Sorbent Properties of Halide-Free Ionic Liquids for Water and CO2

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Sorbent Properties of Halide free Ionic liquids for Water and CO Perfusion Thorge Brünig, Martin Maurer, and Rudolf Pietschnig ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01468 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on July 10, 2017

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ACS Sustainable Chemistry & Engineering

Sorbent Properties of Halide free Ionic liquids for Water and CO2 Perfusion Thorge Brünig, Martin Maurer, Rudolf Pietschnig* Universität Kassel, Institut für Chemie und CINSaT, Heinrich-Plett-Straße 40, 34132 Kassel, Germany. E-mail: [email protected] ABSTRACT: Aqueous solutions of lithium halides are state-of-the-art sorbents in sorption based heat pump and heat storage devices, with the drawbacks of high corrosivity and limited natural resources of lithium. Halide free ILs could be well suited alternative sorbents and have been explored with isothermal titration calorimetry showing superior thermochemical performance. Here we have explored the performance of selected halide free ILs in perfusion measurements monitoring the uptake of water vapor and CO 2 from an air stream with well-defined flow and humidity as a model for open sorption processes. We have determined the saturation vapor pressure of water in the corresponding waterIL mixtures and moreover determined the diffusion coefficients of molecular species in binary [EMIM][Ac]-water mixtures via pulsed-field-gradient nuclear magnetic resonance (PFG-NMR) measurements. These, together with variation of the absorption rates with calorimetric monitoring, indicate that diffusion is approx. three orders of magnitude faster than absorption and that the fast absorption process is not diffusion limited for [EMIM][Ac] in the used concentration range (D = 5.046 ± 0.021*10-11 m2/s; T=25°C). All measurements have been carried out in comparison to LiCl which corroborates the superior performance with regard to reducing water vapor pressure, heat of dilution and absorption rate of acetate based ILs and [EMIM][Ac] in particular.

KEYWORDS: ionic liquid • isothermal titration calorimetry • water sorption • CO 2 sorption • thermal energy storage • diffusion coefficient

INTRODUCTION Thermal energy storage is a successful concept for heating systems in housing and industrial applications1-2. The high efficiency of transforming solar energy to heat is one of the reasons for the interest in applications for heating purposes based on renewable energy sources. The storage of heat, especially over prolonged periods such as in seasonal heat storage, is currently limited mainly by the materials available for this purpose. Besides sensible heat storage, phase change materials3-4 and sorption processes are the most prominent storage strategies.5-6 Sorption processes using liquid sorbents are especially attractive and have already been implemented in commercial devices.7 Most of them rely on concentrated aqueous lithium halide solutions as sorption media which can be regenerated by heat.8-13 Besides the increasing demand for lithium for other purposes (batteries) also the corrosivity of the currently employed solutions has technological disadvantages.14 In the light of these problems Ionic liquids (ILs) have been suggested as alternative sorption media for thermal energy storage and have been explored theoretically and experimentally for closed sorption systems.15-17 Many ILs

however, involve weakly coordinating counter-anions such as BF4- or PF6- 18-19 which have been shown to undergo hydrolytic cleavage with liberation of HF being unfavorable with respect to corrosion.20-22. To minimize the corrosive impact, halide free ILs containing organic anions such as acetate23-25, EtSO4- 26, DEP27, DMP28 or lactate29 have been developed. Very recently we presented a screening of a series of halide-free ILs with carboxylate based anions which revealed excellent performance in terms of heat of dilution upon addition of water, superior to that of lithium halide solutions currently in use. 30 Especially for water sorption in thermal applications, an open sorption strategy is feasible as well, in which water is released to the environment during desorption or taken from the surrounding air for the absorption step. Such an open setup offers the benefits of a direct air conditioning with a simpler construction design taking advantage of the difference in temperature and humidity between indoor and outdoor settings to improve performance.7, 31-34 Previous investigations suggest a savings potential of about 50% in terms of primary energy consumption by using sorption to dehumidify the air of air conditioning systems.35 For the use of ILs in an open sorption system the interaction of the sorbent not only with water but in

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addition with other reactive constituents of air such as oxygen or carbon dioxide needs to be considered. While most ILs are stable towards oxygen, it is well known that imidazolium based ILs are prone to significant uptake of CO2.13, 36-41 Evidence has been provided, that for ILs with basic anions such as carboxylates, the formation of intermediate carbenes is likely which subsequently may undergo CO2 addition in a normal (carbene centered) or abnormal (backbone centered) fashion.36, 42-43 Besides the experimental work performed in inert conditions, the behavior of NHC-CO2 adducts and the closely related imidazolium-hydrogen carbonates has been studied in the presence of water as well, which is especially relevant for the envisaged open sorption process.44-45 Improving the performance of an open sorption storage system requires knowledge of the main experimental parameters. Especially a low water vapor pressure, which offers the possibility of an efficient dehumidification is one of the key aspects.1, 23 Furthermore, high heat of dilution and fast absorption of water enable an increase of the released energy, which is the direct contribution in addition to the heat of condensation which is a constant value. These data in hand offer the possibility to simulate the performance of an open sorption system as a further step into technological implementation. For this purpose, we set out to investigate the sorption of water from the gas phase via perfusion into halide free ILs with nanocalorimetry as a measure for the heat releasing step in sorption based thermal energy storage. In addition, we explored the aspect of diffusion as potential rate determining step. Finally, we studied the thermal effects of CO2 sorption to extend the applicability of our results to open sorption processes in which ILs would be exposed to both water and at least traces of carbon dioxide.

