Microscopic Characterization of CO2 and H2S Removal by Sulfolane

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Microscopic Characterization of CO2 and H2S Removal by Sulfolane Mert Atilhan, Alberto Gutiérrez, and Santiago Aparicio Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01577 • Publication Date (Web): 27 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017

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Microscopic Characterization of CO2 and H2S Removal by Sulfolane Alberto Gutiérrez,a Mert Atilhan,b and Santiago Aparicio*a a b

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

Department of Chemical Engineering, Texas A&M University at Qatar, Doha, Qatar

*

Corresponding author: [email protected]

ABSTRACT: The properties of sulfolane as a physical absorbent for CO2 and H2S acid gases are studied by using density functional theory (DFT) and molecular dynamics (MD) computational chemistry coabsorption of undesired compounds such as hydrocarbons methods with the objective of obtaining a microscopic picture on the use of this solvent for natural gas sweetening purposes. The reported results herein provide the information on intermolecular forces between the absorbent and acid gas molecules, structural changes upon absorption and the behaviour of acid gases at solvent – gas interfaces. Likewise, the cytotoxicity of sulfolane is analyzed through molecular dynamics studies of its interaction with model lipid biomembranes.

Keywords: sulfolane; natural gas sweetening; molecular dynamics; density functional theory; cytotoxicity.

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INTRODUCTION Acid gas removal from natural gas has great technological and economical relevance.1-4 The absorption of CO2, H2S, COS and mercaptans from unprocessed natural gas has been done mostly by using the so-called SULFINOL process, in which a mixture of alkanolamines (diisopropanolamine (30-45%) or methyl diethanolamine (MDEA), 30-45 %), water (5-15 %) and sulfolane (SFL, 40-60 %) is used as solvent for gas sorption.5,6 In this process, chemical (by the alkanolamine) and physical (by SFL) absorption is combined, which leads to several advantages such as higher gas loading and lower energy requirements for regeneration. Considering the elevated pressure conditions of the acid and sour natural gases,7 physical absorption by SFL molecules has a pivotal role in total acid gas capture apart of chemical absorption by alkanolamines. These characteristics make SULFINOL process suitable for other gas treatment operations such as further capture and removal of CO2 in LNG plants.8 Therefore, considering the pivotal role of physical absorption by SFL on the whole performance of the SULFINOL process,4,9,10 it is necessary to obtain detailed information on the mechanism of physical absorption of CO2 and H2S by SFL, which would allow to infer the physicochemical basis of the contribution of SFL molecules to the whole performance of the SULFINOL process in order to impose retrofitting of the processing plants as well as to implement further improvements for developing new methods or solvents with better performance for gas sweetening.11-13 Likewise, it is a well-known technological challenge that the use of alkanol amines for acid gas capturing purposes that leads to issues such as corrosion of process equipment, solvent degradation or losses,14-16 and thus increasing the role of SFL for acid gas capture in order to decrease the load on the alkanolamines in a classical absorber/desorber units. Although COS and mercaptans are also absorbed by SULFINOL process, the analysis of CO2 and H2S absorption is of larger relevance considering that these compounds are in the 3-6 % and 1-3% ranges in a typical natural gas feed, respectively, for raw natural gas whereas the other sulfur compounds are remarkably below these limits. In this work, the properties of SFL regarding to CO2 and H2S physical capture purposes are analyzed from a microscopic viewpoint using a computational chemistry approach according to Density Functional Theory (DFT) and classic molecular dynamics (MD) methods. The objective of this work is to provide a detailed picture of the physical absorption 2


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mechanism of CO2 and H2S by SFL in terms of intermolecular forces, SFL microscopic structuring and molecular changes upon acid gas absorption. Considering the extensive use of SULFINOL process in the natural gas industry, and thus of SFL, it is remarkable that there are no systematic studies on the microscopic properties of SFL as solvent for acid gases in open the literature. There are only a handful of studies that tackles CO2 solubility in SFL via Monte Carlo methods,12,13 but limiting the theoretical studies to the prediction of absorption isotherms without analyzing the microscopic fluid`s structuring. Therefore, there is a scientific need to obtain the physical basis of the mechanism of acid gas absorption by SFL. Computational chemistry has been successfully used for the characterization of other acid gas physical solvents such as ionic liquids,17,18 deep eutectic solvents19 or complex mixed solvents.20 Therefore, previously utilized theoretical and computational approach was extended to this work for the characterization of SFL. The reported results in this manuscript show a characterization of liquid SFL in absence of acid gases, as a reference framework, and of SFL+CO2 or +H2S mixtures at different temperatures and pressures. The mechanism of SFL+CO2 or SFL+H2S intermolecular interactions was firstly analyzed using DFT methods, which provides an accurate description of short-range interactions between the considered molecules. Bulk liquid phase and interfacial properties were analyzed using MD as a function of pressure and temperature. The theoretical results were analyzed in terms of intermolecular forces between SFL molecules, and changes upon acid gases absorption, structural factors, such as available free volume, or behaviour at SFL – acid gases interfaces. Finally, considering that large amounts of SFL used as solvent for natural gas sweetening, the environmental and toxicological profile of this solvent is of pivotal importance. Available results have showed certain toxicity for this solvent,21 but the mechanism of its action on cells is still unknown. Therefore, to advance in the knowledge of the SFL cytotoxicity, MD studies on the SFL interaction with model lipid biomembranes are reported herein, which provide the information on the possibility of SFL crossing cell membranes and its relationship with SFL toxicity.

METHODS All DFT calculations were done at B3LYP-D3/6-311++G(d,p) level22-25with ORCA program.26 Interaction energies, ΔE, for the studied molecular clusters were calculated with Basis Set Superposition Error (BSSE) correction using the counterpoise procedure.27 3


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Molecular dynamics simulations were carried out with the MDynaMix v.5.2 molecular modelling package.28 Forcefield parameters for SFL are reported in Table S1 (Supporting Information) whereas those for CO2 and H2S molecules were obtained from the literature.29,30 Atomic charges for SFL were obtained from optimized structures of isolated molecules calculated at B3LYP-D3/6-311++G(d,p) theoretical level according to CHELPG method.31 Pure SFL and SFL + CO2 and SFL + H2S systems were studied using MD for the compositions, temperatures and pressures reported in Table S2 (Supporting Information). The mole fraction for SFL + CO2 and SFL + H2S systems at each temperature and pressure were obtained from the experimental results by Jalili et al.10 Regarding MD simulations for SFL + CO2 and SFL + H2S systems, MD studies were carried out along two isotherms (303 and 363) K, in order to study the behavior at low and high temperature in a suitable pressure range according to available matching experimental isotherms and related results.10 Initial simulation boxes, with cubic geometry, were built with the Packmol program.32 Periodic boundary conditions were applied for all MD studies. The simulation procedure was developed in two steps i) starting from the initial simulation boxes, 10 ns NPT simulations at the corresponding pressure and temperature conditions for each system were carried out for equilibration purposes (equilibration assured by the time evolution of the total potential energy), ii) from the output configuration of the previous step 20 ns NPT simulations were carried out for production purposes. The interfacial properties of liquid SFL in contact with gas phases containing acid gases were studied using MD simulations in the NVT ensemble, with MDynamix 5.2 program,28 at 303 K. For this purpose, simulation boxes containing 1000 SFL molecules (previously equilibrated at 303 K and 1 bar) were put in contact (above and below) with gas phases containing i) 2000 CO2 molecules and ii) 2000 H2S molecules; Figure S1 (Supporting Information). 20 ns simulations in the NVT ensemble were carried out for production purposes. MD simulations of inhomogeneous systems, such as those containing interfaces, would require long-range corrections when truncation of dispersion interactions is included;33 nevertheless, the reported simulations of interfaces with acid gases were carried out using a large Lennard-Jones cutoff (20 Å), and thus, long range corrections were not included. All MD simulations with MDynamix 5.2 program were carried out using the Nose– Hoover method for controlling control of pressure and temperature. Ewald summation 4


