Understanding the Separation Mechanism of C2H6/C2H4 on Zeolitic Imidazolate Framework ZIF-7 by Periodic DFT Investigations

The selective adsorption of light alkane over alkene on ZIF-7 provides a new insight for the separation of small-sized molecules with similar physical properties. To gain a comprehensive understanding of the separation selectivity of alkane over alkene on ZIF-7, we systematically investigated the adsorption and diffusion processes of C2H6 and C2H4 on ZIF-7 by periodic density functional theory calculations with a standard generalized gradient approximation functional [Perdew-Burke-Ernzerhof (PBE)], PBE with Grimme's D2 and D3 corrections for dispersion, and four van der Waals density functionals (vdW-DFs). We found that because of a stronger dispersion interaction and more number of C-H center dot center dot center dot pi/r/N/H bonds of C2H6 in the ZIF-7 framework compared to that of C2H4, C2H6 has always obviously more favorable adsorption energy than C2H4 at potential adsorption sites and needs to conquer a lower free energy barrier than C2H4 during the transport in ZIF-7. The stronger dispersion interaction between C2H6 and ZIF-7 could be ascribed to the larger molecular surface of C2H6 and the fitness of its molecular shape to the cavity structure of ZIF-7. These findings indicate that the preferential adsorption and quicker diffusion of C2H6 on ZIF-7 determine that ZIF-7 with structural flexibility can be utilized for the separation of C2H6/C2H4 mixture. Our calculations highlight the principle of structural matching for the preferential adsorption: the threefold symmetry of C2H6 fits favorably with the symmetry of the local structure of the adsorption site to form a more stable complex, which is mainly bound through dispersion force. It may provide useful guidance for the rational design of porous materials for the separation of small guest molecules.