EXPERIMENTAL SECTION Synthesis All substances used in this paper are synthesized by ourselves and characterized with NMR and Karl-FischerTitration to ensure high purity (see SI). Standard literature known techniques, were used to purify the Ionic Liquids and other chemicals. A distillation of the reactants was carried out to ensure a high purity of our substances. If necessary, the syntheses were performed under standard inert conditions. For the anion exchange of the ILs a resin (Amberlyst A-26) in the OH-form was used purchased from Sigma Aldrich. The synthesis of the imidazoliumbased ionic liquids can be divided into two steps. In the first step 1-methylimidazole reacts with the corresponding halide (XR) in a quarternization reaction to the imidazolium salt.46-50 Depending on the substance, the addition of the halide is carried out into a dilute solution of 1-methylimidazole or without solvent directly into the liquid reactant. In the second step, the anion was exchanged by using an ion exchange column. The column and the exchange procedure were according to Alcalde et al.51 The completeness of the anion exchange was ensured firstly by integration of the 1H-NMR spectra between anion and cation and on the other hand proven by a silver test. 52

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The NMR data of the ILs and CO2-adducts are shown in the supporting Information. The nomenclature of the compounds follows established conventions, where MMIM refers to N,N’-dimethyl imidazolium, EMIM to Nethyl-N’-methyl imidazolium, PMIM to N-methyl-N’propyl imidazolium, MOEMIM to N-ethoxymethyl-N’methyl imidazolium, IsoMIM to N-isopropyl-N’-methyl imidazolium, EOHMIM to N-hydroxyethyl-N’-methyl imidazolium and [2.3DiolMIM] to N-methyl-N’-2,3dihydroxypropyl imidazolium cations, respectively, with the organic anions acetate (Ac), lactate (Lac) and formate (Form). A survey of abbreviations combined with molecular structures is depicted in the supporting information. NMR Spectroscopy The 1H-and 13C-NMR spectra were recorded on a Varian 400-MRS spectrometer (measurement frequencies: 1H: 400 MHz; 13C: 100.5 MHz. The chemical shift is reported in ppm and the coupling constants in Hz. Solvents were, CDCl3, DMSO-d6 CD2Cl2 and D2O. For 1H- and 13C-NMR measurements the respective solvent peaks were used as internal standard. The diffusion coefficients for the IL [EMIM][Ac] and water were determined by using a Bipolar Pulse Pair Stimulated Echo with the description “DgcsteSL” in VNMRJ 3.1 1H-NMR technique on a Varian VNMRS 500 spectrometer with a Z-axis gradient All the experiments were performed at the concentrations and temperatures (approx. ±0.5°C) summarized in Table 4. Some of the measurements were carried out both with extensive shimming and without shimming. A comparison revealed no observable difference therefore the subsequent measurements were carried out without shimming To calibrate the gradient field strength a HDO in D2O sample at 25°C with a known diffusion coefficient of 19.02*1010 m2/s was used. The maximum gradient strength, which can be achieved with our gradient driver was 65 G/cm. The diffusion time Δ was set to a range of 50 ms and 400 ms, for the length of the gradient pulse δ a value of 2 ms was used. For practical reasons it was necessary to adjust the diffusion time by increasing the concentration or lowering the measurement temperature. In order to determine the

diffusion coefficient, the peak intensity was plotted against the gradient field strength. Subsequently, the data were fitted using the Stejskal-Tanner equation. 𝐼 𝛿 = exp⁡(−𝛾 2 𝑔2 𝛿 2 (∆ − ) 𝐷) 𝐼0 2 In this equation, I and I0 are the signal intensities with and without gradient field, γ is the gyromagnetic ratio, g is the gradient field intensity, δ is the length of the gradient pulse, Δ is the diffusion time between the gradient pulses, and D is the diffusion coefficient. Concentration of the ILs The ionic liquids [MMIM][Form], [MMIM][Ac], [EMIM][Form], [(EOH)MIM)][Ac] and [Choline][Ac] are solids in high purity. To compare the measurements and due to the fact, that a regeneration up to 95 wt% is difficult


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to achieve, the measurements were carried with mixtures containing 95 wt% ± 0.35 wt% of the substance and 5 wt% ± 0.53 wt% water (solubility permitting). For [MMIM][Ac]/[Form] and [EMIM][Form], a lower concentration of 92 wt% ± 0.35 wt% of the substance and 8 wt% ± 0.53 wt% of water is used. Water vapor pressure The measurements of the water vapor pressure were performed in a 100 ml glass flask with connection to the gauge Edwards ASG2 1000 and recorded by the instrument controller TIC 100 (1-1000 mbar ±0.2%). The equipment was calibrated against saturated LiCl and NaCl solutions (Data see SI). Before the measurement was started, the solution was frozen with liquid nitrogen and evacuated (103 mbar). Subsequently, the solution was defrosted and the temperature was adjusted by a thermostat (Julabo F34 ME T = ± 0.02°C). This was done partly for different concentrations. The data were then plotted against the temperature. Calorimetry (Titration and Perfusion) The thermal analysis of the heat of dilution and the heat of the solutions process was carried out with a TAM III from TA Instruments, equipped with a nanocalorimeter which can be used in connection with either a titration module for ITC or a perfusion module with adjustable (T, RH, flow rate) gas stream. Calorimeter specifications: Thermostat temperature range