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method34 (15 Å for cut-off radius) was applied for the treatment of Coulombic interactions. The equations of motion were solved according to the Tuckerman–Berne double time step algorithm35 (1 and 0.1 fs for long and short time steps). Lorentz-Berthelot mixing rules were applied for Lennard-Jones cross terms. The toxicity of SFL molecules was studied through MD simulations for the interaction of SFL with lipidic biomembranes using ACEMD36 program running on GPUs. Lipid bilayers containing POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) were considered and built using CHARMM-GUI membrane builder37 containing 150 POPC molecules (75 + 75 molecules in upper and lower leaflets respectively) being solvated with 20500 TIP3P water molecules,38,39 corresponding to water layers of ∼ 62 Å width above each leaflet. 16 SFL molecules were placed above upper and lower leaflets (at 20 Å) corresponding to 60 mM concentration. The bilayer + water + SFL system was assembled using Packmol software.32 POPC molecules were modelled according to CHARMM36;40,41 SFL molecules using CHARMM general force field (CGenFF) to maintain homogeneity with lipid molecules parameterization, but with atomic charges as those used for MDynamix 5.2 simulations (Table S3, Supporting Information).42 These ACEMD simulations of lipid bilayers were carried out in the NPT ensemble at 303 K and 1 bar with periodic boundary conditions applied in the three space directions with non-bonded cross interaction terms calculated using Lorentz-Berthelot mixing rule, particle mesh Ewald method applied for handling electrostatic interactions,43 temperature controlled with a Langevin type thermostat44 (0.1 ps-1 damping constant), and pressure with a Berendsen barostat45 (0.4 ps relaxation time). 200 ns production runs were carried out with data analysis performed using the Membrane Analysis Tool (MEMBPLUGIN)46 in VMD. The ACEMD implementation of Berendsen barostat allows an efficient handling of lipid membranes; likewise, Berendsen’s one has been used successfully for MD simulations involving lipid biomembranes.47 The reason for using two different MD codes for the reported simulations (MDynaMix and ACEMD) stands on the very large systems (around 85,000 atoms) and very long simulations (200 ns) required for simulations involving lipid biomembranes, whcih can be carried out very efficiently using GPU-based clusters and ACEMD software, which is specially designed for running on these systems.

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RESULTS AND DISCUSSION Pure Sulfolane The properties of pure SFL (in absence of CO2 or H2S molecules) were firstly analyzed. SFL is a dipolar aprotic molecule (μ = 5.6839 D, in gas phase calculated at B3LYP-D3/6-311++G(d,p) theoretical level; μ = 4.7 D in liquid phase21). The globular shape of SFL molecule (calculated ovality, defined as the molecular surface are to volume ratio,48 for SFL in gas phase at B3LYPD3/6-311++G(d,p) is 1.13) determines its mechanism of interaction with other molecules,49 considering these interactions are mainly developed through the oxygen atoms (negatively charged). The short-range SFL-SFL interactions were analyzed through DFT calculations of SFL clusters containing n molecules, Figure S2 (Supporting Information). The potential energy surface for SFL-SFL intermolecular interactions was explored for SFL clusters with n = 2 at 16 different molecular relative orientations, which interaction energies, ΔE, are reported in Table 1. These results show that SFL molecules develop strong dipolar interactions as the large ΔE values show. Likewise, these results show that SFL-SFL interactions for n = 2 (gas phase dimmers) may be developed through different orientations with minor differences for the resulting ΔE, but interactions involving -SO2 groups in both molecules lead to stronger interactions than those between -SO2 and -CH2 groups; e.g. ΔE for SFL-SFL clusters in positions P12 and P13 (with interactions involving -SO2 group in one molecule and -CH2 groups in the other molecule) have roughly half ΔE than the remaining interactions, Figure S2 (Supporting Information) and Table 1. Therefore, it may be expected that SFL-SFL shortrange interactions in liquid state be mainly developed through -SO2 sites. ΔE for larger clusters, n = 3 and 4 Figure S2 (Supporting Information) and Table 1, show that strong interactions are maintained beyond the limit of the dimer, and thus large clustering in liquid SFL may be inferred from these DFT calculations. The topological reasons for the large ΔE values for SFL-SFL interactions reported in Table 1 were analyzed according to the Atoms in a Molecule (AIM, Bader’s theory),50 using the Multiwfn code.51 Results in Figure 2 show the AIM analysis for SFL-SFL cluster (n = 2) for the P5 position of interaction (analogous results were obtained for the remaining dimmers). SFL-SFL intermolecular interactions in dimmers are characterized by the formation of several bond critical points (BCPs), accompanied by ring critical points (RCPs) and a single cage critical point (CCP) at the center of the interaction region. Likewise, BCPs between SFL molecules are characterized by large values of electron density, ρ, and Laplacian of electron density, ∇2ρ. It should be remarked that according to 6


