Double bond carboxylate ionic liquids for reversible SO2 capture

Chemical Physics Letters 724 (2019) 67–72

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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

Research paper

Double bond carboxylate ionic liquids for reversible SO2 capture Liang Wei, Yilin Wang, Zhongjiao Fan, Rujia Wang, Yuan Wang, Jianqiu Chen, Yun Xu

⁎

T

School of Engineering, China Pharmaceutical University, Nanjing 211198, China

H I GH L IG H T S

of SO in [N ][Met] was measured for the first time. • Solubility absorption enthalpy, the fast absorption process, recycling four times. • Low • RETM was used to correlate the experimental data. 2

2224

A R T I C LE I N FO

A B S T R A C T

Keywords: Ionic liquids SO2 Absorption RETM equation

Solubility of SO2 in [N2224][Met] was measured at temperature from 313.2 to 333.2 K and pressure up to 1.8 bar. The [N2224][Met] absorbed SO2 at high speed and could be recycled for four times. The Reaction Equilibrium Thermodynamic Model (RETM) was used to correlate the experimental data, Henry’s constant (H), reaction equilibrium constant (K0), and thermodynamic properties (Δr Hm0 , Δr Gm0 , Δr Sm0 , ΔHphy and ΔHchem ) were derived. Considering the absorption capacity and low absorption enthalpy, as well as the fast absorption process, [N2224] [Met] is thought to be a promising absorbent to ordinary solvent as a desulfurizer.

1. Introduction Fossil is still main source of energy in current industry. During the combustion of fossil, SO2 which is one of the causes of acid rain will be released. Flue gas desulfurization (FGD) for removal of SO2 has been paid more attention on environmental protection due to the restricted emission limit of contamination [1,2]. Many efforts have been put into FGD, but some drawbacks should be conquered. The limestone scrubbing method leads to a large scale of solid waste [3,4], the ammonia method produces volatile organic compounds (VOCs) leading to secondary pollution [5], and the seawater method only is restricted in the coastal areas [6]. Additionally, the absorbents can’t be recycled for FGD process aforementioned methods. Therefore, environmentally friendly and economically methods for SO2 capture are still challenged in the face of efficient absorbents. Ionic liquids (ILs) which are kinds of room temperature salts with many tremendous properties, such as high efficiency, easy recycling property, and non-volatility, are showing their great potential in the application of acidic gas capture. In comparison to conventional ILs, the functional ILs which is designed with multiple-site in the structure displaying advantages for SO2 capture [7–10]. The tetramethylguanidinium lactate ([TMG]-[L]) has been first reported by Han et al [11]. After that TMG-bases ILs [12–14], imidazolium ILs [15–17],

⁎

and cyano-based ILs [10,18,19] were all reported by different groups and played a wonderful role in the SO2 absorption capacities. We noticed that carboxylate-ILs have special affinity to the SO2. Dicarboxylic acid salts and aqueous solutions of mixed hydroxylammonium dicarboxylate were used as task-specific ILs for SO2 absorption reported by Wu’s group [20,21]. Halogenated carboxylate ionic liquids were found to be highly efficient to SO2 capture and the capacity reached up to 4.34 mol mol−1 ILs reported by Cui’s group [22]. [NH2emim][OAc] was mixed with low-viscosity imidazoliumbased ILs [bmim][OH]/[bmim][BF4] to improve SO2 absorption performance in flue gas in Liu’s group [14]. Furoate ILs/PEG200 mixtures showed high performance in SO2 capture reaching at 7 mol kg−1 ILs [23]. Double bond comprised carboxylate-ILs (DB-ILs) were found and showed good performance in CO2 capture [24]. Here we further explored their capacities in SO2 capture. [N2224][Met] and [N2224][Cro] were synthesized according to the reported procedure [24]. The structures of [N2224][Met] and [N2224][Cro] are displayed in Scheme 1. 2. Experimental section 2.1. Materials SO2 (99.99 mol%) were supplied from Nanjing Tianze Gas Co.

Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Xu).

https://doi.org/10.1016/j.cplett.2019.03.034 Received 27 February 2019; Received in revised form 15 March 2019; Accepted 17 March 2019 Available online 26 March 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.

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L. Wei, et al.

313.2 K to 333.2 K and elevate pressures up to about 1.8 bar. The results were shown in Fig. 1b. It was clearly shown that the solubility of SO2 in [N2224][Met] was influenced by temperature and pressure. The solubility of SO2 in ILs decreased with increasing temperature, while the capacity of SO2 in ILs increased with increasing pressure. Lower temperature and higher pressure are in favor of higher solubility of SO2 in ILs. For example, the per kilogram [N2224][Met] to SO2 was 5.82 mol kg−1 ILs at 313.2 K under 1 bar, while it was 8.10 mol kg−1 ILs at the same temperature under 1.8 bar and 5.19 mol kg−1 ILs at 323.2 K under 1 bar.

