Double Salt Ionic Liquids Based on Ammonium ... - ACS Publications

Subscriber access provided by the University of Exeter

Article

Double Salt Ionic Liquids Based on Ammonium Cations and Their Application for CO Capture 2

Mert Atilhan, Baraa Anaya, Ruh Ullah, Luciano T. Costa, and Santiago Aparicio J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05987 • Publication Date (Web): 19 Jul 2016 Downloaded from http://pubs.acs.org on July 21, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 43

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

The Journal of Physical Chemistry

Double Salt Ionic Liquids Based on Ammonium Cations and Their Application for CO2 Capture Mert Atilhan,*a Baraa Anaya,a Ruh Ullah,a Luciano T. Costa,b and Santiago Aparicio*c a b

Department of Chemical Engineering, Qatar University, P.O. Box 2713, Doha, Qatar

Instituto de Química - Departamento de Físico-Química, Universidade Federal Fluminense, 24020-141 Niterói, Brazil c

Department of Chemistry, University of Burgos, 09001 Burgos, Spain

*

Corresponding authors:

e-mail: [email protected] ; phone: +974 4403 4142 (M.A.) e-mail: [email protected] ; phone: + 34 947 258 062 (S.A.)

ABSTRACT: Simple ionic liquids (containing one type of cation with one type of anion) and complex mixed ionic liquids (containing several types of anions and cations, double salts) based on ammonium cations were studied in this work using a combined computational and experimental approach. Theoretical studies were carried out using classical molecular dynamics simulations. The properties and structure of these fluids and their changes upon CO2 absorption were analysed. The fluids' structural, energetic and dynamic properties were considered as a function of the type of ions composing the ionic liquids together with their changes when CO2 is present as a function of CO2 concentration. Likewise, experimental measurements analyse carbon capturing abilities for the studied mixed ionic liquids as a function of pressure and temperature. The reported results show that mixing two neat ammonium-based ionic liquids does not change remarkably the properties of the involved neat ionic liquids, and also the affinities for CO2 are also similar in the mixed ionic liquids. Therefore, vastly different ions should be considered when designing mixed ionic liquids for stimulating CO2 physisorption by increasing the available volume and tuning affinity toward CO2. This work provides a nanoscopic and macroscopic characterization of complex ionic liquids and their ability for carbon capturing for the first time.

1

ACS Paragon Plus Environment


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

1. INTRODUCTION The suitability of using ionic liquids, ILs, as alternative compounds for a wide range of technologies1-6 has led to a systematic screening of their properties using both experimental and computational approaches.7-11 This interest on ILs for environmental and energy related challenges have also been accompanied by possible problems,12 including toxicity13 and poor biodegradability14 for some types of IL, high cost of many IL15 and difficulties for scaling up IL-based processes for industrial applications.16,17 Nevertheless, the vast majority of available studies consider one cation:one anion ILs in spite of the fact that the number of possible ILs can be extended up to 1018 when mixtures of ILs are considered,18,19 which would allow to advance in the development of task specific ILs. Chatel et al.19 reviewed the state-of-the-art of the available literature on ILs mixtures, proposing the use of 'Double Salt Ionic Liquids (DSILs)' term for ILs composed by more than two types of ILs.19 In the DSILs framework every combination of ions should be treated as a specific IL instead of a mixture of ILs leading to a new strategy for ILs design based on the selection of suitable ions and proportions.19 Most of the available literature have treated mixed ILs in the same way of mixtures of molecular solvents, analyzing their deviations from ideality and measuring the changes in the most relevant physicochemical properties,20,21 showing close to ideal mixing behavior,22-24 although several examples of ILs mixtures with large excess properties have also been reported.19 Likewise, the analysis of the literature have showed that an IL mixture can be ideal or non-ideal depending on the considered property.19 Likewise, for many IL mixtures unexpected physicochemical properties are obtained upon mixing, and even relevant features at the nanoscopic level are obtained although apparently ideal behavior is obtained for certain macroscopic properties.19,25 Therefore, ILs mixtures should be systematically studied searching for ions combinations leading to highly non-ideal behavior, which could provide different and tunable properties to those of pure ILs.19,26 For this purpose, computational chemistry is a valuable tool that allows the screening of DSILs properties and nanoscopic behavior.27,28 One of the most relevant applications considered for ILs is their use as agents for CO2 capture from flue gases, 1,2,29-31 and although a wide collection of studies have been published several drawbacks have been reported such as large cost, moderate capturing ability, or not suitable physicochemical properties such as large viscosity, which currently hinder the application of ILs for CO2 capture at large scale.29,30 Nevertheless, most of the IL drawbacks may be surpassed by the ability of tuning their properties through suitable combinations of

2


Page 2 of 43

Page 3 of 43

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


ions, and thus, the use of mixed ILs for CO2 capture would have great advantages over considering only pure IL as it has been done in most of the available literature. Nevertheless, the studies considering mixed ILs for carbon captures purposes are very scarce in the literature.32-37 A rational approach for selecting ions for DSILs design for CO2 capture is to consider those with suitable properties from environmental, toxicological and economical approaches.15,38,39 Ammonium-based cations have been used for developing ILs40-42 because of their low toxicity43,44 and their suitable physicochemical properties such as moderate viscosity.45-47 Therefore, ammonium based ILs were selected in this work for developing mixed ILs (DSILs in the Chatel et al.19 terminology) for CO2 capturing purposes. Four ammonium

-

based

ILs

were

bis(trifluoromethylsulfonyl)imide

selected

in

[BTMA][Tf2N]

this

work:

(IL#1),

butyltrimethylammonium methyltrioctylammonium

trifluoromethanesulfonate [MTOA][OTf] (IL#2), diethylmethylammonium methanesulfonate [DEMA][METS] (IL#3), diethylmethylammonium trifluoromethanesulfonate [DEMA][OTf] (IL#4), Figure 1. Binary mixtures were selected considering the six possible combinations between the four selected IL, all considering equimolar mixtures, pure ILs were also studied for comparison purposes. The properties of the mixed ILs + CO2 systems were studied as a function of pressure and temperature. All the studied systems were studied using classical molecular dynamics simulations to obtain a detailed picture of their nanoscopic properties in systems containing several types of ions48 and CO2 molecules. Likewise, absorption isotherms were experimentally extend theoretical results to macroscopic properties.

2. THEORETICAL AND EXPERIMENTAL METHODOLOGY 2.1. Theoretical Methods The forcefield parameterizations for ions used along this work are reported in Table S1 (Supporting Information). Intramolecular forcefield parameters (bond, angle and torsional) and Lennard-Jones parameters for ammonium – based cations were obtained from a previous work from our group49 and from Chang et al.50; [Tf2N]- parameters come from Logotheti et al.51 and for [OTf]- and [METS]- from Chang et al.50 and from Sunda et al.52 Atomic charges were calculated from optimized structures for isolated ions, and thus totalling +/- 1 for cations/anions, using Density Functional Theory at B3LYP/6-311++g(d) level according to ChelpG method53 with Gaussian 09 (Revision D.01) package.54 CO2 molecules were described according to the parameterization by Shi and Maggin.55 Cubic simulation boxes were used for all the systems which were initially built using Packmol program.56 The number

3



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 4 of 43

of molecules used for simulating pure and mixed IL is reported in Table S2 (Supporting Information). All mixed IL (DSILs) are equimolar binary mixtures of the corresponding pure IL. All the simulations for pure and binary mixed IL were carried out in the NPT ensemble at 298 K and 1 bar. Regarding pure IL + CO2 and mixed ILs + CO2 systems, simulations in the NPT ensemble at 298 K and pressures of 1, 4, 7 and 10 bar were carried out. In order to build the simulation boxes, the mole fraction of CO2 at each pressure and temperature, i.e. CO2 solubility, was predicted using COSMOthermX57 program using the optimized structures for ions calculated at B3LYP/6-311++g(d) level, for which COSMOthermX input files were calculated at BVP86/TZVP/DGA1 level. The BP_TZVP_C21_0111 COSMOthermX parameterization was used. The composition and number of molecules for systems containing CO2 molecules are also reported in Table S2 (Supporting Information). COSMOtherm method has been widely used for predicting CO2 solubilities in ionic liquids showing reasonable deviations with experimental results,58-60 and thus it can be used for developing ionic liquid + CO2 simulation systems as done in this work in case CO2 solubility data were not available. Molecular dynamics simulations were carried out using the MDynaMix v.5.2 molecular modelling package.61 Pressure and temperature were controlled using the Nose– Hoover thermostat. Coulombic interactions were handled with the Ewald summation method,62 with cut-off radius of 15 Å. Tuckerman–Berne double time step algorithm,63 with long and short time steps of 1 and 0.1 fs, was considered for solving the equations of motion. Lorentz-Berthelot mixing rules were used for Lennard-Jones cross terms. Heating and quenching steps were applied in the 298 to 500 K range to the initially built simulation boxes for all the considered systems and then 5 ns equilibration runs were carried out with equilibration being assured through the analysis of the total potential energy. Production runs of 10 ns were done after equilibration steps for all the systems and were used for the calculation of structural, energetic and dynamics properties reported in the following sections.

2.2. Experimental Methods Ionic

liquids

methyltrioctylammonium

butyltrimethylammonium diethylmethylammonium

trifluoromethanesulfonate

bis(trifluoromethylsulfonyl)imide trifluoromethanesulfonate

[MTOA][OTf], [BTMA][Tf2N],

[DEMA][OTf]

and

diethylmethylammonium methanesulfonate [DEMA][METS] were purchased from Iolitech

4


Page 5 of 43

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


(Germany) and used as received. A molar mixing ratio of 1:1 mixtures were prepared and total of 4 pure and 6 mixture ILs were used for gas solubility measurements in this work. Density of the samples were measured by Anton Paar DMA 4500M apparatus. The density meter uses the oscillating U-tube sensor principle and it has a volume requirement of 1 mL of sample. The density meter has reported accuracy of 0.00005 g/m3 in density and ± 0.05 °C in temperature. Density measurements for the samples was collected at 25 °C. Magnetic sorption apparatus (MSA) from Rubotherm Prazisionsmesstechnik® was used to for CO2 adsorption/desorption experiments at pressures up to 10 bars at 25 °C. Temperature of the sample was maintained through external temperature constant temperature thermo-stating bath (from Polyscience Inc.). MSA is equipped with automated Teldyne Isco 260D pump for the automated compression of CO2 in the measurement chamber. Further details of the experimental setup are given elsewhere.64-66 In a typical procedure, known amount (0.1 to 0.2) g of IL sample is placed in a sample holder in MSB high pressure cell and it is vacuumed for about 5 hours at 150 °C. The maximum set pressure (10 bars) is reached via stepwise increments by increasing the pressure gradually from 1 bar up to 10 bars. Each pressure point takes about (60 to 75) minutes to reach pressure and temperature equilibrium, after which different sets of weight measurements along with pressure and temperate data are collected. MSB system is fully automated and the pressure goes to next higher point after completing the previous measurement point. Pressurizing (or absorption sequence) is followed by depressurising (or desorption sequence), which is essential to observe the physisorption or chemisorption nature of the sorbents being measured in the MSB system. Details on the findings of the sorption measurements are given in the section 3.