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Popelier et al.52 ρ and ∇2ρ are in the 0.002 to 0.040 a.u and 0.014 to 0.139 a.u ranges, respectively, for BCPs describing hydrogen bonds, and thus BCPs reported in Figure 2 show strong SFL-SFL interactions in agreement with the large ΔE reported in Table 1. DFT results provided information on short-range SFL-SFL interactions, but liquid phase for SFL should be characterized also by long-range interactions, volumetric effects and larger molecular aggregates, which were studied using MD simulations. The forcefield parameterization used along this work (Table S1, Supporting Information) was validated against available experimental relevant physicochemical properties. Density for pure SFL predicted in this work using MD is slightly larger than the reported experimental data, but deviations from the experimental data are around 0.7 %, and the temperature evolution of density is properly described by MD (∂ρ/∂T = -0.898 10-3 and -0.900 10-3 g cm-3 K-1 for experimental and MD density, respectively), Figure 3a. Nevertheless, in spite of the good MD density predictions, further physicochemical properties need to be analyzed considering that density is a mean-field property.53 For this purpose, dynamic properties are suitable for validation purposes considering that their prediction from MD is a difficult task for complex fluids.54,55 Therefore, self-diffusion coefficients, D, and dynamic viscosity, η, were predicted for SFL using MD and compared with experimental literature data in the studied temperature range, Figures 3b and 3c. Einstein’s equation was used for D calculation from mean square displacements, msd (Figure S3, Supporting Information), and, Green-Kubo method was used for η. SFL is a moderately viscous fluid (η = 10.3 mPa s at 303.15 K),10 thus diffusive regime was reached during MD simulations (log-log plots of msd vs simulation time with 1.00 slope in the studied temperature range). The comparison of D values for pure SFL predicted with MD was not possible because no experimental data is available in the literature, nevertheless, D = 1.09·10-10 m2 s-1 from MD shows a moderately viscous fluid.56 Regarding η predictions, results in Figure 3c show that η values from MD are roughly the half of experimental data, although the temperature evolution from MD resembles the experimental trend. The description of η according to Arrhenius model leads to activation energy of 19.4 and 18.2 kJ mol-1 for experimental and MD data, thus confirming a reasonable ability of the used forcefield parameterization for predicting SFL dynamic properties. Finally, vaporization enthalpy, ΔHvap, of SFL was predicted using MD using equation (1): ΔHvap=(Ugas - Uliq) + RT

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(1)

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where Ugas and Uliq stand for the internal energy of SFL gas (low density phase with noninteracting SFL molecules) and liquid phases, respectively. Calculated ΔHvap = 60.2 kJ mol-1 at 303 K is in good agreement with the experimental value of 66.76 kJ mol-1 at 303 K.57 Therefore, the SFL forcefield parameterization used along this work describes with reasonable accuracy the most relevant physicochemical properties of this fluid, and thus, it may be used for the analysis of the microscopic features of this fluid. Th structural properties of pure SFL were firstly analyzed using radial distribution functions, RDFs, for selected atomic pairs, Figure 4. RDFs for SS-SS atomic pairs are reported in Figure 4a and show that around a central SFL molecule at least three solvation spheres are developed as the three peaks in RDFs plot show. The solvation numbers, N (i.e. the number of SFL molecules around a central one), for the first solvation sphere show that up to roughly twelve molecules are placed in this shell, showing that SFL-SFL intermolecular interactions beyond the dimers. Likewise, residence times, tres (calculated as previously reported residence times58) for SFL molecules in the first solvation sphere are also large. Regarding the possible interaction between OS and HS atoms, RDFs (Figure 4b) show a weak peak in agreement with the mechanisms of SFL-SFL interactions from DFT studies (Figure S2, Supporting Information). This OS-HS weak interaction is characterized by its lability as the corresponding tres show. Regarding the temperature effect on RDFs, results reported in Figure 4 show almost negligible effects in the (303 to 343) K range, with a slight decrease in the solvation numbers and a decrease in the tres, and thus pointing to the fluid’s structure remaining unchanged for this temperature range with an increase in the ability of solvation shells, and thus in the intermolecular interactions due to the increase in the available thermal energy. The spatial distribution functions, SDFs, around a central SFL molecule, Figure 5, confirms the trend of SFL to be placed in a ring around another SFL molecule, with molecules interacting one on top of the other one, as the dimers DFT structures showed. Regions with lower SDFs are placed opposite to the -SO2 group, corresponding to the weaker interactions OS-HS interactions. The strength of SFL-SFL intermolecular forces was also computed from MD simulations and shown in Figure 6. These results are in good agreement with the results obtained via DFT studies and shown in Table 1, and they show slightly weakened interactions with increasing temperature. Nevertheless, SFL-SFL intermolecular forces are strong enough to maintain the SFL liquid structuring in the studied temperature range without remarkable 8


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changes, Figure 4, and the slight decrease in the strength of SFL-SFL interactions leads to an increase in molecular mobility: lower tres and increase in msd (Figure S3, Supporting Information). Nevertheless, although MD predicted viscosity decreases from 5.5 mPa s (at 303 K) to 2.4 mPa s (at 343 K), even at the higher studied temperature SFL is a moderately viscous fluid with low diffusion coefficient (2.86·10-10 m2 s-1) because of the large interaction energy and the localized mechanism of SFL-SFL interaction as reported in Figure 5.

Sulfolane – Acid Gas Interfaces The absorption of acid gases (CO2 and H2S) by SFL should develop through an initial stage in which the interface between liquid SFL and gas phase has a pivotal role. Acid gas molecules may be adsorbed at this interface and from this interfacial region migrate to bulk liquid regions, thus this initial adsorption stage would determine the kinetics of the gas capture. Therefore, the properties of SFL – acid gases interfaces were analyzed using MD with the objective of obtaining a microscopic picture of the initial stages of acid gases capturing by SFL. It should be remarked that i) MD simulations for the reported interfaces were carried out without any restriction, i.e. the interfaces are not fixed and ii) evaporation of SFL molecules was not observed during the simulations at the studied pressures and temperatures. The time evolution of intermolecular interaction energies and number density profiles for SFL + CO2 or SFL + H2S interfaces are reported in Figure 7. The mechanism of adsorption for both acid gases is very similar, an adsorbed layer is quickly developed on top of liquid SFL. On top of the initially adsorbed acid gas layer additional molecules are adsorbed, which is showed by the widening and increase in intensity of number density peaks reported in Figure 7. Once the acid gas adsorbed layers are developed, CO2 or H2S molecules begin to cross the interface and move toward bulk liquid regions, which is showed by the widening of the density peaks with the corresponding number of molecules absorbed in bulk liquid regions, Figures 7b and 7c. The development of adsorbed acid gas layers on top of SFL is quantified by the time evolution of CO2-CO2 or H2S-H2S intermolecular interaction energies, Figures 7a and 7b. In both cases, these interaction energies are remarkably large, being larger for H2S-H2S. Nevertheless, the case of CO2-CO2 shows an increase in intermolecular interaction energy (in absolute value) followed by a sudden increase at roughly 4 ns and then values almost 9