Scheme 1. Chemical structure of the ILs investigated in this work.

(Nanjing, China). Silver oxide (AR grade, 99.5 wt%), Methacrylic acid (AR grade, 99.5 wt%), and crotonic acid (99 wt%) were purchased from Aladdin Chemical Reagent Co., and used without further purification.

3.2. Recyclability of [N2224][Met] In order to illustrate the reversible absorption of SO2 in the ILs, the SO2-saturated [N2224][Met] was heated to 358.2 K and the equilibrium cell was vacuumed about 12 Pa to the system. Then recovered ILs was recovered with the same amount of SO2 (with an initial partial pressure of about 2.3 bar) and the capacity of SO2 was recorded under the same conditions. The results were shown in Fig. 2. It was found that [N2224] [Met] captured SO2 in a quite rapid speed. It took less than 90 s for [N2224][Met] to arrive at absorption equilibrium. The final SO2 partial pressure in Fig. 2 is 1.63 bar and the equilibrium absorption amount of SO2 is 2.65 mol kg−1 ILs. As can been seen, the [N2224][Met] could be recovered after four absorption/desorption cycles and the amount of absorption SO2 wasn’t obviously declined. The results suggested that the capacity of SO2 in [N2224][Met] was highly reversible and the SO2saturated [N2224][Met] could be easily recovered by heating and vacuuming.

2.2. General procedures for determination of SO2 absorption The apparatus for the determination of gas absorption in DB-ILs is the same as that in our previous work [24]. There are two 316 L stainless steel chambers whose volumes are 126.13 cm3 (V1) and 51.73 cm3 (V2), respectively. The bigger chamber, used as a gas reservoir, restores the gas before it goes into the smaller chamber through needle valve. The smaller chamber called as the equilibrium cell is equipped with a magnetic stirrer. Both chambers are put in the water bath. The temperatures (T) are controlled by an automatic thermoregulatory with the uncertainty of ± 0.1 K. The pressures are monitored using two pressure transducers of ± 0.5% uncertainty. The pressure transducers are connected to a computer through an absolute pressure controller using RS232 communication interface to record the pressure changes online. In a general procedure, a known mass of DB-ILs was put into the equilibrium cell, and the air of the two chambers was evacuated. The residual pressure of the equilibrium cell was recorded to be P0. The gas reservoir was fed to a pressure of P1 from the cylinder. The needle valve was turned on to let some of gas be introduced to the equilibrium cell. When the two chambers remained constant for at least 1 h, absorption equilibrium was reached. The pressures of gas reservoir and equilibrium cell were denoted as P1′ and P2 respectively. The partial pressure of the equilibrium cell was Pg = P2 − P0 . The uptake of gas n(Pg) was calculated by the following equation

n(Pg) = ρg (P1, T ) V1 − ρg (P1', T ) V1 − ρg (Pg, T )(V2 −

ω ) ρIL

3.3. Comparison of solubility of SO2 in the different ILs In order to have a comparison of capacity for SO2 capture between DB-ILs and other normal or functional investigated pure ILs, a summary of the solubility of SO2 in different kinds of ILs was presented in Table 1. It was found that solubility of SO2 in two DB-ILs exhibited relatively large advantage than that of [TMG][Tf2N], [E3MIm2][Tf2N]2 and [P66614][PhO] under 1 bar and be comparable to that of [TMG] [BF4] and [N2224][FA]. It should be noticed that [N2224][Met] could show more competitive capacity for SO2 capture reach up to 8.10 mol kg−1 partial pressure of SO2 under 1.8 bar. Due to relatively high performance for SO2 under 1.8 bar, [N2224][Met] was considered to be potential alternative for capturing SO2.

(1)

where ρg (P1, T ) represents the density of gas in mol/cm3 at Pi (i = 1, g) , ω is the weight of DB-ILs and ρIL is the density of DB-ILs in g/cm3 at T . V1 and V2 represent the volumes in cm3 of the two chambers, respectively. In order to get solubility data at elevated pressures, more gas was introduced into the equilibrium cell to reach a new equilibrium. Duplicate experiments for each sample were run to obtain the averaged values of SO2 solubility. The averaged reproducibility of the solubility data in this work was with 1%. After determinations, residual gas in the chambers was successively introduced to an off-gas absorber containing an aqueous solution of NaOH to prevent gas leakage into atmosphere. SO2 is toxic gas. All of the experiments must carefully carry out in a fume hood (see Scheme 2).