3. RESULTS AND DISCUSSION The results obtained in this work comprise both computational and experimental studies. In section 3.1 to 3.4, the nanoscopic characterization of the studied ionic liquids from molecular dynamics results will be presented. These results will be supported by experiments reported in section 3.5.

3.1. Pure Ionic Liquids A study on the properties of the pure ILs was carried out initially as a reference framework for comparison

purposes

when

DSILs

systems

are

considered.

The

most

relevant

physicochemical properties of the pure ILs calculated from MD are reported in Table 1. The

5



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

comparison of MD predicted properties with experimental data is commonly done in the literature for the validation of the used force field and the applied computational methodology. Density data predicted from MD simulations is in very good agreement with experimental data with deviations lower than 1 % on average, with the largest deviations (4.1 %) for [MTOA][OTf] Table 1.67-70 Dynamic viscosity was calculated using Green – Kubo method, and results are in good agreement with available experimental data. There are not additional experimental data for comparison purposes but results obtained from MD are in reasonable agreement with experiments, thus the force field applied in this work seems to be suitable for describing the main properties of the studied fluids. The main structural features of the studied ammonium-based ILs are analysed using the center-of-mass radial distribution functions, RDF, reported in Figure 2. Cation-cation RDFs reported in Figure 2a shows a peak at 6.5 Å followed by additional peaks, which are different for each IL. In the case of [BTMA][Tf2N] two additional peaks in cation-cation RDFs are obtained, which shows a large degree of ordering for [BTMA] cation even for distances larger than 15 Å, this cationic ordering is not so well defined for the remaining cations. Likewise, RDF for a type of cation does not change remarkably for different counterions; e.g.

RDFs for [DEMA] are almost the same in [DEMA][METS] and

[DEMA][OTf], Figure 2a. In the case of anion-anion RDFs, Figure 2b, the arrangement of anions is dependent on the type of counterion involved in each ionic liquid; e.g. anion – anion RDFs for [MTOA][OTf] and [DEMA][OTf] are remarkably different whereas those for [DEMA][METS] and [DEMA][OTf] are very similar even these are different anions, Figure 2b. Regarding anion-cation RDFs, Figure 2c, ILs containing the same cation show the same RDFs, confirming the pivotal role of the ammonium ions on the development of fluids’ structuring with minor role for the anions. Likewise, the very intense peak for anion-cation RDF in [MTOA][OTf] is justified considering that the large size of this cation leads to anions placed at a specific site in the first solvation sphere around the cation center-of-mass. The running integrals for the corresponding anion-cation RFDs reported in Figure 2 c leads to 6.1, 3.3, 7.0 and 6.5 anions around the corresponding cation in the first solvation shell for [BTMA][Tf2N], [MTOA][OTf], [DEMA][METS] and [DEMA][OTf], respectively. This is in agreement with anions being placed around the large [MTOA] cation in specific places, whereas for the other cations a wider region of anion distribution around the cations is obtained. This is confirmed by spatial distribution functions around the ammonium cations reported in Figure 3. Results in Figure 3b show that [OTf] anions are placed around the nitrogen site in the cation avoiding the regions dominated by the long octyl chains, whereas 6


Page 6 of 43

Page 7 of 43

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


for the other cations anions are distributed around the whole cation because the short alkyl chains allows a most effective solvation of the cation by the corresponding anions. [DEMA] cations has one hydrogen bond donor site (N1-Hn, Figure 1), thus hydrogen bonding with acceptor sites in the anions could be developed. The analysis of the trajectories showed and average number per ion pair of 0.51 ± 0.04 and 0.60 ± 0.05 hydrogen bonds in [DEMA][METS] and [DEMA][OTf], respectively, all of them between Hn (cation) and Os (anion) sites, Figure 1. Hydrogen bonding between through the Fc sites in [OTf] anion was discarded. The strength of ion-ion interactions in the studied IL are quantified through the intermolecular interaction energies, Einter, obtained as the sum of Lennard-Jones and coulombic contributions, Table 1. The calculation of Einter for each interacting pair was done from the definition of the forcefields (Table S1, Supporting Information) and the cut-offs considered in section 2.1. Regarding Einter for repulsive cation-cation interactions, results in Table 1 show that this repulsive term decreases with increasing length of alkyl chains. This is justified considering the RDFs reported in Figure 2a, the size of the cations with long alkyl chains, especially for [MTOA], avoids the contact between neighbour cations, thus decreasing repulsive terms. This behaviour is also inferred for anion-anion interactions, those anions in IL with big counterions lead to lower anion-anion repulsive Einter, e.g [OTf] – [OTf] Einter in [MTOA][OTf] are 2.5 times lower than in [DEMA][OTf]. Nevertheless, although increasing cation size leads to fluids’ stabilization through decreasing Einter repulsive terms, it also decreases (in absolute value) Einter anion-cation attractive terms. In this way, the total Einter, summing up all ion-ion contributions, follows the ordering (in absolute value) [DEMA][METS]>[BTMA][Tf2N]>[DEMA][OTf]>[MTOA][OTf]. Therefore, there is not a direct relationship between total Einter and fluids’ viscosity, [MTOA][OTf] shows the lowest Einter for the studied ILs but it is the more viscous fluid. The total interaction energy for the studied ILs is in the range of previously studied ILs and in agreement with Density Functional Theory results.71Therefore, these results that geometric factors rising from the size and shape of the involved ions, especially for alkylammonium cations, controls viscous behaviour: the larger the chain the more viscous IL. Only in those cases for which the same cation is considered, the strength of ion-ion interactions develops the pivotal role, e.g. [DEMA][OTf] and [DEMA][METS]. The residence time, tres, of anions around the corresponding cations was also calculated and reported in Table 1. tres was defined as the time a molecule remains within a given distance around another one, defined each one by the position of their centersof-mass. tres was calculated from the exponential decay of conditional probability for a 7



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

molecule to remain within a sphere of radius R + δr around another one after entering the sphere of radius R − δr at time t = 0. The size of the sphere, R, was defined from the first minimum of the corresponding radial distribution functions (first solvation spheres) and δr = 0.25 Å was considered. The results in Table 1 show a linear relationship between tres and dynamic viscosity (R2 = 0.87) showing that the larger tres the larger the dynamic viscosity, but surprisingly lower anion-cation Einter leads to larger tres, which confirms the prevailing role of steric effects on ions dynamic. Regarding the self-diffusion coefficients reported in Table 1, these values show high correlation between anion and cation mobility, showing almost the same coefficients for both ions for all the studied ILs. Therefore, a linear relationship (R2 = 0.93) between tres and average self-diffusion coefficient is inferred.

3.2. DSILs The main physicochemical properties of the DSILs systems are reported in Table 2. The same properties for the neat IL shown in Table 1 were calculated for the DSIL. The linear combination of density and viscosity data for those pure ILs were carried out and the values are shown in parenthesis, Table 2. We obtain a linear relationship between the calculated densities for DSILs and those ones obtained by the linear combination of pure ILs (R2 = 0.96), although a more complex, non-linear, behavior is obtained for viscosity, Figure S1 (Supporting Information). The most important feature to be highlighted from these values is that the mixing of the two neat ILs with higher ion mobility and therefore lower viscosity resulted in a system with the same trend, as shown by the values for the mixture [BTMA][DEMA][Tf2N][OTf] in Table 2. The structural characterization of the DSILs mixtures are provided by the cation(bold)-anion(non bold) center-of-mass RDF for each ion pair in the mixture, as reported in Figure 4. Regarding those DSILs with 2 different cations and 2 different anions, Figures 4a to 4d, complex structural trends are inferred. The comparison of results for [BTMA][MTOA][Tf2N][OTf], Figure 4a, with [BTMA][DEMA][Tf2N][OTf], Figure 4c, shows that replacing an ammonium-based cation containing a short alkyl chains ([DEMA]) by one with long chains ([MTOA]) has a great effect on the ions arrangements around all ions. The structural patterns for [BTMA][DEMA][Tf2N][OTf] show similar trends for both anions around both cations whereas for [BTMA][MTOA][Tf2N][OTf] a densification of [OTf] anions around both cations is obtained. These structural changes are more remarkable for anion-cation RDFs than for cation-cation or anion-anion ones, which suffer minor changes on going from [DEMA] to [MTOA] DSILs, Figure S2 (Supporting Information). This 8