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constant, which points to a two stages adsorption, which is different than for H2S-H2S for which a single step is inferred. The reason for this behavior should be that stationary values for acid gas – acid gas interaction energies are reached around 2 ns before for H2S that for CO2, because of the larger affinity of SFL molecular for H2S as it showed by the acid gas – SFL interaction energies. Likewise, the adsorption of acid gas molecules at the liquid-gas interface leads to changes in SFL structuring, for both gases SFL-SFL interaction energy decreases (in absolute value) when acid gas molecules are initially adsorbed (roughly in the first 2 ns), specially for H2S interface, and then, once molecules start to migrate toward bulk liquid phases, this energy increases again (in absolute value) but leading to lower values than in absence of acid gas molecules. Therefore, SFL molecules at the interface are clearly disrupted by the presence of adsorbed gas molecules, decreasing SFL-SFL interactions for accommodating CO2 and H2S molecules and for allowing to migrate across the interface boundary toward bulk liquid SFL. Number density profiles showed that the CO2 and H2S capturing mechanism is developed in two stages i) adsorption at the interface, with the development of a highly dense acid-gas layer on top of liquid SFL, ii) interfacial crossing and migration toward bulk liquid SFL. To quantify the second stage of the process, the number of acid gas molecules in a central layer corresponding to liquid SFL was calculated as a function of simulation time and reported in Figure 8a. These results confirm that both CO2 and H2S molecules can cross the interface very quickly, with a remarkable number of acid gas molecules in the central bulk liquid layer even in the first ns of the simulations. Likewise, an stationary number of absorbed molecules (roughly 2.7 times larger for H2S than for CO2, in agreement with experimental results10) is reached after 3 and 5 ns for H2S and CO2, respectively, showing that interface does not hinder interfacial crossing and acid gas molecules tend to migrate toward bulk liquid regions to maximize acid gas – SFL interactions, which are remarkably stronger than acid gas – acid gas ones, especially for H2S molecules as reported in Figure 7. Nevertheless, snapshots in Figures 8b and 8c and number density profiles in Figures 7b and 7c show that saturation of liquid SFL is reached (as stationary values in Figure 8a show) together with highly dense adsorbed layers at the corresponding interfaces, and thus, acid gas capturing of these two gases would evolve through these complementary adsorption – absorption mechanisms.

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Sulfolane + CO2 or Sulfolane + H2S Mixtures Results in Figure 7a showed that the absorption of acid gases led to changes in SFLSFL interaction energies, and thus structuring of liquid SFL should change upon interaction with the considered acid gas molecules, because of the remarkable strength of acid gas – SFL intermolecular forces. Therefore, the microscopic properties of SFL + acid gas mixtures were studied as a function of mixture composition, temperature and pressure following the experimental results (absorption isotherms) reported in the literature,10 Table S2 in the Supporting Information section of this manuscript. The absorption of acid gases in different types of solvents is frequently accompanied by a rearrangement of available free volume in the liquid for accommodating absorbed molecules, and thus leading to fluids’ expansion,59-63 especially for organic solvents.62,64 In order to quantify this effect in SFL, volume expansion (% Vexp, as defined by Gallagher et al.65,66) was calculated for SFL + CO2 and SFL + H2S mixtures, Figure 9. These results should be analyzed considering that the upper limit studied for SFL + CO2 mixtures is x(CO2) = 0.15 whereas for SFL + H2S mixtures it was studied up to x(H2S)=0.45 (because of the larger solubility of H2S in SFL10) at 303 K, with x being the corresponding mole fraction. Nevertheless, SFL + H2S mixtures leads to much larger expansions than SFL + CO2 for the same content of acid gas, e.g. for x(acid gas) = 0.15, % Vexp = 0.85 and 1.97 were obtained for CO2 or H2S mixtures, respectively, at 303 K. This expansion can be related with the changes in SFL-SFL intermolecular interaction energies upon acid gases absorption reported in Figures 7a and 7c. Nevertheless, % Vexp for SFL upon CO2 (and H2S) absorption are remarkably low, in the range of those obtained for highly associated fluids such as ionic liquids, which are well-known for being able to fit absorbed gas molecules in the interstices of the fluid.59,60 Therefore, in spite of the changes in SFL-SFL intermolecular interaction energies upon acid gases absorption (Figure 7), results in Figure 9 confirm that SFL is able to fit CO2 and H2S molecules without a large disruption in its volumetric properties and this shall result in low foam formation and less swelling in actual process conditions. This effect was additionally quantified through the calculation of the cavities size distribution in SFL (according to the method reported in a previous work60), Figure 10. The cavities size distribution for pure SFL is characterized by four peaks corresponding to spherical cavities with radii roughly 0.2, 0.6, 1.5 and 2.1 Å. Although the larger cavities could fit some CO2 or H2S molecules, results in Figure 10 shows that absorption of acid gases is accompanied by a 11


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rearrangement of cavities (i.e. free space), with an increase of the number of smallest cavities (i.e. increase in the peak intensity corresponding to cavities in the 0.1-0.2 Å range) and a decrease for the larger ones, being especially remarkable for CO2 mixtures, whereas H2S mixtures are characterized by a widening of the peaks corresponding to the larger cavities, which would justify the larger expansion upon H2S absorption. The strong SFL-acid gas interactions in liquid SFL reported in Figure 7 showed that acid gas molecules can develop intense intermolecular forces with SFL molecules. Therefore, the quantification of these interactions was accurately done using DFT for SFL + m CO2 or + r H2S clusters, Figures S4 and S5 (Supporting Information). Regarding SFL + 1 acid gas clusters, several spatial orientations were studied and the corresponding interaction energies were calculated, Table 1. First, it should be remarked that all SFL + H2S interactions are stronger than SFL + CO2 ones, which stands on the possible development of hydrogen bonding between OS and HH sites. Regarding SFL + CO2 interaction, results in Table 2 show that interaction trough the -SO2 site is clearly favored, with very minor differences depending on the CO2 orientation (∼ 2 kJ mol-1, Table 2). The interaction between CO2 molecules and HS atoms in SFL molecule (e.g. P5 in Figure S4, Supporting Information) is clearly less favorable that interactions through the -SO2 site, Table 2. Likewise, clusters containing larger number of CO2 molecules show that several CO2 molecules can be arranged around a single -SO2 site without weakening SFL-CO2 one by one interactions. In the case of SFL – H2S interactions (Figure S5, Supporting Information), interaction arrangements allowing OS-HH are clearly favored over those involving SH-HS interactions (e.g. positions P5, P7 and P8, Table 1). The analysis of H2S-SFL clusters reported in Figure S5 (Supporting Information) shows that a single H2S molecule may develop 1 (P1, P2, P3) or 2 (94, P6, P9, P10, P11) hydrogen bonds with oxygen atoms in SFL -SO2 group, but interaction energies reported in Table 1 show very similar values (∼23 kJ mol-1) both when 1 and 2 hydrogen bonds are developed (except for P3). This behavior can be justified with the AIM results reported in Figure 11, which shows how the formation of 2 hydrogen bonds (leading to a cycle) increases the donor-acceptor distances (∼2.5 Å for the cyclic structure) in comparison with the situation with a single hydrogen bond (∼2.1 Å), thus weakening the hydrogen bonds. The formation of two hydrogen bonds do not compensate the weakening of each hydrogen bond, the values of ρ and ∇2ρ reported in Figure 11b are slightly larger than the sum of ρ and ∇2ρ for the two

