Computational characterization of halogen vapor attachment, diffusion and desorption processes in zeolitic imidazolate framework-8.

Computational simulation methods are used for characterizing the detailed attachment, diffusion and desorption of halogen vapor molecules in zeolitic imidazolate framework-8 (ZIF-8). The attachment energies of Cl, Br and I are -55.2, -48.5 and -43.0 kJ mol, respectively. The framework of ZIF-8 is disrupted by Cl, which bonds with Zn either on the surface or by freely diffusing into the cage. A framework deformation on the surface of ZIF-8 can be caused by the attachment of Br, but only reorientation of the 2-methylimidazolate linkers (mIms) for I. In diffusion, the halogen molecules have a tendency to vertically permeate the apertures of cages followed with swing effect implemented by the mIms. Larger rotation angles of mIms are caused by Br because of its stronger interaction with mIms than I. A maximum of 7 Br or 5 I molecules can be accommodated in one cage. Br are clinging to the mIms and I are arranged as crystal layout in the cages, therefore in desorption processes molecules attached to the surface and free inside are desorbed while some remained. These results are beneficial for better understanding the adsorption and desorption processes of halogen vapors in the porous materials.