Page 8 of 43

Page 9 of 43

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


rearrangement leads to a weakening for all the anion-cation interactions when [DEMA] is replaced by [MTOA], as depicted by the intermolecular interaction energies in Table 2. The effect of replacing a type of anion may be inferred by comparing results for [BTMA][DEMA][Tf2N][METS], Figure 4b, and [BTMA][DEMA][Tf2N][OTf], Figure 4c. This leads to very minor changes, which shows that a full fluorination of the anion only leads to a slight weakening of anion-cation interactions (Table 2), but the structural properties are almost the same for all the interacting pairs, Figure S2 (Supporting Information). In the case of [MTOA][DEMA][OTf][METS], intermolecular interaction energies are similar to those for [BTMA][MTOA][Tf2N][OTf], Table 2, which shows that the presence of [MTOA] cation weakens all anion – cation interactions when more than one type of cation is present. RDFs reported in Figure 4d show the presence of intense and narrow peaks around [MTOA] cation and two solvation shells around [DEMA]. Likewise, an increase in cation-cation interactions is inferred as showed in Figure S2j (Supporting Information). The comparison of results in Figures 4d with 4e and 4f shows the effect of removing one anion and one cation on the DSILs structuring. Removing [METS] anion from [MTOA][DEMA][OTf][METS] leads to very minor structural changes (Figures 4d and 4e and Figure S2, Supporting Information). Nevertheless, these minor structural rearrangements leads to large changes in intermolecular interaction energy of [OTf] anion with both cations, which are almost twice than in the case of DSIL containing both [OTf] and [METS] anions. On the contrary removing [MTOA] cation from [MTOA][DEMA][OTf][METS] DSIL leads to larger structural changes for all the interacting pairs (Figures 4d and 4f and Figure S2, Supporting Information). Likewise, intermolecular interaction energies for [DEMA][METS][OTf] are the largest ones (in absolute value) for all the studied DSILs, which shows that removing large cations and considering small and similar anions ([METS] and [OTf]) improves ionic interaction for these ammonium-based DSILs. Therefore, this behavior suggest the relation with the interaction strength between the ions in the system and, as a consequence, a possibility for tuning the fractional free volume in DSILs.72 The main structural features inferred from RDFs in Figure 4 may be refined considering the Spatial Distribution Functions (SDFs) reported in Figure 5. These results confirm the poor anionic arrangement around large [MTOA] cations, Figures 5a and 5h, only when a single anion is present in the DSIL a higher anionic density around the [MTOA] core is obtained, Figure 5j (in agreement with the stronger intermolecular forces reported in Table 2), although for all the cases the long alkyl chains are excluded for interactions with any anion, confirming their disruptive role on DSIL structuring for any anion-cation combination. 9



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

Regarding the distribution around middle size cations ([BTMA]), similar patterns are obtained for all the anions and cations considered, Figures 5b, 5d and 5f. Regarding the small [DEMA] cation, all the studied anions are concentrated in similar regions around the cation, Figures 5c, 5e, 5g, 5i and 5l. The results inferred from RDFs and SDFs and the data for intermolecular interaction energies reported in Table 2 follow complex trends. The first conflicting results is the fact that larger intermolecular interaction energies do not lead to large viscosity. For example, [DEMA][METS][OTf] has 3.5 times larger anion-cation interaction energies than [BTMA][MTOA][Tf2N][OTf] whereas its viscosity is 2.8 times lower. A similar behavior is inferred for the calculated self-diffusion coefficients, which obviously increase for DSILs with decreasing viscosity but follow an inverse trend for anion-cation interaction energy. Results in Table 2 show that the presence of [MTOA] cation decreases ionic mobility, not only for this cation but also for all the ions, and thus, these lower self-diffusion coefficients lead to larger viscosity as a rule. Likewise, it is remarkable that the nanoscopic dynamics of the studied DSILs, quantified through the calculated residence times around each ions, seems to be well correlated with the behavior of self-diffusion coefficients, and thus with macroscopic viscosity. Therefore, the lower the residence times, the larger the diffusion coefficients and thus lower viscous fluids. These results points to ionic diffusion controlled not only by the strength of interionic interactions but also to very relevant effects on the ions dynamic caused by the arrangement of ions around other ones, as reported in Figure 5, which may hinder or favor ionic mobility. Nevertheless, the results reported in Figure 6 show the linear relationship between the deviations between calculated viscosity or interaction energy with regard to the corresponding linear combinations from pure ILs forming DSILs is reported. Therefore, the largest deviations from linear additive behavior are in agreement with the largest deviations in the same additive behavior for intermolecular interaction energy, showing the new effects upon DSILs formation from the corresponding ILs, which modify interaction energy by the structural rearrangements. 3.3. Pure Ionic Liquids with CO2 The characterization of IL+CO2 mixtures requires the knowledge of gas solubility in the corresponding IL, which in this work was predicted using COSMOthermX method at 298.15 K and for pressures lower than 10 bar, Figure S3 (Supporting Information). These results show moderate CO2 solubility in the four studied ammonium-based ILs, but with the largest solubility obtained for [MTOA][OTf], which may be justified considering the largest size of 10


Page 10 of 43

Page 11 of 43

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


the cation allowing a better fitting of absorbed gas molecules and in good agreement with the weaker interaction strength between ions the higher free volume available to absorbing gas, as highlighted by the intermolecular energies in Table 1 and previously discussed in the literature.72 This is confirmed when [MTOA] cation is replaced by [DEMA], both with [OTf] anion, leading to a remarkable decrease in CO2 solubility. The simulation boxes used for MD studies of IL+CO2 mixtures were prepared resembling those compositions reported in Figure S3 (Supporting Information) and were carried out for the same temperature and pressures for each CO2 concentration. The absorption of CO2 molecules in the studied ILs should lead to changes in the physicochemical properties of the ILs, which should be especially relevant for the volumetric properties.73

For this purpose, density and percentage change of volume upon CO2

absorption, %Vexp, which was defined according to the criterion by Gallagher et al.,74 are reported in Table 3 as a function of pressure for the studied ILs. These results show an increase of density for [BTMA][Tf2N] with increasing amounts of CO2, and thus contraction upon gas absorption (negative %Vexp) whereas expansion upon absorption is obtained for the remaining ILs. Nevertheless, these volumetric changes are very minor with expansions or contractions lower than 2 % for all the cases, with remarkably low values for [DEMA][METS]. This points to a situation in which the cavities available in the ILs are able to fit the absorbed CO2 molecules without large changes in the fluids’ structuring as a consequence of the thermodynamics of the systems. The structuring of ILs+CO2 mixtures was initially analysed using RDFs for the centerof-mass of CO2 molecules around the corresponding ions, Figure 7. For all the studied ILs, the arrangement of CO2 molecules around the ammonium-based cations is very similar in spite of the size of the considered alkyl chains, with a first peak at roughly 5.5 Å, followed by two weaker bands, Figures 7a, 7c, 7e and 7g. The intensity of this first peak tends to decrease with increasing pressure in most of the cases. Regarding the CO2 arrangement around anions, for [Tf2N], Figure 7b, a wide band with a minor feature at short distance is inferred. In the case of [OTf], the distribution of CO2 molecules is dependent on the type of counterion, for ILs with [MTOA] cation a narrow peak with a shoulder is inferred, whereas for [DEMA] the intensity of the peaks decreases remarkably. These structural conclusions are refined when considering SDFs reported in Figure 8. SDFs for CO2 molecules around cations shows distributions around the whole cation structures, with the exception of [MTOA] in which the long alkyl chains are avoided by gas molecules and being specially remarked for the small and symmetrical [DEMA] cation. The distribution around [Tf2N] anion, Figure 8a, shows 11



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

CO2 concentration around the fluorine atoms and above the nitrogen atom, avoiding oxygen atoms. In the case of [METS] and [OTf], low density regions around the oxygen atoms are also inferred but with a densification upon anion fluorination in the vicinity of fluorine atoms, especially for ILs containing small counterions ([DEMA][OTf]). The strength of ion-CO2 interactions is quantified through the corresponding intermolecular interaction energies reported in Figure 9. The values of CO2 – ion interaction energies are an order of magnitude lower than ion-ion ones, Table 1, which is reasonable considering the mechanism of physisorption and are in the range of previous theoretical studies for ion – CO2 interactions.75 Cation-CO2 interactions are larger than anion-CO2 ones in most of the cases, with the exception of [DEMA][METS], which leads to the stronger anion-CO2 interactions. The effect of pressure, and thus of larger CO2 concentrations, on ionCO2 interactions is very minor for the studied range because most of the CO2 molecules are placed in the first solvation shell around the ions, Figure 7, and the increase of CO2 concentration does not hinder the interaction with the ions, Figure 8. Likewise, the absorption of CO2 molecules does not weaken remarkably the anion-cation interactions, the minor volumetric changes upon gas absorption, Table 3, does not lead to a rearrangement of ion-ion orientation and organization, and thus the strength of ion-ion intermolecular forces is maintained for the studied CO2 concentration range. The dynamics of CO2 molecules around the ions is analysed using the residence times reported in Table 4. CO2 molecules stay longer times around ions for ILs containing [DEMA] cation, which can be justified by the structure of the solvation spheres reported in Figure 8 being more continuous for [DEMA]-ILs than for other ILs and allowing the movement of CO2 molecules inside that region without leaving the solvation sphere around the ions. [BTMA][Tf2N] leads to the shortest residence times and [DEMA][METS]. Likewise, the increasing pressure (increasing CO2 concentration) decreases residence times showing that the largest concentration of CO2 molecules around the ions improves ion mobility through CO2 CO2 interactions. This is especially remarkable for [DEMA][METS], which showing the largest residence times also show the largest decrease with increasing CO2 concentration. The results for volumetric properties reported in Table 3 showed minor changes upon CO2 absorption, this effect may be quantified at the nanoscopic level through the cavity size distribution reported in Figure 10. Cavities distributions were calculated by creating a map of randomly distributed points in the simulation boxes and measuring for each point the distance to the edge of the closest atom defined by its van der Waals sphere. In this approach the complex geometry of the cavities is not considered and it may be considered as a first 12


Page 12 of 43

Page 13 of 43

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


approach but it provides information about fluid’s cavities and the changes upon gas absorption. Results in Figure 10 shows almost negligible changes in cavities distribution upon CO2 absorption for all the studied ILs, and thus the available cavities are able to fit absorbed gas molecules without remarkable changes, in agreement with the low expansion or contractions reported in Table 3 and the minor changes in ion-ion interactions reported in Figure S4 (Supporting Information). It is also remarkable that the cavity distribution for [DEMA]-containing ILs are very different to the other ILs, especially for [DEMA][METS], with cavities of smaller size than in the other ILs. On the contrary, ILs with larger cations ([BTMA] and [MTOA]) show larger cavities and cavities distribution extending towards larger sizes in comparison with narrow distributions for [DEMA]-ILs. Regarding the interaction between CO2 molecules, RDFs for CO2-CO2 pairs are reported in Figure 11. It should be remarked that the trend of CO2 molecules is almost negligible in the studied ILs, intermolecular interaction energies for CO2-CO2 interactions are lower than 1.5 kJ mol-1 for all the systems at the highest pressure (highest CO2 concentration). Therefore, although RDFs reported in Figure 11 show intense peaks at roughly 4.5 Å, the integration of these RDFs for the first solvation shell (first minima at roughly 6.8 Å) show that 1 to 2 CO2-CO2 molecules surround each central CO2, and the extension toward the second shell shows 3 to 5 molecules for distances up to 11 Å. These results discard the formation of regions rich in CO2 molecules, and confirm that they are evenly distributed around all the ions.

