Journal of Membrane Science 428 (2013) 546–553 Contents lists available at SciVerse ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci Water-solubility-driven separation of gases using graphene membrane Joonho Lee, N.R. Aluru n Department of Mechanical Science and Engineering, Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA article info abstract Article history: We investigate separation of gas-mixtures using a graphene membrane. By introducing a water slab Received 5 May 2012 between a gas-mixture and the graphene membrane, we show that the gas-mixture can be separated Received in revised form based on the water-solubility of the gas molecule. By considering various gas-mixtures we show that 28 September 2012 the separation ratio follows the water-solubility ratio of the gas molecules in the mixture. We also Accepted 2 November 2012 demonstrate the water-solubility-driven separation of gas-mixtures using a carbon nanotube, but we Available online 10 November 2012 show that the graphene membrane provides higher selectivity ratio because of its single-atom Keywords: thickness. Gas separation & 2012 Elsevier B.V. All rights reserved. Molecular dynamics Graphene 1. Introduction membrane and comes out to the other region [12]. Ever since the liquid membrane (or membrane contactor) was introduced Separation of CO2 from the flue gas is an important step [13,14], separation methods based on the absorption liquid and towards reducing the global emissions of CO2. Many processes the membrane have improved over a couple of decades and such as adsorption, absorption and membrane technology have researchers have primarily focused on various systems and been used to remove CO2 from the flue gas [1,2]. A popular diverse materials in the liquid membrane system. To achieve industrial method to separate CO2 is to use amine solvents in the efficient gas separation through the liquid membrane, the type of absorption process. In spite of high product yields and purities, high absorption liquid used in the system is important. Water as an energy consumption and liquid loss in desorption are regarded as absorption liquid has steadily received much attention for gas disadvantages of the approach [2,3]. On the other hand, membrane separation [11,15,16]. In addition, different materials for the technology has been considered as an energy efficient method for membrane have been used instead of the conventional polymeric CO2 separation for a couple of decades, as membranes are thin and membrane, to reduce the overall diffusion thickness. easy to scale-up [4]. Recent studies on gas separation through Graphene is a promising material for various applications membranes have focused on inorganic membranes such as zeolites because of its exceptional physical properties and atomic scale and carbon-based membranes for high-temperature environments thickness [17]. Graphene is a single-atom-thick planar sheet [5–8]. The thickness of these membranes ranges from tens of made of carbon atoms arranged in a hexagonal lattice. Graphene, nanometers to several micrometers. without defects, does not have pores and is impermeable, but Taking advantage of both the absorption and the membrane atomic scale pores can be generated in a graphene sheet by technologies, a couple of methods have been proposed. The first chemical or thermal treatment [18,19]. Porous graphene has been one is a membrane contactor in which a membrane acts as an investigated as a membrane material and it has been shown to be interface between the feed gas and the absorption liquid [9,10]. a fast transporter of water [20]. In addition, porous graphene has CO2 is solvated in the absorption liquid through the membrane also been shown to be a high performance membrane material for and this solvated CO2 is separated later by using stripper or the ion, gas and nanoparticle separation [21–23]. Jiang et al. consid- same type of membrane contactor called desorber [11]. The ered graphene with subnanometer size pores as a membrane for second approach uses a liquid membrane, where an absorption gas separation and observed that porous graphene can be a highly liquid and polymeric membranes are placed between two gas efficient membrane for gas separations [22]. regions. CO2 in the feed gas region penetrates through the liquid In this paper, we investigate water solubility-driven separation of CO2 from CO2/O2,CO2/N2 and CO2/CH4 mixtures using a porous graphene membrane. Molecular dynamics simulation is an impor- n tant tool to understand the vapor–liquid and liquid–solid inter- Corresponding author. Tel.:þ1 217 333 1180; fax: þ1 217 244 4333. E-mail address: [email protected] (N.R. Aluru). face effects on the separation procedure and the coupled URL: http://netfiles.uiuc.edu/aluru/www/ (N.R. Aluru). interaction of gas with the graphene pore and the water slab [24]. 