Molecular mechanism of CO2 absorption in phosphonium amino acid ionic liquid

Prabhat Prakash and Arun Venkatnathan*
Department of Chemistry and Centre for Energy Science, Indian Institute of Science Education and Research, Pune 411008, India. E-mail: arun@iiserpune.ac.in; Fax: +91-20-2586-5315; Tel: +91-20-2590-8085

Received 13th April 2016 , Accepted 1st June 2016

First published on 2nd June 2016


Abstract

In this investigation, we examine the molecular mechanism of high pressure CO2 absorption in tetra-butylphosphonium lysinate Ionic Liquid (IL) using molecular dynamics simulations. The calculations show that on CO2 absorption, a structured bilayer of charges (in opposite phases) at the CO2–IL interface is formed. The interface formation starts within a short span of tens of picoseconds, and attains saturation around ten nanoseconds, where CO2 molecules remain absorbed in the bulk IL layers. The simulations predict a 0.9 molar absorption of CO2 at T = 298 K and pCO2 = 20 bar, with a further increase of 17% at a reduced temperature. The structural properties show that CO2 molecules strongly and preferentially interact with the two terminal amine sites of the anions, where the presence of carboxylate group further enhances CO2 absorption. The interaction and absorption of CO2 molecules leads to enhanced mobility of the cations and anions of the IL. The mobility of anions is slightly higher than cations due to the preferential interaction of CO2 molecules with anions. The molecular mechanism examined in this work can be used as a predictive tool to develop efficient amino acid functionalized ILs for CO2 absorption.


1. Introduction

Carbon dioxide (CO2) emissions from post-combustion pose strong challenges to the environment and health and this has motivated the search for materials for its efficient capture.1,2 The current generation of materials which demonstrate high CO2 uptake are amine based compounds.3 Unfortunately, the use of amines lead to poor recyclability (due to the large enthalpy of CO2 absorption4) and high corrosion of steel5 (due to the active participation in oxidation). The use of Room Temperature Ionic Liquids (RTILs) as materials for CO2 capture6–8 have been proposed as a probable alternative, where the choice of cations and anions influence the amount of CO2 absorption. Compared to cations, anions play a more significant role in absorption9 and their functionalization to enhance absorption has been attempted.10–13 Among them, amino acid ionic liquids (AAILs) show potentially high14,15 CO2 uptake, due to the presence of amine groups. In one of the earliest attempts, Bates et al.16 employed FTIR spectroscopy and observed a 0.5 molar absorption of CO2 (at room temperature) in an amine functionalized imidazolium based IL. Similar absorption capacity was reported by Zhang et al.17 on phosphonium AAILs, where the authors also observed enhanced absorption of CO2 on addition of water in IL. Carvalho et al.18 used bubble point determination approach and found that phosphonium ILs have higher CO2 solubility compared to imidazolium ILs. Using calorimetric and React-IR spectroscopy methods, Gurkan et al.19 observed an equimolar CO2 absorption (at room temperature) in phosphonium based methioninate and prolinate AAILs. Wang et al.20 measured enthalpy of absorption for a series of phosphonium based ILs and concluded that anions with large pKa, show higher CO2 absorption. Seo et al.21 synthesized a range of aprotic heterocyclic anion based phosphonium ILs, and using volumetric measurements reported an equimolar CO2 absorption (at room temperature). Recently, Eisinger et al.22 reported a phase change (from solid to liquid) in a tetraethylphosphonium based IL. The authors concluded that heat evolved during the phase change leads to reduced power consumption associated with CO2 capture in post combustion industrial process.

