High uptakes of methane in Li-doped 3D covalent organic frameworks.

By using a multiscale theoretical method, which combines the first-principles calculation and grand canonical Monte Carlo (GCMC) simulation, we studied storage capacities of methane in 3D covalent organic frameworks (COFs) and their Li-doped compounds at T = 243 and 298 K. Our results predicted that, at T = 298 K and 35 bar, the excess gravimetric capacities of COF-102 and COF-103 reach 17.72 and 16.61 wt % (corresponding to 302 and 285 cm(3) (STP)/g)), which are in good agreement with experimental data, while the excess volumetric capacities of COF-102 and COF-103 reach 127 and 108 v (STP)/v, respectively. The high methane storage capacity of the COFs can be attributed to their ultrahigh surface areas and low densities. To further enhance the methane capacity, we investigated the impact of Li-doping on the methane storage performance of the COFs. Our first-principles calculations show that the Li cation doped in the COFs can enhance the binding of methane to the substrate significantly because of the London dispersion and the induced dipole interaction, due to the strong affinity of Li cation to methane molecules. At T = 298 K and relatively low pressures (p < 50 bar), the Li-doping method nearly doubles the methane uptakes of the COFs, compared to the nondoped materials. In particular, at T = 298 K and p = 35 bar, the methane volumetric uptakes of Li-doped COF-102 and COF-103 reach 303 and 290 v (STP)/v, respectively, which is more than 2 times those in the nondoped (127 and 108 v (STP)/v). To the best of our knowledge, the Li-doped 3D COFs show the largest methane storage uptakes at room temperature to date.