Effects of Functionalization, Catenation, and Variation of the Metal Oxide and Organic Linking Units on the Low-Pressure Hydrogen Adsorption Properties of Metal−Organic Frameworks

The dihydrogen adsorption isotherms of eight metal−organic frameworks (MOFs), measured at
77 K up to a pressure of 1 atm, have been examined for correlations with their structural features. All
materials display approximately Type I isotherms with no hysteresis, and saturation was not reached for
any of the materials under these conditions. Among the six isoreticular MOFs (IRMOFs) studied, the
catenated materials exhibit the largest capacities on a molar basis, up to 9.8 H2 per formula unit. The
addition of functional groups (−Br, −NH2, −C2H4−) to the phenylene links of IRMOF-1 (MOF-5), or their
replacement with thieno[3,2-b]thiophene moieties in IRMOF-20, altered the adsorption behavior by a minor
amount despite large variations in the pore volumes of the resulting materials. In contrast, replacement of
the metal oxide units with those containing coordinatively unsaturated metal sites resulted in greater H2
uptake. The enhanced affinities of these materials, MOF-74 and HKUST-1, were further demonstrated by
calculation of the isosteric heats of adsorption, which were larger across much of the range of coverage
examined, compared to those of representative IRMOFs. The results suggest that under low-loading
conditions, the H2 adsorption behavior of MOFs can be improved by imparting larger charge gradients on
the metal oxide units and adjusting the link metrics to constrict the pore dimensions; however, a large pore
volume is still a prerequisite feature.