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The Equilibrium in Protic Ionic Liquids: The Degree of Proton Transfer and Thermodynamic Properties Kaizhou Chen, Yongtao Wang, Jia Yao, and Haoran Li J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b10671 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017

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The Journal of Physical Chemistry

The Equilibrium in Protic Ionic Liquids: The Degree of Proton Transfer and Thermodynamic Properties Kaizhou Chen†, Yongtao Wang†, Jia Yao* †, Haoran Li †, ‡ †

Department of Chemistry, ZJU-NHU United R&D Center, Zhejiang University, Hangzhou, 310027, P. R. China ‡

State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310027, P. R. China.

* To whom correspondences should be addressed: Jia Yao: Email: [email protected], Fax: +86-571-87951895

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ABSTRACT: The degree of proton transfer and thermodynamic parameters of six acetate-based protic ionic liquids (PILs) were measured using nuclear magnetic resonance (NMR) spectroscopy and calculated by van’t Hoff analytical method, respectively. The degree of proton transfer of these PILs at 298 K spread over a large range, which is from 39.6% (1-methylpyrrolidinium acetate, [MpyrH][AcO]) to 94.4% (1-butylimidazolium acetate, [BuimH][AcO]). The calculated standard enthalpy change of the reaction (∆ ) is from −23.30 to − 7.80 kJ mol and the standard entropy change of the reaction (∆ ) is

from −42.70 to − 8.07 J mol K . The correlation between the degree of proton transfer and aqueous ∆p or ∆ was investigated as well. Furthermore, in some special cases, ∆ , especially the entropy change of symmetry, also plays an important role in affecting

the degree of proton transfer.


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INTRODUCTION Ionic liquids (ILs) are usually defined as fused salts with a melting point near room temperature.1 The interest in ILs has been boosted over the past decades in both industry and academia due to their unusual properties, such as non-flammability, non-volatility, increased thermal stability and high ionic conductivity.2-3 Protic ionic liquids (PILs),4 as a significant subclass of ILs, have emerged as a new kind of materials for applications such as catalysts,5-8 fuel cells,9 chromatography,10 organic synthesis,11 and greenhouse gas absorption.12 PILs are differentiated for they are formed in general by the transfer of a proton from a Brønsted acid, HA, to a Brønsted base, B.13-14 HA + B ⇌ BH ! + A

(1)

Due to the incomplete proton transfer, neutral acid and base species exist in PILs.2, 4, 15-16 To some extent, the equilibrium in a PIL, which largely depends on the choice of cation and anion,3 will affect its properties, such as density, viscosity and conductivity.2, 14, 17 Therefore, the exact degree of proton transfer in PILs, which quantitatively reflect the ionicity (“ionic nature” or “how ionic they are” etc.) 18, is appreciated. In some applications, the ionicity of PILs is a key factor. When PILs are applied in the extraction of lignin from lignocellulosic biomass, Achinivu et al. discovered that PILs with higher ionicity are helpful in increasing the lignin extraction efficiency.19 However, when PILs are used for drug delivery, Stoimenovski et al. pointed out that PILs with lower ionicity are more desirable because they can cross the membrane barrier more easily.20-21 Consequently, knowing the ionicity of PILs seems crucial. Up to now, one widely used way to assess the ionicity is still based on the Walden rule,1 which relates the ionic


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conductivity and the fluidity of the medium. Whether the ionic liquid is “good’ or “poor” depends on how closely it corresponds with the ideal line. Generally, the extent of proton transfer should be at least 99% when a PIL is considered as a “good” ionic liquid.22 Angell et al. indicated in an investigation13 that the difference between acid dissociation constants of the respective acid and base in aqueous solution (∆p ), can be an important parameter to roughly estimate how complete the proton transfer is. It pointed out the PILs with ∆p > 8 have nearly ideal Walden behavior.23 However, solvents can play an important role in proton exchange, as discussed in several researches that the p values of both acid and base show large differences in various media,24-26 thus bringing the validity of using aqueous p values for pure PIL systems into focus.27 Therefore, researchers are still working to estimate the ionicity of PILs in a relatively precise and convenient way. Thermodynamic properties of PILs like the enthalpy or entropy change of the synthetic reaction can be quite useful in exploring which factor (or factors) has the biggest influence on the

properties

of

PILs.

