Solvation Thermodynamic Properties of Hydrogen Sulfide in [C4

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Solvation Thermodynamic Properties of Hydrogen Sulfide in [Cmim][PF], [Cmim][BF] and [Cmim][Cl] Ionic Liquids, Determined by Molecular Simulations 4

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Joel Sanchez-Badillo, Marco Gallo, Sandra Alvarado, and Daniel Glossman-Mitnik J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b06525 • Publication Date (Web): 03 Aug 2015 Downloaded from http://pubs.acs.org on August 9, 2015

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The Journal of Physical Chemistry

Solvation Thermodynamic Properties of Hydrogen Sulfide in [C4mim][PF6], [C4mim][BF4] and [C4mim][Cl] Ionic Liquids, Determined by Molecular Simulations Joel Sánchez-Badillo1, Marco Gallo1*, Sandra Alvarado2, Daniel Glossman-Mitnik3 1

Facultad de Ciencias Químicas, UASLP, Av. Manuel Nava No. 6, Zona Universitaria San Luis Potosí, S.L.P. 78210, México. 2 Centro de Investigación en Alimentación y Desarrollo (CIAD), A. C. , Av. Cuarta sur No. 3820, Fracc. Vencedores del Desierto., 33089 Unidad Delicias, Chihuahua, México. 3 Laboratorio Virtual NANOCOSMOS, Departamento de Medio Ambiente y Energía, Centro de Investigación en Materiales Avanzados, Chihuahua, Chih. 31136, México *Corresponding author: Marco Gallo, Facultad de Ciencias Químicas, UASLP, San Luis Potosí, S.L.P. 78210, México, Telephone: 52-444-826-2300, Fax: 52-444-826-2300, e-mail: [email protected]

Abstract Removal of hydrogen sulfide (H2S) and acid gases from natural gas is accomplished by absorption processes using a solvent. The gas solubility in a liquid can be used to measure the degree of removal of the gas and is quantified by the Henry´s constant (kH), the free energy of solvation at infinite dilution or the excess chemical potential (µex). In

this work, Henry´s constants and thermodynamic properties of solvation of H2S were calculated in three ionic liquids [C4mim][PF6], [C4mim][BF4] and [C4mim][Cl] ([C4mim]: 1-butyl-3- methyl imidazolium). The first step in this work was the evaluation of the force fields (FF) for the gas and condensed phases in order to obtain accurate values for the excess chemical potential for H2S on each ionic liquid using free energy perturbation techniques (FEP). In the H2S-[C4mim][PF6] and H2S-[C4mim][BF4] systems the results obtained by molecular simulation agree with the experimental values reported in the literature. However, the solvation free energy calculated for the H2S-[C4mim][Cl] system can be considered predictive because of the lack of experimental data at the simulated conditions. Based on these results, the best solvent for removing H2S is [C4mim][Cl] since it has the highest affinity for this specie (lowest value of the Henry´s constant). Also, solvation thermodynamic properties such as enthalpy and entropy were calculated in order to evaluate their contribution to the free energy of solvation.

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Introduction Natural gas has become an alternative source of energy worldwide. Methane (CH4) is the main component of natural gas; however, other contaminant acidic gases such as carbon dioxide (CO2), hydrogen sulfide (H2S) and nitrogen oxides (NOX) also are present in natural gas. Acid gases must be removed to avoid emissions into the atmosphere. Hydrogen sulfide is a toxic and corrosive compound and prolonged exposure is proven to cause severe health problems.1 The sweetening of natural gas is the process where acidic gases (CO2, H2S) are absorbed using amine solutions.2,3 Amines are organic compounds that evaporate at room temperature (Volatile Organic Compounds VOCs), causing air pollution. It is extremely important to design new absorbents that are friendly with the environment, i.e. Ionic liquids, which are considered as green solvents.4 Ionic liquids (IL) consist of a cation and an anion and have melting points below 100°C,5 low vapor pressures and are in a liquid-like state below 25°C. Recently, ionic liquids have caught the attention of the scientific community because changes in the structure and type of ion permits the design of compounds with precise physicochemical properties for a wide range of applications, ranging from sensors to medicine.5,6 There are many databases reporting limited properties of ionic liquids7-9 It also has been shown that ionic liquids can be used in chemical reactions as catalysts.10,11 Several authors have found that changing the structure and size of the anion in IL substantially modifies the interaction energy with gaseous solutes.12-14 This finding has permitted the synthesis and design of ionic liquids that can function as solvents for specific tasks, such as the sweetening of natural gas. There are several theoretical and experimental studies of the solubility of gases (CO2, SO2, CO, N2, O2, H2, N2O, CH4, Ar, Xe, H2O, etc.) employing the ionic liquid cation 1-butyl-3-methylimidazolium ([C4mim]). Within the experimental studies we can find the following ionic liquids: Hexafluorophosphate, [C4mim][PF6];4,13-23