3.4. DSILs with CO2 The predicted solubilities of CO2 in the studied DSILs calculated according to COSMOthermX are reported in Figure S5 (Supporting Information). These results are in agreement with the behavior of pure ILs reported in Figure S3 (Supporting Information), i.e. DSILs rising from the combination of two ILs with high affinity for CO2 show high affinity for CO2 and vice versa. For example, the DSIL with highest CO2 affinity, [BTMA][MTOA][Tf2N][OTf], rises from the combination of ions in ILs [BTMA][Tf2N] and [MTOA][OTf], which are those with the highest CO2 solubilities, Figure S3 (Supporting Information). An analogous behavior is obtained for [DEMA][METS][OTf], which stands from the combination of ILs with poorest CO2 solubility. Nevertheless, the formation of DSIL does not reinforce the CO2 solubility in comparison with those in pure ILs, in fact the highest CO2 solubility in DSILs is lower than for the highest solubility in pure ILs, and thus the formation of DSILs decreases the capacity of CO2 capturing for the studied systems. 13



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

The changes in volumetric properties upon CO2 absorption are also very minor for the studied DSILs, with the only exception of [MTOA][DEMA][OTf], Table 5. For most of the DSILs, CO2 absorption is accompanied by volume expansions lower than 1.4 %, whereas for [MTOA][DEMA][OTf] it leads to volume contraction for the whole studied composition range. This behaviour shows that the cavities distribution available in pure ILs, which allowed an efficient inclusion of absorbed gas molecules without remarkable volume changes as previous discussed, are maintained when DSILs are formed. RDFs for CO2 molecules around ions in DSILs are reported in Figure 12. The comparison of these RDFs with those in pure ILs, Figure 7, shows excellent agreement, i.e. the arrangement of CO2 molecules around each ion is almost the same independently if the ion is part of a pure IL or if it is in a DSIL, which would justify the similarities between calculated gas solubilities in pure ILs and DSILs as reported in Figures S3 and S4 (Supporting Information). Moreover, the coordination numbers around each ion are almost the same for each ion, and they are also equal around anions and cations in each DSIL. Therefore, a very homogeneous distribution of absorbed molecules around all available ions is obtained in the DSILs. Nevertheless, intermolecular interaction energies between CO2 molecules and ions reported in Table 5 for most of the DSILs are slightly weaker than the values obtained in pure ILs, Figure 9. Therefore, a slightly lower affinity for CO2 molecules in DSILs is obtained when compared with those ILs forming them. Likewise, the minor changes in volumetric properties upon CO2 absorption in DSILs previously mentioned are also accompanied by very minor weakening in all anion – cation interactions for all the pairs when compared with the respective pure IL as reported in Table 6. This may be also justified considering the almost null changes in cavities distribution reported in Figure 13 when CO2 is absorbed by DSILs. The slight weakening of CO2-interactions in DSILs in comparison with those in pure ILs leads to a decrease in CO2 residence time around most of the ions, with some exceptions around [DEMA] cations, Tables 4 and 7. Therefore, CO2 molecules are slightly more mobile in the DSILs than in the corresponding ILs, although these differences are minor.

3.5. Experimental CO2 solubility in pure ILs and DSILs. Room temperature CO2 solubilities were experimented at pressures up to 10 bars via MSA as described in details in section 2.2. Initially pure ILs were measured for their CO2 solubility capacity, which was followed by the CO2 solubility experiments of 6 possible DSILs combinations obtained via 1:1 molar mixing ratios. Both the ILs and DSILs are compared among each other to see the effect of the cations and anions. Experimental density values are 14


Page 14 of 43

Page 15 of 43

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


given in Table 8. Regarding to solubility measurements, experimental results showed only physisorption behaviour for both the ILs and DSILs and no trace of chemisorption was observed. Moreover, there has been no unrealistic and unusual increase in buoyancy during the weighing measurements at the sorption process, which can be concluded as the elimination of the swelling effect at the mentioned experimental pressure conditions for these ILs and DSILs. According to the results for the experimental CO2 solubility given in Table 8 and Figure 14; and further details of the sorption data in supporting information Figure S6-S9. When pure ILs are compared, [MTOA][OTf] has the highest solubility with 0.701 mmol/g of CO2. This is followed with [DEMA][METS] with 0.660 mmol/g, [DEMA][OTf] with 0.631 mmol/g and the least performing one as [BTMA][Tf2N] with 0.610 mmol/g. The ranking of the solubilities can be written as [MTOA][OTf] > [DEMA][METS] > [DEMA][OTf] > [BTMA][Tf2N]. This result shows an agreement with the simulation data presented in section 3.3 regarding to the highest CO2 solubility performance ILs as [MTOA][OTf]. The change in solubilities is not remarkable in terms of experimental numerical values but there is 10% decrease in the CO2 solubility when [MTOA] cation was replaced with [DEMA]. When the [DEMA] cation is kept same and anion is altered from [METS] to [OTf], [METS] resulted in higher anion-CO2 attraction than the [OTf] and [DEMA][METS] had only 5.9% less CO2 sorption capacity in comparison with [MTOA][OTf]. Despite the high affinity of CO2 with fluorine atoms, CO2 solubility in [BTMA][Tf2N] appeared to be the least in the measurements with 13% reduction in CO2 capacity in comparison with [MTOA][OTf]; which supports the idea of cation-CO2 interactions are larger than anion-CO2 as mentioned in section 3.3. On the other hand, CO2 solubilities of 6 DSILs were given in Table 8 and Figure 14; and further details of the sorption data in supporting information Figure S10-S15. The results obtained for the binary mixtures of ILs are in agreement with both the experimental data for pure ILs and the simulation data that shows DSILs rising from the combination of two ILs with high affinity for CO2 show high affinity for CO2 and vice versa, with the slight exception of one mixture out of the investigated six. The highest CO2 solubility DSIL obtained by the 2 highest CO2 solubility capable pure ILs as [MTOA][OTf] + [DEMA][METS] with 0.669 mmol/g, which is 4.6% less than the highest CO2 solubility data obtained by [MTOA][OTf]. Keeping [MTOA][OTf]+[DEMA][METS] at the top of the list, the overall ranking of the DSILs are as follows:

15



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 16 of 43

[MTOA][OTf]+[DEMA][METS]

>

[MTOA][OTf]+[BTMA][Tf2N]

>

[DEMA][METS]+[DEMA][OTf]

>

[MTOA][OTf]+[DEMA][OTf]

>

[DEMA][OTf]+[BTMA][Tf2N] > [DEMA][METS]+[BTMA][Tf2N]. All the DSILs appeared within the expected feasibility regions considering that the lowest and the highest possible solubility data is restricted by the pure ILs constituents of the obtained DSILs with the exception of only [MTOA][OTf]+[DEMA][OTf]. It was expected to obtain the CO2 solubility of [MTOA][OTf]+[DEMA][OTf] between 0.701 mmol/g (CO2 solubility data for [MTOA][OTf]) and 0.631 mmol/g (CO2 solubility data for [DEMA][OTf]). However experimental data for the [MTOA][OTf]+[DEMA][OTf] DSIL fall 1.2% less than the pure [MTOA][OTf].

4. CONCLUSIONS The properties of six different complex double salt ionic liquids based on alkylammonium cations are analysed as well as those for the corresponding pure ionic liquids, for neat and CO2 absorbed mixtures, using classical molecular dynamics simulations and the results were supported with the experimental densities and solubilities obtained from state-of-the-art gravimetric sorption experiments on both pure ILs and DSILs. The structural and energetic properties of the double salt ionic liquid are strongly related with those of corresponding pure ionic liquids because cations belonging to the same molecular family are considered. Mixing two neat ionic liquids for forming a double salt ionic liquid maintains most of the properties of the involved neat ionic liquids, although with some changes in the strength of anion-cation interactions. Regarding the behaviour of double salt ionic liquids upon CO2 absorption, it should be remarked that they also maintain most of the properties of the forming neat ionic liquids regarding CO2 affinity, CO2 arrangement around ions and minor volume changes. Therefore, the properties of the studied double salt ionic liquids should be close to ideal in the thermodynamic behaviour regarding forming neat ionic liquids, and the affinity for CO2 molecules and the mechanisms of absorption are not changed or improved upon the formation of the double salt ionic liquid. The similar molecular characteristics of the ions involved in double salt ionic liquid make that although up to four different ions are considered, the behaviour is not very different to that in pure ionic liquids. Therefore, more dissimilar ions are required for changing remarkably the properties of the double salt ionic liquids when compared with the constituent pure ionic liquids both from the thermodynamics and structural viewpoints. On the other hand, experimental results show the CO2 capture performance and

16


Page 17 of 43

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


viability of the pure ILs and DSILs for low pressure applications up to 10 bars and low temperatures at 25 °C. Results showed a total physisorption behaviour, which makes both pure ILs and DSILs suitable for pressure driven CO2 capture processes that might lead to reduction in solvent regeneration cost when compared with the current state of the art amine based systems. results obtained for the binary mixtures of ILs are in agreement with both the experimental data for pure ILs and the simulation data that shows DSILs rising from the combination of two ILs with high affinity for CO2 show high affinity for CO2 and vice versa. There has not been a remarkable change in the maximum CO2 sorption performance in the case of DSILs, this could be explained due to the similar salt effect as the cations are from the same family of ILs, which shall be altered by using vastly different cations in order to stimulate the CO2 physisorption by increasing the excess volume of the DSILs when two vastly different ILs are mixed, in this way inducing a porosity in the liquid.76

ASSOCIATED CONTENT Supporting Information Table S1 (force field parameterization), Table S2 (simulation boxes), Figure S1 (relationship for physicochemical properties of DSILs and linear combinations of properties for pure ILs), Figure S2 (radial distribution functions in DSILs), Figure S3 (CO2 solubility in ILs), Figure S4 (anion-cation intermolecular interaction energy in IL-CO2 systems), and Figure S5 (CO2 solubility in DSILs by COSMOthermX method), Figures S6-S14 (Experimental CO2 absorption and desorption data for ILs and DSILs). ACKNOWLEDGEMENTS This work was made possible by the support of Qatar National Research Fund, National Priorities Research Program grant (NPRP 6 – 330 – 2 – 140), Ministerio de Economía y Competitividad (Spain, project CTQ2013-40476-R) and Junta de Castilla y León (Spain, project BU324U14). We also acknowledge The Foundation of Supercomputing Center of Castile and León (FCSCL, Spain) and Computing and Advanced Technologies Foundation of Extremadura (CénitS, LUSITANIA Supercomputer, Spain) for providing supercomputing facilities. The statements made herein are solely the responsibility of the authors.

