Hydrogen and methane adsorption in metal-organic frameworks: A high-pressure volumetric study

We report hydrogen and methane adsorption isotherms in two prototypical metal-organic framework compounds (i.e., MOF5 and ZIF8) over a large temperature (30-300 K) and pressure (up to 65 bar) range using a fully computer-controlled Sieverts apparatus. We find that, in a volumetric method, a proper choice of real gas equation of state is critical for obtaining reliable isotherm data. The widely used van der Waals equation of state (EOS) is not adequate to describe H-2 and CH4, while the modified Benedict-Webb-Rubin (MBWR) EOS works well, even at very low temperatures and high pressures. With the known sample mass and bulk density, the skeleton density and the specific pore volume of MOF5 and ZIF8 were also measured. In addition to excess and absolute adsorption isotherms, we also introduce an "effective adsorption" which compares the amounts of gas adsorbed in a container with and without the adsorbent. At low temperatures, the maximal excess adsorption capacities of H-2 and CH4 in MOF5 are found to be 10.3 wt % and 51.7 wt %, respectively, while they are only 4.4 wt % and 22.4 wt % in ZIF8. From the temperature-dependent isotherm data, the isosteric heat of adsorption (Q(st)) was also estimated. The excess Q(st)'s for the initial H-2 and CH4 adsorption in MOF5 are similar to 4.8 kJ/mol and similar to 12.2 kJ/mol, respectively. We obtained similar Q(st)'s for ZIF8. We hope that the detailed isotherm curves reported here over a large temperature and pressure range will be a critical test for future grand canonical Monte Carlo simulations and force-field models.