Effect of water on high-pressure ternary phase equilibria of CO2 + H2O + alkanolamine based ionic liquid

Journal Pre-proof Effect of water on high-pressure ternary phase equilibria of CO2 + H2O + alkanolamine based ionic liquid

Murilo L. Alcantara, Paulo H.R. Silva, Lucienne L. Romanielo, Lúcio Cardozo-Filho, Silvana Mattedi PII:

S0167-7322(19)36909-0

DOI:

https://doi.org/10.1016/j.molliq.2020.112775

Reference:

MOLLIQ 112775

To appear in:

Journal of Molecular Liquids

Received date:

16 December 2019

Revised date:

19 February 2020

Accepted date:

24 February 2020

Please cite this article as: M.L. Alcantara, P.H.R. Silva, L.L. Romanielo, et al., Effect of water on high-pressure ternary phase equilibria of CO2 + H2O + alkanolamine based ionic liquid, Journal of Molecular Liquids(2020), https://doi.org/10.1016/j.molliq.2020.112775

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© 2020 Published by Elsevier.

Journal Pre-proof

Effect of water on high-pressure ternary phase equilibria of CO2 + H2O + alkanolamine based ionic liquid Murilo L. Alcantara1,5, Paulo H. R. Silva2, Lucienne L. Romanielo2, Lúcio CardozoFilho3,4 Silvana Mattedi¹.

1

Chemical Engineering Graduate Program, Polytechnic School, Universidade Federal da Bahia, Salvador-BA, Brazil. 2

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School of Chemical Engineering, Universidade Federal de Uberlândia -MG, Brazil

3

Brazil. 4

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Chemical Engineering Department, Universidade Estadual de Maringá, Maringá-PR,

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São Paulo State University, Campus of São João da Boa Vista, São João da Boa

re

Vista-SP, Brazil. 5

Abstract

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Chemical Engineering, Universidade de São Paulo, São Paulo-SP, Brazil.

Some ethanolamine or hydroxylammonium based ionic liquids have shown great potential in CO2 separation processes. However, not much is known about the influence of water on the CO2 solubility of these liquids. In this paper, we study the high-pressure ternary liquid-vapor phase equilibria. The systems were composed of carbon

dioxide,

water,

and

an

alkanolamine

based

IL:

N-methyl-2-

hydroxyethylammonium propionate [m-2HEA][Pr]. We studied systems whose water contents varied from 6.7% to 52.5 %, temperatures from 313 K to 353 K, and pressures from 0.6 MPa up to 22 MPa. The phase equilibria experiments showed that the amount of ionic liquid in the mixture controls the solubilization of CO2 and is not significantly affected by changes in the water content within the studied range. These data were simulated from a molecular point of view using the Gibbs ensemble Monte Carlo method. A force field was estimated to describe the Ionic Liquid (IL)

Journal Pre-proof density at a wide range of water content values using the isothermal−isobaric Monte Carlo (NPT-MC). The calculated densities presented a good agreement with experimental data, indicating that the proposed force field parameters are suitable to describe the densities of the IL mixtures and, therefore, can be used to simulate the ternary phase equilibria. The software package CassandraV1.2 was employed to simulate the ternary phase equilibria, resulting in reasonably low deviations when compared to the experimental data. However, the predicted solubility of carbon dioxide in N-methyl-2-hydroxyethylammonium propionate [m-2HEA][Pr] was slightly

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higher when compared with the experimental values.

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Keywords: Phase equilibrium, water, ionic liquid, carbon dioxide, molecular

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simulation.

[bmpyrr][dca] [emim][dep] [cprop][Tf2N] [cprop][dca] [BHEA][Bu] CO2 [DEA][Bu] ILs [m-2HEA][Pr] [toa][Tf2N] x NMR T K [tes][Tf2N] [thtdp][phos] [thtdp][dca] VLE

lP

[bmpip][Tf2N]

Nomenclature

1-Allyl-3-methylimidazolium dicyanamide 1-Butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide 1-Butyl-1-methylpyrrolidinium dicyanamide 1-Ethyl-3-methylimidazolium diethylphosphate 1,2,3-Tris(diethylamino)cyclopropenylium bis(trifluoromethylsulfonyl) imide 1,2,3-Tris-(diethylamino)cyclopropenylium dicyanamide bis-(2hidroxy-ethylammonium) butanoate Carbon dioxide diethylammonium butanoate Ionic liquids N-methyl-2-hidroxy-ethylammonium propionate Methyltrioctylammonium bis(trifluoromethylsulfonyl)imide Mole fraction Nuclear magnetic resonance Temperature Temperature unit, kelvin Triethylsulfonium bis(trifluoromethylsulfonyl)imide Trihexyltetradecylphosphonium bis-(2,4,4-trimethylpentyl) phosphinate Trihexyltetradecylphosphonium dicyanamide Vapour–liquid equilibrium