Computational investigations using Molecular Dynamics (MD) simulations to model interactions between supercritical CO2 and [Bmim][PF6] IL were employed by Huang et al.23 The authors calculated structural properties and concluded that while CO2 molecules can easily permeate into the IL phase, the ions of IL do not diffuse to the CO2 phase. Subsequently, Perez-Blanco and Maginn24 investigated absorption/desorption of CO2 in [Bmim][Tf2N] IL at various temperature and pressure conditions. The authors concluded that CO2 molecules quickly absorb at the IL interface, and further diffuse to the bulk IL at longer timescales. Xing et al.25 reported that the amine tethering on anion of an imidazolium IL enhances CO2 absorption. The authors suggested that amine functionalization on the imidazolium cation leads to high interaction between cation and anion which results in low interaction of CO2 with anion of IL, while amine functionalization on anion of IL weakens intra-IL interactions and facilitates accommodation of CO2 absorption. Morganti et al.26 and García et al.27 compared the relative absorption of CO2 and SO2 in non-amine functionalized ILs and found preferential absorption of SO2 over CO2 in such non-amine ILs. Izgorodina et al.28 showed that ILs containing large size anions (e.g. sulfonic, carboxylic and other bulky functional groups) are favorable candidates for CO2 physisorption. Klähn and Seduraman29 investigated several imidazolium-based ILs (with varying alkyl chain length of cation) and concluded that only the voids available in IL facilitate CO2 absorption. The authors also concluded that ions slightly rearrange to accommodate CO2 molecules in the bulk IL, and CO2–IL interactions do not play any significant role.

Existing studies have provided very limited mechanistic details on CO2 capture and for a limited type of ILs. The theoretical studies so far have not thoroughly examined the timescale of CO2 absorption at IL interface and bulk. The actual process of CO2 absorption in the bulk IL at very long timescales (∼100 ns) has never been explored. To examine these effects and propose a general mechanism of CO2 absorption, we have chosen tetra-butylphosphonium lysinate [P4444][Lys] IL (see Fig. 1) which has large thermal stability, and the presence of multiple amine sites on the lysinate anion which can facilitate CO2 capture. The computational details are presented in Section 2. Section 3 explores the mechanism associated with CO2 absorption from MD simulations (using all atom force-fields). The properties calculated from MD simulations are: IL charge density, time dependent particle densities of CO2 (at the interface and interaction with bulk IL), absorption isotherms at varying pressure and temperature, characterization of important molecular interactions between sites in IL and CO2 via Radial Distribution Functions (RDFs)30 and Spatial Distribution Functions (SDFs)31 and effect of CO2 absorption on dynamics of cations and anions of IL. A summary of important findings concludes this paper.


image file: c6ra09577a-f1.tif
Fig. 1 Structure of tetra-butylphosphonium lysinate (atom types defined for RDFs).

2. Computational details

The computational model required for MD simulations was initially created using a well equilibrated slab of [P4444][Lys] molecules, at T = 298 K and PIL = 1 bar. A box of 512 ion pairs of IL was initially created followed by annealing and equilibration using the NpT ensemble. Various initial templates of CO2 molecules were created and equilibrated using NpT ensemble to maintain the pressure and temperature conditions. The equilibrated IL slab was centered in a cuboidal box of dimensions 9.0 × 9.0 × z nm, where ‘z’ is derived as the length of the box (see Fig. S1 in the ESI) required to contain a certain number of equilibrated CO2 molecules (based on the required initial partial pressure and temperature). The IL slab was sandwiched between two layers using equilibrated CO2 templates, where CO2 molecules were initially separated by a vacuum layer of 1.0 nm from the surface of IL on each side of the box (see Fig. S1 in the ESI). The initial vacuum was created to maintain an initial non-interacting behavior between CO2 and IL based on earlier theoretical studies32 on other ILs. All MD simulations for CO2 absorption were performed at T = 298 K and 278 K; pCO2 = 1, 10 and 20 bar. However, in order to examine the mechanism of CO2 absorption, unless explicitly mentioned, all results are focused on simulations performed at T = 298 K, and pCO2 = 20 bar in this article. The GROMACS 4.6.7 (ref. 33) program was used for MD simulations. The CO2 molecules were modeled using popularly used flexible TraPPE force field.34 The force field parameters for [P4444][Lys] IL were taken from the work of Zhou et al.35 The force field parameters of the neat IL were benchmarked by performing MD simulations and the calculated structural and dynamical properties like density, RDFs and conductivity were found to be in good agreement with previously reported experimental17 and simulation35 studies (see Table S1 in the ESI). All simulations for CO2–IL systems were carried out for 100 ns with a time-step of 0.5 femtoseconds at the NVT ensemble with temperature maintained using the Nosé–Hoover chain thermostat36,37 and a 1.4 nm cut-off for calculation of non-bonding interactions.