Theoretical

thermodynamic

study

of

1,5-diazabicyclo[4.3.0]non-5-enium acetate28 and experimental thermochemistry study of 1-methylimidazolium ethanoate29-31 revealed the difference in thermodynamic properties aroused by the distinction between two cations. And what is more, acquiring the specific thermodynamic data of PILs is necessary for modelling engineering processes used in the industrial production and processing.29 In our previous work,16 we quantitatively measured the degree of proton transfer in a PIL: n-propylammonium acetate (N3HAc) by spectroscopic methods (NMR and ATR-IR). Herein, we further studied six acetate-based PILs with various aqueous ∆p values. It turns out


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that the degree of proton transfer in these PILs do not correlate well with aqueous ∆p , instead, they are shown to be related to the types of hybridisation of the cationic center. To get the thermodynamic properties, we measured the equilibrium constants within a temperature range from 298 to 323 K. Their standard enthalpy and entropy changes of the reaction, ∆ and ∆ can be acquired according to van’t Hoff analysis at the same

time. These measurements can be associated with the degree of proton transfer, proving ionicity is influenced by various factors. Interestingly, by taking symmetric entropy change into consideration, we gave an explanation for why 1-methylimidazolium acetate ([MimH][AcO]) is an exception in correlating ∆ with the degree of proton transfer and

offered suggestions about designing a desirable PIL with proper ionicity from one point of view.

EXPERIMENTAL SECTION

General. All materials were stored and handled inside a glove-box under an oxygen and water free nitrogen atmosphere. Karl-Fischer titrations were carried out with a Mettler-Toledo DL32 coulometric KF titrator. The NMR experiments were carried out on Bruker DMX 500-MHz and Agilent DD2 600-MHz spectrometer.

Materials.

Acetic

acid

(AcOH,

Macklin,

≥99.99%),

1-methylimidazole

(Mim,

Sigma-Aldrich, ≥99%), 1-vinylimidazole (Vim, J&K Chemical, 99%), 1-butylimidazole (Buim, J&K Chemical, 99%), and pyridine (Py, Macklin, 99.5%) were used as received. 1-Methylpyrrolidine (Mpyr, Macklin, 98%) and N,N-dimethylbutylamine (DMBuA, Energy Chemical, 98%) were stored over molecular sieves (Energy Chemical, 4Å, 1.6 mm-2.5 mm;


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activated in vacuum at ~473 K during at least 6h) before using. The water contents in all materials were determined by Karl-Fischer titration to be less than 500 ppm.

Synthesis. All ionic liquids were synthesized by dropwise addition of acetic acid to form different mole ratio mixtures with the base. Considering the decomposition of any of the material during the course of the exothermic proton transfer reactions, each synthesis was performed in a hollow glass vessel which was full of refrigerant. A typical synthesis was carried out following the procedure: acetic acid was added dropwise into the flask which was charged with precisely weighed precursor base while stirring vigorously. The precise amount of acetic acid added into the flask was calculated by the difference in weight before and after adding. In order to ensure the proton exchange had been brought to equilibrium, each sample was left to stir for more than 24 h after complete addition. The water contents in these ionic liquids were determined by Karl-Fischer titration to be less than 700 ppm. 1

H NMR measurements. Samples were loaded into standard 5 mm NMR tubes, each of

which contained a coaxial capillary filled with DMSO-d6, so the influence of solvent was eliminated. All spectra are referenced to TMS. The variable temperature range is from 298 K to 323 K.

RESULTS AND DISCUSSION

Quantitative Measurement of the Degree of Proton Transfer In this work, the investigated six PILs are formed by a common anion, acetate, and various nitrogenous cations. The molecular structures of the materials are shown in Scheme 1 along with their abbreviations.


