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)]center dot 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 [Cu-2(COO)(4)] paddlewheel SBUs (secondary building units) and C-3-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 N-2, H-2, and CH4. SDU-6 with -OH as the functional group possesses high CH4 uptake (172 cm(3) cm(-3) at 35 bar), which is very close to DOE target of 180 cm(3) cm(-3).