17



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

REFERENCES (1) MacFarlane, D. R.; Tachikawa, N.; Forsyth, M.; Pringle, J. M.; Howlett, P. C.; Elliott, G. D.; Davis, J. H.; Watanabe, M.; Simon, P.; Angell, C. A. Energy Applications of Ionic Liquids. Energy Environ. Sci. 2014, 7, 232-250. (2) Lei, Z.; Dai, C.; Chen, B. Gas Solubility in Ionic Liquids. Chem. Rev. 2014, 114, 1289-1326. (3) Somers, A.; Howlett, P. C.; MacFarlane, D. R.; Forsyth, M. A Review of Ionic Liquid Lubricants. Lubricants 2013, 1, 3-21. (4) Ho, T. D.; Zhang, C.; Hantao, L. W.; Anderson, J. L. Ionic Liquid in Analytical Chemistry: Fundamentals, Advances, and Perspectives. Anal. Chem. 2014, 86, 262-285. (5) Marrucho, I. M.; Branco, L. C.; Rebelo, L. P. N. Ionic Liquids in Pharmaceutical Applications. Ann. Rev. Chem. Biomol. Eng. 2014, 5, 527-546. (6) Federov, M. V.; Kornyshev, A. A. Ionic Liquids at Electrified Interfaces. Chem. Rev. 2014, 114, 2978-3036. (7) Aparicio, S.; Atilhan, M.; Karadas, F. Thermophysical Properties of Pure Ionic Liquids: Review of Present Situation. Ind. Eng. Chem. Res. 2010, 49, 9580-9595. (8) Wang, X.; Chi, Y.; Mu, T. A Review on the Transport Properties of Ionic Liquids. J. Mol. Liq. 2014, 193, 262-266. (9) Paduszynski, K.; Domanska, U. Viscosity of Ionic Liquids: An Extensive Database and a New Group Contribution Model Based on a Feed-Forward Artificial Neural Network. J. Chem. Inf. Model. 2014, 54, 1311-1324. (10) Freitas, A.; Shimizu, K.; Lopes, J. N. C. Complex Structure of Ionic Liquids. Molecular Dynamics Studies with Different Cation-Anion Combinations. J. Chem. Eng. Data 2014, 59, 3120-3129. (11) Zhan, S.; Brehm, M.; Brüssel, M.; Holloczki, O.; Kohagen, M.; Lhmann, S.; Malberg, F.; Pensado, A. S.; Schöppke, M.; Weber, H.; Kirchner, B. Understanding Ionic Liquids from Theoretical Methods. J. Mol. Liq. 2014, 192, 71-76. (12) Cevasco, G.; Chiappe, C. Are Ionic Liquids a Proper Solution to Current Environmental Challenges? Green Chem. 2014, 16, 2375-2385. (13) Egorova, K. S.; Anaikov, V. P. Toxicity of Ionic Liquids: Eco(cyto)activity as Complicated, but Unavoidable Parameter for Tak-Specific Optimization. ChemSusChem. 2014, 7, 336-360. (14) Neumann. J.; Steudte, S.; Cho, C. W.; Thöming, J.; Stolte, S. Biodegradability of 27 Pyrrolidinium, Morpholinium, Piperidinium, Imidazolium and Pyridinium Ionic Liquid Cations Under Aerobic Conditions. Green Chem. 2014, 16, 2174-2184. (15) Chen, L.; Sharifzadeh, M.; MacDowell, N., Welton, T.; Shah, N.; Hallett, J. P. Inexpensive Ionic Liquids: [HSO4]- - Based Solvent Production at Bulk Scale. Green Chem. 2014, 16, 3098-3106. (16) Daillo, A. O.; Len, C.; Morgan, A. B., Marlair, G. Revisiting Physico-Chemical Hazards of Ionic Liquids. Separ. Purif. Sci. Technol. 2012, 97, 228-234. (17) McCrary, P. D.; Rogers, R. D. 1-Ethyl-3-methylimidazolium Hexafluorophosphate: from Ionic Liquid Prototype to Antitype. Chem. Commun. 2013, 49, 6011-6014. (18) Plechkova, N. V.; Seddon, K. R. Applications of Ionic Liquids in the Chemical Industry. Chem. Soc. Rev. 2008, 37, 123-150. (19) Chatel, G.; Pereira, J. F. B.; Debbeti, C.; Wang, H.; Rogers, R. D. Mixing Ionic Liquids - "Simple Mixtures" or "Double Salts". Green Chem. 2014, 16, 2051-2083. (20) Niedermeyer, H.; Hallett, J. P.; Villar – Garcia, I. J.; Hunt, P. A.; Welton, T. Mixtures of Ionic Liquids. Chem. Soc. Rev. 2012, 41, 7780-7802. (21) Clough, M. T.; Crick, C. R.; Gräsvick, J.; Hunt, P. A.; Niedermeyer, H.; Welton, T.; Whitaker, O. P. A Physicochemical Investigation of Ionic Liquid Mixtures. Chem. Sci. 2015, 6, 1101-1114. (22) Lopes, J. N. C.; Cordeiro, T. C.; Esperanca, J. N. S. S.; Guedes, H. J. R.; Huq, S.; Rebelo, L. P. N.; Seddon, K. R. Deviations from Ideality in Mixtures of Two Ionic Liquids Containing a Common Ion. J. Phys. Chem. B 2005, 109, 3519-3525. (23) Stoppa, A.; Buchner, R.; Hefter, G. How Ideal are Mixtures of Room-Temperature Ionic Liquids? J. Mol. Liq. 2010, 153, 46-51. (24) Navia, P.; Troncoso, J.; Romani, L. Viscosities for Ionic Liquid Binary Mixtures with a Common Cation. 2008, 37, 677-688. (25) Aparicio, S.; Atilhan, M. Mixed Ionic Liquids: The Case of Pyridinium-Based Fluids. J. Phys. Chem. B. 2012, 116, 2626-2637. (26) Serra-Moreno, J.; Jeremias, S.; Moretti, A.; Panero, S.; Passerini, S.; Scrosati, B.; Appetecchi, G. B. Ionic Liquid Mixtures with Tunable Physicochemical Properties. Electrochim. Acta 2015, 151, 599-608. (27) Shimizu, K.; Tariq, M.; Rebelo, L. P. N.; Lopes, J. N. C. Binary Mixtures of Ionic Liquids with a Common Ion Revisited: A Molecular Dynamics Simulation Study. J. Mol. Liq. 2010, 153, 52-56. 18


