Adsorption of Gases in Microporous Organic Molecular Crystal, a Multiscale Computational Investigation

The grand canonical Monte Carlo (GCMC) method and high-level first-principle calculations are performed to investigate the role of a constrained channel of microporous organic molecular crystal in separating H-2 from binary mixtures containing N-2, CH4, or CO2. GCMC simulations show that the selectivity of N-2, CH4, or CO2 over H-2 is in the order of N-2/H-2 < CH4/H-2 < CO2/H-2, which is consistent with the order of isosteric heats of adsorption. Particularly at low pressure the selectivity is very high because CO2, CH4, or N-2 initially occupies the preferential site in the channel with less sites left for H-2. In addition, dispersion corrected density functional theory (DFT-D) is introduced to study the interaction energies and structural properties of the conjugated channel and gases. By comparing with the benchmark data of the coupled-cluster calculations with singles, doubles, and perturbative triple excitations [CCSD(T)] estimated at the complete basis set (CBS) limit, the proper functional is selected. The first-principle calculations confirm that the heterogeneous channel can hold CO2, CH4, or N-2 much stronger than H-2, suggesting the microporous organic molecular crystal is a good candidate for potential hydrogen purification.