Tausif Altamasha,
Mert Atilhan*a,
Amal Aliyana,
Ruh Ullaha,
Gregorio Garcíab and
Santiago Aparicio*b
aDepartment of Chemical Engineering, Qatar University, Doha, Qatar. E-mail: mert.atilhan@qu.edu.qa
bDepartment of Chemistry, University of Burgos, Burgos, Spain. E-mail: sapar@ubu.es
First published on 7th November 2016
The properties of choline chloride plus phenylacetic acid deep eutectic solvents in neat liquid state and upon absorption of CO2 are analyzed using a theoretical approach combining quantum chemistry using Density Functional Theory and classic molecular dynamics methods. This study investigates the physicochemical properties, structuring, dynamics and interfacial behavior of the selected deep eutectic solvent from the nano-size point of view to infer its viability for effective CO2 capture. DFT results provided information on the mechanism of short-range interactions between CO2 and the studied DES, showing a better performance than previously studied DES. The mechanism of CO2 capture is analyzed considering model flue gas, showing a two-stage process with water, CO2 and N2 molecules developing adsorbed layers at the interface but in different regions. Water adsorbed layers would delay the migration of CO2 molecules toward bulk liquid regions, which should be considered for developing large-scale applications.
Deep Eutectic Solvents (DES) have been recently proposed as possible platforms for developing carbon capture sorbents.29–32 DES are mixtures with of two, or more, compounds with melting temperature lower than either of the individual compounds.33–35 These eutectic mixtures are formed by combining Lewis or Brønsted acids and bases which can contain different types of ionic species.35 The depression of the melting point upon mixing of the DES compounds are produced by the hydrogen bonding interaction between the DES compounds, and thus, the mixing of an hydrogen bond acceptor (HBA), frequently a quaternary ammonium salt, and an hydrogen bond donor (HBD), is the common approach for developing DES.
The main DES properties and their suitability for carbon capture purposes have been recently reviewed,30 and although the number of studies is still limited the interest on these systems has increased recently. The main advantages of DES rise from the large number of compounds leading to DES, which produce a large library for selecting the most suitable DES according to their physicochemical properties and affinity for CO2. Likewise, DES synthesis carried out with 100% atom economy,36 the null toxicity and full biodegradability37,38 of most of used HBA and HBD, the possibility of developing DES based on fully natural molecules,39 and their low cost, are remarkable advantages for DES.
The developing of DES-based technologies require to find the most suitable HBA–HBD combinations, leading to the best physicochemical properties (e.g. low viscosities, large thermal stability) and high affinity for CO2 molecules. For this purpose, understanding the microscopic behavior of DES, its relationship with macroscopic physicochemical properties, and the mechanism of interaction between CO2 and DES-molecules is of pivotal importance. This information can be obtained using computational chemistry methods such as Density Functional Methods (DFT)40,41 and classic molecular dynamics (MD).42–44 The use of computational chemistry for DES have led to information on the structuring, dynamics and CO2-capturing mechanism for selected systems, although its systematic application to understand DES properties is still in its infancy.40–44 For this purpose, the properties of DES based on choline chloride as HBA (CHCl) and phenylacetic acid (PhOAc) in 1:
2 mole ratios have been studied in this work using both DFT and MD approaches. Our group has published previous studies in which the properties of DES based on natural products, such as levulinic acid, were analyzed.42 The objective of the present work is to extend systematically the results of previous studies to DES based on PhOAc as HBD and to infer the changes in the microscopic properties of the DES in relationship with CO2 capture. The selection of PhOAc was done as a model of aromatic carboxylic acids as a platform for DES development. The presence of the bulky phenyl ring in PhOAc should contribute to increase the available free volume in the fluid, which could favor CO2 solubility combined with the presence of the carboxylic acid group. The purpose of this work is to characterize this type of material as part of the research on the suitability of DES for carbon capturing in order to find the most suitable molecular combinations and their relationship with molecular features and nano-sized liquid properties.