15°C - 150°C

accuracy

< ± 0.1°C

long term stability

< ± 100 μK/24 h

short term stability

< ± 10 μK/ (p-p)

scanning rate

< ± 2°C/h (20°C - 150°C)

Nanocalorimeter short term noise

< ± 10 nW

baseline drift

< 40 nW/24 h

accuracy

< 1%

precision

± 100 nW

Since the samples were measured and handled at isobaric conditions in ambient air with relative humidity (RH) between 30 % RH – 50 % RH, a minimal absorption of water before the start of the experiment could not be avoided. The preparation took about 30 minutes. The maximum measured rate of absorption of water from the

air by using the ionic liquid [EMIM][Ac] is 2.1 mg (H2O)/h, at an area of 0.95 cm2 (corresponding to the bottom surface of the ampoule), at a temperature of 23°C and a relative humidity of 54%. The average of weighed mass within the measuring ampoule is 500 mg. This results in an error of the water content of about 0.42 wt%, starting from an initial water content in the 1000 ppm range. The Isothermal Titration Calorimetry (ITC) was carried out in 1 mL and 4 mL glass ampoules. The ITC measurements were carried out with the appropriate solutions in water, with stirring frequencies between 100 rpm and 200 rpm using a gold propeller. For the titrations the sample is transferred into the glass ampoule which is then attached to the titration module and inserted into the calorimeter following the standard procedure of the manufacturer. Upon successful calibration and recording of a constant baseline titration steps are performed by addition of water (2 µL-10 µL, error 90 wt%., however, lower than we measured for [EMIM][Ac] (19.1 mbar, 40°C). It needs to be noted that the data from the literature refer to commer cially obtained ILs with limited purity, while our compounds have a documented purity of >99% and water traces are limited to the ppm range (cf. SI). In general, it can be derived from the data that the vapor pressure depends on the structural features of cations and anions in combination. However, no simple structure property relationships can be established from the collected data. The longer alkyl chain entails a liquid state for [PMIM][Form] at conditions where the shorter chain cations [MMIM]+ and [EMIM]+ form solids with the same anion. In case of the corresponding acetates, the [EMIM] compound is also liquid in contrast to the [MMIM] analog. Based on these observations, the [EMIM][Ac] show the lowest total vapor pressure of the three alkyl-substituted Imidazoles. The [MMIM] variety, on the other hand, must be measured at a higher dilution to obtain a liquid, which in turn leads to an increase in vapor pressure owing to the higher water content. Table 1 Vapor pressure above concentrated mixtures of selected ILs and inorganic salts with water at three different temperature levels. (Measuring range and accuracy p = 1-1000 mbar ±0.2%; T = ± 0.02°C) p [mbar] (T=25°C) 4.6 4.9

p [mbar] (T=40°C) 11.0 12.3

p [mbar] (T=80°C) 71.2 114.1

[MMIM][Ac] 92 wt%

2.3

4.8

42.3

[MMIM][Form] 92 wt%

2.3

3.0

25.3

[MMIM][Lac] 95 wt% [EMIM][Ac] 95 wt% [EMIM][Ac] 91 wt% [EMIM][Ac] 80 wt% [EMIM][Form] 92 wt%

