TMG][DOD] respectively. 3.2 SO2 absorption and desorption behavior The dependence of molar ratio of absorbed SO2 to the original ILs on the reaction time is demonstrated in Figure 4. In the beginning stage, the SO2 solubility in ILs increased linearly with time. After 40 min, the absorption process on [TMG][SUC] reached the equilibrium sage, whereas the equilibrium times of [TMG][SUB] and [TMG][DOD] were 130 and 150 min, respectively. The saturated molar ratios of SO2 to [TMG][SUC], [TMG][SUB], and [TMG][DOD] were 4.76, 5.96, and 5.96, respectively. Compared with [TMG][SUC], the increasing alkyl chain length of [TMG][SUB] led to a relatively improved SO2 capacity, this result is similar to the result reported by Yang[20]. The absorption capacity of SO2 by ILs was significantly affected by the interaction of SO2 and ILs. From the perspective of molecular structure, the tetramethylguanidine dicarboxylic acids ILs in this paper have the similar active sites with SO2. However, the molar absorptions amount of [TMG][SUB] and [TMG][DOD] is larger than that of [TMG][SUC]. For [TMG][SUC], given the desorption of the IL accompanied by decomposition, no dicarboxylic acid occurred in the solid products after desorption. Thus, [TMG][SUC] was unable to absorb SO2 repeatedly. By comparison, [TMG][SUB] also exhibited poor recyclability. As indicated in Figure S1, approximately 1 mol of SO2 in [TMG][SUB] remained undesorbed after five cycles of absorption-desorption. In addition, it was unfortunate that the higher temperature could result in increased decomposition of [TMG][SUB] because of low thermal stability

[12]

. The recycle experiment on

[TMG][DOD] demonstrated even poorer recyclability. In the first absorption–desorption cycles, approximately 1 mol of SO2 in [TMG][DOD] remained undesorbed. After the second absorption process, the absorption capability of [TMG][DOD] was significantly reduced by 1.7 mol of SO2/mol of IL. The third absorption process showed that [TMG][DOD] had nearly lost its absorption capability. The results indicated that the 4


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stability of [TMG][DOD] was inferior to that of [TMG][SUB]. Amine groups of guanidinium-based ILs could chemically combine SO2 by forming amine sulfate, and 1 mol [TMG][L] are used to chemically absorb 0.5 mol SO2 [21]. Thus, 1 mol [TMG][SUB] or [TMG][DOD], which have two effective chemical active sites with SO2, could chemically absorb 1 mol SO2. The high chemical absorption enthalpy made the regeneration procedure difficult [9, 12] under lower temperatures. As a result, 1 mol of SO2 absorbed in [TMG][SUB] or [TMG][DOD] cannot be desorbed completely at 80 °C. Figure 5 shows the molar ratio of the absorbed SO2 to the original ILs as a function of absorption temperature. Temperature has a strong effect on SO2 absorption of ILs. The absorption capacity decreased gradually with the temperature increased. As an example, the equilibrium absorption amount of [TMG][DOD] reached 6.15 mol/mol SO2 at 20 °C and reduced to 2.77 mol/mol SO2 at 80 °C. Therefore, a high temperature is not favorable for SO2 removal efficiency, but is conducive to desorption [20]. It is consistent with the previous work of [TMG][SUB] [12]. The partial pressure dependence of SO2 absorption was also essential. However, the reaction rate of absorbing SO2 under low SO2 partial pressure was very slow because of their high viscosity. With the increase of the absorption time, the viscosity increases too much for their chemical reaction with SO2. At last, ILs condenses into solid state and cannot continue to absorb SO2. This phenomenon is consistent with that reported in literature

[21]

. Therefore, the effect of pressure on SO2 absorption by this type of ILs

cannot be performed, and so the pure SO2 absorption by ILs was discussed in this paper. The viscosity of ionic liquid before and after SO2 absorption was measured at 40oC. The viscosity of pure [TMG][SUB] is 367.0 mPa•s, while the viscosity of the IL saturated with SO2 decreased to 299.2 mPa•s. 3.3 Mechanism of SO2 absorption The FTIR spectra of [TMG][SUC] and [TMG][DOD] were shown in Figure 6 and Figure S2, respectively. In the FTIR spectra of [TMG][SUC], the ILs saturated by SO2 showed new absorption bands at 1323 and 955 cm-1 compared with the SO2-free [TMG][SUC], which can be assigned to the extension vibration of S=O and S–O···H groups, respectively, as described in previous reports [7, 8]. In addition, a new strong hydroxyl