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[amim][dca]

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Abbreviations

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Nomenclature

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1. Introduction The removal of carbon dioxide from pre-combustion can enhance the stream heat potential, reduce equipment corrosion, and avoid its supersizing, resulting in economic gains. For over eight decades, the use of aqueous alkanolamine based solutions has remained as one of the leading CO2 capturing processes [1]. These solvents remove CO2 by exothermally reacting with this molecule, producing soluble

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carbamate/carbonate salts [2]. High evaporation rates, low thermal stability, and

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equipment corrosion are some of the main drawbacks of the use of these materials. The incorporation of reactive and volatile alkanolamine molecules within the stable

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structures of low volatile ionic liquids (ILs) can produce ILs with moderate thermal

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stability [3,4], low toxicity [5], but also with lower CO2 selectivities when compared to the original alkanolamine structures [2,6–9]. Thermodynamic studies showed that the

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ethanolamine based ILs still present high selective CO2 solubility. This feature, however, still lower than that of the original structures [10,11]. Furthermore, the

na

interaction mechanism between these ionic liquids and carbon dioxide could be of chemical [8,12] or physical [9,13] nature, which has paved the way for further

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applications, such as physical absorption [14,15] and membrane separation processes [16–19].

The ethanolamine-based (also known as Hydroxylammonium-based) ILs present high hygroscopic [4] and viscosity [4,9,20,21] properties, and the addition of water has a positive influence, facilitating the transport in polymeric membranes [18,22]. Therefore, these ILs are unlikely to be applied without a trace of water in CO2 separation processes [12]. To date, only a few controversial studies focused on the effect of water on the CO2 solubilization in ionic liquids, mainly formed by imidazolium ions [23–25]. Although the reported remarks are controversial, most studies have reported a reduction of CO2 solubilization with the increase of the water content [25], possibly because water and CO2 compete for empty the spaces between the ions [23].

Journal Pre-proof Carbon dioxide solubilization in imidazolium-based ILs suggests that the addition of water might reduce the Coulomb interactions between the ions, and, at low water concentrations, the CO2 solubility might be slightly higher when compared with the water-free IL [25]. Ethanolamine based ionic liquids are more viscous and less conductive than imidazolium-based ones. That might be due to the presence of strong hydrogen bonding between the involved ions [26]. Small amounts of water might increase the cation-anion strong polar interactions and, possibly, increase the mixture viscosity [21,26]. If the CO2 solubilization occurs by the formation of chemical bonds with these ILs, the water concentration can change the reactions involved [12].

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The wide variety of ILs and molecular interactions hinder the definition of a general

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effect of water on the CO2 solubilization in ILs. Therefore, each family of IL structures should be individually analyzed.

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Few studies have addressed the effects of CO2 absorption and ethanolamine based

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ILs. To the best of our knowledge, there are no studies in the literature concerning ILs synthesized by the neutralization of alkanolamines and carboxylic acids, despite high

CO2

selectivities

[9,10,13,14,27].

and

apparent

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their

CO2

chemical

bond

freeness

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The present paper's primary goals are to study the effect of water on the absorption of CO2 in an ethanolamine/carboxylic-acid-based IL, which can be performed by

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providing and analyzing consistent ternary phase equilibria data at a wide range of temperatures, pressures, and compositions. These data were also simulated through a reliable and robust method.

The modeling of the phase equilibria of these ILs has been a matter of study of several papers, and to this end, various models have been employed, such as the empirical predictive correlation proposed by Carvalho and Coutinho [28]; PengRobinson EoS; Peng-Robinson EoS coupled with Wong-Sandler/NRTL or two parameters mixing rules [13,29], three-parametric Redlich Kwong – Peng Robinson equation of state (RKPR-EoS) [27], and even robust models such as the predictive ternary sPC-SAFT [30]. The average relative deviations and number of adjusted parameters of these models can be observed in Table 1.

Journal Pre-proof Table1. Comparison among several models to predict/adjust high-pressure phase equilibria of CO2 + alkanolamine based ILs [31]. Models

Adjusted parameters

PR 2 par. [31]

PRRK [27]

PRWS/NRTL [9]

sPC-SAFT [9]

0

2

8

Two by each isotherm

1a or 0b

2.3

.

29

25.0

8.6

139.9

47.4

190.6

54.5

30.0

25.8

13.1

7.2

.

.

.

.

26.2

155.6b

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24.5

.

11.5

119.2b

.

7.5

16.1b

.

.