5.8 1.3 2.0 8.5 2.5

12.2 2.4 4.2 19.1 3.8

66.8 12.7 37.7 147.6 21.5

[EMIM][Lac] 95 wt%

3.2

6.7

49.0

[PMIM][Ac] 95 wt%

1.5

2.5

17.5

[PMIM][Form] 95 wt%

1.6

2.4

10.7

[PMIM][Lac] 95 wt%

1.8

3.0

23.3

[IsoMIM][Ac] 95 wt%

2.0

5.2

49.3

[IsoMIM][Form] 95 wt%

2.1

6.4

53.2

[(MOE)MIM][Ac] 95 wt%

3.6

7.1

78.1

[(MOE)MIM][Form] 95 wt%

4.6

8.5

81.4

[(MOE)MIM][Lac] 95 wt%

6.2

10.1

85.2

[(2,3Diol)MIM][Ac] 95 wt%

6.1

13.0

59.4

substances LiCl 44 wt% NaOH 40 wt%


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[(EOH)MIM][Ac] 95wt%

5.7

12.7

71.9

[Cholin][Ac] 95 wt%

5.0

8.2

49.3

A modification of the substituent at the imidazolium cation to alcohol or ether groups, changes the interaction between the cation and the anion, as well as between the two ions and water. The introduction of a hydrophilic alcohol group strengthens the interaction via H-bonds between the cation and the anion and thereby reduces the IL’s affinity to water.57 Similarly, imidazolium cations with short-chain ether substituents exert a reduced repulsion between two cations as well as a reduced attraction between cation and anion.50 For such a scenario a reduced vapor pressure would be anticipated, since the main part of the interaction with water is usually attributed to the anion.58-59 However, we could not observe such an effect and in turn found even slightly higher vapor pressures possibly owing to competition between ether and water dipoles for solvation of ions. A comparison of the measured values of the acetate and formate with the lactate ILs shows that the hygroscopic nature is controlled by multiple factors and not only by the direct interaction between ions. Alternative features such as the formation of solvated clusters (micro-heterogeneities) have been described in the literature based on calculations and are in agreement with experimental findings in terms of molecular volume.30, 60-61

Heats of dilution In addition to the reduction of the water vapor pressure, the heat of dilution represents a further important parameter for the optimization of sorption media. Since only a few literature values for the heat of dilution are available at high concentration, these were determined in ITC measurements for aqueous solutions at concentrations given in table 2. The LiCl benchmark (44 wt%) has a dilution heat of -306 kJ/kg (H2O) at 25°C, which corresponds to approximately 13.6% of the condensation heat of water. This ratio can be increased substantially up to 25% by the use of ILs ([EMIM][Ac] -805 kJ/kg (H2O)). A summary of the titration results is given in table 2. The acetate species offer the advantage of a lower melting point compared to the formate-based ILs. This allows them to be used as neat substances. In addition, they release the largest amounts of heat during dilution. Thus the titration of [PMIM][Ac] compared to [PMIM][Lac] results in a 260 kJ/kg higher dilution energy (H2O). This trend is also reflected in the study of the cation [(MOE)MIM]+. Similar to [EMIM]+, [MMIM]+, and [PMIM]+, the order of decreasing dilution heat is: [Ac] > [Form] > [Lac] For this reason, only the [Ac]-salts of the cations [(2,3Diol)MIM]+, [(EOH)MIM]+ and [Cholin]+ were measured. Both the [Cholin]- and the [(EOH)MIM]acetates are not liquid at 25°C and moreover interact over a hydrogen bond with the anion resulting in a higher melting point which limits the applicable concentration range. As a further drawback the latter ILs and also the diol show a lower heat of dilution compared to the alkyl

substituted analogs which leads to the conclusion that additional hydrophilic substituents do not necessarily improve the sorption properties of ILs. This is in agreement with the stronger anion cation interaction in hydrophilic ILs previously discussed, which reduces the released energy.57 By comparison, [EMIM][Ac] can provide a particularly high heat of dilution (-805 kJ/kg (H2O) liberated over a concentration range covering a change of 25 wt%. The useful concentration range for LiCl as a benchmark is limited to a change of 9 wt% (p < 10 mbar at T = 35°C)23, 62 which highlights the superior sorption capacity of ILs and [EMIM][Ac] in particular. The considerably higher dilution heats of the ILs compared to the salts LiCl (-306 kJ/kg (H2O)) and NaOH (414 kJ/kg (H2O)) can be attributed to the high concentration of the ILs. Table 2: Different heat of dilutions of ILs in the relevant concentration range compared to the benchmark LiCl and the hygroscopic NaOH electrolyte. Heat of dilution [-kJ/kg H2O] Concentration [wt%]: LiCl

44 306

NaOH Concentration [wt%]: [MMIM][Ac]

40

37

Ref.

200 414

100

33

247

95

90

605

500

28

430

28

[MMIM][Form] [MMIM][Lac]

560

400

290

28

[EMIM][Ac]

805

620

479

28

440

28

[EMIM][Form] [EMIM][Lac]

467

360

230

28

[PMIM][Ac]

760

610

440

28

[PMIM][Form]

640

460

378

28

[PMIM][Lac]

500

345

268

28

[IsoMIM][Ac]

734

540

400

[IsoMIM][Form]

577

491

380

[(MOE)MIM][Ac]

579

381

276

[(MOE)MIM][Form]

321

225

175

[(MOE)MIM][Lac]

276

196

157

[(2,3Diol)MIM][Ac]

328

271

227

[(EOH)MIM][Ac]

308

270

[Cholin][Ac]

426

400

28

As a result, the ratio of water to ions is much smaller and the energy released by the strong interaction is much larger. A detailed discussion of the thermochemical results based on isothermal titration calorimetry in connection with structural aspects of ILs has been reported recently. 30 The efficient reduction of the vapor pressure combined with the heats of dilution at different concentrations corroborate the huge potential of ILs as sorbents compared with conventional inorganic salts.