3. Results and discussion

CO2 absorption at the interface and bulk

A time averaged charge density of [P4444][Lys] IL shows, on absorption of CO2, a highly ordered and structured bilayer (of opposite phase) at the surface is formed (see Fig. 2a). The cations and anions of IL reorient themselves to facilitate the formation of the CO2–IL interface. The area under the curve of positively charged outer region of the CO2–IL interface is smaller than the negatively charged inner region. The existence of this bilayer facilitates the absorption of CO2 molecules to the surface of IL. The charged bilayer formation at the CO2–IL interface is a characteristic feature of CO2 absorption and its formation is independent of thermodynamic conditions (see Fig. S2 in the ESI). In comparison to vacuum–IL interface, the charge density is found more structured in CO2–IL interface (see Fig. S3 in the ESI). The charge bilayer formation (near the interface) supports the observations of Perez-Blanco et al.24 and Dang et al.38 on vacuum–[Bmim][NTf2] IL interface.
image file: c6ra09577a-f2.tif
Fig. 2 (a) Charge densities of IL before (t = 0) and after absorption (averaged from t = 5 ns to t = 10 ns). (b) Particle densities of CO2 in the z direction of the box, where the average width of the IL layer is 4.8 nm. (c) Snapshot of CO2–IL interface, surficial and bulk CO2 absorption at t = 30 ns, color scheme is: green (CO2), red (P+4444 cation) and blue (Lys anion).

Perez-Blanco et al.24 calculated the particle density profile of CO2 molecules on absorption in [Bmim][NTF2] IL. The authors observed the interfacial crossing of CO2 occurs with an exchange time (between CO2 and IL phase) of 2.5 ps at the interface. Following a similar approach, we calculated the time averaged particle densities of CO2 to examine its distribution during the processes of absorption. The particle densities are calculated using a 100 ps time window (for densities calculated till t = 1.5 ns) and 1000 ps time window, from the average time (for densities calculated after t = 2.0 ns) (see Fig. 2b). The results show that within the first few picoseconds; several layers of CO2 molecules are absorbed near the surface of IL on both sides of the box. The intensity at the CO2–IL interface increases till 100 picoseconds. A slight decrease in CO2 particle density at the interface starts around 1.0 ns, as CO2 molecules can enter the bulk IL region. Around 10.0 ns, the densities at the interface attain saturation. The time of saturation can be defined as the time beyond which particle densities at the interface remain almost constant. On saturation at the interface, existing CO2 molecules present at the CO2–IL interface either could move towards bulk CO2 or could diffuse into the bulk IL layers and remains trapped. At this point, a dynamic movement of CO2 is observed, however, due to favourability of the absorption, CO2 molecules eventually diffuse towards the IL layer. The particle densities of CO2 at the interface and in bulk IL remains almost constant between t = 30 ns and 100 ns, suggesting a total saturation of CO2 density in all phases (see Fig. S4 in the ESI). Similar features and trends in particle densities at other pressures and temperature are shown in Fig. S5 in the ESI. A representative snapshot of absorption at t = 30 ns (see Fig. 2c) shows the presence of CO2 molecules in the bulk IL layers. The quantitative measure of CO2 uptake is seen from an absorption isotherm discussed in the next section. A video of simulation of CO2 absorption in [P4444][Lys] IL at T = 298 K and pCO2 = 20 bar is provided as a ESI Movie.