6.6*

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a

118

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Average relative deviation (%)

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ILs [m-2HEA][Pr] [27] [BHEA][Bu] [27] [2HEA][Bu] [13] [m-2HEA][Bu] [13] [e-2HEA][Bu] [13] [DEA][Bu]c [9]

Corr. Carv. Cout. [28]

sPC-SAFT applied to binary equilibria, adjusted with k ij (IL+CO2). sPC-SAFT predictive, applied to ternary phase equilibria (IL + CO 2 + H2O). c Only at data with XCO2 below 0.5.

na

b

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Since the aforementioned models could neither predict nor be adjusted to most of the experimental data with acceptable deviations, a new approach had to be adopted in the present study.

The solubilization of CO2 in the water-IL solution was calculated from a molecular point of view using the Gibbs ensemble isothermal−isobaric Monte Carlo (NPT-MC) method. A simplified force field was proposed, where only the hydrogen atoms bonded to oxygen and nitrogen atoms are explicitly taken into account. Since the previously applied models (Table 1) were not able to satisfactorily describe the experimental data, we proposed a new approach based on molecular simulation. The use of molecular simulation has been growing in all fields of science and engineering since it can bring new insights into the phenomena under study. However, the success of this methodology depends on the accuracy of the model and, in the case of the study of ILs, on the employed force field. Although many force fields have been proposed for describing aprotic ionic liquids [32–35], there is no

Journal Pre-proof systematic study of force fields for protic ionic liquids. Diel and Cabral [36] proposed an

atom

force

field

to

describe

the

equilibrium

properties

of

methyl-2-

hydroxithethylammonium cation [m-2HEA]+. The large number of atoms and, therefore, bonds, angles, and dihedrals result in a large number of parameters and, consequently, higher computational costs. In this work, a simplified force field was proposed to simulate the ternary phase equilibria of the ILs under study.

2. Materials and methods

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2.1 Choosing the alkanolamine based IL.

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The alkanolamine/carboxylic-acid-based IL studied in this paper was the N-methyl-2hydroxyethylammonium propionate [m-2HEA][Pr]. This protic IL presents high

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CO2/CH4 selectivity [10,27], moderate viscosity [21], thermal stability [3], and toxicity [5]. Furthermore, it is one of the most studied alkanolamine/carboxylic-acid-based IL

re

in the literature [37,38]. Figure 1 shows the [m-2HEA][Pr] molecular structure, its CO2/CH4 ideal selectivity, as well as a comparison with other ILs that do not interact

na

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with CO2 through a chemical reaction.

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CO2/CH4 Ideal Selectivity

90

60

30

0 313

323

[m-2HEA][Pr] [BHEA][Bu] [DEA][Bu] [thtdp][phos] [thtdp][dca] [amim][dca] [bmpyrr][dca] [cprop][dca] [cprop][Tf2N] [bmpip][Tf2N] [tes][Tf2N] [toa][Tf2N] [emim][dep]

333 343 353 Temperature (K) Figure 1. Comparison of CO2/CH4 ideal selectivity among ILs that do not interact with CO2 through significant chemical reaction [9].

Journal Pre-proof 2.2 Water fraction range to be studied.

The [m-2HEA][Pr] used in the present study was collected from the same synthesizing batch of that used on a previous study [27] (0.860 ± 0.076 % gwater/g). In the present study, this mass fraction was converted into a molar fraction basis. The molecular weights of the IL and water were considered as ~149.2 gmol and ~18.0 gmol, respectively. The significant difference between these molecular weights promoted a significant increase in the water percentage, from 0.860 ± 0.076 % (gwater/g) to 6.70 ± 0.59 % mol.water/mol.

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Four different initial water content values were studied: 0.860 %(gwater/g), 3.5684 %

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(gwater/g), 5.7012 %(gwater/g), and 11.774%(gwater/g). These water fractions were obtained from the addition of small weighted amounts of water to the synthesized IL.

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In terms of molar fractions, the water fractions correspond to 6.70%, 23.47%,

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33.38%, and 52.52%.

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2.3 Synthesizing route.

The [m-2HEA][Pr] used in the present study was obtained from the same batch of

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that presented in a previous study [27]. The water content, 1H, and

13C

NMR

analyses were applied to the synthesized IL. These analyses can also be found in

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the supplementary material.

The studied alkanolamine based IL was synthesized through the neutralization of equimolar propanoic acid with N-methyl-2-hydroxyethyl amine. These precursors were purchased from Sigma Aldrich with mass purity of at least 99%. In summary, the synthesizing route consists of the neutralization reaction within a three-necked glass reactor in a nitrogen environment. The flask was mounted in a thermal bath at 283.1 K. First, and N2 flow was used to sweep away most of O2 and CO2. Then, the amine precursor was injected in the reactor followed by a slow dropwise addition of carboxylic acid. The final viscous product was purified in a rotary evaporator under a moderate vacuum (20 MPa) and a temperature of 333.15 K for 48 h.