3.4. Thermodynamic analysis According to reported work on the SO2 absorption in ILs, it was supposed that one [N2224][Met] molecule could chemically absorb one SO2 molecule through the functional site of carboxylate in the ILs. To understand the binding mechanism, reaction equilibrium thermodynamic model (RETM) equation is applied based on the two aspects of absorption - physical and chemical absorption. The related equation could be expressed as following Eqs. (2) and (3), respectively.

3. Results and discussion

SO2 (g) → SO2 (l)

(2)

SO2 (l) + IL (l) → SO2 IL (l)

(3)

where g and l in Eqs. (2) and (3) represent the current states gas or liquid of a species. The physical absorption part is described by Henry's law, which is defined in terms of molality, is expressed as Eq. (4)

3.1. SO2 solubility and absorption capacity in ILs The capacity of [N2224][Cro] and [N2224][Met] for SO2 were measured as a function of time at 313.15 K according to our reported procedure [24]. The results were graphically described in Fig. 1a. As we can see, [N2224][Cro] showed slightly superiority than that of [N2224] [Met] for absorption SO2 while for absorption CO2 [N2224][Cro] Vs [N2224][Met] was 0.30 mol mol−1 IL Vs 0.18 mol mol−1 IL. The [N2224] [Met] was chosen to investigate solubility of SO2 at temperature from

P = Hm γSO2

m SO2 m0

(4)

where P is the partial pressure of SO2 in kPa, Hm is Henry’s law constant in kPa, γSO2 is the activity coefficient of the physically dissolved SO2, and m SO2 is the concentration of physically dissolved SO2 in mol·kg−1, 68

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V1 WB WT

P1

V2

V3 P2

VP GR

GC

PC

EC

MS

TC

Scheme 2. Apparatus for the determination of gas solubility. (GC – gas cylinder; V1, V2, V3, V4 – valves; VP – vacuum pump; WT – washing tower; WB – water bath; TC –thermal controller; GR – gas reservior; EC – equilibrium cell; P – pressure transducer; MS – magnetic stirrer; NI – numerical instrument; PC – personal computer).

m0 is the standard molality (1 mol kg−1). The chemical absorption part is described by chemical reaction equilibrium of Eq. (5) K0

The mass balance for SO2 is expressed as Eq. (7), where mt is the total concentration of SO2 in the liquid phase. The liquid phase was assumed as ideal condition, the value of γSO2,γSO2 IL and γIL are all assumed as 1.0. After deducted according to equations from Eqs. (4)–(8), the following equation is obtained to correlate the total solubility of SO2 in ILs and partial pressure of SO2.

γSO2 IL m so2 =

m0

P m γ IL P 0 IL m0

(5)

K0

where is the equilibrium constant, γSO2 IL and γIL are the activity coefficients of the SO2 (l) and SO2IL (l) complex in the liquid phase, respectively, m so2 andmIL are the concentrations of the SO2 (l) and SO2IL (l) complex after absorption equilibrium in mol·kg−1, and P° is the standard pressure (100 kPa). The mass balance of ILs is expressed by Eq. (6)

m IL0 = mIL + m SO2 IL

mt = m IL0

The mass balance of SO2 is expressed by Eq. (7) (7)

1 MIL

(8)

m IL0 =

Eq. (6) is the mass balance for the absorbent, where mIL0 is the initial concentration of absorbent and is a constant that can be calculated from Eq. (8) where MIL is the molar mass of the absorbent in g mol−1. 1.6

10

1.4

8

1.2

Solubility of SO 2 mol.kg -1

SO2 solubility mol / mol ILs

(9)

The experimental data are graphically illustrated in the Fig. 1(b) in comparison with fitting results from the RETM Eq. (9). It clearly shows that the absorption curve slightly deviates from the ideality (R2 > 0.99). The RETM equation is a proper model for SO2 solubility in [N2224][Met]. In addition, the calculated values of K 0 and H for [N2224][Met] are presented in Table 2. It can be seen that [N2224][Met] has relatively smaller values of K0 than those reported in the literature [23], indicating that [N2224][Met] has an interaction with SO2 weaker than [N2224][FA] both chemically and physically. On the other hand, that is the reason for [N2224][Met] and could be easily recovered. To further understand the gas dissolution and reaction equilibrium between SO2 and [N2224][Met], the thermodynamic properties including molar reaction enthalpy (Δr Hm0 ), molar Gibbs energy of reaction (Δr Gm0 ), and molar reaction entropy (Δr Sm0 ) are also summarized in Table 2. The