Quantitative effects of pressure on CO2 absorption

The time dependent molar absorption ratio of CO2[thin space (1/6-em)]:[thin space (1/6-em)]IL is calculated by an integration of CO2 particle densities between two CO2–IL interfacial boundaries. The absorption isotherm (see Fig. 3) shows a sharp blip at very short timescales (in few picoseconds) due to CO2 absorption at the initially formed CO2–IL interface. A slight decrease in absorption at timescale of 1 ns arise as CO2 molecules at the interface will diffuse into the bulk IL layers, where absorption reaches an asymptotic value within a timescale of 10 ns. At T = 298 K and pCO2 = 20 bar, a 0.9 molar absorption ratio (calculated as an average using time dependent molar absorption values from t = 10 ns to t = 30 ns) is achieved, and is ∼13 and 1.5 times higher than the absorption at pCO2 = 1 bar and 10 bar respectively. At T = 278 K and pCO2 = 20 bar, a maximum of 1.08 molar absorption of CO2 per IL is achieved. The average molar absorption ratio values at other thermodynamic conditions are shown in Table S2 of ESI. Due to the choice of a different cation, the theoretical values of molar absorption in this study are lower than the experimentally reported values of Goodrich et al.39 (1.4 CO2[thin space (1/6-em)]:[thin space (1/6-em)]IL molar absorption) and Saravanamurugan et al.40 (1.6 CO2[thin space (1/6-em)]:[thin space (1/6-em)]IL) with similar class of ILs.
image file: c6ra09577a-f3.tif
Fig. 3 Molar CO2 absorption in IL.

Structural view of interactions between CO2 and IL

The interaction between CO2 molecules and various interaction sites of the ILs are examined using calculation of RDFs and SDFs. The P[P4444], N1[Lys], N2[Lys] interaction sites on the IL and C[CO2] sites are used for examination of structural features. The RDFs between the P site of the cation and N1, N2 sites of the anion for CO2–IL and vacuum–IL systems show that interaction between cations and anions of IL decreases with CO2 absorption (see Fig. S6 in the ESI). A time dependent N1,N2–C RDF (see Fig. 4a) shows that structural features are similar at different times. However, the intensities of these RDFs increase till t = 10 ns, and then remains invariant till t = 30 ns, due to the saturation of CO2 molecules at the CO2–IL interface. Using the time period of t = 10 to t = 30 ns, we characterize various RDFs which correspond to possible interactions between cation/anionic sites of the IL and CO2 (see Fig. 4b). The cation–CO2 RDF show a sharp first peak with a small secondary peak. However, the first peak position appears at larger distances from the interaction site (0.56 nm) and with relatively low intensity. This is because CO2 molecules have less preference to interact with the cation, due to a large positive charge on the phosphorous atom and steric hindrance from the bulky alkyl (butyl) group. In contrast, the anion–CO2 RDFs show the following features: the N1–C RDF show three distinct solvation shells, a sharp first peak with the highest intensity, a first minimum at shortest distance of 0.48 nm, secondary and a tertiary shell which can accommodate CO2 molecules. Approximately 11 CO2 molecules interact with the N1 site within the cut-off of 1.1 nm. The N2–C RDF shows the existence of two solvation shells, which are relatively less distinct, and smaller compared to structural features of N1–C RDFs. Further, ∼4.5 CO2 molecules can interact with the N2 sites within a cutoff of 1.1 nm. This suggests that CO2 prefers to interact with the N1 site of the anion over N2. Our results support previous studies on conventional and AAILs. For e.g., CPMD calculations of Bhargava and Balasubramanian41 on [Bmim][PF6] IL and Shi et al.42 on [Emim][OAc] IL also have shown that CO2 molecules preferentially interact with anion. Gurkan et al.19 provided spectroscopic evidences of CO2 binding with [P66614] based AAILs, a signature of COO formation (due to reactive amine–CO2 interactions). Recently Luo et al.43 observed multiple site cooperative interactions can enhance CO2 capture in a pyridinium anion based IL.
image file: c6ra09577a-f4.tif
Fig. 4 (a) Time dependent RDFs of anion–CO2 interactions, (b) averaged RDFs of anion–CO2 and cation–CO2 interactions, (c) SDF of C[CO2] around N1,C,O[Lys], (d) SDF of C[CO2] around the N2[Lys] site.