Tetrafluoroborate

[C4mim][BF4];12-14,19,21,22,24-26

Acetate,

[C4mim][Ac];27 Trifluoroacetate [C4mim][TFA];22,27 Bis(trifluoromethyl-sulfonyl)imide, [C4mim][Tf2N];12,17,19,21,22,25,28-31

Dicyanamide

[C4mim][DCA];31

Methyl

sulfate,

[C4mim][CH3SO4];23,32 and Trifluoromethane sulfonate, [C4mim][CF3SO3].33 Nevertheless, the experimental information regarding the solubility of H2S in ionic liquids is still 2 ACS Paragon Plus Environment

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scarce.18,19 An excellent review of the solubility of gases in ionic liquids has been published by Lei et al.34 In the H2S-[C4mim][PF6] system at 298.15, K Jou and Mather18 reported a Henry´s constant value of 14.3 bar. These authors also obtained the enthalpy of solvation at infinite dilution ( ) and the entropy at infinite dilution ( ). For the systems H2S-

[C4mim][PF6] and H2S-[C4mim][BF4] at 303.15 K, Jalili et al.19 obtained a Henry´s constant of 18.6 ± 0.2 and 15.5 ± 0.1 bar respectively, along with solvation thermodynamic properties, however, the free energies of solvation reported by these authors are positive, indicating that the solvation process is unfavorable, difficult to occur. Molecular dynamics simulations have been used to understand the energetic and structural phenomena at the molecular level in IL and to obtain thermodynamic and transport properties in the bulk phase.35-44 Nevertheless, the precision of the thermodynamic properties obtained using molecular simulations depends strongly on the quality and the transferability of the force field employed.45,46 Hanke et al.47 carried molecular dynamics simulations to obtain the solubility of benzene with and without charges in dimethyl-imidazolium chloride and dimethylimidazolium hexafluorophosphate ionic liquids. The molecules were modeled using their own force fields. The authors found that the local solute-solvent electrostatic interactions largely influence the solubility of the gaseous species. Shah and Maginn38 performed NpT Monte Carlo simulations and used Widom´s test particle insertion or TPI-Widom48 to calculate the solubility of carbon dioxide in [C4mim][PF6] with united-atom (UA-FF) and all-atom (AA-FF) force fields.35,36 The calculated solubility of CO2 was lower than the experimental value, the authors argued that the strength of solute-solvent interactions was not correctly assessed by these force fields. Urukova et al.49 calculated the solubility of CO2, CO and H2 in [C4mim][PF6] using Monte Carlo simulations with the united-atom force field proposed by Shah et al.35 The calculated Henry´s constants for CO2 and CO present large deviations with respect to the experimental values. In the same year (2005), Shah and Maginn50 published a study on the solubility of gases such as CO2, N2, O2, C2H6 in the [C4mim][PF6] IL. These authors modeled the IL with a united atom force field38 and used TPI-Widom and Expanded Ensemble (EE) techniques to calculate the excess chemical potential and subsequently the 3 ACS Paragon Plus Environment


