Ab initio based grand canonical Monte-Carlo simulations of CH_4 uptake in covalent-organic frameworks (COFs)

Alternatives routes towards new energies sources have become a main stream research worldwide. Methane has the potential to become a future fuel, however there have been longstanding problems related to transport and storage making it not an economically viable path. Attempts to overcome these issues include conversion to methanol, better compression techniques and sorption into porous materials. The latter is of special interest because of the recent discovering of new class of materials called Covalent Organic Frameworks (COF) that are: tailored materials, highly crystalline, have a high surface area (≥2000 m^2/g) with a high pore volume and are made of just light atoms (C, Si, B,O and H). These properties allowed COFs to have the lowest crystalline densities among solid state materials with promising properties for storage. In order to investigate CH4 sorption phenomena in COF an ab initio study was performed. Accurate second order Møller-Plesset perturbation theory (MP2) calculations were executed using doubly-polarized valence quadruple-ζ(QZVPP) basis sets in order to develop the correct force fields (FFs) between CH4 and COF structure as well as CH_4 and CH_4. With the developed FFs, statistical mechanics concepts were implemented in a Grand Canonical Monte Carlo algorithm to simulate methane sorption in COFs. The approach was also tested comparing the densities of methane obtained by simulation with experimental ones at various pressures. Also experimental data for COF-5 and COF-8 were well reproduced with this theoretical approach. From this validation the method was extended to predict methane uptake in COF-1, COF-6, COF-8, COF-10, COF-102, COF-103, COF-105, COF-108 and COF-300. COF-1 is predicted to reach Department of Energy of United States of America target for methane storage in porous solids of 180 v(STP)/v. Using reticular chemistry new hypothetical structures are proposed to be synthesized, which are: COF-28, COF-300 and COF350