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

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Research Article pubs.acs.org/journal/ascecg

Sorbent Properties of Halide-Free Ionic Liquids for Water and CO2 Perfusion Thorge Brünig, Martin Maurer, and Rudolf Pietschnig* Universität Kassel, Institut für Chemie und CINSaT, Heinrich-Plett-Straße 40, 34132 Kassel, Germany S Supporting Information *

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 CO2 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 water−IL 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 approximately 3 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, CO2 sorption, Thermal energy storage, Diffusion coefficient

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INTRODUCTION Thermal energy storage is a successful concept for heating systems in housing and industrial applications.1,2 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), 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, halidefree ILs containing organic anions such as acetate,23−25 EtSO4−,26 DEP,27 DMP,28 or lactate29 have been developed. © 2017 American Chemical Society

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 Specifically, 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 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 addition with other reactive constituents of air such as oxygen or carbon dioxide needs to be considered. While most ILs are stable toward oxygen, it is well known that imidazoliumbased 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 Received: May 10, 2017 Revised: June 20, 2017 Published: June 26, 2017 7228

DOI: 10.1021/acssuschemeng.7b01468 ACS Sustainable Chem. Eng. 2017, 5, 7228−7239

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ACS Sustainable Chemistry & Engineering

spectrometer with a Z-axis gradient. All experiments were performed at the concentrations and temperatures (approximately ±0.5 °C) summarized in Table 5. 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 × 10−10 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 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.

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. Specifically, low water vapor pressure, which offers the possibility of efficient dehumidification, is one of the key aspects.1,23 Furthermore, high heat of dilution and fast absorption of water enable an increase in 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 heatreleasing step in sorption-based thermal energy storage. In addition, we explored the aspect of diffusion as a potential ratedetermining 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.

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⎛ ⎛ δ⎞ ⎞ I = exp⎜− γ 2g 2δ 2⎜Δ − ⎟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 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 to achieve, the measurements were carried with mixtures containing 95 ± 0.35 wt % of the substance and 5 ± 0.53 wt % water (solubility permitting). For [MMIM][Ac]/[Form] and [EMIM][Form], a lower concentration of 92 ± 0.35 wt % of the substance and 8 ± 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 a connection to a 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 (for data, see SI). Before the measurement was started, the solution was frozen with liquid nitrogen and evacuated (10−3 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). Thermal analysis of the heat of dilution 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 an adjustable (T, RH, flow rate) gas stream. Specifications are shown in Table 1. Gravimetric Tracking. The mass uptakes of water or CO2, which are absorbed in the perfusion measurements, were measured with an analytical balance of the type Kern ABJ 320-4 (accuracy ± 0.0002 g). Karl Fischer Titration. The determination of the water content was performed in a coulometric titration using the device 7500

EXPERIMENTAL SECTION

Synthesis. All substances used in this paper are synthesized by ourselves and characterized with NMR and Karl Fischer titration to ensure high purity (Supporting Information). 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 imidazolium-based ionic liquids can be divided into two steps. In the first step, 1methylimidazole 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 first by integration of the 1H NMR spectra between anion and cation and, on the other hand, proven by a silver test.52 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 N-ethyl-N′-methyl imidazolium, [PMIM] to N-methyl-N′-propyl imidazolium, [MOEMIM] to N-ethoxymethylN′-methyl imidazolium, [IsoMIM] to N-isopropyl-N′-methyl imidazolium, [EOHMIM] to N-hydroxyethyl-N′-methyl imidazolium, and [2.3DiolMIM] to N-methyl-N′-2,3-dihydroxypropyl 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. 1H- and 13C NMR spectra were recorded on a Varian 400-MR spectrometer (measurement frequencies: 1H: 400 MHz; 13C: 100.5 MHz). Chemical shifts are reported in ppm and 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 the 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

Table 1. Calorimeter Specifications temperature range accuracy long-term stability short-term stability scanning rate short-term noise baseline drift accuracy precision 7229

Thermostat 15−150 °C < ±0.1 °C < ±100 μK/24 h < ±10 μK/(p-p) < ±2 °C/h (20−150 °C) Nanocalorimeter < ±10 nW 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 the case of the corresponding acetates, the [EMIM] compound is also liquid in contrast to the [MMIM] analog. On the basis of these observations, the [EMIM][Ac] shows 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. 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