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Henry's constant. These authors noticed that polar solutes like water or CO2 have strong interactions with the anion, because of the solute-anion interactions, unlike non-polar solutes as ethane where the solvent-solvent interactions hinders the solute-solvent interactions causing low solubility. Pomelli et al.51 performed both theoretical and experimental studies of the solubility of H2S in various ionic liquids at 298.15 K and 14 bar, and they found that the acid gas does not react with the IL used. The hydrogen sulfide is more soluble in [C4mim][Cl], and the solubility of H2S decreases in the order [C4mim][Cl] > [C4mim][BF4] > [C4mim][TfO] > [C4mim][Tf2N] >> [C4mim][PF6], this experimental trend also was corroborated by first principle calculations of the interaction energy of H2S with ionic liquids. Kerlé et al.52 carried molecular simulations to determine the excess chemical potential of carbon dioxide in [Cnmim][Tf2N] with n = 2, 4, 6 and 8. The ionic liquid was simulated with the all-atom force field (AA-FF) proposed by Canongia-Lopes et al.40 The authors noticed that the dominant interactions are anion-CO2, in addition that the greater the side chain, the greater the solubility of CO2. They proposed the term “cavity contribution” to explain this effect, the space that must be created to insert the CO2 molecule between the anions and cations of the ionic liquid, as well as the repulsive energy involved. Ghobadi et al.53 performed molecular simulations to determine the solubility of CO2 and SO2 in a series of ionic liquids, including [C4mim][PF6] and [C4mim][BF4]. These authors found that the gas-anion interactions are important when the charge density on the anion is large, but that the order of the anion-gas interaction does not explains solely the solubility. They defined a new solubility parameter as the ratio of the solute-ionic liquid interaction energy over the cation-anion interaction energy density. Prasad and Senapati,54 by carrying first principles calculations, noticed that the gascation interaction energy was lower than the gas-anion interaction energy – a key aspect to understand the solubility of combustion gases such as SO2, N2 and CO2 in the ionic liquid [C4mim][NO3]. However, they indicated that free volume can play an important role in gas solubility. Jalili et al.55 carried a theoretical and experimental study about the solubility of CO2, H2S and mixtures in [C8mim][Tf2N]. The authors found that H2S is two times more soluble than CO2. To understand this result, they carried out ab-initio calculations and 4 ACS Paragon Plus Environment

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found that the H2S-IL and H2S-[Tf2N]- energetic interactions are higher than the CO2-IL and CO2-[Tf2N]- interactions, concluding that the strength of gas-anion energy interaction is responsible for the solubility order found. Based on free volume considerations, Damas et al.,56 by performing ab-initio calculations, noticed that the solubility of the gases CO2, H2S or SO2 increases with the size of the cation in the [Cnmim][Tf2N] ionic liquid. This behavior occurs because the charge of the cation is re-distributed with the number of carbons in the chain, decreasing the energetic interaction with the anion and thereby increasing the free volume and therefore the solubility. The space to accommodate the gas in the IL is created by the weakening of the anion-cation energetic interactions.56 Liu et al.57 carried Free Energy Perturbation58 calculations (FEP) to obtain the Henry´s constant of various gases (H2S, CO2, N2, H2) in [C2mim][Tf2N]. The authors found that polar gases show higher solubility compared to non-polar gases. The Ionic liquids were simulated with an all-atom force field based on the generalized force field from AMBER (GAFF).37 Wu and Maginn59 obtained by molecular simulations the Henry's constant of water in five different ionic liquids. The Ionic liquids had in common the tetrabutylphosphonium ([P4444]+) cation combined with aprotic heterocyclic anions such as 2-Cyanopyrrolide ([2CNpyr]-). The objective of the study was to understand the effect of water on ionic liquids properties for CO2 capture. Also the solubility, enthalpies and entropies of solvation were calculated, indicating that the solubility process is mainly enthalpy driven. The amount of information (theoretical or experimental) for the H2S-IL system is limited and exists only for some ionic liquids such [C4mim][PF6], [C4mim][BF4], [Cnmim][Tf2N] and [C4mim][Cl].18,19,51,55,57 In contrast the amount of information available for CO2 is large and continues to grow. 4,12,13,15,16,23-31,33,38,49,50,52,53-57,59 The solubility of gases in liquid solvents can be obtained by molecular simulation calculations by determining the Henry's constant (kH). A function of the solute excess chemical potential µex (isothermal-isobaric conditions) in the system.60 The lower the value of the Henry´s constant, the greater the solubility of the gas in the condensed phase. The Henry's constant is obtained from the excess chemical potential using the following equation:60 5 ACS Paragon Plus Environment


