Understanding Gas Storage in Cuboctahedral Porous Coordination Cages

Porous molecular solids are promising materials for gas storage
and gas separation applications. However, given the relative dearth
of structural information concerning these materials, additional studies
are vital for further understanding their properties and developing
design parameters for their optimization. Here, we examine a series
of isostructural cuboctahedral, paddlewheel-based coordination cages,
M24(tBu-bdc)24 (M = Cr, Mo, Ru; tBu-bdc2– = 5-tert-butylisophthalate),
for high-pressure methane storage. As the decrease in crystallinity
upon activation of these porous molecular materials precludes diffraction
studies, we turn to a related class of pillared coordination cage-based
metal–organic frameworks, M24(Me-bdc)24(dabco)6 (M = Fe, Co; Me-bdc2– = 5-methylisophthalate;
dabco = 1,4-diazabicyclo[2.2.2]­octane) for neutron diffraction studies.
The five porous materials display BET surface areas from 1057–1937
m2/g and total methane uptake capacities of up to 143 cm3(STP)/cm3. Both the porous cages and cage-based
frameworks display methane adsorption enthalpies of −15 to
−22 kJ/mol. Also supported by molecular modeling, neutron diffraction
studies indicate that the triangular windows of the cage are favorable
methane adsorption sites with CD4–arene interactions
between 3.7 and 4.1 Å. At both low and high loadings, two additional
methane adsorption sites on the exterior surface of the cage are apparent
for a total of 56 adsorption sites per cage. These results show that
M24L24 cages are competent gas storage materials
and further adsorption sites may be optimized by judicious ligand
functionalization to control extracage pore space.