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Scheme 1. Structures and abbreviations of materials and formed PILs being investigated in this work.

As all the bases applied are tertiary amines, imidazoles and pyridine, the data processing for different PIL systems are similar. Therefore, [MimH][AcO] as a sample is emphatically introduced. Since the theory and proving process to calculate the degree of proton transfer using 1H NMR has been elaborated in our previous work,16 herein, a brief introduction is given to illuminate the calculation process in [MimH][AcO] which is not exactly the same as that in N3HAc of that paper. The equilibrium in [MimH][AcO] can be summed as follow:

Mim + AcOH ⇌ 'MimH('AcO( I *+, *-./0 0 E *+, − *'+,0( *-./0 − *'+,0( *'+,0(

where I represents the initial concentration and E denotes the equilibrium concentration. As Mim has no exchangeable proton, the observed chemical shift δ value is only contributed by two parts which are the unreacted molecule AcOH and ionic species [MimH].


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The chemical shift of the exchangeable proton observed in [MimH][AcO] can be described as eq 2. 2 = 24567 × (*4567 − *':;? , with its aqueous ∆p is shown in Figure 2. In this work, we use lnK >? to represent ionicity, because it has a positive correlation with the degree of proton transfer (shown in eq 4) and its magnitude matches with other parameters studied.

Figure 2. The logarithm of equilibrium constant of each acetate-based protic ionic liquid lnGH versus the difference between acid dissociation constants of the respective acid and base in aqueous solution, ∆p . The solid line represents the tendency. ■ and ▲ symbols represent sp2 and sp3 nitrogen centred cations, respectively. For ionic liquids whose cationic center is sp2 nitrogen, the lnK >? shows an upward trend towards the increasing of the base strength. However, PILs with sp3 nitrogen centred cations do not match the trend. Though the fact that the degree of proton transfer cannot be judged


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just by aqueous ∆p has been stated in many literatures,8, 27, 31 it is interesting to find the example that there is difference between PILs applied sp2 nitrogen centred cations and sp3 ones. Thermodynamic Study by Van’t Hoff Analytical Method The Gibbs free energy change for the reaction, ∆ I , is related to the corresponding

equilibrium constant K >? at temperature T as ∆ I = −RTlnK >?

(5)

where R is the universal gas constant. Then, according to the Gibbs-Helmholtz equation ( ∆ I = ∆ − T∆ ), the equilibrium constant K >? and temperature T can be

associated, which is so called the van’t Hoff equation:

∆r H mθ ∆r S mθ lnKeq = − + RT R

(6)

∆ and ∆ are the standard enthalpy and entropy changes of the reaction. In this

work, all the plots of lnK >? versus 1/T are linear, which means the heat capacity change of the reaction is negligible. Therefore, ∆ and ∆ can be calculated by the slope and

intercept in the plot, respectively. Taking [MimH][AcO] as an example again, the same fitting procedure introduced in the previous part was applied to handle the data obtained at 303, 308, 313, 318 and 323 K, and the fitting curves are shown in Figure 3. The obtained equilibrium constants and calculated degree of proton transfer can be seen in Table 2. When [MimH][AcO] is formed at stoichiometric acid/base ratio, its degree of proton transfer decreases as the temperature increases. This phenomenon coincides with the


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exothermic process during the synthesis, which indicates that the degree of proton transfer should be studied at a stabilized testing temperature.

Figure 3. Fitting results of the chemical shift of exchangeable proton in the Mim/AcOH system as a function of the mole ratio between Mim and AcOH at a certain temperature.

Table 2 Summary of the equilibrium constants and the degree of proton transfer of [MimH][AcO] at each temperature. T/K

Keq

n

298

164.93

92.8%

303

158.20

92.6%

308

151.18

92.5%

313

142.09

92.3%

318

132.51

92.0%

323

123.37

91.7%

Using the data in Table 2, the van’t Hoff plot of [MimH][AcO] can be drawn, shown in Figure 4. The results show that in this reaction, ∆ = −9.33 kJ mol , ∆ =

11.29 J mol K and this value of enthalpy change is comparable to the literature value29


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determined by Calvet-drop microcalorimetry which is (−13.1 P 0.7) kJ mol. Since the interaction between polar solvent and ions is stronger than that between polar solvent and neutral molecule, ∆ here is less negative than that in aqueous solution (−(28.07 P

0.03) kJ mol).29

Figure 4. Van’t Hoff plot of [MimH][AcO]. Solid line represents linear regression.

