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Mechanism of Molecular Permeation through Nanoporous Graphene Membranes
Chengzhen Sun,†‡ Michael S. H. Boutilier,† Harold Au,† Pietro Poesio,§ Bofeng Bai,‡ Rohit Karnik,† and Nicolas G. Hadjiconstantinou*,†
†Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachussets 02139, United States
‡State Key Laboratory of Multiphase Flow in Power Engineering, Xian Jiaotong University, Xian, Shaanxi 710049, China
§Department of Mechanical and Industrial Engineering, University of Brescia, Brescia 25123, Italy
Summary
This paper presents the transport of He, H2, N2, and CH4 across nanoporous graphene. This study showed that the flux for the gases that do not adsorb (He, H2) on the surface of graphene was high than those who adsorb (N2, CH4) on graphene.
Classical molecular dynamics simulation was used in this study. The graphene membrane had an area of 3 x 3 nm2 and the height of the simulation box was 18 nm which was divided into two equal chambers with nanoporous graphene in its middle. Each chamber contained 50 molecules of the gas. Reflective boundary condition was used in z-direction while periodic boundary condition was used in x and y-directions. The flux was tested with 10 different pore sizes and diameter of the pore was described by (Ap/π)1/2 with Ap as the pore area. NVT ensemble was used with a total simulation time of 2 x 108 fs with a time step of 0.067 fs. Temperature was fixed at 300 K using Nose-Hoover thermostat algorithm. For C-C, H-H, and C-H interaction AIREBO potential was used and for N2 bond, harmonic potential while for other gases Lennard-Jones potential was used.
Flux was computed for the molecules crossing in both directions. It was found that pore geometry was more likely permeable to H and He than N2 and CH4. The permeance of H and He was found to be 106 GPU and it is greater than state of the art gas separation processes. It was found that permeation depends on molecular mass and kinetic surface of the gas. In addition, adsorption of the gas on the membrane surface is also responsible for gas permeation. The flux was divided into direct and surface flux. Surface flux was found negligible for non adsorbing gases. So, direct flux played a major role in transport of the gas across the membrane. The surface flux is under study by this research group.
Reviewed by: Aamir Alaud Din