Adsorption Sites of Hydrogen in Zeolitic Imidazolate Frameworks

Grand canonical Monte Carlo simulations were employed to investigate hydrogen adsorption in zeolitic imidazolate frameworks [ZIFs, a new category of metal-organic frameworks (MOFs), here ZIF-8 used as an example] at 77 K and pressures increasing from 10 to 8000 kPa. A modified OPLS-AA force field was applied for the imidazolate frameworks and Buch's model for hydrogen molecules. It shows good performance compared with the experimental measurements. The adsorption sites in the ZIF-8 materials were explored by a previously proposed technique named "computer tomography for materials (mCT)". The mCT images suggest that the first adsorption site locates at both sides of the imidazolate ring and it is close to the imidazolate C=C bond. The hydrogen molecules then are adsorbed in the pore channel which forms the secondary adsorption site. The difference between the ZIF's adsorption sites and the MOF's was analyzed, and it may be helpful to design new ZIF materials with improved hydrogen adsorption capability.