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hydrogen bonds in the cycle structure reported in Figure 11a, and thus interaction energies for single hydrogen bonds are slightly larger than for two hydrogen bonds (Table 1). The structural properties in liquid SFL containing CO2 and H2S at different mole fractions were firstly analyzed using RDFs, Figure 12. Results in Figures 12a and 12b shows that SFL-SFL intermolecular interactions suffer very minor changes upon absorption of acid gas molecules, only the second and third solvation shells for high H2S mole fractions suffer certain modifications. Therefore, minor volume expansion reported in Figures 9 and 10 are based on the minor changes in SFL-SFL interactions because liquid SFL can fit absorbed acid gas molecules without large perturbations of SFL-SFL dipolar interactions. Likewise, the trend of acid gas molecules to self-associate in SFL is also confirmed by RDFs in Figure 12c and 12d, the narrow and intense first peaks combined with relevant peaks for SFL-acid gas interactions (Figures 12e and 12f) confirm that several acid gas molecules can interact simultaneously with a single central SFL molecule, in agreement with DFT results in Figures S4, S5 (Supporting Information) and Table 2. This is confirmed by solvation numbers in the first solvation shell reported in Figure S6 (Supporting Information). The dynamics of acid gas molecules in the SFL solvation shells was quantified with the corresponding residence times, Figure 13. Larger tres were obtained for CO2 and for H2S, despite the stronger H2S-SFL tan CO2-SFL interactions (Table 1) interactions the larger lability of shells containing H2S is justified by the lower number of H2S molecules around a central SFL (Figures S6e and S6F, Supporting Information), which leads to a less sterically hindered region around SFL in the case of H2S molecules. DFT results showed the development of H2S-SFL hydrogen bonding, which extension was quantified in liquid SFL. Results in Figure 14a for OS-HH (SFL- H2S possible hydrogen bonds) shows the narrow and intense peaks, the first one corresponding to H2S hydrogen bonded to SFL and the two next ones to H2S placed in the first solvation’s sphere around a central SFL molecule but not hydrogen bonded to SFL but to neighbor H2S molecules in the same shell. These three peaks for OS-HH RDFs correspond to the features reported for SS-SH RDFs in Figure 12f for the first solvation shell (a wide peak containing two shoulders). The number of OS-HH hydrogen was quantified and results in Figure 14b shows that roughly half of H2S molecules are hydrogen bonded to SFL, with the other half hydrogen bonded to neighbor H2S molecules. The percentage of H2S hydrogen bonded to SFL molecules slightly

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decreases with increases mole fraction, but SFL can maintain extensive hydrogen bonding with H2S up to large H2S mole fractions. Spatial distribution of absorbed acid gas molecules was analyzed using SDFs, Figure 15. The comparison of SFL distribution around another SFL molecule in absence (Figure 5) and presence (Figure 14) of acid gas molecules shows negligible changes. SFL molecules tend to be placed in the ring region around other SFL molecules (top to bottom interactions) and this arrangement is maintained upon interaction with acid gas molecules. The almost free region in from around -SO2 group in SLF molecules is occupied by the CO2 or H2S molecules, without remarkably disrupting SFL structuring (Figures 9 and 10), and some additional acid gas molecules are placed close to HS atoms in SFL (with weaker interactions, Table 1). The dynamics of SFL in presence of acid gases was analyzed through self-diffusion coefficients (Figure 16) obtained from mean square displacements (Figure S7, Supporting Information). The comparison of the dynamics for acid gas – SFL mixtures shows faster diffusion of acid gas tan for SFL molecules both for CO2 and H2S, with larger differences for H2S. Regarding D for SFL molecules, it shows very minor changes with increasing acid gas mole fraction (decreasing for CO2 and increasing for H2S) thus confirming again the minor disruption of SFL liquid structuring upon CO2 or H2S absorption. Likewise, D values for H2S are larger tan for CO2 which can be related with the larger residence times in the first solvation spheres reported in Figure 13. These effects are maintained upon increase of temperature (Figures 16c and 16d) which leads to faster diffusion for all the considered molecules but the trends are the same as in the lower temperature (Figures 16a and 16b). Finally, although DFT results reported in Table 2 allowed the quantification of the strength of acid gas – SFL intermolecular forces, these results were obtained for small clusters and thus effects of long range interactions, which are remarkable according to results in Figure 12, need to be analyzed from MD simulations. Therefore, intermolecular interaction energies are reported in Figure 17 for acid gas – SFL mixtures. Regarding SFL-SFL interactions, they are more weakened by H2S than by CO2 molecules, because of the larger amount of absorbed H2S, but SFL-SFL interaction remains even close to equimolar mixtures and high temperatures, confirming that they develop the pivotal role in acid gas – SFL mixtures and thus in acid gas capturing by SFL. Likewise, acid gas – SFL interactions are also remarkably strong (even larger tan DFT results reported in Table 1 because of cooperativity), and stronger for H2S than for CO2, and although slightly weakened with increasing mole 14


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fraction and temperature they show very efficient interaction between acid gases and SFL, which would also justify the large acid gases capturing ability of SFL.

Sulfolane Molecules Interacting with Lipidic Bilayers: Cytotoxicity The use of SFL for industrial scale treatment of gas mixtures (especially for natural gas) requires the use of large amounts of this solvent, and thus it may have a large impact on environment. The toxicity of this fluid has been experimentally studied21 but the interaction of SFL with living organisms is not fully clarified. To model the interaction of SFL with cells and to analyze the possibility of this molecule to cross cell membrane, a lipid bilayer containing POPC phospholipids was used as a model and MD simulations of its interaction with SFL were carried out. POPC is a component of eukaryotic cell membranes, and thus, it is suitable for modeling SFL – cell interactions. MD simulations for 200 ns allowed the characterization of SFL-POPC membrane in a reasonable time frame. Results in Figure 18a show that SFL molecules can penetrate in the inner membrane region crossing the phosphate groups boundary. This penetration is very fast and efficient, in 200 ns 90 % of the available SFL molecules are placed inside the bilayer, Table 2. Therefore, the first conclusion is that SFL molecules can cross the cell membranes very efficiently, thus leading to cytotoxicity. The number density profiles in Figure 18b show two large peaks in regions below the phosphate groups but also a remarkable population in the central region of the bilayer surrounded by POPC alkyl chains. The SFL molecules inserted in the bilayer adopt preferential orientations as reported in Figure 19: i) molecules below the phosphate groups (corresponding to the two number density peaks in Figure 18b) are skewed, with -SO2 group pointing toward the center of the bilayer, ii) molecules in the center of the bilayer are parallel to the interface. The insertion of SFL molecules in POPC bilayer should change its more relevant properties, Table 2. Surprisingly, despite the large number of inserted SFL molecules, the thickness of the bilayer does not change, but the area per lipid increases, and thus, the insertion of SFL molecules leads to a dilatation of the bilayer in the plane of the membrane without changes in its thickness. This can be related with the SFL molecular orientation reported in Figure 19, molecules orientated almost parallel to the interface thus leading to changes in this direction not in bilayer thickness. Order parameters increase both for POPC palmitoyl and, especially, oleoyl chains, thus leading to slightly more ordered bilayers upon 15


