Effect of Li Doping on Diffusion and Separation of Hydrogen and Methane in Covalent Organic Frameworks

We systematically investigate the diffusion and separation behavior of hydrogen and methane in covalent organic frameworks (COFs) and Li-doped counterparts. The results indicate that the self-diffusivities of hydrogen and methane in COFs decrease monotonically with the increase of pressure. Interestingly, after Li doping into COFs, the self-diffusivities first increase at low pressure, and then reach a plateau, and finally decrease slightly at high pressure. This phenomenon stems from the fact that the Li atom has a strong affinity to the gas molecules. In addition, it is also found that the COFs show a larger self-diffusivity than most of the metal organic frameworks (MOFs), and the permselectivities of the COFs for H-2/CH4 are 1 order of magnitude higher than the Li-doped COFs. In particular, the adsorption selectivity of Li-doped COFs for CH4/H-2 gets a significant improvement, compared to undoped ones. To further understand the effect of Li doping on diffusion and separation of gases, the isosurface and the contour plots of the center of mass and radial distribution functions of gases are also explored. In short, the Li doping into COFs can increase the adsorption selectivity of COF materials significantly, while for a kinetic separation process, the undoped COF-based membranes might be a very promising material.