(6)

mt = m SO2 + m SO2 IL

K 0P P + K 0P + 1 Hm

1.0 0.8 0.6 0.4 0.2

[N2224][Cro] [N2224][Met]

0.0

(a)

6

4

2

313.2 323.2 333.2

0

(b)

-0.2 0.0

0.2

0.4

0.6

0.8

1.0

0.0

SO2 partial pressure / bar

0.5

1.0

1.5

SO2 partial pressure / bar

Fig. 1. SO2 absorption in [N2224][Cro] (at 313.2 K) and [N2224][Met] (at different temperature). 69

2.0

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3.0

2.5

SO2 solubility mol / Kg ILs

SO2 solubility mol / Kg ILs

3.0

2.0 1.5 1.0 0.5

2.5 2.0 1.5 1.0 0.5

0.0 0.0 0

2

4

6

8

10

12

14

1

2

Time / min

3

4

5

Recycle

Fig. 2. SO2 absorption equilibrium time in [N2224][Met] and recycle times at 313.2 K. Table 1 Comparison of the solubility of SO2 in DB-ILs and other ILs. Entry

a

Absorbents

T/K

Mt/mol mol

1

[N2224][Cro]

313.2

1.41

2

[N2224][Met]

313.2

3 4 5 6 7

[Bmim][BF4] [Hmim][ BF4] [TMG][BF4] [TMG][Tf2N] [E3MIm2] [Tf2N]2 [TMG][L] [N2224] [disuccinate] [Bmim][Ac] [Bmim] [MeSO4] [P66614][PhO] [P66614][Tetz] [P66614][Im] [N2224][FA]

8 9 10 11 12 13 14 15 a b

−1

Mt/mol kg

−1

Δr Sm0 = Mw

Ref

5.79

243.22

1.39

5.71/8.10b

243.22

293.2 293.2 293.2 293.2 293.2

1.50 1.41 1.27 1.18 1.60

6.63 8.34 6.25 2.98 1.74

226.13 169.06 203.12 396.04 918.06

This work This work [25] [25] [25] [25] [26]

313.2 313.2

1.70 1.83

8.29 6.64

205.14 275.21

[11] [20]

313.2 313.2

1.91 2.11

9.63 8.43

198.26 250.10

[27] [27]

293.2 293.2 293.2 313.2

3.02 3.72 4.80 –

5.23 6.73 8.75 5.94

576.54 552.53 548.51 241.17

[28] [29] [29] [23]

Δr Hm0 − Δr Gm0 T

(12)

As shown in Fig. 3, the linear relationship between lnK 0 and 1/T were very good and the value of R2 was 0.99. The results of Δr Hm0 ,Δr Gm0 and Δr Sm0 at 313.2 K were presented in Table 2. It is known that the enthalpy of absorption is another important value for the estimation of the interaction between the gas and liquid component, which assumes for major energy consumption for gas absorption and desorption process. As can been seen, the absolute value of Δr Hm0 for SO2 in [N2224][Met] was lower than that of [N2224][FA] which was 46.39 kJ mol−1 as discussed before. We notice that K. Huang [30] calculated Δr Hm0 for SO2 in [N2222][diglutarate] and [DMEA][diglutarate], the value of that were −53.73 kJ mol−1 and −47.15 kJ mol−1 respectively. The two absolute values of Δr Hm0 were near twice times larger than that of [N2224][Met]. However, [N2222][diglutarate], [DMEA][diglutarate] and [N2224][FA] didn’t show huge advantages than [N2224][Met] in SO2 capture. In order to figure out this phenomenon, physical absorption enthalpy (ΔHphy ) was calculated from the Herry’s constant at different temperature in form of linear fit between lnH and 1/T (R2 > 0.98). Chemical absorption enthalpy (ΔHphy ) was calculated according to this formula,

Solubility of SO2 in ILs under 1 bar. Solubility of SO2 in [N2224][Met] under 1.8 bar.

Δr Hm0 = ΔHphy + ΔHchem

(Δr Hm0 )

molar reaction enthalpy is obtained from the Vant’t Hoff equation by the linear fitting between ln K0 and 1/T,

Δr Hm0 ∂lnK 0 =− ∂ (1/ T ) R

The results were shown in Table 2. The calculate values of ΔHphy and ΔHchem in [N2224][Met] for SO2 capture were –23.1 kJ mol−1 and −3.74 kJ mol−1, respectively. The comparison clearly revealed that physical absorption takes more advantage than that of chemical absorption in [N2224][Met] for SO2 capture. Normally, chemical process dominates at low SO2 partial pressure and plays a key role in the high capacity of ILs, and then physical process plays a role step by step at relatively high partial pressure. In the case of [N2224][Met] for SO2 capture, the chemical process ended at very short partial pressure. The clue could be found from the absorption curve. [N2222][diglutarate],

(10)

The absorption Gibbs free energy (Δr Gm0 ), and entropy (Δr Sm0 ) are calculated from the following equations respectively.