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Perfusion of Water into ILs ITC is a relatively fast method to assess a set of sorbents with respect to heat of dilution. Together with reduction of water vapor pressure this parameter provides a guideline to establish trend for the suitability of the substances as sorbent. In a real sorption process additional aspects such as diffusion, surface tension and viscosity will affect the dehumidification at the gas-liquid interface as well. By the use of a perfusion equipment, the dehumidification of an airstream can be tracked thermally by measuring the absorption enthalpy. Based on the previous assessment of relevant parameters, the ILs [EMIM][Ac], [EMIM][Lac], [PMIM[Ac], [PMIM][Lac] and [MOEMIM][Ac] have been chosen for perfusion measurements in addition to the benchmark LiCl. The formate ILs have not been taken into consideration owing to their limited thermal stability. 30 As a start the structurally least complex LiCl has been chosen (44 wt%) for which a standard perfusion measurement is shown in figure 1. The heat flow (black line), by gradually increasing the relative humidity (blue line) of the air stream is plotted against time. As a result of the increasing relative humidity, the heat flow increases gradually as well. At a relative humidity of 13±1.5% (cf. 11.3±0.3% according to ref.53), the heat flow changes from endothermic to exothermic. Furthermore, the maximum heat flow and the released energy (by integration; gray area) can be used to determine the absorption rate. It should be noted that this requires a previously performed titration measurement. For example in the case of LiCl the heat flow is 553 μW/cm2 at a relative humidity of 60% liberating heat of -1.97 J/h*cm2. In general such perfusion measurements provide information about the development of the heat flow with increasing relative humidity. In order to compare the properties of the ILs selected, such perfusion measurements have been performed for each sorbent and the maximum heat flows obtained at given RHs were plotted against the relative humidity of the airstream (Figure 2). It can be easily derived from this plot, that there is a linear relation between the rise of the RH and the increasing heat flow. Compared with LiCl (44 wt%) as a benchmark, all ILs generate a higher heat flow up to a RH of 60%. At 100% RH only the ILs [EMIM][Ac], [PMIM][Ac] and [PMIM][Lac] show a higher heat flow than LiCl (1403 μW, 1274 μW, 1048 μW vs. 994 μW). In the case of the [EMIM][Ac], the heat flow at 100% RH is approximately 40% higher than that of LiCl (1403 μW vs. 994 μW) and thus approximately 20% above the value which results from the sum of the condensation (-2444 kJ/kg) and dilution heat (-805 kJ/kg [EMIM][Ac] vs. -306 kJ/kg LiCl), which are measured in the ITC-experiments. Since there is no other energy releasing step, this indicates a faster absorption rate. In addition, it can be concluded that all selected ILs generate an exothermic signal even at low relative humidity, which makes them superior to LiCl in this RH range, for which an endothermic signal is detected owing to evaporation of water. This point could be relevant for

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open sorption at low RH, e.g. in winter time. The perfusion data presented above allow a qualitative overview suitable to compare different sorbents. However, changing the RH over various steps changes the composition of the sorbent over time which limits the quantitative analysis in terms of energy for the second RH step and beyond. For a quantitative interpretation at higher humidity we performed additional measurements in order to determine the maximum absorption rates at constant humidity levels of 100% RH and 55% RH (cf. SI). Moreover, the water uptake during these measurements has been tracked gravimetrically which allows a sound correlation between mass and heat transfer. Table 3 summarizes the results of these measurements for the sorbents [EMIM][Ac], [MOEMIM][Ac], [PMIM][Ac] and LiCl. The data show that at both humidity levels all ILs liberate more energy than LiCl in terms of maximum heat flow as well as energy per amount of absorbed water.

Figure 1 Perfusion experiment with LiCl as sorbent (44 wt%, T=25°C, V=0.15 mL in ampoule with glass inlet). The RH (blue line) increases step by step over time. The heat flow (black line) rises following the trend of the RH. The released energy is determined via integration of the heat flow for a given step (grey area).

The highest values have been obtained for [EMIM][Ac] which liberates a 17% larger energy per absorbed amount of water (at 100% RH) compared with LiCl and also has a faster absorption rate than the latter. At the lower humidity level of 55% RH, the advantage of the IL is even more pronounced. Thus, [EMIM][Ac] absorbs 37% more water in a similar period of time and generates a 75% higher heat flow. Both the faster absorption rate and the higher energy can be attributed, on one hand, to the high concentration of the IL as a liquid and, on the other hand, to the high propensity to form hydrogen bonds The ether functionalized IL [MOEMIM][Ac] has a lower absorption rate compared with LiCl. Nevertheless, this IL also generates a higher heat flow due to the higher dilution enthalpy particularly at a 55% RH level. The other alkyl imidazole [PMIM][Ac] shows a similar sorption characteristic such as [EMIM][Ac] at 55% RH. In addition to the dependence on relative humidity, the absorption characteristic at different temperatures were investigated as well. It should be noted that several factors influence the


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absorption rate. On one hand, warmer air can absorb more moisture, which is the reason why we used the absolute humidity in plotting the flow. On the other hand, a higher temperature entails a higher water vapour pressure at the liquid/gas interface. Figure 3 shows a plot of the heat flow against the absolute humidity for [EMIM][Ac]. Up to a value of 0.002 g(H2O)/200 mL(air), which corresponds to 43.5% RH at 25°C, the heat flows measured at 25°C and 40°C do not differ significantly. Even with increasing humidity, the absorption at both temperatures shows similar characteristics.