Page 18 of 43

Page 19 of 43

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


(28) Paschek, D.; Golub, B.; Ludwig, R. Hydrogen Bonding in a Mixture of Protic Ionic Liquids: a Molecular Dynamics Simulation Study. Phys. Chem. Chem. Phys. 2015, 17, 8431-8440. (29) Karadas, F.; Atilhan, M.; Aparicio, S. Review on the Use of Ionic Liquids (ILs) as Alternative Fluids for CO2 Capture and Natural Gas Sweetening. Energ. Fuel. 2010, 24, 5817-5828. (30) Ramdin, M.; de Loos, T. W.; Vlugt, T. J. H. State-of-the-Art of CO2 Capture with Ionic Liquids. Ind. Eng. Chem. Res. 2012, 51, 8149-8177. (31) Zhou, Y.; Liu, J.; Xiao, M.; Meng, Y.; Sun, L. Designing Supported Ionic Liquids (ILs) within Inorganic Nanosheets for CO2 Capture Applications. ACS Appl. Mater. Interfaces. 2016, 8, 5547-5555. (32) Finotello, A.; Bara, J. E.; Narayan, S.; Camper, D.; Noble, R. Ideal Gas Solubility Selectivities in a Binary Mixture of Room-Temperature Ionic Liquids. J. Phys. Chem. B 2008, 112, 2335-2339. (33) Shifflett, M.; Yokozeki, A. Phase Behavior of Carbon Dioxide in Ionic Liquids: [emim][Acetate], [emim][Trifluoroacetate], and [emim][Acetate]+[emim][Trifluoroacetate] Mixtures. J. Chem. Eng. Data 2009, 54, 108-114. (34) Lei, Z.; Han, J.; Zhang, B.; Li, Q.; Zhu, J.; Chen, B. Solubility of CO2 in Binary Mixtures of RoomTemperature Ionic Liquids at High Pressures. J. Chem. Eng. Data 2012, 57, 2153-2159. (35) Pinto, A.; Rodríguez, H.; Colón, Y. J., Arce, A.; Arce, A.; Soto, A. Absorption of Carbon Dioxide in Two Binary Mixtures of Ionic Liquids. Ind. Eng. Chem. Res. 2013, 52, 5975-5984. (36) Pinto, A. M.; Rodríguez, H.; Arce, A.; Soto, A. Carbon Dioxide Absorption in the Ionic Liquid 1Ethylpyridinium Ethylsulfate and its Mixtures with Another Ionic Liquid. Int. J. Greenh. Gas Con. 2013, 18, 296-304. (37) Pinto, A.; Rodríguez, H.; Arce, A.; Soto, A. Combined Physical and Chemical Absorption of Carbon Dioxide in a Mixture of Ionic Liquids. J. Chem. Thermodyn. 2014, 77, 197-205. (38) Ranke, J.; Stolte, S.; Störman, R.; Arning, J.; Jastorff, B. Design of Sustainable Chemical Products - The Example of Ionic Liquids. Chem. Rev. 2007, 107, 2183-2206. (39) Socha, A. M., Parthasarathi, R.; Shi, J.; Pattathil, S.; Whyte, D., Bergeron, M.; George, A.; Tran, K.; Stavila, V.; Venkatachalam, S.; Hahn, M. G.; Simmons, B. A.; Singh, S. Efficient Biomass Pretreatment Using Ionic Liquids Derived from Lignin and Hemicellulose. Proc. Nat. Acad. Sci. 2014, 111, E3587-E3595. (40) Li. H.; Zhao, G.; Liu, F.; Zhang, S. Physicochemical Characterization of MFm- - Based Ammonium Ionic Liquids. J. Chem. Eng. Data 2013, 58, 1505-1515. (41) Blundell, R. K.; Licence, P. Quaternary Ammonium and Phosphonium Based Ionic Liquids: a Comparison of Common Anions. Phys. Chem. Chem. Phys. 2014, 16, 15278-15288. (42) Lethesh, K. C.; Dehaen, W.; Binnemans, K. Base Stable Quaternary Ammonium Ionic Liquids. RSC Adv. 2014, 4, 4472-4477. (43) Stolte, S.; Matzke, M.; Arning, J.; Böschen, A.; Pitner, W. R.; Welz-Biermann, U.; Jastorff, B.; Ranke, J. Effects of Different Head Groups and Functionalized Side Chains on the Aquatic Toxicity of Ionic Liquids. Green Chem. 2007, 9, 1170-1179. (44) Biczak, R.; Pawlowska, B.; Balczewski, P.; Rychter, P. The Role of the Anion in the Toxicity of Imidazolium Ionic Liquids. J. Hazard. Mater. 2014, 274, 181-190. (45) Machanova, K.; Boisset, A.; Sedlakova, Z.; Anouti, M.; Bendova, M.; Jacquemin, J. Bis{(trifluoromethyl)sulfonyl}imide Ionic Liquids: Volumetric and Transport Properties. J. Chem. Eng. Data 2012, 57, 227-2235. (46) Chotaray, P. K.; Gardas, R. L. Thermophysical Properties of Ammonium and Hydroxylammonium Protic Ionic Liquids. J. Chem. Thermodyn. 2014, 72, 117-124. (47) Bhattacharjee, A.; Coutinho, J. A. P.; Freire, M. G.; Carvalho, P. J. Thermophysical Properties of Two Ammonium-Based Protic Ionic Liquids. J. Sol. Chem. 2015, 44, 703-717. (48) Fumino, K.; Bonsa, A. M., Golub, B.; Paschek, D.; Ludwig, R. Non-Ideal Mixing Behavior of Hydrogen Bonding in Mixtures of Protic IonicLiquids. ChemPhysChem 2014, 16, 299-304. (49) Aparicio, S.; Atilhan, M. A Computational Study on Choline Benzoate and Choline Salicylate Ionic Liquids in the Pure State and After CO2 Absorption. J. Phys. Chem. C 2012, 116, 9171-9185 (50) Chang, T. M.; Dang, L. X.; Devanathan, R.; Dupuis, M. Structure and Dynamics of N,N-Diethyl-Nmethylammonium Ionic Liquid, Neat and with Water, from Molecular Dynamics Simulations. J. Phys. Chem. A 2010, 114, 12764-12774. (51) Logotheti, G. E.; Ramos, J.; Economou, I. G. Molecular Modeling of Imidazolium – Based [Tf2N-] Ionic Liquids: Microscopic Structure, Thermodynamic and Dynamic properties, and Segmental Dynamics. J. Phys. Chem. B 2009, 113, 7211-7224. (52) Sunda, A. P.; Venkatnathan, A. Molecular Dynamics Simulations of Triflic Acid and Triflate Ion / Water Mixtures: A Proton Conducting Electrolytic Component in Fuel Cells. J. Comput. Chem. 2011, 32, 33193328.

19



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

(53) Breneman, C. M., Wiberg, K. B. Determining Atom-Centered Monopoles from Molecular Electrostatic Potentials. The Need for High Sampling Density in Formamide Conformational Analysis. J. Comput. Chem., 1990, 11, 361-373. (54) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; et al. Gaussian 09, revision D.01; Gaussian, Inc.: Wallingford, CT, 2010. (55) Shi, W.; Maginn, E. Atomistic Simulation of the Absorption of Carbon Dioxide and Water in the Ionic Liquid 1-n-Hexyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]. J. Phys. Chem. B 2008, 112, 2045−2055. (56) Martínez, L.; Andrade, R.; Birgin, E. G.; Martínez, J. M. Packmol: A Package for Building Initial Configurations for Molecular Dynamics Simulations. J. Comput. Chem. 2009, 30, 2157-2164. (57) Eckert, F.; Klamt. A. COSMOthermX, Version C21_0111; COSMOlogic GmbH & Co. KG: Leverkusen, Germany, 2010. (58) Zhang, X.; Liu, Z.; Wang, W. Screening of Ionic Liquids to Capture CO2 by COSMO-RS and Experiments. AIChE J. 2008, 54, 2717-2728. (59) Gonzalez-Miguel, M.; Teja, M.; Ethier, A. L.; Flack, K.; Switzer, J. R.; Biddinger, E. J., Pollet, P.; Palomar, J.; Rodriguez, F.; Eckert, C. A.; Liotta, C. L. COSMO-RS Studies: Structure–Property Relationships for CO2 Capture by Reversible Ionic Liquids. Ind. Eng. Chem. Res. 2012, 51, 16066-16073. (60) Mortazavi-Manesh, S.; Satyro, M. A.; Marriott, R. A. Screening Ionic Liquids as Candidates for Separation of Acid Gases: Solubiliy of Hydrogen Sulfide, Methane, and Ethane. AIChE J. 2013, 59, 2993-3005. (61) Lyubartsev, A. P.; Laaksonen, A. MDynaMix - A Scalable Portable Parallel MD Simulation Package for Arbitrary Molecular Mixtures. Comput. Phys. Commun. 2000, 128, 565-589.. (62) Essmann, U. L.; Perera, M. L.; Berkowitz, T.; Darden, H.; Lee,H.; Pedersen, L. G. A Smooth Particle Mesh Ewald Method. J. Chem. Phys. 1995, 103, 8577-8593. (63) Tuckerman, M.; Berne, B. J.; Martyna, G. J. Reversible Multiple Time Scale Molecular Dynamics. J. Chem. Phys. 1992, 97, 1990-2001 (64) Karadas, F.; Köz, B.; Jacquemin, J.; Deniz, E.; Rooney, D.; Thompson, J.; Yavuz, C. T.; Khraisheh, M.; Aparicio, S.; Atihan, M. High Pressure CO2 Absorption Studies on Imidazolium-Based Ionic Liquids: Experimental and Simulation Approaches. Fluid Phase Equilibr. 2013, 351, 74-86. (65) Patel, H. A.; Karadas, F.; Canlier, A.; Park, J.; Deniz, E.; Jung, Y.; Atilhan, M.;. Yavuz, C. T. High Capacity Carbon Dioxide Adsorption by Inexpensive Covalent Organic Polymers. J. Mat. Chem. 2012, 22, 8431-8437. (66) Karadas, F.; El-Faki, H.; Deniz, E.; Yavuz, C.T.; Aparicio, S.; Atilhan, M. CO2 Adsorption Studies on Prussian Blue Analogues. Micropor. Mesopor. Mat. 2012, 162, 91-97. (67) Kilaru, P.; Baker, G. A.; Scovazzo, P. Density and Surface Tension Measurements of Imidazolium-, Quaternary Phosphonium-, and Ammonium-Based Room-Temperature Ionic Liquids: Data and Correlations. J. Chem. Eng. Data 2007, 52, 2306-2314. (68) Bhhattacharjee, A.; Luís, A.; Santos, J. H.; Lopes-da-Silva, J. A.; Freire, M. G.; Carvalho, P. J.; Coutinho, J. A. P. Thermophisical Properties of Sulfonium- and Ammonium-Based Ionic Liquids. Fluid Phase Equilibr. 2014, 381, 36-45. (69) Jacquemin, J.; Husson, P.; Padua, A. A. H.; Majer, V. Density and Viscosity of Several Pure and WaterSaturated Ionic Liquids. Green Chem. 2006, 8, 172-180. (70) Nguyen, N. L.; Rochefort, D. Electrochemistry of Ruthenium Dioxide Composite Electrodes in Diethylmethylammonium – Triflate Protic Ionic Liquid and its Mixtures with Acetonitrile. Electrochim. Acta 2014, 147, 96-103. (71) Gupta, K. M.; Jiang, J. Systematic Investigation of Nitrile Based Ionic Liquids for CO2 Capture: A Combination of Molecular Simulation and ab Initio Calculation. J. Phys. Chem. C 2014, 118, 3110-3118. (72) Lourenco, T. C.; Coelho, M. F. C.; Ramalho, T. C.; van der Spoel, D.; Costa, L. T. Insights on the Solubility of CO2 in 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide from the Microscopic Point of View. Environ. Sci. Technol. 2013, 47, 7421-7429. (73) Huang, X.; Margulis, C. J.; Li, Y., Berne, B. J. Why Is the Partial Molar Volume of CO2 So Small When Dissolved in a Room Temperature Ionic Liquid? Structure and Dynamics of CO2 Dissolved in [Bmim+] [PF6]. J. Am. Chem. Soc. 2005, 127, 17842-17851. (74) Gallagher, P. M.; Coffey, M. P.; Krukonis, V. J.; Klasutis, N. In Supercritical Fluid Science and Technology; Johnson, K. P., Penninger, J. M. L., Eds.; American Chemical Society (ACS):Washington, D.C., 1989; ACS Symposium Series, Vol. 406, Chapter 22, pp 334-354. (75) Izgorodina, E. I.; Hodgson, J. L.; Weis, D. C.; Pas, S. J.; MacFarlane, D. R. Physical Absorption Of CO2 in Protic and Aprotic Ionic Liquids: An Interaction Perspective. J. Phys. Chem. B 2015, 119, 11748-11759.