Δr Gm0 = −RTlnK 0

(13)

(11)

Table 2 Thermodynamic properties of SO2 absorption in [N2224][Met]. Solvent [N2224][Met]

Properties Hm/100 kPa kg mol K0 R2 0 Δr Hm /kJ mol−1

−1

313.2/K

323.2/K

333.2/K

0.38 ± 0.01 3.49 ± 0.25 0.9947 −26.84 ± 0.14

0.48 ± 0.01 2.53 ± 0.17 0.9933

0.65 ± 0.03 1.88 ± 0.16 0.9932

0 Δr Gm /kJ mol−1

3.25 ± 0.18

0 Δr Sm /kJ mol−1 ΔHphy /kJ mol−1 ΔHchem /kJ mol−1

−96.09 ± 1.04 –23.10 ± 2.20 −3.74 ± 2.20

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1.3 -0.4

1.2

-0.5

1.1

-0.6

lnH

lnK0

1.0 0.9

-0.7 -0.8

0.8 0.7

-0.9

0.6

-1.0

0.00300

0.00305

0.00310

0.00315

0.00300

0.00320

0.00305

0.00310

0.00315

0.00320

1/ T

1/T Fig. 3. Linear fit of lnk0, lnH and 1/T.

4. Conclusion

[DMEA][diglutarate] and [N2224][FA] have the similar situation as same as [N2224][Met] from their SO2 absorption curves. This’s the reason the difference in SO2 capacity among the three ILs isn’t obvious.

[N2224][Cro] and [N2224][Met] were investigated for SO2 absorption. Take [N2224][Met] for example, the solubility of SO2 were systematically measured under the pressure range of 0–1.8 bar and the temperature range of 313.15–333.15 K. The results demonstrated that [N2224][Met] could efficiently capture SO2 under 2 bar. Based on the RETM equation to evaluate the SO2 performance in [N2224][Met], Herry’s law constants, reaction equilibrium constants and thermodynamic function were all calculated. It was illustrated that physical absorption SO2 instead of the chemical process took a key role in [N2224][Met]. [N2224][Met] could be easily recycled four times. Owing to the advantages of high absorption capacity and low absorption enthalpy, as well as the fast absorption process, we believe that [N2224] [Met] can be regarded as a potential alternative to some volatile absorbent to be applied in SO2 absorption. The authors declare no competing financial interest.

3.5. The mechanism between the SO2 and ILs The mechanism of [N2224][Met] for SO2 capture was investigated and 13C NMR was applied to characterize the [N2224][Met] and [N2224] [Met] + SO2 (Fig. 4a). As shown in Fig. 4, the chemical shift of eCOO– in [N2224][Met] moved upfield from 172.8 to 171.3 ppm, indicating the interaction of SO2 with eCOO–. As the double bond carbon atom adjacent to carboxyl group, the π…S interaction between double bond and SO2 resulted in the downfield shifting of one carbon atom from 118.2 to 126.9 and upfield shifting of the other carbon atom from 144.3 to 136.1 ppm. From the above thermodynamic discussion, we knew that chemical uptake SO2 didn’t dominate process. Even so, the double bond and carboxylate mainly interact with SO2 during the chemical process. The further poof was obtained from comparing the FT-IR spectra between the [N2224][Met] and [N2224][Met] + SO2 (Fig. 4b). It was clean shown that the σC]C stretching vibration from 1652 to 1707 cm−1 and the peak at 3488 cm−1 shift to 3470 cm−1 which was assigned to SeO stretch absorption peak due to the interaction of SO2 and [N2224][Met]. The new band at 1034 cm−1 was attributable to symmetric vibration of S]O bond.

Acknowledgments This work was supported by the National Natural Science Foundation of China (no. 21808246), the Fundamental Research Funds for the Central Universities (no. 2632017PY01), the College Students Innovation Project for the R＆D of Novel Drugs (no. J1310032) and Jiangsu Overseas Visiting Scholar Program for University Prominent Young ＆ Middle-aged Teachers and Presidents. [N2224][Met]-SO2

1707 1034

[N2224][Met]

3470

(b)

4000

3500

3000

2500

2000

1500

Wavenumber/cm-1 Fig. 4.

13

C NMR (a) and FT-IR (b) spectra of [N2224][Met] before and after t SO2 absorption. 71

1000

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