0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.11.006 J. Lee, N.R. Aluru / Journal of Membrane Science 428 (2013) 546–553 547 Fig. 1. (a) Snapshot of the system (CO2/O2 separation though water/graphene membrane). (b) Porous graphene membrane. By performing extensive molecular dynamics simulations, we used for the L–J potential. Electrostatic interactions were com- understand separation of gases. To investigate the merits of porous puted by using the Particle Mesh Ewald method with a grid graphene for gas separations, we also perform molecular simula- spacing of 0.15 nm. Periodic boundary conditions are applied in tions using carbon nanotubes (CNT). Studies on CNT have shown all the three directions. Time integration was performed by using fast transport of water and gases through the tubes [25–27], so the leap-frog algorithm with a time step of 1.0 fs. The tempera- comparison studies between porous graphene and CNT can provide ture of the system is maintained at 300 K by Nose´–Hoover insights on the effect of the length of the pore on gas separations. thermodynamics with a characteristic time of 0.1 ps [39]. Simula- We envision that our work will lead to the use of porous graphene tions were performed using GROMACS 4.0.7 [40]. as an efficient membrane for gas separations. 3. Results and discussion 2. Methods 3.1. Solubility of gases in water We performed molecular dynamics simulations to investigate water solubility-driven separation of gases through a porous Before presenting results on the separation of gases through graphene membrane. A snapshot of the CO2/O2 mixture and the the porous graphene membrane, we discuss the solubility of the water slab separating the mixture and the porous graphene various gases considered in this work (CO2,O2,N2 and CH4)in membrane is shown in Fig. 1(a). The volume of the system bulk water at 300 K. The solubility results will not only explain 3 (3.9 Â 3.8 Â 14.0 nm ) is fixed throughout the simulation. The the separation properties, but also show the limitations, if any, of gas mixture is initially positioned at both ends of the system. the force-fields in accurately representing experimental solubility During simulation, the gas molecules penetrate through the water data. We estimated the solubility of each gas using the method slab and the porous graphene membrane and enter into the discussed in [36]. We simulated the water slab system, as shown middle region, which is initially empty. The 0.99 nm diameter in Fig. 2(a), with 98 gas molecules and 1600 water molecules at graphene pore is shown in Fig. 1(b). The thickness of the water 300 K. Using the averaged value of gas concentration along the z- slab is approximately 2.4 nm. The bond length of CO2,O2 and N2 is axis, as shown in Fig. 2(b), we calculated gas concentration in the maintained by the shake constraint scheme. The bond stretching water slab. Since the concentration of gas in water is proportional and angle bending of CH4 is controlled by the OPLS-AA force field to gas vapor pressure, we can compare the gas solubility by [28]. The non-bonded potential energy between molecules is calculating the Henry’s constant (pressure unit) as shown in given by Table 1. The gas vapor pressure is calculated by van der Waals equation and Henry’s constant is calculated based on Henry’s law Unonbond ¼ ULJ þUCoulomb ð1Þ hH ¼ p=x , where p is the pressure of gas in the vapor region and x where is the mole fraction of gas in the water slab. If the gas molecule "# X s 12 s 6 has a smaller Henry’s constant, then it has a higher solubility. We U ¼ 4E ij À ij ð2Þ LJ r r note that CO2 has higher solubility compared to that of O2,N2 and i,j ij ij CH4. The solubility of O2 is about two times bigger than that of N2 X and the solubility of CH4 is between that of O2 and N2. Even qiqj UCoulomb ¼ ð3Þ though there is some discrepancy between experimental solubi- rij i,j lities and our estimated values, the ratios of various gas solubi- The Lennard–Jones (L–J) and Coulomb potential parameters for lities are reasonable compared to the experimental values and we the various gas molecules (CO2,O2,N2 and CH4) are taken from will use this to explain gas separations. [28–30]. We estimated the solubility of CO2 [29,31,32], O2 [30,33,34], N2 [29,30,33] and CH4 models [28,35] by the method 3.2. Separation of gas mixtures discussed in Ref. [36] using SPC/E water slab at 300 K.