2.4 NMR spectroscopy and water content.

Journal Pre-proof The NMR spectra were used to identify the product of the reaction and quantify the water-free purity of this material. The ionic liquid was previously dried under a high vacuum (10-4 Pa) for a period of at least 48 h. The 1H and

13C

NMR spectra were

acquired at 298 K on a Bruker Advance III 500 spectrometer operating at 11.75 T (500 MHz for 1H) and using D2O as the solvent. The acquisition parameters were: 64 and 1024 scans (NS), 64 and 32 k data points (TD), spectral window (SW) of 12.02 and 248.47 ppm, acquisition time (AQ) of 5.45 and 0.54 s, and relaxation delay (d1) of 10 and 0.5 s for 1H and

13C,

respectively. The results were analyzed in the Bruker

TopSpin software.

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The water contents were determined with a Metrohm 831 Karl Fischer titrator using a Metrohm coulometer. A total of 10 samples were collected and analyzed from

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recently synthesized [m-2HEA][Pr]. The uncertainty was calculated according to

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GUM [39], and the reported values are based on a standard uncertainty multiplied by a coverage factor k=2, providing a level of confidence of approximately 95%.

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Additional information about the method can be observed in a previous paper [27].

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2.5 Static-synthetic visual method in a variable volume cell.

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Phase equilibria experiments were conducted applying a static-synthetic visual method to a high-pressure variable-volume cell. The apparatus and the methodology used were fully described and validated in previous studies [27,40–43].

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The experimental apparatus consisted of a cell that was divided by a piston that could freely move to equalize the pressure of front and rear chambers. Two rubber O-rings guaranteed the sealing of this piston. The samples were added into the front cell, while CO2 was injected in the rear one to control the system pressure. The front cell temperature was measured by a thermocouple (resolution of ± 1 K). A CO 2 syringe pump (ISCO 260D) was used to control the system pressure or inject gas into the system. An absolute pressure transducer with a resolution of ± 0.01 MPa resolution (SMAR, LD 301) was employed to measure the pressure. A handheld programmer (SMAR, HT 201) controlled the pressure with a syringe pump. A water bath coated the syringe pump system with the help of a PID controller (MEC DIGI, model SHM 112). The pressure calibration was performed with the help of an HP34401A digital multimeter. Two sapphire windows were coupled to this chamber to allow the cell to be exposed

Journal Pre-proof to light and be observed by the operator. An inner stirring magnet is always added to the front compartment to promote the homogenization of mixtures. The system can achieve pressures up to 30 MPa and temperatures up to 423 K. More severe conditions would not be safe and could promote the degradation of the O-ring. The CO2 syringe pump can be connected to either the front or rear cell, depending on the valve’s configuration. The experiment procedure started with a CO2 flow through all lines to remove residual air. We collected 10-13g IL samples from the flask with a syringe. The samples were weighted and then injected into the front VLE cell. After that, the

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syringe was weighted to compute the total amount of IL that was injected. Another weighted syringe was used to perform a similar procedure with distilled water. After

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the injection of both weighted liquids, the cell was sealed. The addition of CO 2

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occurred under constant pressure and temperature. The amount of injected CO 2 can be calculated by the contained volume variation, according to the literature [44]. The

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CO2 used in the experiment was purchased from White Martins (>99 % purity by CG).

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The expanded uncertainties of molar fraction, system temperature, and equilibrium pressure were considered to be ± 5 x 10⁻⁴ mol/mol, ± 1.6 K, and > ± 0.01 MPa.

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These values were estimated according to GUM [39], based on a standard uncertainty multiplied by a coverage factor k=2, providing a level of confidence of

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approximately 95%. Additional information about these considerations and calculations can be found in the supplementary material.