Table 3 For each electrolyte a perfusion measurement at two fixed relative humidity levels (100 % and 55 %) were performed for which the concentration change during the perfusion experiment has been tracked which can be converted to water uptake and energy release. [EMIM][Ac] relative humidity [%]

start concentration [wt%] end concentration [wt%] m(absorbed H2O) [g] n(absorbed H2O) [mol] energy [-J] energy/H2O [-kJ/kg]

1500

LiCl

[(MOE)MIM][Ac]

[PMIM][Ac]

100 100.00

55 100.00

100 44.00

55 44.00

100 100.00

55 100.00

100 100.00

55 100.00

98.14

90.90

42.46

41.68

92.10

95.35

91.90

98.31

0.0035

0.0187

0.0078

0.0108

0.0164

0.0082

0.0104

0.0031

0.00019

0.00104

0.00043

0.00060

0.00091

0.00046

0.00058

0.00017

10.3

49.4

19.6

27.1

43.3

21.9

30.0

9.1

2949

2642

2513

2509

2641

2671

2880

2942

LiCl [EMIM][Ac] [EMIM][Lac] [PMIM][Ac] [PMIM][Lac] [MOEMIM][Ac]

1000

heat flow [W]

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

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500

0

0

10

20

30

40

50 RH [%]

60

70

80

90

100

Figure 2 Plot of heat flow obtained vs. relative humidity (RH) of air stream for different ILs and LiCl.


8

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3500 3000

600

500

400

2500

heat flow [W]

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


2000

300

200

results for [EMIM][Ac] 15°C 25°C 40°C

100

1500 0 0,0000 0,0005 0,0010 0,0015 0,0020

1000 500 0 0,0000

0,0015

0,0030

0,0045

0,0060

0,0075

0,0090

0,0105

abs. moisture [g/200mL] Figure 3 Plot of the heat flow over the absolute moisture measured with [EMIM][Ac] for three different temperatures. The magnified insert on the right shows the start of the measurement from 0 g/200 mL – 0.0015g/200 mL. By contrast, the measurement at 15°C shows a reduced heat flow which is roughly 25% lower than at higher temperatures for a humidity of 0.0015 g(H2O)/200 mL(air). This lower heat flow/water uptake may be related to the increasing viscosity63 of the sorbent as well as to slower diffusion at this temperature. In addition to the assessment of hindered diffusion, the plot linear slopes in Figure 4 further may be used to determine the concentration of a sorbent above which an exothermic heat flow is generated by extrapolating the linear slopes to a heat flow of 0 μW. These onset concentrations derived this way are 35 wt% for [EMIM][Ac], 42 wt% for [PMIM][Ac], 53 wt% for [MOEMIM]][Ac] and 55 wt% for [Cholin][Ac] which compares with 11 wt% for LiCl. For the ILs [EMIM][Ac] shows the lowest onset concentration and [Cholin][Ac] the highest. By comparison LiCl has a much lower onset concentration, however a much smaller concentration range suitable for liquid sorption owing to its limited solubility. ILs in turn may be used as liquid sorbents up to a concentration of 100 wt%.

Diffusion of water in selected ILs For the sorption process of water from the gas phase into ILs models have been suggested which involve a water rich surface layer and diffusion controlled transport of water into the less hydrated bulk IL.25, 59, 64 In the light of reports of slow water sorption behavior for certain ILs in the literature,18, 65-66 we investigated the water uptake and diffusion for the ILs [EMIM][Ac], [PMIM][Ac],

[(MOE)MIM][Ac], [Cholin][Ac] and LiCl as a benchmark to assess the relevance of this potential issue for the compounds under investigation in our study. In a first step, we explored the correlation between water uptake of the sorbent and the resulting heat flow. As outlined in the previous sections, the hygroscopic behavior and the heat of dilution are both a function of the sorbent concentration. Water uptake by the sorbent from the gas phase leads to a continuous dilution of the sorbent, which means in turn that for a more diluted sorbent less energy, is released per water molecule absorbed. To elucidate any deviation from this ideal behavior, we tracked the heat flow for the perfusion of water at a constant air stream (200 mL/h, 25°C) of 100% RH for the ILs [EMIM][Ac], [PMIM][Ac], [(MOE)MIM][Ac], [Cholin][Ac] (100 wt%, 25°C)and LiCl (44 wt%, 25°C). In addition, for each sorbent we gravimetrically tracked the mass increase from the start (+) to the end (0) of the perfusion experiment, which summarizes the total water uptake of all intermediate steps and allows to track the dilution by calculating the concentration. Figure 4 shows the linear relationship between heat flow and concentration obtained by these measurements for all sorbents under investigation. As an example, for LiCl we observe a reduced heat flow of 1681 μW at a concentration of 37 wt% at the end of a measurement which started at a concentration of 44 wt% with a heat flow of 1995 μW.