Geometry optimizations for molecular clusters composed of 1 CHCl ionic pair + 2 PhOAc molecules and for the same system + 1 CO2 molecule at different positions were developed with the Berny algorithm using GEDIIS.51 The optimized structures were checked through their vibrational frequencies, discarding the presence of negative frequencies. Computed energies were corrected (to avoid basis set superposition error) according to counterpoise procedure.52 Atomic charges were computed to fit the electrostatic potential according to the ChelpG scheme.53 All calculations were carried out with Gaussian 09 (Revision D.01) package.54 Interaction energies (ΔE) for the studied processes related with binding energy for [CH][Cl] salt (ΔEIP), DES formation (ΔEDES) and CO2 capture by the selected solvent (ΔEDES–CO2) were defined as:
ΔEIP = EIP − (ECH + ECl) | (1) |
ΔEDES = EDES − (EIP + 2EPhOAc) | (2) |
ΔEDES·CO2 = EDES·CO2 − (EDES + ECO2) | (3) |
The second objective of MD studies was to characterize the interfacial properties of CHCl_PhOAc_1_2 in contact with vacuum layers and with acid gas phases. For this purpose, previously equilibrated simulation boxes (with the same amount of molecules used for the study of pure DES properties) were put in contact with a vacuum layer (200 Å long in the z-direction), with a gas phase containing 500 CO2 molecules (in a box 190 Å long in the z-direction, corresponding to CO2 vapor density equal to 0.066 g cm−3 equal to that experimentally obtained for vapor CO2 at 3 MPa and 298 K),56 or with a gas phase emulating the typical composition of a flue gas, which is the mixture of gases being emitted from fossil-fueled power plants, (50CO2 + 375N2 + 50H2O + 25O2 molecules, in a box 190 Å long in the z-direction). These simulations for the study of interface behavior were carried out in the NVT ensemble at 298 K for CO2 and flue gas interfaces, and at 298, 318, 338 and 358 K for vacuum interfaces.
MDynaMix v.5.2 software was used to carry out all the simulations reported in this work.57 Simulations in the NVT and NPT ensembles were carried out with pressure and temperature controlled with the Nose–Hoover method. Ewald method58 was applied for handling coulombic interactions. The equations of motion were solved using the Tuckerman–Berne double time step algorithm (1 and 0.1 fs, for long and short time steps, respectively).59 Lennard-Jones cross terms were calculated using Lorentz–Berthelot mixing rules.
The forcefield parameterization used along MD simulations is reported in Table S2 (ESI†). According to our previous studies for other types of DES,43 the atomic charges develop a pivotal role for DES MD simulations, and thus, they were obtained from DFT calculations of 1 [CH][Cl] + 2 PhOAc clusters optimized at B3LYP-D2/6-31+G** theoretical level using ChelpG charges.53 In agreement with previous results for other DES,42 the 1:
2 stoichiometry of the studied DES leads to two types of PhOAc molecules with slightly different atomic charges because of different interactions with the salt (Table S2, ESI†). Likewise, used total charges are +0.6941 for [CH]+, −0.6707 for Cl−, and +0.0261 and −0.0497 for the two different types of PhOAc molecules. Lennard-Jones parameters for [CH]+ and Cl− were obtained from a previous work,44 whereas those for PhOAc were obtained from SwissParam.60 Forcefield parameters for gas molecules were obtained from a previous work.61
Fig. 1 shows the optimized structures for CHCl_PhOAc_1_2⋯CO2. For a first approximation, CO2 capture at the molecular level would be related with the strength of the interactions between the solvent (CHCl_PhOAc_1_2) and the gas molecule, which has been assessed through interaction energies defined above (eqn (3)). Aimed at obtaining the most relevant information on the potential energy surface for the interaction between CHCl_PhOAc_1_2 and CO2 molecule, several starting points for CHCl_PhOAc_1_2 and one CO2 molecule in different relative positions were tested, selecting those molecular arrangements with lower total energies. As seen in Fig. 1, nine arrangements were found for the interaction between the DES and the gas molecule. This figure also