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SFL insertion, Table 2 and Figure 20a. The arrangement of lipid chains is not remarkably changed upon SFL insertion, larger interdigitation with minor changes (∼1o) in orientation of alkyl chains regarding interface. The main effect of SFL insertion stands on the increase (68 %) of POPC lateral diffusion, Table 2 and Figure 20b. Therefore, MD simulations of SFL-POPC membrane interactions shows large insertion ability of SFL molecules inside the bilayer, adopting well-defined molecular orientations with disruption of membrane area but not thickness, and thus, increase in membrane lateral mobility. These results point to remarkable cytotoxicity induced by SFL molecules

CONCLUSIONS The properties of SFL for acid gas capturing purpose were studied using a theoretical approach. Pure SFL is characterized by strong interaction between -SO2 groups with SFL molecules adopting prevailing top-bottom orientation orientations, leaving the -SO2 groups almost free for inserting acid gas molecules. The liquid SFL – acid gas interface is characterized by fast adsorption of both types of acid gases and migration toward bulk liquid regions in short times, and once bulk liquid saturation is reached an adsorbed acid gas layer remains in the liquid surface, thus providing combined adsorption-absorption mechanisms for acid gas capturing. The properties of acid gas – SFL liquid mixtures were characterized by strong SFL-acid gas, both for CO2 and H2S, leading to small volume expansions because of free volume rearrangements and insertion of acid gas molecules in regions not affecting SFLSFL dipolar interactions, thus leading to minor changes in SFL liquid structuring in the studied composition ranges and both for low and high temperatures. Likewise, SFL molecules are able form hydrogen bond with SFL without disrupting SFL-SFL interactions, and thus leading to the large solubility of H2S IN SFL. Finally, MD studies on the interaction of SFL molecules with model lipid membranes have showed easy and fast insertion of SFL molecules into the bilayer, thus pointing to remarkable cytotoxicity which should be taken into account considering the extensive use of this solvent and its possible, environmental and toxicological problems.

ACKNOWLEDGEMENT This work was funded by Junta de Castilla y León (Spain, project BU324U14). We also acknowledge The Foundation of Supercomputing Center of Castile and León (FCSCL, Spain) 16


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for providing supercomputing facilities. The statements made herein are solely the responsibility of the authors.

ASSOCIATED CONTENT Supporting Information Table S1 (SFL forcefield parameterization); Table S2 (system used for MD simulations); Table S3 (CGenFF forcefield for SFL used for simulation of lepidic bilayers); Figure S1 (initial simulation boxes used for MD stuy of SFL + acid gas interfaces); Figure S2 (SFL clusters from DFT calculations); Figure S3 (mean square displacements in pure SFL); Figure S4 (SFL+CO2 clusters from DFT calculations); Figure S5 (SFL+H2S clusters from DFT calculations); Figure S6 (solvation numbers in SFL+CO2 and SFL+H2S mixtures); Figure S7 (mean square displacements in SFL+CO2 and SFL+H2S mixtures). This material is available free of charge via the Internet at http://pubs.acs.org.

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Table 1. Interaction energies, ΔE, for (a) n SFL clusters containing n molecules; (b) 1 SFL + m CO2 clusters containing m CO2 molecules; and (c) 1 SFL + r H2S clusters containing r H2S molecules. Structures reported in Figures S2, S3 and S4 (Supporting Information) n 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 3 4

n SFL -1 position ΔE / kJ·mol P1 -49.39 P2 -49.28 P3 -46.13 P4 -49.28 P5 -51.28 P6 -51.22 P7 -51.17 P8 -49.52 P9 -37.89 P10 -51.17 P11 -51.15 P12 -25.63 P13 -22.29 P14 -49.39 P15 -51.25 P16 -51.22 – -84.38 – -135.53

m 1 1 1 1 1 1 1 1 1 1 1 2 3 4

1 SFL + m CO2 -1 position ΔE / kJ·mol P1 -16.20 P2 -17.78 P3 -17.83 P4 -17.78 P5 -4.12 P6 -18.72 P7 -18.30 P8 -17.83 P9 -18.30 P10 -17.07 P11 -17.72 – -36.00 – -58.21 – -75.80

r 1 1 1 1 1 1 1 1 1 1 1 2 3 4

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1 SFL + r H2S -1 position ΔE / kJ·mol P1 -22.97 P2 -23.00 P3 -19.88 P4 -23.37 P5 -7.40 P6 -23.76 P7 -7.30 P8 -8.77 P9 -23.74 P10 -23.76 P11 -22.61 – -45.21 – -68.58 – -90.55

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Table 2. Properties for POPC-water and POPC-water-SFL systems obtained from MD simulations for the last 20 ns of the simulation (200 ns total simulation time). NSFL stands for the total number of SFL molecules per leaflet; INSERT subscript stands for the number of the SFL molecules inserted in the lipidic bilayer; APOPC stands for the area per lipid; h stands for the thickness of the bilayer; DL stands for the POPC lateral diffusion (xy plane with z-coordinate being perpendicular to bilayer surface); -sn-1 and sn-2 for the average order parameters of sn-1 (palmitoyl) and sn-2 (oleoyl) tails; ωρ stands for the width of the interdigitation region between upper and lower leaflets; ϴPN, ϴsn-1 and ϴsn-2 stands for the angle formed between PN (vector joining P and N atoms in POPC), sn-1 (vector along palmitoyl chain) and sn-2 (vector along oleoyl chain) vectors, and bilayer normal (z-axis). All values from MD at 303 K and 0.1 MPa NSFL system POPC-water POPC-water-SFL

NSFL – 32

INSERT

– 29

8

2

APOPC / nm 0.644±0.017 0.664±0.018

h / nm 3.940±0.196 3.940±0.178

10 DL / 2 -1 cm s 13.0±0.4 21.8±0.4

-sn-1 0.160±0.054 0.164±0.056

-sn-2 0.123±0.055 0.137±0.015

22


ωρ / nm 0.464±0.044 0.496±0.045

ϴPN / deg 74.3±1.9 74.0±1.9

ϴsn-1 / deg 148.5±1.5 149.1±1.4

ϴsn-2 / deg 146.3±1.6 147.1±1.6

Page 23 of 45

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

Energy & Fuels

Figure Captions.