9

ACS Sustainable Chemistry & Engineering The linear slope of all data points for each sorbent demonstrates that the reduced heat flow after prolonged perfusion is most of all a consequence of the dilution encountered during this process and not of hindered diffusion for the compounds we investigated, i.e. [EMIM][Ac], [PMIM][Ac], [(MO)EMIM][Ac], [Cholin][Ac] and LiCl. These findings do not support the formation of a water or water rich surface layer at the gas liquid interface with hindered diffusion to the bulk sorbent in our case. In fact such a more diluted surface layer should generate a significantly lower heat flow, for which a deviation from the heat flow at unhindered diffusion at equal water uptake would be anticipated. This should entail a deviation from linearity at different water supply rates for a given concentration or at constant water supply rates for different hygroscopic properties, e.g. owing to dilution. Moreover, such a pre-diluted surface layer should show a reduced water uptake (be less hygroscopic) therefore further reducing the heat flow owing to the lower heat of dilution for more diluted sorbents which again is not observed in our experiments (cf. also SI). Beyond this qualitative assessment of hindered diffusion we also measured the diffusion coefficients in solution using PFG-SSE NMR for the best performing IL, [EMIM][Ac].67-71 The field gradient NMR technique is a way to determine the diffusion coefficient of a substance in solution. An advantage of this method is that it can be measured without destruction and without the use of a marker, which may influence the property of the solution. Since, the sorption takes place at different temperatures, the diffusion coefficient was determined at different temperatures, namely 15°C, 25°C, 35°C and 45°C. The results are summarized in Table 4.

At a temperature of 25°C, the water molecules diffuse approximately 2.5 times faster than the ions of the IL. As expected, the diffusion rate of all species in solution increases with increasing concentration of water and increasing temperature. In pure water, the diffusion coefficient of a water molecule is approximately 45-times larger. The slower diffusion rate of water in IL can be attributed to the strong H-bond interaction between water and the IL as evident form large heat of dilution. The diffusion coefficients determined in our measurements are in agreement with available data from the literature(D(H2O):2.88*10-11M2/s).70 While our samples were freshly prepared under inert atmosphere yielding analytically pure ILs, the literature data have been obtained for purchased ILs with only 97% purity. Commercial ILs frequently contain water, further anions such as chloride or bromide and also acetic acid as impurities which will affect the precision of the diffusion coefficient determination. In addition, the halide content is known to influence the viscosity ILs. 65 With the diffusion coefficient in hand we have performed model calculations in order to estimate water diffusion from the surface to the bulk (cf. SI for details). Also from these calculations arises that diffusion is approx. three orders of magnitude faster in relation to the absorption process. In summary, thermal analysis of the absorption process revealed a non-diffusion-controlled absorption for [EMIM][Ac], [PMIM][Ac], [(MOE)MIM][Ac], [Cholin][Ac] and LiCl. In addition the relevant diffusion coefficients have been determined for the diffusion of water in [EMIM][Ac] via PFG-NMR measurements, which further corroborate that diffusion does not hinder absorption during perfusion of ILs with moist air.

3500 [EMIM][Ac] [PMIM][Ac] [MOEMIM][Ac] LiCl [Cholin][Ac]

3000 2500

heat flow [W]

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 10 of 17

2000 1500 1000 500 0 0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 concentration [wt%]


10

Page 11 of 17

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Figure 4 Plot of the heat flow over the concentration for five electrolytes. The linear relationship between heat flow and concentration matches from the start to the end of the measurements.


11


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Page 12 of 17

Table 4 Diffusion coefficients D for molecular species in [EMIM][Ac]-water mixtures at different concentrations and temperatures. Values are given as multiples of 10-11m2s-1. Temperature [±0.5°C]

15°C

25°C

35°C

45°C

m% [±0.35%]:

98

95

90

80

cation

1.185 ± 0.004

/

/

/

anion

1.144 ± 0.006

/

/

/

H2O

2.218 ± 0.040

/

/

/

cation

1.953 ± 0.014

2.589 ± 0.034

3.431 ± 0.028

5.909 ± 0.051

anion

1.812 ± 0.017

2.611 ± 0.027

3.757 ± 0.037

6.391 ± 0.069

H2O

5.046 ± 0.021

7.037 ± 0.225

8.746 ± 0.243

14.508 ± 0.376

cation

4.226 ± 0.022

5.588 ± 0.034

7.448 ± 0.044

12.185 ± 0.048

anion

3.958 ± 0.043

5.467 ± 0.069

8.138 ±0.128

13.072 ± 0.083

H2O

7.893 ± 0.121

10.578 ± 0.178

14.906 ± 0.369

24.607 ± 0.551

cation

8.255 ± 0.086

8.853 ± 0.231

11.921 ± 0.204

24.138 ± 0.216

anion

8.231 ± 0.091

9.199 ± 0.157

11.675 ± 0.272

22.186 ± 0.134

H2O

12.116 ± 0.153

14.721 ± 0.385

24.108 ± 0.309

38.967 ± 0.818

Perfusion of CO 2 into ILs As already mentioned in the introduction, the reaction of several ILs with CO2 has been studied with theoretical and analytical means.13,36, 42-43 In addition, several NHCs were reacted with CO2 and analyzed with X-ray crystallography, IR and NMR.72 For imidazolium ILs with a basic anion, a mechanism was proposed by Maginn in 2005, involving the deprotonation of the cation. 73 Additional evidence for the involvement of NHCs in ILs provided the crystal structure of the CO 2 adduct of 1,3dialkylimidazolium acetate36 by Gurau et al., as well as the direct reaction of chalcogens with [EMIM][Ac]. 74 Concerning the thermochemistry of the CO2 sorption of ILs Huang et al. reported the enthalpy for CO2 physisorption in ILs to approx. 20 kJ/mol.75 For phosphonium based ILs with a basic anion a value of 56 kJ/mol could be calculated.76 Owing to the relevance of CO2 uptake during open sorption with an IL-water working pair we investigated the absorption enthalpy for the CO2 sorption for several ILs experimentally and performed NMR controls after sorption to identify the sorption products. The results of the CO2 sorption are shown in Figure 5 (Data also included in the SI). The plot shows a near linear relationship between the absorbed amount of CO2 and the energy