20


Page 20 of 43

Page 21 of 43

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


(76) Carvalho, P.J., Kurnia, K.A.; Coutinho, J.A.P. Dispelling Some Myths About the CO2 Solubility in Ionic Liquids. Phys. Chem. Chem. Phys. 2016, 18, 14757-14771.

21



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 22 of 43

Table 1. Properties for pure ILs calculated from molecular dynamics simulations at 298 K and 1 bar. Density, ρ. Residence time, tres, of anion (A) around cation (C); center of mass self-diffusion coefficients for A, D(A), and C, D(C), calculated from mean square displacements and Einstein's relationship; dynamic viscosity, η, calculate using Green-Kubo method; intermolecular interaction energy, Einter, as the sum of Lennard-Jones and coulombic contributions. Error for properties obtained from molecular dynamics are reported only for D and η, for the remaining properties errors are lower than the reported data. Parenthesized values for self-diffusion coefficients show the slope of log-log plots of mean square displacement as a function of simulation time, β parameter, for the time interval used in the calculation. -Einter / kJ×mol-1

IL [BTMA][Tf2N]

-3

ρ / g cm 1.40

1.3747

a

1.3942

b

1012 D(A)

1012 D(C)

2

2

-1

η / mPa s

-1

tres / ps

/ m ×s

/ m ×s

134

18±1.1

17±1.0

(0.99)

(0.98)

3±0.2

4±0.2

(0.96)

(0.96)

19±1.1

18±1.1

(0.98)

(0.98)

31±1.7

30±1.6

C-C

A-A

A-C

741

752

-2012

428±17.1

302

399

-1186

148±6.0

1400

1365

-3307

54±2.3

1201

1201

-2914

138±5.5 112

c

1.3918 c 1.349 d [MTOA][OTf]

1.05 1.095

[DEMA][METS]

1.15 1.135

[DEMA][OTf]

183 d

103 d

1.30 1.291

76 d

(1.00) a

b

c

(1.00)

42.2

e

d

Experimental data from: Ref. 67; Ref. 68; Ref. 69; this work; e Ref. 70.

22


Page 23 of 43

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


Table 2. Properties for DSILs calculated from molecular dynamics simulations at 298 K and 1 bar. Density, ρ; residence time, tres, of anion (A1 or A2) center of mass around cations (C1 or C2) center of mass; center of mass self-diffusion coefficients for A, D(A1 or A2), and C, D(C1 or C2); dynamic viscosity, η; A-C intermolecular interaction energies, Einter, as the sum of Lennard-Jones and coulombic contributions. tres calculated as indicated in Table 1 caption with R obtained from the first minima in radial distribution functions reported in Figure 4. C1, C2 and A1, A2 stand for the first and second cations, and for the first and second anions, with numbering defined in the ordering of the DSIL name indicated in the first column (see Table S2, Supporting Information, for labeling details). The parenthesized values in the density and viscosity columns shows the data calculated from the linear combination of density or viscosity data for those pure ILs (Table 1) leading to each DSIL. Error for properties obtained from molecular dynamics are reported only for D and η, for the remaining properties errors are lower than the reported data. Parenthesized values for self-diffusion coefficients show the slope of log-log plots of mean square displacement as a function of simulation time, β parameter, for the time interval used in the calculation. 1012D / m2×s-1

tres / ps DSIL [BTMA][MTOA][Tf2N][OTf]

ρ / g cm-3

A1-C1

A1-C2

A2-C1

A2-C2

A1

A2

C1

C2

η / mPa s

A1-C1

A1-C2

A2-C1

A2-C2

1.16

144

166

154

162

13±0.7 (0.96)

14±0.7 (0.97)

13±0.7 (0.97)

12±0.6 (0.97)

257±10.1 (283)

779

729

861

714

106

109

100

113

21±1.1 (0.97)

21±1.1 (0.98)

21±1.1 (0.97)

21±1.1 (0.97)

117±4.9 (143)

1228

1174

1243

1343

29

27

27

28

25±1.3 (0.98)

25±1.3 (0.98)

25±1.3 (0.98)

25±1.3 (0.98)

77±3.0 (96)

1199

1157

1184

1238

118

109

125

97

16±1.0 (0.98)

18±1.0 (0.98)

14±0.7 (0.97)

18±1.0 (0.98)

146±6.0 (288)

794

933

794

996

131

124

–

–

23±1.2 (0.98)

–

22±1.1 (0.98)

24±1.2 (0.98)

132±5.5 (241)

1554

1815

–

–

81

–

89

–

26±1.3 (0.99)

26±1.3 (0.98)

27±1.4 (0.98)

–

92±4.0 (101)

3082

–

3051

–

(1.23) [BTMA][DEMA][Tf2N][METS]

1.33 (1.28)

[BTMA][DEMA][Tf2N][OTf]

1.40 (1.35)

[MTOA][DEMA][OTf][METS]

1.06 (1.10)

[MTOA][DEMA][OTf]

1.10 (1.18)

[DEMA] [METS] [OTf]

-Einter / kJ×mol-1

1.22 (1.23)

23



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 24 of 43

Table 3. Volumetric properties of IL + CO2 mixtures at 298 K as a function of pressure, p, and CO2 mole fraction, xCO2. ρ stands for density, and % Vexp for volume change upon CO2 absorption according to the criterion by Gallagher et al.74 [BTMA][Tf2N] -3

p / bar

xCO2

ρ / g cm

1 4 7 10

0.02 0.08 0.13 0.19

1.45 1.47 1.51 1.49

%V

[MTOA][OTf] exp

-1.13 -1.35 -1.79 -1.23

-3

xCO2

ρ / g cm

0.03 0.12 0.20 0.27

1.05 1.03 1.01 1.04

[DEMA][METS]

%V

exp

0.93 1.88 1.12 1.19

-3

xCO2

ρ / g cm

0.02 0.06 0.11 0.15

1.15 1.14 1.17 1.17

%V

[DEMA][OTf] exp

0.42 0.12 0.42 0.44

xCO2

ρ / g cm-3

% Vexp

0.01 0.04 0.07 0.10

1.30 1.28 1.27 1.27

0.42 0.87 0.93 1.20

Table 4. Residence time, tres, of CO2 molecules around cations (C) or anions (A) of IL + CO2 mixtures at 298 K as a function of pressure. The corresponding CO2 mole fractions for each pressure are reported in Table 3

tres / ps [BTMA][Tf2N] P / bar around C

[MTOA][OTf]

[DEMA][METS]

[DEMA][OTf]

around A

around C

around A

around C

around A

around C

around A

1 4

32 31

31 30

37 33

36 32

62 57

57 51

52 51

45 44

7 10

30 29

29 28

32 27

31 25

51 40

45 38

49 43

42 37

24


Page 25 of 43

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


Table 5. Density, ρ, % Vexp for volume change upon CO2 absorption according to the criteria by Gallagher et al.,74 % Vexp, intermolecular interaction energies, Einter, as the sum of Lennard-Jones and coulombic contributions, between CO2 molecules and the corresponding ions, with numbering defined in the ordering of the DSIL name indicated in the first column (see Table S1, Supporting Information, for labeling details), for DSIL + CO2 mixtures at 298 K as a function of pressure and CO2 mole fraction -Einter / kJ mol-1 DSIL [BTMA][MTOA][Tf2N][OTf]

[BTMA][DEMA][Tf2N][METS]



[MTOA][DEMA][OTf]

[DEMA] [METS] [OTf]

p / bar

xCO2

ρ / g cm-3

% Vexp

CO2-C1

CO2-C2

CO2-A1

CO2-A2

1

0.02

1.17

0.18

5

13

3

2

4

0.12

1.16

0.28

7

17

4

2

7

0.20

1.17

0.55

7

18

4

3

10

0.27

1.18

0.75

9

18

6

5

1

0.01

1.33

0.13

10

9

1

2

4

0.06

1.32

0.53

12

10

5

3

7

0.11

1.31

0.97

13

13

8

4

10

0.15

1.31

1.49

16

10

10

10

1

0.01

1.39

0.11

13

7

5

1

4

0.06

1.38

0.39

13

11

7

3

7

0.11

1.38

0.96

13

11

11

3

10

0.15

1.37

0.99

14

13

12

5

1

0.02

1.07

-0.52

16

6

1

1

4

0.10

1.05

-0.35

20

6

3

3

7

0.17

1.06

0.02

21

7

2

2

10

0.23

1.06

0.46

21

9

5

5

1

0.02

1.10

-2.73

16

5

–

1

4

0.10

1.11

-2.67

18

5

–

1

7

0.17

1.10

-2.05

20

6

–

2

10

0.24

1.11

-1.99

22

8

–

2

1

0.01

1.22

0.10

21

–

4

1

4

0.05

1.22

0.37

24

–

7

2

7

0.08

1.21

0.70

24

–

9

7

10

0.12

1.22

0.82

25

–

11

9

25



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 26 of 43

Table 6. Percentage variation in ion – ion intermolecular interaction energy, % ∆Einter, in DSIL upon CO2 absorption at 298 K and 10 bar, for CO2 mole fractions reported in Table 5 % ∆Einter a DSIL

A1-C1

A1-C2

A2-C1

A2-C2

[BTMA][MTOA][Tf2N][OTf]

-2

-2

-4

-1


-2

-4

-5

-1


-5

-1

-1

-4

-2

-1

-1

-3

-1

-1

–

–

[MTOA][DEMA][OTf][METS] [MTOA][DEMA][OTf] [DEMA] [METS] [OTf] a

-2 – -2 – % ∆ = 100 , − , , , where , and , stand for the

corresponding in DSIL and DSIL with CO2 systems, respectively. Table 7. Residence time, tres, of CO2 molecules around cations (C1 or C2) or anions (A1 or A2) of DSIL + CO2 mixtures at 298 K. All the values calculated for 10 bar with the corresponding CO2 mole fractions reported in Table 5. C1, C2 and A1, A2 stand for the first and second cations, and for the first and second anions, with numbering defined in the ordering of the DSIL name indicated in the first column (see Table S1, Supporting Information, for labeling details) tres / ps DSIL

around C1

around C2

around A1

around A2


24

21

22

25


27

29

26

27


42

45

39

37


39

47

33

39

[MTOA][DEMA][OTf]

23

27

25

–

[DEMA] [METS] [OTf]

55

–

45

46

Table 8. Maximum CO2 solubility data obtained from experiments (ranked in descending order) IL or DSIL [MTOA][OTf] [DEMA][METS] + [MTOA][OTf] [DEMA][METS] [MTOA][OTf] + [BTMA][Tf2N] [DEMA][METS] + [DEMA][OTf] [DEMA][OTf] [DEMA][OTf] + [MTOA][Otf] [BTMA][Tf2N] + [DEMA][OTf] [BTMA][Tf2N] + [DEMA][METS] [BTMA][Tf2N]

density / g cm-3 1.095 1.106 1.135 1.196 1.211 1.291 1.200 1.196 1.292 1.349

maximum sorption / mmol CO2 grIL-1 0.701 0.669 0.660 0.649 0.643 0.631 0.624 0.616 0.611 0.610

26


Page 27 of 43

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


Figure Captions.