5


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band appeared at 3160 cm−1, which can be assigned to a hydrogen bond

Page 6 of 16

[22–24]

. Similar

phenomenon has also been observed in the FTIR spectrum of [TMG][DOD] (Figure S2). In the UV spectra, SO2 presented a characteristic absorption peak within the range from 250 to 320 nm

[25, 26]

. Compared with the UV spectrum of pure [TMG][SUC], the

SO2-absorbed sample present a new peak at 275 nm. This peak means the formation of new bonds between SO2 and NH2, which rendered the transition from n to п* difficult for SO2

[27]

. The results mentioned above were consistent with our previous work of

[TMG][SUB]

[12]

. FTIR and UV analysis showed that there is interactions existed

between the ILs and SO2. The interaction of ILs and SO2 was also investigated via NMR spectroscopy. The peaks associated with 1H, 2H in 1HNMR spectra as well as the peaks associated with1C, 2C, 4C in

13

CNMR spectra shifted downfield respectively for these three ILs, that

illustrated the interactions appeared in ILs and SO2. The new chemical shift at 7.7-7.9 ppm corresponds to the formation of S=O···H (Table S1). In addition, after SO2 absorption, there are apparent displacements of hydrogen on N-CH3 in 1H NMR of [TMG][SUC]. Why do these series of tetramethylguanidine dicarboxylic acid ILs with short chains in this paper possess such properties and exhibit less capacity for SO2 capture? Interstitial spaces in ILs are favorable for SO2 absorption. It is the asymmetry of the anionic structure that affects SO2 absorption. The increase of chain length in anion led to the increase of interstitial spaces [29]. It is proposed that the sizes of positive and negative ions are different and asymmetric. Consequently, the cation and anion are difficult to tightly pack from the microcosmic point of view. Thus, many interstitial caves are generated among positive and negative ions. When the growth of alkyl chain is present in anion, the dispersion force between alkyls increases, the asymmetry of ILs is further enhanced, and the interstitial space among ILs also increases. More interstitial spaces enable SO2 to be effectively absorbed, avoids SO2 crowding in the interstice, and decreases the energy of SO2 absorption. Therefore, the more interstice increases the capacity of absorbing SO2, this is analogous to Sistla’s conclusion[30] which reported that interstitial spaces is conducive to the absorption of CO2. It is demonstrated that asymmetric ions usually result in weak cation-anion interactions/intermolecular forces of attraction and create many interstitial spaces; such spaces are favorable for CO2 6


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absorption. Therefore, when the [SUC] anion was replaced by [SUB] or [DOD] in this study, the absorption capacity of ILs improved obviously. To further investigate the mechanism of SO2 absorption by [TMG][SUC] and [TMG][SUB], the geometry optimization for the free ILs and the SO2-treated ILs was calculated at the B3LYP/6-31 ++ G(d,p) level of theory[31]. The optimized structures of [TMG][SUC] indicated that the hydrogen-bonding interaction via N···H···O with distance of 1.46 and 1.09 Å is presented (Figure 10), and the particular spatial distribution of N···H···O leads to a strong hydrogen-bonding interaction between the cation and anion, which resulted in increasing viscosity and numerous electrons being concentrated in the corresponding N atom, that is conducive for combination with SO2. As shown in Figure 11, there are two main interaction modes present on the optimized structures of the stable [TMG][SUC]-SO2 complexes. The first mode is the relatively strong association between the sulfur atom of SO2 and the nitrogen atom in the -NH2 group. The distances of S and N are 1.962 and 2.065 Å, respectively. The second mode is the interaction between sulfur atoms of SO2 and the oxygen atom in carbonyl as well as the interaction between oxygen atoms of SO2 and the hydrogen atom in methylene groups which are bonded to the carbonyl carbon. The interaction distances are 2.621, 2.639, 2.781, and 2.710 Å for S4···O11, O8···H2, S3···O10, and O6···H3, respectively. The calculations of the interaction among these atoms are in agreement with the experimental results. In addition, some of the oxygen atoms in SO2 are close to the hydrogen atoms in the N-CH3 group, and the interactions of the oxygen atoms and hydrogen atoms occurred. This can reasonably explain the apparent chemical displacement of hydrogen in N-CH3 in 1H NMR after SO2 absorption. The distances between O9···H6, O3···H9, O6···H3, and O1···H8 are 2.466, 2.274, 2.710, and 2.671 Å, respectively. Because the succinate in [TMG][SUC] has a short chain length, the limited interstitial spaces limit the capability of [TMG][SUC] to absorb SO2. Therefore, compared with [TMG][SUB] and [TMG][DOD], the desulfurization capacity of [TMG][SUC] is unsatisfactory. For [TMG][SUB], Figure 12 shows a relatively strong association between the sulfur atom of SO2 and the nitrogen atom in the -NH2 group. The distances between S⋯N are 7