reports interaction energies related with CO2 capture, which are ranged between 53.65 kJ × mol−1 (structure 9) and 22.78 kJ × mol−1 (structure 6). For structure 9, the CO2 molecule is mainly linked (concretely the central carbon atom) to the Cl atom with a bond length equal to 3.342 Å and to the O atom (from COOH motif) with a bond length equal to 2.902 Å. Structures 5 (ΔEDES·CO2 = 42.87 kJ × mol−1) and 7 (ΔEDES·CO2 = 39.94 kJ × mol−1) yields a similar interaction mechanism between DES motif and CO2 molecule. For structures 8 (ΔEDES·CO2 = 42.80 kJ × mol−1), 2 (ΔEDES·CO2 = 36.08 kJ × mol−1), 3 (ΔEDES·CO2 = 31.92 kJ × mol−1) and 4 (ΔEDES·CO2 = 31.92 kJ × mol−1), CO2 molecule is placed in the vicinity of an oxygen atom (from COOH motif), with a bond length ≈ 2.834 Å. Considering that the hydrogen atom of the hydroxyl group in PhOAc is interacting with chlorine anion, the interaction of CO2 molecules for structures 2, 3, 4 and 8 is in agreement with the available DFT studies on the interaction of CO2 with organic acids.62 Energy differences could be related with the strength of C(CO2)⋯O(COOH) interactions as well as the presence of some weak hydrogen bond between CO2 molecule (through H atoms) and methyl H (from choline cation). For structures 1 (ΔEDES·CO2 = 43.70 kJ × mol−1) and 6 (ΔEDES·CO2 = 22.78 kJ × mol−1), the main interaction is related with and CO2–Cl intermolecular bond. Nonetheless, the absence of additional H-bond between CO2 molecules and choline cation (there is only a weak H-bond between CO2 and phenyl motif) in structure 6 leads to the lowest |ΔEDES·CO2| values. The interaction energies obtained for CHCl_PhOAc_1_2 from DFT would agree with a better performance for CO2 capture in comparison with other deep eutectic solvents previously reported by our group, such as the one based on levulinic acid as hydrogen bond donor.42
The structure of CHCl_PhOAc_1_2 at the nano-sized level is firstly analyzed using the radial distribution functions, RDFs, reported in Fig. 2. Results for center-of-mass RDFs reported in Fig. 2a confirm the strong anion–cation interaction in the DES together with the preferential interaction of PhOAc with chlorine anion. The anion–cation interaction is developed through the hydroxyl group in [CH]+ (O1–H4 sites) and PhOAc interacts with Cl− through the hydroxyl group (O2–H5 sites). No hydrogen bonding between PhOAc and [CH]+ is inferred, whereas hydrogen bonding between neighbor PhOAc molecules is inferred, Fig. 2b. The spatial distribution around [CH]+ reported in Fig. 3a shows anionic concentration around the head hydroxyl group with additional distribution around the methyl groups bonded to nitrogen but discarding hydrogen bonding. In the case of distribution around PhOAc, Fig. 3b, Cl− is placed above the –OH group and neighbor PhOAc molecules in close regions around the –COOH group.
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Fig. 3 Spatial distribution functions for the reported sites around (a) [CH]+ and (b) PhOAc obtained from molecular dynamics simulations for CHCl_PhOAc_1_2 at 298 K and 1 bar. Isodensity values reported for 4-times bulk density. Atom names as in Fig. 2. |
The dynamics of solvation shells around each molecule are quantified using the residence times of one molecule around other, as defined in a previous work,42 Fig. S3 (ESI†). These results show that PhOAc molecules stay longer times in the vicinity of Cl− than [CH]+, which confirm the strong affinity of PhOAc for the anion. The residence times of one PhOAc molecule around another neighbor PhOAc molecule are also large confirming the development of self-association between these molecules. This is in agreement with the strength of the intermolecular forces reported in Fig. S4 (ESI†), which show strong anion–cation interactions (as expected from their prevailing coulombic character), but also strong PhOAc–Cl− interactions. Likewise, the average number of hydrogen bonds reported in Fig. 4 confirms the development of hydrogen bonding between PhOAc and Cl−, which is surprisingly even slightly reinforced with increasing temperature and the minor extension of anion–cation and PhOAc–PhOAc interactions.