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RESULTS AND DISCUSSION Vapor Pressure of Water−IL Mixtures. The proclivity of a substance to reduce the water vapor pressure is an important criterion for identifying suitable sorbents for use in open sorption storage systems. This thermodynamic behavior has been treated as the most important parameter in a closed system. Although the water vapor pressure of some ionic liquid−water mixtures has already been measured, the experimental conditions differ with respect to temperature and concentration range which makes them hard to compare.9,19,23,26,54,55 Neat ionic liquids show a negligible vapor pressure, and therefore, the vapor liquid equilibrium of binary water−IL mixtures is composed exclusively by water in the gas phase. For the sake of comparison, we measured the vapor pressure above a series of concentrated IL−water mixtures at three different temperatures (Table 2). In addition we also measured the vapor pressure above concentrated solutions of LiCl and NaOH at the same conditions since these have been implemented successfully in sorption devices which require a liquid sorbent. As shown from Table 2, the vapor pressures of these ILs and the two other sorbents range between 1.3 and 6.2 mbar at room temperature, indicating the strongly hygroscopic nature of all of these compounds. These data are in reasonable agreement with literature values, although they are difficult to compare owing to differences in temperature, concentration, impurities, or method. For instance, the water vapor pressure of LiCl has been reported by Johnson et al. with 16.1 mbar at a concentration of 45.8 wt % (30 °C) using an indirect calorimetric method.56 All investigated ILs obtained a reduced or comparable water vapor pressure referring to LiCl as a benchmark. Compared to the halide-containing [MMIM][Cl], 7230


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ACS Sustainable Chemistry & Engineering 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 (microheterogeneities) 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 3. The LiCl benchmark (44 wt %) has a dilution heat of

[(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.30 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. 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. 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 a 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 perfusion equipment, the dehumidification of an airstream can be tracked thermally by measuring the absorption enthalpy. On the basis of 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

Table 3. Heat of Dilution of ILs in Relevant Concentration Range Compared to Benchmark LiCl and Hygroscopic NaOH Electrolyte heat of dilution [−kJ/kg H2O] concentration [wt %]

44

LiCl NaOH concentration [wt %]

306

[MMIM][Ac] [MMIM][Form] [MMIM][Lac] [EMIM][Ac] [EMIM][Form] [EMIM][Lac] [PMIM][Ac] [PMIM][Form] [PMIM][Lac] [IsoMIM][Ac] [IsoMIM][Form] [(MOE)MIM][Ac] [(MOE)MIM][Form] [(MOE)MIM][Lac] [(2,3Diol)MIM][Ac] [(EOH)MIM][Ac] [Cholin][Ac]

100

40

37

33

200 414 95 605

560 805

400 620

467 760 640 500 734 577 579 321 276 328

360 610 460 345 540 491 381 225 196 271 308 426

247 90

ref

500 430 290 479 440 230 440 378 268 400 380 276 175 157 227 270 400

28 28 28 28 28 28 28 28 28

28

−306 kJ/kg (H2 O) at 25 °C, which corresponds to approximately 14% 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 3. The acetate species offer the advantage of a lower melting point compared to the formatebased 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 7231


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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, 1274, 1048 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 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 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 4 summarizes the results of these measurements for the sorbents [EMIM][Ac],

Figure 1. Perfusion experiment with LiCl as sorbent (44 wt %, T = 25 °C, V = 0.15 mL in ampule 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 (gray area).

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

Figure 2. Plot of heat flow obtained vs relative humidity (RH) of air stream for different ILs and LiCl. 7232


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Table 4. 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]

100 100.00 98.14 0.0035 0.00019 10.3 2949

LiCl 55 100.00 90.90 0.0187 0.00104 49.4 2642

100 44.00 42.46 0.0078 0.00043 19.6 2513

[(MOE)MIM][Ac] 55 44.00 41.68 0.0108 0.00060 27.1 2509

100 100.00 92.10 0.0164 0.00091 43.3 2641

55 100.00 95.35 0.0082 0.00046 21.9 2671

[PMIM][Ac] 100 100.00 91.90 0.0104 0.00058 30.0 2880

55 100.00 98.31 0.0031 0.00017 9.1 2942

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 0−0.002 g/200 mL.