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1.902 and 2.123 Å. The interaction also occurred between sulfur atoms of SO2 and the oxygen atom in carbonyl as well as between oxygen atoms of SO2 and hydrogen atoms of methylene groups which is adjacent to the carbonyl carbon. The interaction distances are 2.699, 2.693, 2.573, 2.711, 4.066, and 3.035 Å for O1···H1, O2···H1, S1···O9, O6···H2, O5···H2, and S3···O10, respectively. The growth of chain length in the anion increased the asymmetry of ILs, decreased the space resistance, avoided SO2 crowding in the interstice, and caused the hydrogen atom in methylene groups to interact with SO2 easily. Therefore, the capacity of absorbing SO2 is enhanced. This result is also in accordance with the results reported by Yang et al.

[29]

and Yamini et al. [30]. Therefore, [TMG][SUB] and

[TMG][DOD] are more favorable for SO2 absorption than [TMG][SUC]. Based on FTIR, UV, and NMR spectra, quantum chemical calculation of SO2-free and SO2-absorbed ILs, the mechanism of guanidinium-based dicarboxylic acid ILs was deduced for the absorption of SO2, as shown in Scheme 2. The mechanism indicated that the ratio of ILs to SO2 was 1 at the state of chemisorption. When ILs absorbed SO2, a new chemical bond was initially formed between the functional group (-NH2) and SO2. Subsequently, the newly formed amino sulfurous acid ester reacted with 1 mol unreacted IL. At the same time, 2 mol dicarboxylic acids were released. Furthermore, SO2 also interacted with the carboxyl and adjacent methylene on the dicarboxylic acid. For [TMG][SUC], the highly symmetrical and compact structure resulted in strong cation-anion interactions and intermolecular forces of attraction, that create fewer interstitial spaces. Such an effect was unfavorable for SO2 absorption. Therefore, absorbing 2 mol SO2 may be difficult for [TMG][SUC]. For [TMG][SUB] and [TMG][DOD], when the alkyl chain grew in anion, the dispersion force between alkyls increased. Furthermore, the asymmetry of ILs was further enhanced, and the interstitial spaces among ILs increased. Thus, 1 mol dicarboxylic acid could easily absorb 2 mol SO2. 4. CONCLUSION The SO2 absorption behaviour of three tetramethyl guanidinium-based ILs was studied. The molar ratios of SO2 to [TMG][SUC], [TMG][SUB] and [TMG][DOD] were 4.76, 5.96, and 5.96, respectively. SO2 could interact with the carboxyl and adjacent methylene on the anion as well as the amino group on the cation. Anion of the ILs greatly influenced

8


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SO2 solubility. When the [SUC] anion was replaced by [SUB] or [DOD], the absorption capacity of the ILs significantly improved. The increased chain length in anion enlarged the asymmetry of ILs, decreased the space resistance, avoided SO2 crowding in the interstice, and caused the hydrogen atom of methylene to interact with SO2 easily. Therefore, the capacity of absorbing SO2 was enhanced. AUTHOR INFORMATION Corresponding Author *Telephone/Fax: +86-0311-81668302. E-mail: [email protected]. *Telephone/Fax: +86-0310-8578760. E-mail: [email protected]. ORCID Yongqi Hu: 0000-0003-4991-1748 Author Contributions ∥ Xiaocai Meng and Jianying Wang contributed to this work equally. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the Science and Technology Foundation of Universities of Hebei Province (ZD2015113 and QN2016138), the Natural Science Foundations of Hebei Province (B2016402030 and B2016208102)

and

International

Collaborative Research of Hebei Province (16391213D).

