| The domain of gas adsorption science has been significantly impacted by the discovery of metal-organic frameworks (MOFs), and these materials have since matured beyond traditional characterization to veritable applications in storage and catalysis. To ascertain key structural features that lead to favorable storage capacity of H2, CO2, H2S and CH4 in these materials, a subset (MOF-2, MOF-74, MOF-177, MOF-505, Cu3(BTC)2, IRMOFs-1, -3, -6, -8, -11, and -14) was examined for adsorption behavior in several temperature and pressure regimes. A study of H2 adsorption reveals that, contrary to other microporous materials, H2 uptake at 77 K and 760 torr among different MOFs does not scale with surface area. Materials having interpenetrated frameworks, pores of 5--10 A diameter, or open-metal sites exhibited higher H2 uptake, with MOF-505 adsorbing a record-breaking 2.47 wt% H2 at 77 K and 760 torr. Low pressure, low temperature adsorption of CO2 on several large-pore MOFs produced fully-reversible stepped isotherms with correlated step location and pore size. The initial linear region of these isotherms was identified as the Henry region, with a calculated 19(1) kJ/mol isosteric heat of adsorption for CO2 on IRMOF-1. Subsequent low temperature in situ SXRD investigations of CO2 adsorbed on IRMOF-1 clearly identified the primary adsorption sites found on the inorganic cluster. Toward more practical applications, nine MOFs were examined for CO2 capacity at room temperature up to 42 bar. Capacity was found to scale roughly with surface area, and Zn4O(O2C)6-type frameworks produced sigmoidal isotherms, with MOF-177 adsorbing a record-breaking 147 wt% at 42 bar. Room temperature, high pressure studies of H2S sorption on several MOFs demonstrated capacities of 20 wt%, but the irreversible uptake was determined to be due to chemisorption rather than physisorption. Nine MOFs were examined for CH4 capacity at room temperature up to 42 bar and revealed similar trends as the high pressure CO2 study, with MOF-177 reversibly adsorbing 20 wt% CH4 at room temperature and 42 bar. |