[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. 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 characteristics at different temperatures were investigated as well. It should be noted that several factors influence the 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 vapor 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 and 40 °C do not differ significantly. Even with increasing humidity, the absorption at both temperatures shows similar characteristics. 7233


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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.

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 diffusioncontrolled 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 the 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 functions 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. 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, 7234


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Table 5. Diffusion Coefficients D for Molecular Species in [EMIM][Ac]−water Mixtures at Different Concentrations and Temperaturesa wt% [± 0.35%] temperature [± 0.5 °C]

a

98

95

90

80

15 °C

cation anion H2O

1.185 ± 0.004 1.144 ± 0.006 2.218 ± 0.040

− − −

− − −

− − −

25 °C

cation anion H2O

1.953 ± 0.014 1.812 ± 0.017 5.046 ± 0.021

2.589 ± 0.034 2.611 ± 0.027 7.037 ± 0.225

3.431 ± 0.028 3.757 ± 0.037 8.746 ± 0.243

5.909 ± 0.051 6.391 ± 0.069 14.508 ± 0.376

35 °C

cation anion H2O

4.226 ± 0.022 3.958 ± 0.043 7.893 ± 0.121

5.588 ± 0.034 5.467 ± 0.069 10.578 ± 0.178

7.448 ± 0.044 8.138 ± 0.128 14.906 ± 0.369

12.185 ± 0.048 13.072 ± 0.083 24.607 ± 0.551

45 °C

cation anion H2O

8.255 ± 0.086 8.231 ± 0.091 12.116 ± 0.153

8.853 ± 0.231 9.199 ± 0.157 14.721 ± 0.385

11.921 ± 0.204 11.675 ± 0.272 24.108 ± 0.309

24.138 ± 0.216 22.186 ± 0.134 38.967 ± 0.818

Values are given as multiples of 10−11 m2 s−1.

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). Inset shows expanded view of the range from 0 to 0.1 mmol CO2.

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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.

e.g., owing to dilution. Moreover, such a prediluted 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, 25, 35, and 45 °C. The results are summarized in Table 5. 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 from a large heat of dilution. The diffusion coefficients determined in our measurements are in agreement with available data from the literature (D(H2O): 2.88 × 10−11 M2/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 of 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 approximately 3 orders of magnitude faster in relation to the absorption process. In summary, thermal analysis of the absorption process revealed a nondiffusion-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. Perfusion of CO2 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 CO2 adduct of 1,3-dialkylimidazolium 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 approximately −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

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CONCLUSION 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 on the basis of 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 be pointed out that the heat flow of the sorption process is based on nonequilibrium 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 7236