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O N N

HO NH

2 N

n

-O

NH2

OH N

O

11


n 2

O

O-

n=1,3,5

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Scheme 1. Formation of the guanidinium-based dicarboxylic acid ionic liquids. O S O

O

O -O N

C+

N

+SO2 O

NH 2

2

-

-SO2

n

2

O

H 2N

C+

N

H 2N

H

H

O

O-

S

O

O

O C

N

S

O

C

+

NH2

O

n

2

N

H2 N H

+SO2

-O N -

2

n=0,1,2

O

O

+

H

n

O S

H2 N

O

H

H

-

-SO2 O

O

S O

O

O N

2

N

H2 N C+

NH 2

C

N

S

O-

H

H

O

H2 N

H O

n

O

H

O S

n=0,1,2 O

Scheme 2. Proposed mechanism of SO2 absorption by ILs.

Figure 1.

Experimental setup for SO2 absorption of ILs.

1. SO2 gas cylinder; 2. gas cylinder pressure regulator; 3. valves; 4. mass flowmeter; 5. N2 gas cylinder; 6. glass tube filled with IL; 7. water bath; 8. safety device of preventing suck-back; 9. absorber ( NaOH solution).

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1.04 [TMG][SUB] [TMG][DOD]

1.03

1.01

.

ρ / g cm

-3

1.02

1.00 0.99 0.98 0.97 290

300

310 320 T/K

330

340

Figure 2. Fitting curve of the densities of ILs with temperature. 0.04 [TMG][SUB] [TMG][DOD]

0.03 0.02 0.01

.

-3

ln(ρ / g cm )

0.00 -0.01 -0.02 -0.03 290

300

310 320 T/K

330

340

Figure 3. Density (ρ) as a function of temperature for ILs. 7 6 Molar Ratio(nSO2/nIL)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

5 4 3 2 [TMG][SUC] [TMG][SUB] [TMG][DOD]

1 0 0

20 40 60 80 100 120 140 160 180 200 220 Time / min

Figure 4. Absorption of SO2 by [TMG][SUC], [TMG][SUB][12], and [TMG][DOD] at different time under atmospheric pressure at 25 °C.

13


Energy & Fuels

7 [TMG][DOD] [TMG][SUB]

Molar Ratio(nSO2/nIL)

6 5 4 3 2 10

20

30

40

50

60

70

80

90

o

T/ C

Figure 5. Absorption of SO2 by [TMG][SUB][12] and [TMG][DOD] at different temperature under atmospheric pressure. 1 4 0 9 5 5

1 2 0

3 1 6 0

1 3 2 3

1 0 0

T %

8 0 6 0 4 0 2 0 0

b e fo re a fte r

5 0 0

1 0 0 0

1 5 0 0

3 0 0 0 λ

/ c m

3 5 0 0

-1

Figure 6. FTIR spectra of [TMG][SUC] before/after SO2 absorption. 4 before after

After 3

Abs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 16

2

1 Befor 0 240

280

320

360

400

λ / nm

Figure 7. UV–vis spectra of [TMG][SUC] before/after SO2 absorption.

14


4 0 0 0

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Energy & Fuels

(a)

(b)

1

13

Figure 8. (a) H NMR and (b) C NMR of [TMG][SUC] before/after SO2 absorption.

(a)

(b)

Figure 9. (a) 1H NMR and (b) 13C NMR of [TMG][DOD] before/after SO2 absorption.

Figure 10. Optimized structures of [TMG][SUC].

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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 11. Optimized structures of [TMG][SUC]-SO2.

Figure 12. Optimized structures of [TMG][SUB]-SO2.

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