2.6 Molecular simulation.

The simulation of gas solubility in solvents, which presents significant intramolecular flexibility, is a challenging task. A large number of atoms and their electrostatic interactions cause the molecular simulation to consume a high amount of computational resources. Therefore, a new force field was proposed to describe [m2HEA][Pr] molecules through a simplified approach. Following the TraPPE idea of prioritizing transferability, it is possible to minimize the number of (pseudo) atoms needed for the simulation. In the proposed force field, only the hydrogen atoms linked to electronegative elements (oxygen and nitrogen, for instance) were explicitly considered as interaction sites. The alkyl groups were considered as pseudo-atoms,

Journal Pre-proof and their Lennard-Jones parameters were taken from the TraPPE force field. The charge distribution for the anion and cation are similar to the force fields proposed by Doherty et al. [34]; Chen et al. [45], and Wick et al.[46]. Eq. 1 calculates the potential energy of the system as the sum of two terms related to intramolecular and intermolecular interactions: 𝑈𝑡𝑜𝑡𝑎𝑙 = 𝑈𝑖𝑛𝑡𝑟𝑎 + 𝑈𝑖𝑛𝑡𝑒𝑟

(1)

The term corresponding to the intramolecular interactions was divided into two other

(2)

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𝑈𝑖𝑛𝑡𝑟𝑎 = 𝑈𝐵 + 𝑈𝑁𝐵

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terms: interactions of bonded (UB) and non-bonded (UNB) atoms (Eq. 2).

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The UB term is related to energetic contributions of stretch bonds, bonds, angles, dihedral angles, and improper angles. In this work, the bond length was kept

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constant. Therefore, the UB term was calculated according to Eq. 3. (3)

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𝑈𝐵 = 𝑈𝑎𝑛𝑔𝑙𝑒 + 𝑈𝑑𝑖ℎ𝑒𝑑𝑟𝑎𝑙 + 𝑈𝑖𝑚𝑝𝑟𝑜𝑝𝑒𝑟

The energy associated with bond angles was calculated by the harmonic potential

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(Eq. 4).

𝑘𝜃 (𝜃 − 𝜃0 )2 2

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𝑈𝑎𝑛𝑔𝑙𝑒 =

(4)

The dihedral and improper angle energy contributions were calculated by an OPLSlike expression, in which all the dihedral parameters for the anion [Pr]- were taken from Doherty, et al. [34] (Eq. 5).

𝑈𝑑𝑖ℎ𝑒𝑑𝑟𝑎𝑙 = 𝑎0 + 𝑎1 (1 + 𝑐𝑜𝑠∅) + 𝑎2 (1 + 𝑐𝑜𝑠2∅) + 𝑎3 (1 + 𝑐𝑜𝑠3∅)

(5)

The expression used to calculate the van der Waals and Coulomb interactions between ij non-bonded atoms (UNB) and also between ij atoms of different molecules (Uinter) is the same and was calculated as shown in Eq. 6. 12

𝑈𝑁𝐵

𝜎𝑖𝑗 = 4𝜀𝑖𝑗 [( ) 𝑟𝑖𝑗

6

𝜎𝑖𝑗 𝑞𝑗 𝑞𝑖 −( ) ]+ 𝑟𝑖𝑗 4𝜋𝜀𝑜 𝑟𝑖𝑗

(6)

The first term is related to van der Waals energy. It was modeled using the pairwise Lennard-Jones (LJ) potential. The second term is related to the Coulomb interactions. The LJ potential requires the σij and εij parameters for each pair of atoms

Journal Pre-proof ij. The proposed force field gives the parameters for the pair of identical atoms (ii; jj). The LJ parameters for unlike pairs (ij) were calculated using the Lorentz-Berthelot combining rule (Eq. 7 and 8). 𝜎𝑖 +σ𝑗 2

(7)

𝜀ij = √𝜀𝑖 .𝜀𝑗

(8)

𝜎ij =

The Ewald summation technique well represents the electrostatic interactions. However, most of the computation costs come from the evaluation of long-range

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electrostatics with this method. McCann and Acevedo [47] presented a review of the pairwise alternatives to Ewald summation for the calculation of long-range

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electrostatic interactions in ionic liquids. Among these alternatives, the damped shifted force (DSF) method proposed by Fennell and Gezelter [48], using a dumpling

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value of 0.2 Å-1, provided a good energetic agreement with Ewald simulations but

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using less computation time. For this reason, this method was chosen to be applied in the present study modeling. A cutoff radius of 14 Å was used for both the anion’s

potential.

lP

and the cation’s contributions, and an analytical tail correction was applied to the LJ The simulations were carried out using the scaling procedure by

na

considering that there were no interactions between atoms separated by up to two bonds. A scaling factor of 0.5 was applied to the interaction of atoms separated by

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three bonds, and full interactions were used for atoms separated by four or more bonds. Tables 2 and 3 present the complete lists of parameters of proposed force fields of [m-2HEA][Pr] anion and cation. The atom charge distribution was evaluated in the prediction of the [m-2HEA][Pr] + H2O density using unscaled charges and also charges scaled by 0.9 and 0.8. The results were compared to the experimental data in the literature [49]. The simulations were performed in the NPT ensemble. The best scale charge was selected and applied to the simulation of the ternary phase equilibria (CO2 + H2O + [m-2HEA][Pr]) using the GEMC-NPT ensemble. The four-site model, called TIP4P [50], was selected for H2O, and the three site TraPPE model [45] was used for CO2 [50]. All simulations were performed with the help of the software package Cassandra v1.2 [51]. For NPT simulations, the moves used were: translation (33%), rotation (33%), regrowth (33%), and volume (1%). For the GEMC simulations, the probabilities used were: 30% translation, 30% rotation, 30% regrowth, 0.5% volume, and 9.5% of

