Comparison of the Effect of Functional Groups on Gas-Uptake Capacities by Fixing the Volumes of Cages A and B and Modifying the Inner Wall of Cage C in rht-Type MOFs

Three porous (3,24)-connected rht-type metal–organic
frameworks
(MOFs), [Cu3L­(H2O)3]·xsolvents
(H6LOH = 4,4′,4″-(hydroxysilanetriyl)­tris­(triphenyl-3,5-dicarboxylic
acid), SDU–6; H6LMe = 4,4′,4″-(methylsilanetriyl)­tris­(triphenyl-3,5-dicarboxylic
acid), SDU–7; H6LiBu = 4,4′,4″-(isobutylsilanetriyl)­tris­(triphenyl-3,5-dicarboxylic
acid), SDU–8), have been successfully prepared
from [Cu2(COO)4] paddlewheel SBUs (secondary
building units) and C3-symmetric Si-based hexatopic carboxylatelinkers.
All porous MOFs are constructed from 3D packing of nanosized cuboctahedral,
truncated tetrahedral, and truncated octahedral cages. SDU–6–8 differ only in the functionality of the central Si atom of the hexacarboxylate
ligands with hydroxyl, methyl, and isobutyl groups, respectively.
Gas adsorption measurements of activated MOFs suggested that decoration
of the cage walls with strong polar groups can enhance the adsorption
capacities for N2, H2, and CH4. SDU–6 with −OH as the functional group possesses
high CH4 uptake (172 cm3 cm–3 at 35 bar), which is very close to DOE target of 180 cm3 cm–3.