released for the acetate-based ILs. For most formate and lactate based ILs lower energies are observed, compared to the acetate ILs. The energy values refer to the sum of the enthalpies released by chemisorption and physisorption. Again [EMIM][Ac] shows the highest sorption activity with the highest CO2 uptake. The NMR control after the sorption experiment confirms the formation of the expected carboxylate in ca. 10 ± 1%(NMR) besides unreacted IL. The ratios for the other ILs are even lower and for the diluted/solid ILs ([MMIM][Ac] and [EMIM][Form]; [2,3DiolMIM][Ac] and 90 wt%: [(EOH)MIM][Ac]) no carboxylate formation could be detected via NMR at all, which is in line with the very low heat flow observed. For all ILs located on the linear slope of this plot the energy released can be extrapolated to ca. 60 kJ/mol(CO2) which is in good agreement with the values obtained by theoretical calculations in the literature mentioned above. Although this energy value is rather high, our experiments show that only partial CO2 sorption is observed under conditions where complete water sorption takes place easily. Moreover, CO2 sorption is limited to very concentrated ILs and is disfavored in relation to H2O in more dilute solutions according to the literature36 and in agreement with our findings above.


12

Page 13 of 17

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Figure 5 Plot of the energy released during CO2 sorption of several ILs vs. the amount of CO2 (T = 25°C; p = atmospheric pressure, concentration of ILs is 100 wt% if not otherwise stated). Insert shows expanded view of the range 0 to 0.1 mmol CO2.


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Conclusion

Page 14 of 17

AUTHOR INFORMATION

In summary, we have studied the water uptake of a selection of ILs as liquid sorbents from an airstream of controlled humidity and flow rate in terms of perfusion experiments using a nanocalorimeter. For comparison we also measured concentrated aqueous LiCl (44 wt%) under identical conditions, owing to its technological importance in sorption based energy storage. Out of these compounds [EMIM][Ac] shows the best performance referring to energy release per mol of water, lowering the water vapor pressure and available concentration range suitable for exothermic water sorption. The aspect of hindered diffusion, which has been suggested for concentrated liquid sorbents in the literature before, has been explored for our substances and based on a correlation of heat flow and concentration and independently via PFG-NMR spectroscopy we could demonstrate that diffusion within the sorbent is not rate limiting. In this context the diffusion constants of the prevailing species in a binary [EMIM][Ac]-water mixture have been determined for different concentrations and temperatures. In this context, it needs to pointed out that the heat flow of the sorption process is based on non-equilibrium conditions of the overall system with the water supply at the gas/sorbent interface being the rate determining factor for the energy releasing steps. With regard to open sorption devices, we also considered the CO2 sorption properties of the sorbents under investigations. We have been able to experimentally determine the enthalpy for this process via calorimetry which agrees well with the theoretically predicted value. For the ILs under investigation CO2 sorption is however far less efficient and moreover limited to very concentrated ILs, which means in turn that it will not negatively affect water sorption over a wide concentration range. Therefore [EMIM][Ac] should be well suited for energy storage applications even in an open sorption system and outperforms the well-established LiCl in the aspects of reducing water vapor pressure (1.3 mbar vs. 4.6 mbar), heat of dilution (-805 kJ/kg vs. -306 kJ/kg) and the absorption performance of water (75% higher energy at 55 %RH). An even more pronounced advantage is observed for very low RH values where LiCl solution in contrast to [EMIM][Ac] shows endothermic effects which may be relevant for open sorption for instance in winter time for seasonal storage applications.

ACKNOWLEDGEMENT Financial support by the BMBF project OpenSorp (T.B.) is gratefully acknowledged. Moreover, we are grateful to Prof. Dr. Ulrike Jordan for helpful discussions.

ASSOCIATED CONTENT Supporting Information. Overview of all ILs used in this study together with their spectroscopic characterization, water content; calibration and details of the calorimetric measurements. This material is available free of charge via the Internet at http://pubs.acs.org.”

Corresponding Author *Rudolf Pietschnig. E-mail: [email protected] Notes The authors declare no competing financial interest.

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For Table of Contents Use Only TOC artwork:

TOC Synopsis: Sorbent Properties of Halide free Ionic liquids for Water and CO2 Perfusion

Thorge Brünig, Martin Maurer, Rudolf Pietschnig* Ionic liquids were compared to aqueous LiCl solution in perfusion experiments for evaluating their feasibility as sorbents in open water sorption systems.


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Sorbent Properties of Halide-Free Ionic Liquids for Water and CO2

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