Journal Pre-proof configurational-biased exchange of molecules between boxes. The regrowth moves were done using the configurational-biased technique with 12 trial positions. In the NPT and GEMC simulations, 150 pairs of ions were used and the number of water molecules was changed to observe the molar water fraction.

ro

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lP

l (Å) 1.250 1.540 1.540 ao (kJmol-1) 0.000 0.000

Non-bonded atoms parameters ε / kb (K) q (e) 52.843 0.7 105.69 -0.8 46.0 -0.1 98.0 0.0 Non-bonded atoms parameters Angle Kθ (K rad-2) 0 (gradus) O–C–O 126.0 40261.7 O–C–CH2 117.0 35229.0 C–CH2–CH3 111.1 31706.0 -1 -1 a1 (kJ mol ) a2 (kJ mol ) a3 (kJ mol-1) 0.000 43.934 0.000 0.000 0.000 0.000

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Bond C–O C–CH2 CH2–CH3 Dihedrals* O–C–CH2–O CH3-CH2-C-O

σ (Å) 3.75 3.02 3.95 3.75

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Atom C O CH2 CH3

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Table 2. Force field parameters proposed for the anion: [Pr]-.

Journal Pre-proof Table 3. Force field parameters proposed for cation: [m2HEA]+.

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Kθ (Krad-2) 25178.0 31250.0 31250.0 21955.0 25200.00 27700.0 a3 (kJmol-1) -2.217 6.401 1.562 -0.873

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Bond CH3N–N+ N+–CY N+–HN CX–CY CX–O O–HO Dihedral (OPLS) CH3–N+–CY–CX N+–CY–CX–O CY–CX–O–HO HN–N+–CY–CX

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Atom CH3N N+ Cy HN CX O HO

Non-bonded atoms parameters σ (Å) ε / kb (K) q (eV) 3.75 98.0 0.485 3.43 84.5 -0.75 3.95 46.0 0.265 0.00 0.00 0.485 3.95 46.0 0.295 3.02 93.0 -0.700 0.00 0.00 0.435 Bonded atoms parameters la(Å) Angleb Θ0 (degrees) + 1.448 CH3N–N –CY 109.5 1.448 CH3N–N+–HN 112.9 1.01 CY–N+–HN 112.9 + 1.54 HN–N –HN 106.4 1.43 CY–CX–O 109.5 0.945 CX–O–HO 108.5 ao (kJmol-1) a1 (kJmol-1) a2 (kJmol-1) 26.096 -10.780 -4.950 0.000 1.468 -0.444 0.000 1.744 -0.242 2.928 0.397 -0.873

3. Results and discussion 3.1 IL characterization.

As reported in our previous study [27], the [m-2HEA][Pr] structure was confirmed by 1H

and

13C

NMR. The purity of water free components was 99.95%. This value was

determined by the difference between the area under the curve of known peaks and unknown ones (impurities). We also assumed that no metal contaminant was present. The water measurements of recent synthesized ILs resulted in 0.860 ± 0.076 %(gwater/g) or, on a molar basis, 6.70 ± 0.59 % (molwater/mol). Additional information about NMR results and water contents can be found in our previous paper [27] and the supplementary material.

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3.2 ELV high-pressure ternary phase equilibria. The high-pressure phase equilibria data (bubble points) of [m-2HEA][Pr] + CO2 +H2O were successfully obtained at different molar fractions, as can be seen in Figures 25. For a fixed system composition, the increase in the temperature promotes an increase of bubble point pressures, which means that the solubility of CO 2 in the

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na

lP

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liquid phase decreases as the temperature increases.

Figure 2. Phase equilibria of [m-2HEA][Pr] + CO2 +H2O at initial water molar fraction of 6.70% [27].

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Figure 3. Phase equilibria of [m-2HEA][Pr] + CO2 +H2O at initial water molar fraction of 23.47%.

Figure 4. Phase equilibria of [m-2HEA][Pr] + CO2 +H2O at initial water molar fraction of 33.38%.

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Figure 5. Phase equilibria of [m-2HEA][Pr] + CO2 +H2O at initial water molar fraction of 52.52%.