The absorption of CO2 molecules should lead to changes in the structure of the studied CHCl_PhOAc_1_2 DES. First, the liquid should show expansion upon gas absorption but this effect is very minor with an expansion of just 12% for xCO2 = 0.48, Fig. 5, which shows that the DES is able to rearrange its structure for fitting the absorbed gas molecules without very remarkable changes in DES liquid structuring. This is confirmed by the intermolecular interaction energies between of CHCl_PhOAc_1_2 molecules reported in Fig. 6a, which suffer very minor changes upon CO2 absorption in the studied range. Moreover, the affinity of PhOAc and [CH]+ molecules for CO2 is very similar, Fig. 6b, but the strength of PhOAc–CO2 interactions is the half of levulinic acid–CO2 interactions as reported in a previous work,20 but CO2 solubility is almost the same both in DES containing PhOAc and levulinic acid (both DES with the same ions and stoichiometry), showing the relevance of other factors such as volume rearrangement upon gas absorption. The arrangement of CO2 molecules in CHCl_PhOAc_1_2 is analyzed from RDFs reported in Fig. 7. RDFs around [CH]+ show that CO2 molecules are preferentially placed around the cation hydroxyl group, Fig. 7a, which leads to a peak in RDFs around Cl−, Fig. 7b. Regarding the distribution around PhOAc, results in Fig. 7c show that CO2 molecules are preferentially placed around the aromatic ring in PhOAc. This is confirmed by the spatial distribution functions reported in Fig. 8. Likewise, results in Fig. S5 (ESI†) show a trend for developing CO2–CO2 clustering with increasing CO2 mole fraction, especially around PhOAc as reported in Fig. 8b. Regarding the dynamics of CO2 molecules, results in Fig. S6 (ESI†) show that in spite of the moderate strength of intermolecular interactions involving CO2 molecules, Fig. 6b, the residence times of CO2 are long enough when compared with other stronger interactions, Fig. S3 (ESI†).
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Fig. 5 Percentage of volume change, % Vexp, upon CO2 absorption, according to the criteria by Gallagher et al.,63 for CHCl_PhOAc_1_2 + CO2 mixtures at 298 K as a function of CO2 mole fraction, xCO2. |
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Fig. 6 Intermolecular interaction energy, Einter, sum of Lennard-Jones and coulombic terms, for CHCl_PhOAc_1_2 + CO2 at 298 K and 1 bar. |
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Fig. 7 Site–site radial distribution functions, g(r), for CHCl_PhOAc_1_2 + CO2 systems at 298 K and 29.91 bar (xCO2 = 0.478). Atom names as in Fig. 12; CD stands for carbon atoms in CO2. |
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Fig. 8 Spatial distribution functions of CO2 center-of-mass around (a) [CH]+ and (b) PhOAc for CHCl_PhOAc_1_2 + CO2 at 298 K and 29.91 bar (xCO2 = 0.478). Values reported for 4 times bulk density. |
The absorption of CO2 molecules from a gas phase is developed in two stages, first gas molecules are adsorbed at the corresponding interface and second, they diffuse from the interfacial region toward the bulk liquid phase. The first stage, adsorption at the interface, is critical for the development of industrial large scale gas capturing operations because very strong adsorption and long residence times at the interface would hinder the diffusion toward the bulk liquid, which should be considered for process design purposes. Therefore, the nano-sized characteristics of CHCl_PhOAc_1_2 interfaces in contact with a CO2 gas phase and with a model flue gas were also studied using MD. Likewise, the CHCl_PhOAc_1_2–vacuum interface was also studied for comparison purposes. The structure of the interface at vacuum is characterized by regions in contact with the vacuum