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= exp

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(1)

= 1/ , and is the ionic liquid density. There are several computational techniques to obtain the excess chemical potential; one method is based on the transformation from an initial state (0) to a final state (1) known as free energy perturbation (FEP) techniques.58,61 FEP gradually changes the value of the atomic variables (charges, Lennad-Jonnes parameters) via a coupling parameter λ that affects the potential energy of the system (or Hamiltonian ℋ) between two consecutive states, as shown in the following equations:58,61 ℋ = ℋ!"# + ∆"# & − ℋ!"# &

(2)

*

∆ = ()* − ()+ = , − ln〈exp!− ℋ ⁄&〉(

(3)

()+

Indexes 0 and 1 represent the initial and final states respectively. Each ∆λi constitutes a window in FEP techniques. In each window it is necessary to perform a system equilibration and a production step in order to determine the free energy of that window. The difference between the free energy at λ=0 (initial state) and λ=1 (final state) gives the total free energy (free energy of solvation). When the system is at λ=0, we have the condensed phase containing a solute molecule, and when λ=1 we have only the pure condensed phase. The alchemical transformation can be performed in both directions: removal of the solute molecule in the forward direction (0 1) or the creation of the solute molecule in the backward direction (1 0). Both results must match numerically with the opposite sign. Techniques such as the Bennett acceptance ratio (BAR) allow the merging of results from both directions in order to generate a statistically correct result.61 The objective of this work is the calculation of Henry's constant for H2S in three imidazolium-based ionic liquids [C4mim][PF6], [C4mim][BF4] and [C4mim][Cl], using free energy perturbation techniques (FEP) in combination with Bennett´s acceptance ratio


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(BAR). We also obtained the thermodynamic excess properties (μex, hex, sex) in each simulated system to elucidate the solubility mechanism.

Computational methodology First the simulations were carried at 298.15 K and 1 atm, using the NpT ensemble with a time step of 1 fs in a cubic cell. The NAMD62 simulation package, along with the Visual Molecular Dynamics package VMD,63 was used in this work. The temperature and pressure were controlled by Langevin Dynamics and Langevin Dynamics Piston respectively62, Lennard-Jones Parameters for cross interactions were calculated using Lorenz-Berthelot mixing rules.64 As the initial step in this work, a simple validation of the FEP technique was performed by obtaining the excess chemical potential of argon in water. Subsequently the force field of the H2S molecule was tested under the same FEP scheme (H2S-Water system). Also structural and energetic properties of pure ionic liquids, such as density (ρ)65 enthalpy of vaporization (∆Hvap) and radial distribution function (RDF), were calculated in order to determine the quality of the ionic liquids force fields. After observing that these force fields reproduced accurately the free energy of hydration for H2S and structural and energetic properties of pure ionic liquids we proceeded next with the determination of the excess chemical potential of H2S in ionic liquids [C4mim][PF6], [C4mim][BF4] and [C4mim][Cl], along with the determination of thermodynamic excess properties (hex and sex).

Determination of the Free energy of solvation of Argon in Water. 800 water molecules were placed in a simulation box containing and modeled with the TIP3P66 force field as shown in Figure 1a. The Lennard-Jones (LJ) parameters for argon were taken from Tobias and Brooks III67, as shown in Figure 1b. The equilibration and production times were 2 and 5 ns respectively, with a 12 34 cutoff. The particle mesh Ewald68 summation (PME) algorithm was used for the coulombic interactions, with an accuracy of 10-6. After simulation of the pure aqueous phase, an argon atom was inserted into the simulation box followed by an equilibration step of 1 ns.



