Adsorption of environmentally significant gases (hydrogen, carbon dioxide, hydrogen sulfide, methane) in metal-organic frameworks.

Abstract

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 <italic> in situ</italic> 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% CH₄at room temperature and 42 bar.