layer being rich in PhOAc, whereas ions are placed in inner regions close to the Gibbs dividing surface, Fig. 9a. The atomic arrangement at vacuum interface, Fig. 9b and c, show that [CH]+ is placed almost parallel to the interface although slightly skewed with hydroxyl groups pointing to the vacuum layer. PhOAc are slightly skewed regarding to the vacuum interface, with –COOH group placed in inner regions and phenyl ring closer to the vacuum layer in parallel to the interface. The contact of CHCl_PhOAc_1_2 with a CO2 gas layer leads to minor changes in the arrangement of molecules at the interface in comparison with vacuum interface, Fig. 9d, and although a certain rearrangement of [CH]+ and PhOAc is produced in contact upon CO2 adsorption the orientation of molecules is very similar, Fig. 9e and f. The contact of CHCl_PhOAc_1_2 with CO2 gas phase leads to the development of a highly dense adsorbed layer in the first stages of the simulation, Fig. S7 (ESI†). This adsorbed layer follows a complex dynamics, first the intensity of the peaks shows increasing number of adsorbed molecules, then the peaks broadening shows that certain CO2 molecules move toward inner regions, and finally the shifting of the peaks show adsorbed layers moving toward inner regions at the interface, Fig. S8 (ESI†). Results in Fig. 9d showed that the outer layers of CHCl_PhOAc_1_2, in contact with CO2 gas phase, are rich in PhOAc molecules, and thus the adsorption of CO2 molecules is characterized by strong PhOAc–CO2 interactions (especially when the adsorbed layer is fully developed, t > 6 ns), and in minor extension interactions with [CH]+ are developed, Fig. 10.
For industrial operations involving CO2 capturing, the gas phase from which CO2 has to be adsorbed is a mixture (flue gas) in which the low partial pressure of CO2 hinders its capturing. Therefore, the properties of CHCl_PhOAc_1_2 in contact with a model flue gas were also studied in this work. The density profiles at the flue gas interface reported in Fig. 11a show that water molecules present in the flue gas develops an adsorbed layer in inner regions when compared with CO2 adsorbed layer. The water adsorbed layer stays in regions close to Cl− whereas CO2 molecules stay close to PhOAc molecules. Therefore, although the adsorption of water molecules does not hinder the development of a CO2 adsorbed layer, because they occupy different regions at the interface, the water layer should difficult the diffusion of CO2 molecules from the interface toward bulk liquid regions. Moreover, an additional adsorbed layer of N2 molecules is developed at outer regions, and thus three consecutive layers are developed at the interface being occupied by water, CO2 and N2 molecules, Fig. 11b. Likewise, the adsorption mechanism from flue gas is characterized by a moderate affinity toward CO2 molecules paired with very strong water–Cl− interactions and non-negligible N2–PhOAc interactions, Fig. S9 (ESI†).
Footnote |
† Electronic supplementary information (ESI) available: Fig. S1 (DFT results); Table S1 (systems used for MD simulations with CO2); Table S2 (forcefield parameterization using along this work); Fig. S2 (comparison between experimental and molecular dynamics predicted thermophysical properties); Fig. S3 (calculated residence times for CHCl_PhOAc_1_2); Fig. S4 (intermolecular interaction energies in CHCl_PhOAc_1_2); Fig. S5 (radial distribution functions in CHCl_PhOAc_1_2 + CO2); Fig. S6 (residence times in CHCl_PhOAc_1_2 + CO2); Fig. S7 (density profiles in CHCl_PhOAc_1_2 + CO2); Fig. S8 (snapshot for CHCl_PhOAc_1_2 + CO2); Fig. S9 (intermolecular interaction energy in CHCl_PhOAc_1_2 + flue gas). See DOI: 10.1039/c6ra22312e |
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