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Figures 6 and 7 show a comparison of phase equilibria data at a fixed temperature (353 K). However, they differ in the way the molar contents were calculated. In Figure

(𝑥𝑖 = 𝑛

𝑛𝑖

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6, the molar contents were calculated considering water as part of the solution 𝐶𝑂2 +𝑛𝐼𝐿 +𝑛𝑤𝑎𝑡𝑒𝑟

). In Figure 7, on the other hand, the molar contents were

calculated on a water-free basis (𝑥𝑖 = 𝑛

𝑛𝑖

𝐶𝑂2 +𝑛𝐼𝐿

), i.e., when the CO2/IL ratio remains

constant regardless of the amount of water present in the solution.

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Figure 6. Phase equilibria of [m-2HEA][Pr] + CO2 +H2O at 353 K. considering water in molar fraction calculus.

(a) (b) Figure 7. Phase equilibria of [m-2HEA][Pr] + CO2 +H2O at (a) 353 K and (b) 313 K. presented with water-free molar fractions. Figure 6 shows that the increase in the water content, within the studied range, promotes a substantial reduction of the CO2 solubility (an increase of equilibrium pressures). However, for a water-free composition (Figure 7), the bubble point pressures remain almost unchanged as the water content increases. It is reasonable

Journal Pre-proof to assume that the CO2 solubility is mainly affected by the amount of IL in the mixture, regardless of the amount of water added to the system. Thus, the reduction of the CO2 solubility, observed in Figure 6, should be related to the overall decrease in the IL molar content. Similar conclusions were also affeered by previous studies on imidazolium-based ILs [25]. Despite the apparent high molar percentage of water in the mixtures (6.7% to 52.5 %), the water fraction present in these mixtures is significantly low on a mass basis (below 12% g/g). It indicates that the addition of a small amount of water to a fixed amount of IL does not significantly change the amount of CO 2 dissolved in the

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mixture. Therefore, it can potentially be used as a solvent, for example, to mitigate the high viscosity of these ILs.

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A previous study suggested that the CO2 solubilization mechanism in non-reactive

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ILs was related to the empty spaces between the ions [23]. In this way, the increase in the water content should have reduced the CO2 solubilization since these

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molecules would be competing for the available empty spaces. Figure 7 proves this was not the case with the [m-2HEA][Pr] in the studied ranges of pressures,

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temperatures, and compositions. However, experimental data of CO2 fractions (XCO2 ~ 0.1) indicate that the increase of water might reduce the gas solubility in the IL. The

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conclusions.

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insufficient amount of data, however, prevents us from coming to any further

3.3 Molecular simulation.

The density of [m-2HEA][Pr] at 313.15 and 333.15K was evaluated for different water contents using the proposed simplified force field (Tables 2 and 3). The results are presented in Figure 8, along with experimental data obtained from the literature [52]. The results reasonably agree with the experimental data, indicating that the proposed force field parameters for the N-methyl-2-hydroxyethylammonium propionate are suitable and can be used to predict the carbon dioxide absorption. The best agreement, in the range 0 < Xwater < 0.6, was obtained using the charge scaled by 0.8. The simulated density at 313.15K and 333.15K using the net charge of ±0.8 were 1048.2±0.7 kg/m3 and 1040 ±1.7 kg/m3, which are in good agreement with experimental values: 1056.2 and 1043.2 kg/m 3, respectively [52]. The value predicted by the proposed force field at 313.15K (1020.759) is also closer to experimental data

Journal Pre-proof than that values predicted by the all-atom force field proposed by Diel and Cabral [36]. Therefore, the proposed force field, using the charge scaled by 0.8, was used to perform the GEMC-NPT simulations and estimate the ternary phase equilibria data

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(CO2 + H2O + [m-2HEA][Pr]).

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Figure 8. Density of [m-2HEA][Pr] with different water content at T=313.15K (a) and T=333.15K (b). Square, cross, and triangle symbols are values predicted by the force field proposed using the unscaled charge, scaled by 0.9 and 0.8, respectively. The full circles are experimental data presented by the literature [52]. GEMC simulations were performed to evaluate the effect of the water content on the phase equilibria of CO2 + H20 + [m-2HEA][Pr] at 313K, 333K, and 353K. The results are presented in Figures 9, 10, and 11, respectively. Throughout the simulation, we observed that the larger deviations between experimental and simulation always occur in the low-pressure region.

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Figure 9. Phase equilibria of CO2+H2O+[m-2HEA][Pr] at 313K. The open symbols are GEMC results using the force field proposed and charge scaled by 0.8, while the filled symbols are experimental data. Triangle, square, and circle symbols represent the water content of 6.7%, 33.3%, and 52.5%, respectively.

