Adsorption and diffusion of hydrogen and methane in 2D covalent organic frameworks

We report computer simulation results for adsorption and diffusion of hydrogen and methane in recently synthesized 2D covalent organic frameworks. Our model, based on classical force fields, is able to reproduce fairly well the experimental adsorption isotherms of argon at T = 87 K. The calculated adsorption isotherms of H-2 and CH4 at T = 77 K and T = 298 K show that the uptake to be expected for these gases is generally half than the one in metal-organic frameworks or 3D covalent organic frameworks both on a gravimetric and a volumetric basis, with the exception of COF-8, which has an uptake comparable to other organic frameworks as far as methane adsorption is concerned. We have investigated the mechanism of diffusion in order to assess similarities and/or differences with the behavior observed in carbon nanotubes, that have a similar pore structure and where transport is known to occur very rapidly. Our results point out that gas diffusion in 2D covalent organic frameworks is one order of magnitude more rapid than in metal-organic frameworks or zeolites, but still not as fast as in carbon nanotubes. The adsorption and diffusion characteristics of these materials are related to the peculiar structure of the solid-fluid potential energy surface. (C) 2008 Elsevier Inc. All rights reserved.