Figure 1. Ions involved and atom labeling in ionic liquids studied in this work.

Figure 2. Center-of-mass radial distribution functions, g(r), for pure ILs at 298 K and 1bar. A and C stand for anion and cation, respectively.

Figure 3. Spatial distribution functions of anion center of mass around the corresponding cation for the reported pure IL at 298 K and 1 bar. Isodensity plots at six times bulk density.

Figure 4. Anion – cation center-of-mass radial distribution functions, g(r), for DSILs at 298 K and 1 bar. The labels of each panel show: (underlined) pure ILs from which the DSILs were obtained (equimolar mixtures), (bold) cations and (non bold) anions for each DSIL. A1 and A2 stand for the first and second anion in the DSIL (numbered as in the label of each panel) and C1 and C2 for the first and second cation in the DSIL (numbered as in the label of each panel).

Figure 5. Spatial distribution functions of anions center of mass around the corresponding cations for the reported DSILs at 298 K and 1 bar. Isodensity plots at six times bulk density. A1 and A2 stand for the first and second anion in the DSIL (numbered as in the bottom label of each panel) and C1 and C2 for the first and second cation in the DSIL (numbered as in the bottom label of each panel). All data calculated at 298 K and 1 bar.

Figure 6. Relationship between percentage deviation of viscosity for the DSILs, ηDSIL, with regard to viscosity calculated from the linear combination of viscosity data for those pure ILs leading to each DSIL and percentage deviation of total anion-cation intermolecular interaction energy for the DSILs, Einter,DSIL, with regard to values calculated from the linear combination of anion-cation intermolecular interaction energy data for those pure ILs leading to each DSIL. All data calculated at at 298 K and 1 bar. Continuous line show linear fit (R2 = 0.92, slope = 0.46).

Figure 7. Radial distribution functions, g(r) (left axes), and running integrals of g(r) (right axes), N, between the center of mass of ions and CO2 molecules for IL + CO2 systems as a function of applied pressure at 298 K. Figure 8. Spatial distribution functions of CO2 center of mass around the corresponding ions for the reported IL + CO2 systems at 298 K. Isodensity plots at six times bulk density. All values calculated for 10 bar corresponding to CO2 mole fractions reported in Table 3. Figure 9. CO2 – ion intermolecular interaction energy (sum of Lennard-Jones and coulombic contributions) for IL + CO2 systems as a function of CO2 mole fraction at 298 K. Figure 10. Distribution of cavity sizes in IL and in IL + CO2 mixtures at 298 K. Values for mixtures containing CO2 calculated for 10 bar corresponding to CO2 mole fractions reported in Table 3.

27



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

Figure 11. Radial distribution functions, g(r) (left axes), and running integrals of g(r) (right axes), N, between the center of mass of CO2 molecules for IL + CO2 systems at 298 K. All values calculated for 10 bar corresponding to CO2 mole fractions reported in Table 3. Figure 12. Radial distribution functions, g(r) (left axes), and running integrals of g(r) (right axes), N, between the center of mass of ions and CO2 molecules for DSIL + CO2 systems at 298 K. A1 and A2 stand for the first and second anion in the DSIL (numbered as in the label of each panel) and C1 and C2 for the first and second cation in the DSIL (numbered as in the label of each panel). All values calculated for 10 bar corresponding to CO2 mole fractions reported in Table 5. Figure 13. Distribution of cavity sizes in DSIL and in DSIL + CO2 mixtures at 298 K. Values for mixtures containing CO2 calculated for 10 bar corresponding to CO2 mole fractions reported in Table 5. Figure 14. Maximum CO2 solubility data obtained from experiments

28


Page 28 of 43

Page 29 of 43

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


Cc,Hc Hn H N1 + N

H3C

Ct,Ht CH3 Cc,Hc

CH3 Cm,Hm

Fc F Os O

S1

S

N1

CH 3

CH 3

N1 N

H 3C N

Cc,Hc

[MTOA]+

[BTMA]+

F

O Os

[OTf]-

Ft F

S St

S

F Fc

Ot O

N2N

O

-

CH3 Cm,Hm

H3C

Cm,Hm

O Os

CH3 +

+

CH 3

[DEMA]+ Fc F Cs

H3C Ct,Ht

Ct,Ht

Cf

O

F F

O Ot

F Ft F Ft

[Tf2N]-

Figure 1.

29


Os O Cs,Hs H3C

S

S1

-

O Os

O Os

[METS]-


2

[BTMA][TF2N] [MTOA][OTf] [DEMA][METS] [DEMA][OTf]

(a) C-C g(r)

1.5 1 0.5 0 2

(b) A-A g(r)

1.5 1 0.5 0 8

(c) A-C 6 g(r)

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 30 of 43

4 2 0 0

5

10 r/Å

15

Figure 2.

30


20

Page 31 of 43

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


Figure 3.

31



10

A1-C1 A1-C2 A2-C1 A2-C2

(a) 1-2: [BTMA][MTOA][Tf2N][OTf]

g(r)

8 6

(b) 1-3: [BTMA][DEMA][Tf2N][METS]

(c) 1-4: [BTMA][DEMA][Tf2N][OTf]

(e) 2-4: [MTOA][DEMA][OTf]

(f) 3-4: [DEMA][METS][OTf]

4 2 0 10

(d) 2-3: [MTOA][DEMA][OTf][METS]

8 g(r)

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 32 of 43

6 4 2 0 0

5

10 r/Å

15

20 0

5

10 r/Å

15

20 0

Figure 4.

32


5

10 r/Å

15

20

Page 33 of 43

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


Figure 5.

33



0 100 (Einter,DSIL - Einter,lin) / Einter,lin

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 34 of 43

-10

-20

-30 -50

-40 -30 -20 -10 100 (ηDSIL - ηlin) / ηlin

Figure 6.

34


0

CO2 - anion

5

0 2.5

N

g(r)

15

2

N

1 5

0

1.5

5

0.5 0

0 0

5

10 r/Å

15

20

N

15

1.5 10 1 5 0 20

(h)

2

15

1

20

(f)

2.5

(g) 10

0

0

20

2

5

0.5

0

2.5

10 1

2.5

1.5

0.5

15

1.5

20

10

20

(d)

0

g(r)

(e)

0

0.5

N

g(r)

2

0

2 g(r)

15

5

2.5

[DEMA][OTf]

0

1

0

5

0.5

1.5

0.5

10 1

2.5

10

15

1.5

0 20

(c)

2

[DEMA][METS]

N

1

g(r)

1.5 10

20

(b)

2

15

N

(a)

0.5

[MTOA][OTf]

2.5

N

20

2 g(r)

[BTMA][Tf2N]

CO2 - cation 2.5

15

1.5 10 1 5

0.5 0

0 0

5

Figure 7.

35


10 r/Å

15

20

N

p = 1 bar p = 4 bar p = 7 bar p = 10 bar

g(r)

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


g(r)

Page 35 of 43


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

Figure 8.

36


Page 36 of 43

Page 37 of 43

0 Einter/ kJ mol-1

(a) CO2 - cation -20 [BMIM][Tf2N] [MTOA][OTf] [DEMA][METS] [DEMA][OTf]

-40

-60 0 Einter/ kJ mol-1

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


-20

-40

(b) CO2 - anion

-60 0

0.1

0.2 xCO2

Figure 9.

37


0.3


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

Figure 10.

38


Page 38 of 43

Page 39 of 43

5

[BTMA][Tf2N] [MTOA][OTf] [DEMA][METS] [DEMA][OTf]

4

25 20

3

15

2

10

1

5

g(r)

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


0

0 0

5

10 r/Å

15

20

Figure 11.

39


0 2.5

(i)

2

0 2.5

10 1

0 5

10 r/Å

15

N

15 10

1 5 0 20

(l)

2

A1 A2

15

1.5 10 1 5

0.5

0 0

0 20

(j)

0 2.5

5

0.5

5

1.5

15

1.5

N

10 1

0.5

0 20

(k)

2

15

2

5

0.5

(h)

0 2.5

N g(r)

10 1

0 20

1.5

15

1.5

5

0.5

0 20

N

10 1

2

5

0.5

15

0 2.5

N g(r)

10 1

(f)

1.5

15

1.5

0 20

0.5

0 20

(g)

2

5

2

5

0.5

N

10 1

0 2.5

N g(r)

10 1

15

1.5

15

1.5

0 20

(d)

N

0 20

(e)

5

0.5

N g(r)

g(r)

10

20

0

0 0

5

Figure 12.

40


10 r/Å

15

20

N

5

2

15

1

2 N g(r)

10

20

A1 A2

(b)

1.5

15

1

CO2 - anion

0 2.5

(c)

1.5

Page 40 of 43

0.5

0 20

0.5

g(r)

N g(r)

5

2

g(r)

15

2

10

0 2.5

g(r)

2.5

1 0.5

g(r)

[BTMA][DEMA][Tf2N][OTf] [MTOA][DEMA][OTf][METS]

20

1.5

0 2.5

[MTOA][DEMA][OTf]

C1 C2

(a)

2

g(r)


CO2 - cation

2.5

0 2.5

[DEMA][METS][OTf]

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 41 of 43

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


Figure 13.

41



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

Figure 14.

42


Page 42 of 43

Page 43 of 43

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


TOC Graphic

43


Double Salt Ionic Liquids Based on Ammonium ... - ACS Publications

Recommend Documents