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The transformations in the forward and backward directions had the same specifications of temperature, pressure, cutoff and PME as the pure condensed phase. The coupling parameter (∆λ) was 0.0625 for the forward direction. After finishing this direction, the output coordinates and a value of ∆λ=-0.0625 were used to simulate the backward direction. Sixteen windows for each FEP transformation step were used. Each window had an equilibration and production steps of 100 and 600 ps respectively. The soft core potential69 (SCP) was used in all FEP simulations. The merging of the results of both directions was calculated with the BAR technique using the plug-in Parse FEP70 included in the VMD software.

Determination of the Free energy of solvation of H2S in Water. In this system, 800 water molecules were placed in a simulation box and modeled with the TIP4P/ε force field71 (Figure 1c). Subsequently one H2S molecule (Figure 1d) was inserted in the simulation box. H2S was simulated with the force field (Model C) reported by Potoff et al.72 The same specifications for the Ar-water system were used in this system except that the equilibration time for the system with the H2S molecule was 2 ns. The FEP calculations used a window size of ∆λ =0.02 and were equilibrated for 100 ps followed by 600 ps of production for each window.


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Figure 1. Structures, nomenclature and force fields for the molecules studied in this work. a) Water, TIP3P force field66 b) Argon67 c) Water, TIP4P/ε force field 71 d) Hydrogen Sulfide e) Anions [PF6]-, [BF4]-, [Cl]- and cation [C4mim]+ in the Ionic liquids [C4mim][PF6] [C4mim][BF4] and [C4mim][Cl]. Oxygen atoms are displayed in red color, Hydrogen in white, Argon in blue, Lone-Pair electrons in orange, Sulfur in yellow, Fluorine in pink, Phosphorus in purple, Boron in light green, Chlorine in dark green, Carbon in gray and Nitrogen atoms in blue color.

Determination of the Free energy of solvation of H2S in Ionic Liquids. For these systems we used 200 ionic pairs of each ionic liquid in our simulation box. The united-atom force fields (UA-FF) were used. The [C4mim][PF6] and [C4mim][BF4] ionic liquids were simulated with the force field proposed by Zhong et al.44 The force field developed by Liu et al.43 was used for [C4mim][Cl], Figure 1e. The



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simulation times were 2 and 5 ns for the equilibration and production for the pure condensed phases respectively, a 16 34 cut off was used, while electrostatic interactions were calculated using PME. The density (ρ), enthalpy of vaporization (∆Hvap) and radial distribution function (RDF) were obtained at 298.15 K and compared against experimental data or previous simulations reported in the literature. After the calculation of the pure phase properties of ionic liquids, one H2S molecule was inserted into the simulation box. The

H2S-[C4mim][BF4],

H2S-[C4mim][BF4]

and

H2S-[C4mim][Cl]

systems

were

equilibrated for 2 ns followed by 4 ns production under the same conditions as the pure condensed phase. To obtain the enthalpy of vaporization, the gas phase for the ionic liquid was simulated using a single ion pair (cation-anion) with the same force field used for the simulation of the pure condensed phase at the same temperature. No periodic boundaries conditions were applied to simulate the ideal gas as proposed by Morrow and Maginn,36 among other authors.43,44 The FEP calculations used a window of ∆λ=0.064 in the range of λ=0.00 to λ=0.96 and a window of ∆λ=0.008 from λ=0.96 to λ=1.00, 20 windows in total. Values of -0.008 and -0.064 were used for the windows in the calculations for the backward direction in the same range as the forward direction. Each window was equilibrated 300 ps followed by 1000 ps of production. In order to obtain thermodynamic excess properties (hex and sex), the IL and then H2S-IL systems were simulated 2 and 5 ns for equilibration and production respectively at another temperature 353.15 K and 1 atm. The final coordinates for the H2S-LI production were used in the FEP calculation in both forward and backward direction at 353.15 with 20 windows with the same equilibration and production time for each window as the FEP at 298.15 K. The BAR method was used also at 353.15 K. We carried numerical temperature derivatives for the excess chemical potential to determine the enthalpy and entropy of solvation according to the following equations52:

ℎ67 = 67 + 8 67 8 67 = − 9

: 67 ; :

Solvation Thermodynamic Properties of Hydrogen Sulfide in [C4

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