Figure 1. Molecules and labelling used along this work. Figure 2. AIM analysis of SFL-SFL interaction for n SFL cluster (n =2) in P5 position (Figure S2, Supporting Information). Bond critical (BCP), ring critical (RCP), and cage critical (CCP) points involving SFL-SFL interactions 2 according to AIM are reported. Electron density, ρ, and Laplacian of electron density, ∇ ρ, for SFL-SFL BCPs are reported. Figure 3. Thermophysical properties of pure SFL (ρ, density, D, self-diffusion coefficient and η, dynamic viscosity ) as a function of temperature from MD simulations and comparison with literature experimental data (Jalili et al.10) when available. Experimental data for D were not available in the literature. Figure 4. Radial distribution functions, g(r), solvation numbers, N, and residence times, tres, for pure SFL as a function of temperature for the reported atomic pairs (labelling as in Figure 1) from MD simulations. N and tres are calculated for the first solvation sphere, defined according to the first minima in g(r) for each atomic pair. Figure 5. Spatial distribution functions of selected atoms (labelling as in Figure 1) around a central SFL molecule for pure SFL from MD simulations at 303 K. Top and side views are reported for the sake of visibility. Figure 6. Intermolecular interaction energy, Einter (sum of Lennard-Jones and Coulombic contributions) in pure SFL from MD simulations at 303 K. Figure 7. (a,c) Intermolecular interaction energy, Einter (sum of Lennard-Jones and Coulombic contributions), and (b,d) number density profiles, ρ, for SFL at (a,b) CO2 and (c,d) H2S interface from MD simulations at 303 K. In panels b and d, profiles at different simulation times are reported and vertical dashed lines show the position of the Gibbs dividing surface. Figure 8. Total number of CO2 or H2S molecules, N, absorbed in SFL+CO2 or SFL+H2S interfaces MD simulations at 303 K. The region for calculating absorption was defined as a central region (shadowed in panels b and c) in SFL below zGDS – 10 Å, where zGDS stands for the z-coordinate of the Gibbs dividing surface. exp

Figure 9. Percentage of volume change, % V , upon (a) CO2 or (b) H2S absorption, according to the criteria by 65,66 Gallagher et al., for (a) SFL + CO2 and (b) SFL + H2S mixtures as a function of CO2 or H2S mole fraction, x, and different temperatures. Pressures for each mixture composition are reported in Table S2 (Supporting Information). Figure 10. Distribution of cavity sizes for pure for x SFL + (1-x) CO2 (x = 0.15) and x SFL + (1-x) H2S (x = 0.45) from MD simulations at 303 K. Pressures for each mixture composition are reported in Table S2 (Supporting Information). Figure 11. AIM analysis of r H2S - 1 SFL interaction for r SFL cluster (r =1) in P1 and P6 positions (Figure S5, Supporting Information). Bond critical (BCP) and ring critical (RCP) points involving H2S-SFL interactions according to AIM are reported. Electron density, ρ, and Laplacian of electron density, ∇2ρ, for H2S -SFL BCPs are reported. Distances between OS and HH atoms are also reported in red. Figure 12. Selected radial distribution functions, g(r), for SFL + CO2 and SFL + H2S mixtures as a function of CO2 or H2S mole fraction, x, at 303 K. Pressures for each mixture composition are reported in Table S2 (Supporting Information). Figure 13. Residence times, tres, of acid gases in the first solvation sphere around SFL molecules for SFL + CO2 and SFL + H2S mixtures as a function of CO2 or H2S mole fraction, x, at 303 K. Pressures for each mixture composition are reported in Table S2 (Supporting Information). First solvation spheres defined by the first minima in radial distribution functions reported in Figure 9.

23


Energy & Fuels

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. (a) Radial distribution functions, g(r), between OS (SFL) and HH (H2S) atomic pairs and (b) average number of OS (SFL) - HS (H2S) hydrogen bonds (per H2S molecule), NH, for SFL + H2S mixtures as a function of H2S mole fraction, x, at 303 K. Pressures for each mixture composition are reported in Table S2 (Supporting o Information). The criteria for defining hydrogen bonding in panel b were 3.5 Å and 60 for donor – acceptor separation and angle. Figure 15. Spatial distribution functions of selected atoms (labelling as in Figure 1) around a central SFL molecule for pure for x SFL + (1-x) CO2 (x = 0.15) and x SFL + (1-x) H2S (x = 0.45) from MD simulations at 303 K. Pressures for each mixture composition are reported in Table S2 (Supporting Information). Figure 16. Self-diffusion coefficients, D, for SFL + CO2 and SFL + H2S mixtures as a function of CO2 or H2S mole fraction, x, at 303 K. Pressures for each mixture composition are reported in Table S2 (Supporting Information). Figure 17. Intermolecular interaction energy, Einter (sum of Lennard-Jones and Coulombic contributions), for SFL + CO2 and SFL + H2S mixtures as a function of CO2 or H2S mole fraction, x, at 303 K. Pressures for each mixture composition are reported in Table S2 (Supporting Information). Figure 18. (a) Snapshot (after 200 ns of MD simulations) of SFL interacting with POPC lipidic bilayer, (b) number density profiles obtained for the 180 to 200 ns of MD simulations at 303 K and 0.1 MPa. In panel a, TIP3 water molecules are omitted for the sake of visibility; in panel b, values for SFL are multiplied by 25 and for P1 atoms by 50. P1 atoms stand for phosphorous atoms in POPC lipid molecules. Figure 19. Orientation of SFL molecules inserted in POPC bilayer as defined by average angle, , formed bewtween C-S and z vectors (as defined in the Figure, with z being a vector perpendicular to the POPC bilayer). A molecular sketch is showed above the Figure indicating the orientation of SFL molecules in the different regions inside the POPC bilayer, which limits are defined by the position of P1 atoms in POPC molecules (indicated by orange lines). Values obtained for the 180 to 200 ns of MD simulations at 303 K and 0.1 MPa. Black line shows polynomial fit of data to show the trend. Figure 20. (a) Deuterium order parameter, SCD, for sn-1 (palmitoyl) and sn-2 (oleoyl) tails of POPC and (b) mean square displacement, msd, for the reported systems from MD simulations at 303 K and 0.1 MPa. Values obtained (a) for the last 20 ns (180 to 200 ns) and (b) for the last 10 ns (190 to 200 ns) of MD simulations.

24


Page 24 of 45

Page 25 of 45

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

Energy & Fuels

HS,CS

HS,CS OD

HS,CS

CD

OD

HS,CS

S SS O

O

OS

OS

HH

H

HH SH

H

S

Figure 1.

25


Energy & Fuels

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 45

BCP (3,-1) RCP (3,+1) CCP (3,+3)

1 5

4 3

ρ / a.u. 0.01245 0.01158 3 0.00569 4 0.00593 5 0.01314 1 2

2

∇ 2 ρ / a.u. 0.04919 0.03893 0.01980 0.01864 0.04456

Figure 2.

26


1.28

ρ / g cm-3

0.8

ρMD

(a)

ρexp

1.26

100(ρexp-ρMD)/ρexp

0.76

1.24

0.72

1.22

0.68

1.2

0.64

1010 D / m2 s-1

3.2 300

310

Dexp

2.8

320 330 T/K

340

320 330 ηMD T/K η

340

350

(b)

2.4 2 1.6 1.2 12 300

310

10.3

10

350-50 (c) -60

exp

100(ηexp-ηMD)/ηexp

8.01

8

-70

6.32

6

5.4

-80

5.16

4.3

4.22

3.5

4

2.8

-90 2.4

2

-100 300

310

320 330 T/K

340

Figure 3.