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GEMC 6.70%

16

Experimental 6.70% GEMC 33.38%

14

Experimental 33.38%

Pressure (MPa)

12 10 8

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6

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0

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0 0.1

0.2

0.3

333 K 0.4

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x CO2

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Figure 10. Phase equilibria of CO2+H2O+[m-2HEA][Pr] at 333K. The open symbols are GEMC results using the force field proposed and charge scaled by 0.8, while the filled symbols are experimental data. Square and circle symbols represent the water content of 6.7% and 33.3%, respectively.

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Figure 11. Phase equilibria of CO2+H2O+[m-2HEA][Pr] at 353K. The open symbols are GEMC results using the force field proposed and charge scaled by 0.8, while the filled symbols are experimental data. Triangle, square, and circle symbols represent the water content of 6.7%, 33.3%, and 52.5%, respectively.

Although the results obtained with the proposed force field are not in full agreement with the entire experimental data set, we can observe that the simulated phase equilibria follow the behavior of experimental data with apparent low deviations. Furthermore, the force field parameters were obtained by transferability without further optimization. A new investigation involving the use of net charge ± 1 and ± 0.9 should be performed for ternary phase equilibria. Marin-Rimoldi et al. [53] found optimal charge scale values of 0.8, 0.9 and 1.0 for describing water solubility in the following 1-n-butyl-3-methylimidazolium-based ionic liquids: [C4min][Cl], [C4min][PF6] and [C4min][Tf2N], respectively. Neither their molecular approach nor the currently applied one could precisely predict the experimental phase equilibria data of the ethanolamine-based IL and CO2. Since several other approaches had also been applied without significantly high precision (Table 1), we believe that the applied considerations should be reviewed. Possibly, the consideration of non-formation of

Journal Pre-proof chemical bonds between these ILs and CO2 should be disregarded in further studies. This assumption was made due to the NMR analyses of IL samples collected before and after the high-pressure CO2 solubilization experiments. The results indicate that no significant degradation nor new components were formed after the carbon dioxide solubilization [9,13,27]. However, a reversible reaction mechanism may be occurring at the high-pressures and moderate temperatures of the ELV experiments. Thus, after the pressure and temperature drop, back to ambient conditions, the products formed may have reversibly turned back into their original ILs, therefore justifying the

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absence of these chemicals in the NMR analyses.

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Conclusions

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The water effect on the carbon dioxide solubilization process in an alkanolaminebased IL [m-2HEA][Pr] was studied. The results indicate that the increase in the

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water content of the IL promotes a substantial reduction of the CO 2 solubility. This behavior is very well described in the literature. However, on a water-free basis, the

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bubble point pressures remain almost unchanged despite the increase in the water content, i.e., the CO2 solubility is mainly related to the amount of IL in the mixture,

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regardless of the amount of water added to the system. Consistent experimental high-pressure phase equilibria data, of about 100 points, were successfully

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measured and reported in the present paper. The proposed simplified force field, based on the force parameters proposed by Doherty et al. [34]; Chen et al. [45], and Wick et al. [46], was adjusted to represent [m-2HEA][Pr] experimental density data at different water content, using different net charges. The adjusted force field, using a 0.8 net charge, was applied to predict the ternary phase equilibria data with reasonably low deviations from experimental data.

CRediT author statement Alcantara M. L.: Experimental data measurements and analyses, Writing – original draft, review & editing, and its corrections. Mattedi, S.: Funding acquisition, resources, conceptualization, and supervision. Cardozo-Filho, L.: Resources, supervision of experimental work Silva, P. H. R.: Software, molecular simulation. Romanielo L.: Software, molecular simulation supervision, Writing - Review &

Journal Pre-proof Editing, and its corrections.

Acknowledgments The authors gratefully acknowledge the support from CNPq (Proc. 452003/2019-9 and Grant PQ 306640/2016-3), CAPES (PDSE-88881.132805/2016-01), and FAPEMIG (APQ-03452-17) are gratefully acknowledged.

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Journal Pre-proof Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. All of the sources of funding for the work described in this publication are acknowledged below:

Financial support from CNPq (Proc. 452003/2019-9 and Grant PQ 306640/2016-3). CAPES (PDSE-88881.132805/2016-01) and FAPESB/SECTI (APP0075/2016) are

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Highlights



Report of high-pressure ternary phase equilibria experimental data of an

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alkanolamine based IL + CO2 +H2O.

CO2 solubility is mainly related to the amount of IL in the system.



An increase in water content promotes the reduction of CO2 solubility by

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

reducing the IL concentration. 

Parameters of the force field were proposed to adjust IL experimental density data.



Monte-Carlo molecular simulation predicted the ternary phase equilibria with moderate-low deviations.