To present a three-dimensional view of density distribution of CO2 molecules around the amine sites of the anion, we calculated Spatial Distribution Functions (SDFs) using the TRAVIS31 program (visualized using VMD44). The SDF of CO2 molecules surrounding the N1 site (see Fig. 4c) shows three clouds of CO2 densities, where two clouds are close to the carboxylate oxygen atoms, and the remaining cloud near the N1 site of the anion. However, only a single cloud of CO2 is observed near the N2 site (see Fig. 4d) which confirms that the N1 site has preferential interaction with the CO2 molecules, where the carboxylate group enhances CO2 absorption.

Effect of CO2 absorption on IL mobility

The influence of CO2 absorption on dynamics of cations and anion of IL is examined by Mean Square Displacement (MSD)30 (see Fig. 5). To compare the effect of CO2 absorption, we simulated a vacuum–IL system, where the MSD of cations and anions are similar. Both cations and anions of IL move faster in CO2–IL system compared to in vacuum–IL system. In comparison to cations, anions move faster and this is due to preferred interaction of anion with CO2 (as seen from RDFs as well). Similar trends are seen at other partial pressures (see Fig. S7 in the ESI). The MD simulations of Perez-Blanco and Maginn32 have shown that mobility of ions of IL remain unaffected with CO2 absorption where they have calculated the dynamical properties up to 1 ns. Our MSDs supports that their observations are valid only at short timescales. At timescales beyond 1 ns (as seen in the present study), the results show that CO2 absorption increases the mobility of cations and anions. This mobility arises due to physical interaction of CO2 molecules at the interface and bulk IL layers. The trends in mobility of cations and anions of IL can be used as a benchmark to explore the dynamical properties of similar class of phosphonium based AAILs. Since a linear regime of MSD could not be found for cations and anions of IL, the diffusion coefficients could not be calculated.
image file: c6ra09577a-f5.tif
Fig. 5 Mean Square Displacement of cations and anions of IL.

To check the reproducibility of CO2 absorption events, we have performed MD simulations with different initial configurations and system sizes (see Fig. S8 in the ESI) used for the CO2–IL system. The simulations show structural and dynamical properties of CO2–IL system remain unchanged by the choice of initial configuration (see Fig. S9–S11 in the ESI).

4. Conclusions

A molecular mechanism of high pressure CO2 absorption in [P4444][Lys] IL is investigated using MD simulations. The charge densities show a structured ordering with opposite charged layers near the CO2–IL interface. The particle densities show that the interface is saturated with CO2 molecules at around 10 ns. A maximum of 1.08 molar absorption of CO2 can be achieved at T = 278 K and pCO2 = 20 bar. The RDFs and SDFs in this study are found to be in agreement with previously reported works on other ILs. The interaction and absorption of CO2 leads to an increase in the mobility of cations and anions of IL, where higher mobility of anions indicate interactions of CO2 molecules largely with anions. This increase in mobility of ions of IL is supported with the decrease in cation–anion interactions after CO2 absorption (as seen from RDFs). An immediate outcome of this study will be an examination of the process of CO2 desorption and the effect of humidification on CO2 absorption/desorption in IL. Such findings can motivate experimental investigations to screen, synthesize and characterize efficient ILs for CO2 capture and also optimize the functionality of anions to enhance absorption.

Acknowledgements

The authors thank Dr Collins Assisi for computational resources. Prabhat Prakash thanks IISER Pune for graduate fellowship. AV thanks DST Nanomission (Grant No: SR/NM/NS-15/2011) and DST (SB/S1/PC-015/2013) for providing financial support in this work.

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Footnote

Electronic supplementary information (ESI) available: Initial configuration of CO2–IL slab, charge densities, particle densities of CO2, MSD of ions, molar absorption ratio and effect of initial configurations on structural and dynamical properties. See DOI: 10.1039/c6ra09577a

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