Ab Initio Study of Hydrogen Adsorption in COF-10

Ab initio predictions of hydrogen adsorption isotherms in covalent organic frameworks-10 (COF-10) were performed based on the calculation of Gibbs free energy changes. First, structures of adsorption products corresponding to seven possible adsorption sites were optimized. Hydrogen tends to be adsorbed along the pore surface rather than between the two neighboring layers of COF-10. Second, thermodynamic data of adsorption processes were calculated. The calculated adsorption enthalpy reproduces the experimental result well (around –4 kJ mol–1). Two schemes have been adopted to calculate adsorption entropies and the corresponding Gibbs free energies of adsorption, and exhibiting big differences between the results of the two schemes. At last, adsorption isotherms have been plotted based on adsorption equilibrium constants. The scheme adopting frustrated translation with an average relative error of 10.50% is much better than the traditional scheme treating all the six degrees of freedom of hydrogen as vibrational modes with an average relative error of –90.24%.