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(11) Riffat, S. B.; James, S. E.; Wong, C. W. Experimental analysis of the absorption and desorption rates of HCOOK/H2O and LiBr/H2O. Int. J. Energy Res. 1998, 22, 1099−1103. (12) Paranjape, D. V.; Chaudhari, S. K.; Eisa, M. A. R.; Holland, F. A. Characteristics of water-calcium chloride and water-lithium bromide absorption heat pumps. AIChE J. 1986, 32 (11), 1924−1927. (13) Shiflett, M. B.; Kasprzak, D. J.; Junk, C. P.; Yokozeki, A. Phase behavior of {carbon dioxide + [bmim][Ac]} mixtures. J. Chem. Thermodyn. 2008, 40 (1), 25−31. (14) Brandt, B. Korrosionsverhalten ausgewählter Werkstoffe in Lithiumbromid-Lösung für den Anwendungsfall Absorptionskältemaschinen. Mater. Corros. 2004, 55 (7), 536−542. (15) Plechkova, N. V.; Seddon, K. R. Applications of ionic liquids in the chemical industry. Chem. Soc. Rev. 2008, 37 (1), 123−150. (16) Gebresilassie Eshetu, G.; Armand, M.; Scrosati, B.; Passerini, S. Aus ionischen Flüssigkeiten hergestellte Materialien für die Energiespeicherung. Angew. Chem. 2014, 126 (49), 13558−13576. (17) Seiler, M.; Kühn, A.; Ziegler, F.; Wang, X. Sustainable Cooling Strategies Using New Chemical System Solutions. Ind. Eng. Chem. Res. 2013, 52 (47), 16519−16546. (18) Cuadrado-Prado, S.; Domínguez-Pérez, M.; Rilo, E.; GarcíaGarabal, S.; Segade, L.; Franjo, C.; Cabeza, O. Experimental measurement of the hygroscopic grade on eight imidazolium based ionic liquids. Fluid Phase Equilib. 2009, 278 (1−2), 36−40. (19) Wang, J.; Zheng, D.; Fan, L.; Dong, L. Vapor Pressure Measurement for the Water + 1,3-Dimethylimidazolium Chloride System and 2,2,2-Trifluoroethanol + 1-Ethyl-3-methylimidazolium Tetrafluoroborate System. J. Chem. Eng. Data 2010, 55 (6), 2128− 2132. (20) Freire, M. G.; Neves, C. M. S. S.; Marrucho, I. M.; Coutinho, J. A. P.; Fernandes, A. M. Hydrolysis of Tetrafluoroborate and Hexafluorophosphate Counter Ions in Imidazolium-Based Ionic Liquids. J. Phys. Chem. A 2010, 114 (11), 3744−3749. (21) Swatloski, R. P.; Holbrey, J. D.; Rogers, R. D. Ionic liquids are not always green. Green Chem. 2003, 5 (4), 361. (22) Uerdingen, M.; Treber, C.; Balser, M.; Schmitt, G.; Werner, C. Corrosion behaviour of ionic liquids. Green Chem. 2005, 7 (5), 321− 325. (23) Popp, S.; Bösmann, A.; Wölfel, R.; Wasserscheid, P. Screening of Ionic Liquid/H2O Working Pairs for Application in Low Temperature Driven Sorption Heat Pump Systems. ACS Sustainable Chem. Eng. 2015, 3 (4), 750−757. (24) Wasserscheid, P.; Seiler, M. Leveraging gigawatt potentials by smart heat-pump technologies using ionic liquids. ChemSusChem 2011, 4 (4), 459−463. (25) Chen, Y.; Cao, Y.; Mu, T. A New Application of Acetate-Based Ionic Liquids: Potential Usage as Drying Materials. Chem. Eng. Technol. 2014, 37 (3), 527−534. (26) Heym, F.; Haber, J.; Korth, W.; Etzold, B. J. M.; Jess, A. Vapor Pressure of Water in Mixtures with Hydrophilic Ionic Liquids - A Contribution to the Design of Processes for Drying of Gases by Absorption in Ionic Liquids. Chem. Eng. Technol. 2010, 33 (10), 1625−1634. (27) Varfolomeev, M. A.; Khachatrian, A. A.; Akhmadeev, B. S.; Solomonov, B. N.; Yermalayeu, A. V.; Verevkin, S. P. Enthalpies of Solution and Enthalpies of Solvation in Water: The Anion Effect in Ionic Liquids with Common 1-Ethyl-3-methyl-imidazolium Cation. J. Solution Chem. 2015, 44 (3), 811−823. (28) He, Z.; Zhao, Z.; Zhang, X.; Feng, H. Thermodynamic properties of new heat pump working pairs: 1,3-Dimethylimidazolium dimethylphosphate and water, ethanol and methanol. Fluid Phase Equilib. 2010, 298 (1), 83−91. (29) Constantinescu, D.; Schaber, K.; Agel, F.; Klingele, M. H.; Schubert, T. J. S. Viscosities, Vapor Pressures, and Excess Enthalpies of Choline Lactate + Water, Choline Glycolate + Water, and Choline Methanesulfonate + Water Systems. J. Chem. Eng. Data 2007, 52 (4), 1280−1285. (30) Brünig, T.; Krekić, K.; Bruhn, C.; Pietschnig, R. Calorimetric Studies and Structural Aspects of Ionic Liquids in Designing Sorption

outperforms the well-established LiCl in the aspects of reducing water vapor pressure (1.3 vs 4.6 mbar), heat of dilution (−805 vs −306 kJ/kg), and 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.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01468. Overview of all ILs used in this study together with their spectroscopic characterization, water content, calibration, and details of the calorimetric measurements. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Rudolf Pietschnig. E-mail: [email protected]. ORCID

Rudolf Pietschnig: 0000-0003-0551-3633 Notes

The authors declare no competing financial interest.

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

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

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