In all six investigated systems, the linearly dependent coefficients of van’t Hoff analysis are higher than 0.96, indicating the reliability of simulation results. Their van’t Hoff plots can be consulted in Supporting Information and their ∆ and ∆ are summarized in Table 3.

Table 3. Summary of thermochemical data obtained in this work PIL

∆ (kJ mol-1)

∆ (J mol-1 K-1)

[MimH][AcO]

-9.33

11.29

[VimH][AcO]

-16.41

-18.11

[BuimH][AcO]

-23.30

-30.93

[PyH][AcO]

-7.80

-9.80

[MpyrH][AcO]

-10.67

-42.70

[DMBuAH][AcO]

-12.15

-13.93


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The ∆ of these systems are all negative and the order of numerical value agrees with the degree of the exothermic process during the synthesis. The reason why the ∆ of

these PILs show such an order may be the comprehensive result brought by two main effects. One is the different electron-donating ability of the groups bonded to nitrogen, the other effect is the enhancing electronegativity of nitrogen as the conjugate degree increases. The heat of neutralization of strong acids and bases in highly dilute aqueous solutions has been measured decades ago33 to be −57.3 kJ mol and it is much more negative than ∆ of

any PILs studied in this work. This result is not hard to understand due to the neutralization reaction (formation of PIL) happens in solvent-free condition and all these PILs are formed by weak acids and weak bases. Though ∆ of these PILs varies in a comparable wide range, they mainly lie between

the entropy change of combination reaction (−90 to − 70 J mol K )34 and proton transfer reaction (−10 to + 14 J mol K ).35 As PILs can be regarded as cations and anions interacting by weak hydrogen bonding, this result is not surprising. These ∆ values show

an increasing trend towards the decreasing in the size of cations, which may be contributed by the enlarged entropy of mixing caused by the slight size differences between cations and anions. Meantime, the entropy of symmetry has a crucial influence as well and this will be one of the focus in the following discussion. Relationship between the Degree of Proton Transfer and Thermodynamic Parameters In addition to the relationship between the degree of proton transfer and aqueous ∆p , in which solution may play an important but interfering role, we emphatically studied the relationship between the degree of proton transfer and thermodynamic parameters (∆


14

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and ∆ ). In Figure 5(a), it seems clear that lnGH of PILs applied Vim, Buim, Py and DMBuA decrease linearly with their ∆ , and in Figure 5(b), the ∆ of these four PILs

distribute in the range from −30 to − 10 J ∙ mol ∙ K regardless of their lnGH , which should be mostly attributed to the decrease of the degree of freedom caused by the process of ionization. All these imply that there is an enthalpy-entropy compensation effect, which could also explain the deviation of PILs applied Mim and Mpry shown in Figure 5(a), because in Figure 5(b), the ∆ of Mpyr is much more negative (−40 J ∙ mol ∙ K ), while that of Mim is unusually positive (+10 J ∙ mol ∙ K ). Thus, it is worth studying ∆ much

deeper.

Figure 5. The relationship between the logarithm of equilibrium constant of each AcOH based ionic liquid and ∆ and ∆ .

