Hydrogen and Methane Adsorption in Metal−Organic Frameworks: A High-Pressure Volumetric Study

Abstract

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 H2 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 H2 and CH4 in MOF5 are found to be 10.3 ...