27


350

100 (ηexp-ηMD)/ηexp

0.8

η / mPa s

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

Energy & Fuels

100 (ρexp-ρMD)/ρexp

Page 27 of 45

Energy & Fuels

1.5 1

12.2 12.1

15.5 Å

N

g(r)

10.6 Å

50

12.3

303 K 313 K 323 K 333 K 343 K

5.7 Å

tres / ps

2

12 0.5

45

40

11.9

(a) SS-SS 5

10 r/Å

15

11.8 12.3 300

20

320

330

340

350

12.1 N

2.8 Å

12 0.5

35 8 300

310

320 330 T/K

340

350

310

320 330 T/K

340

350

T/K

12.2

1.5 1

310

tres / ps

0 2 0

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 28 of 45

7

6

11.9

(b) OS-HS

0 0

5

10 r/Å

15

11.8

20

5 300

310

320 330 T/K

340

Figure 4.

28


350

300

Page 29 of 45

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

Energy & Fuels

os ss

os ss

Figure 5.

29


Energy & Fuels

59

58 - 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

Page 30 of 45

57

56

55 300

310

320 330 T/K

340

Figure 6.

30


350

Page 31 of 45

0

(a)

CO2-CO2

(b)

SFL-SFL

Einter / kJ mol-1

SFL-CO2 interface

SFL-CO2

-20

-40

-60 0

Einter / kJ mol

-1

5

10

15

t / ns

0

SFL-H2S interface

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

Energy & Fuels

(c)

(d) (d)

-20

-40 H2S-H2S SFL-H2S SFL-SFL

-60 0

5

10

15

t / ns

Figure 7.

31


Energy & Fuels

zGDS

300

(a)

H2S

zGDS

(b)

CO2

200 N

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 45

(c)

100

0 0

5

10

15

t / ns

Figure 8.

32


H2 S SFL CO2

Page 33 of 45

1

10

x CO2 + (1-x) SFL @ 303 K

x H2S + (1-x) SFL @ 303 K x H2S + (1-x) SFL @ 363 K

x CO2 + (1-x) SFL @ 363 K

0.8

8

0.6

6

% Vexp

% Vexp

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

Energy & Fuels

0.4

4

0.2

2

(a)

(b)

0

0 0

0.1 x

0.2

0

0.1 0.2 0.3 0.4 0.5 x

Figure 9.

33


Energy & Fuels

0.12

pure SFL x SFL + (1-x) CO2, x=0.15 x SFL + (1-x) H2S, x=0.45

0.08 P(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 34 of 45

0.04

0 0

1

2 r/Å

Figure 10.

34


3

Page 35 of 45

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

Energy & Fuels

BCP (3,-1) RCP (3,+1) (a) r =1, P1

(b) r =1, P6 1

2.47 Å

2 3 1

2.54 Å

2

3 2.10 Å

ρ / a.u. 1 0.00457 2 0.01039 3 0.00930

∇ 2ρ / a.u. 0.01315 0.03021 0.02701

1 2 3

ρ / a.u. ∇ 2 ρ / a.u. 0.00271 0.01523 0.00567 0.01536 0.01935 0.06442

Figure 11.

35


Energy & Fuels

x CO2 + (1-x) SFL 0.01 0.03 0.07 0.11 0.15

1

0.5

(b) SS-SS

(a) SS-SS 5

10 r/Å

15

0 4 0

20

5

10 r/Å

15

20

3 g(r)

g(r)

x x==0.07 0.07 x x==0.15 0.15 x x==0.24 0.24 0.33 x x==0.33 0.45 x x==0.45

1

3 2

2 1

1 0 2.5 0

(c) CD-CD 5

2

10 r/Å

15

0 2.5 0

20

(d) SH-SH 5

10 r/Å

15

20

2

1.5

g(r)

g(r)

x =x0= 0

1.5

0.5 0 4 0

GAS-GAS

= = = = =

g(r)

x x x x x

x H2S + (1-x) SFL

2

x=0

1.5 g(r)

SFL-SFL

2

SFL-GAS

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 36 of 45

1.5 1

1

0.5

0.5

(e) SS-CD

0 0

5

10 r/Å

15

20

(f) SS-SH

0 0

Figure 12.

36


5

10 r/Å

15

20

Page 37 of 45

65

CO2 (CD) around SFL (SS) H2S (SH) around SFL (SS)

55 tres / ps

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

Energy & Fuels

45

35 0

0.1

0.2

0.3

0.4

x

Figure 13.

37


0.5

Energy & Fuels

4

(a)

x x x x x

2.6 Å

= = = = =

0.48

0.07 0.15 0.24 0.33 0.45

(b)

3 0.46

2

NH

3.4 Å

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 38 of 45

4.4 Å

0.44

1

0.42

0 0

5 r/Å

0

10

0.1 0.2 0.3 0.4 0.5 x

Figure 14.

38


Page 39 of 45

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

Energy & Fuels

x CO2 + (1-x) SFL (a)

x H2S + (1-x) SFL OS CD

(b)

Figure 15.

39


OS SH

Energy & Fuels

x CO2 + (1-x) SFL

8

x H2S + (1-x) SFL

8

H2S

SFL

6

1010 D / m2·s-1

1010 D / m2·s-1

303 K

CO2

4 2

SFL

6 4 2

(a)

(b)

0 0.1 XCO2

0.2 1010 D / m2·s-1

1010 D / m2·s-1

18 0

363 K

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 40 of 45

12 6

(c)

0 0

0.1 x

0.2

0 18 0

0.1

0.2

0.3

0.4

0.5

XH2S 12 6

(d) 0 0

0.1

0.2

0.3 x

Figure 16.

40


0.4

0.5

Page 41 of 45

-20 x SFL + (1-x) CO2

-30 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

Energy & Fuels

-40 x SFL + (1-x) H2S

-50

-60 0

0.1

0.2

0.3

0.4

0.5

x

Figure 17.

41


Energy & Fuels

(a)

-3

0.16

ρ/Å

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 42 of 45

SFL P1 POPC TIP3

0.12

(b)

0.08 0.04 0 -80

-40

0 z/Å

40

Figure 18.

42


80

O

O

S

O S

120

90 / deg

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

Energy & Fuels

O

Page 43 of 45

60

C-S

φ z

30 -20

-10

0 z/Å

10

Figure 19.

43


20

Energy & Fuels

0.24

1

(a)

0.2

(b)

POPC-water POPC-water-SFL

0.8 msd / nm2

0.16 -SCD

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 44 of 45

0.12

0.6 0.4

0.08 SCD,sn-1 (POPC-water)

0.2

SCD, sn-1 (POPC-water-SFL)

0.04

SCD,sn-2 (POPC-water) SCD,sn-2 (POPC-water-SFL)

0 0

2

4

6

8

0

10 12 14 16 18

0

2

4

6 t / ns

Cn

Figure 20.

44


8

10

Page 45 of 45

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Energy & Fuels

O=C=O S H

O

H S

TABLE OF CONTENTS GRAPHIC

45


O

Microscopic Characterization of CO2 and H2S Removal by Sulfolane

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