The reason why the ∆ of [MimH][AcO] is unexpectedly positive may be the

stabilizing effect of entropy of symmetry, suggested by 1H NMR spectrum. Figure 6 shows the comparison of 1H NMR of 1-R-imidazole (R = Methyl, Vinyl, Butyl) in molecular and ionic states, and just covers the chemical shifts of hydrogens binding to C4 and C5 for the sake of simplicity. The values of H-, Methyl-, Vinyl- and Butyl- increase in order, and the


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difference between the values of H- and Methyl- is the slightest. For Vim and Buim, the hydrogens at C4 and C5 show separate peaks, and the same is true at ionic state due to asymmetry kept. On the contrary, the two hydrogens of Mim combine into a single peak in [MimH][AcO] at 1:1 mole ratio, might contributed by the exchangeable proton which has similar size to methyl and makes [MimH] a C2v-like structure. Similarly, Mpyr can be regarded as a D3h–like structure and after proton transferring, [MpyrH] is more like a C3v structure. This configuration change should make great contribution to the ∆ of

[MpyrH][AcO] though it cannot be simply verified via the peaks combining in 1H NMR spectrum.

Figure 6. The change of chemical shift of protons on imidazole. Therefore, the unexpectedly positive or too negative ∆ of [MimH][AcO] or

[MpyrH][AcO] may be the result of entropy of symmetry. The effect of entropy of symmetry


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as a thermodynamically driven has been discussed by Lin,36 and the entropy of symmetry could be shown as a function of symmetry number (S) of the structure considered.37 TU< = RlnS

(7)

Using this equation, we got the difference of TU< of some base between its molecular and ionic states, ∆ VW , shown in Table 4, where the point group was determined according to the volume of proton and groups binding to nitrogen, approximately. The extremely positive ∆ VW of [MimH] contributes to positive ∆ of [MimH][AcO] got from van’t Hoff

analysis, while the negative ∆ VW of

[MpyrH] contributes to extraordinary negative

∆ of [MpyrH][AcO]. Due to the symmetry is kept in other PILs during the reaction, the

entropy change of symmetry has little influence on them. Table 4. Entropy of symmetry for bases and PILs. Structures

Point group

σ

Ssym (J mol-1 K-1)

∆ VW (J mol K )

Mim

[

1

0

+11.5

[MimH]

~[]^

4

11.5

Vim, [VimH]

[

1

0

0

Py, [PyH]

[]_

4

11.5

0

Mpyr

~`ab

12

20.7

-5.8

[MpyrH]

~[a^

6

14.9

CONCLUSION The degree of proton transfer in six acetate-based PILs were determined using 1H NMR spectroscopy. Though there is a remarkable difference between their degrees of proton


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transfer, they should all be regarded as “poor” PILs. Within the scope we studied, the degree of proton transfer and their aqueous ∆p has good correlation for PILs with sp2 nitrogen centered cations, however, not for that with sp3 nitrogen centered cations. It might be due to the difference of p in aqueous solution and ILs. Furthermore, by van’t Hoff analysis, thermodynamic parameters of the proton transfer reactions in these PILs, such as ∆ and ∆ were calculated. Compared to the heat of neutralization of strong acids and bases in highly dilute aqueous solutions, the calculated ∆ values of PILs in this work are much less negative. The ∆ was proved to be a main factor to determine the ionicity of a PIL,

except for whose molecular symmetry would change after proton transferring. A dramatic effect of entropy of symmetry which is thermodynamically driven was verified by the relatively complete proton transfer in [MimH][AcO]. To sum it up, the conjugative effect, substituent effect and symmetry effect on the cations have a comprehensive influence on the ionicity of the PILs. Those effects on the anions have not been investigated in this work but are worth studying deeply in the future. All in all, it could be imagined that after taking all these effects into consideration, there will be emerging ideas for designing PILs with proper ionicity to satisfy different requirements.

ASSOCIATED CONTENT

Supporting Information Fitting plots and van’t Hoff plots of [VimH][AcO], [BuimH][AcO], [PyH][AcO], [MpyrH][AcO] and [DMBuAH][AcO] (PDF)


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AUTHOR INFORMATION

Corresponding Author

Jia Yao: Email: [email protected], Fax: +86-571-87951895 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (No. 21573196), the Fundamental Research Funds of the Central Universities, and the National High Technology Research and Development Program (863 Program) of China (Grant No. SS2015AA020601). REFERENCES (1) Walden, P. Über organische lösungs-und ionisierungsmittel. Z. Phys. Chem. 1906, 55 (1), 207-249.

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