Polar Triptycene-Based Nonmetal Organic Frameworks Show Enhanced Hydrogen Adsorption

Porous nonmetal organic frameworks (N-MOFs) are an emerging
class
of porous materials that can have desirable properties such as stability
for iodine capture, proton conductivity, and xenon/ krypton separation.
Here, we present the first experimental evidence of polymorphism in
porous N-MOFs through the identification of two stable porous phases
of a triptycene framework (T.Cl-α and T.Cl-β). We also
report single-crystal structures for two isostructural porous N-MOFs
(T.Cl-α and T.Br-α). All three polymorphs are porous to
carbon dioxide and nitrogen, and T.Cl-α exhibits a remarkable
hydrogen uptake of 7.2 mmol g–1 at 77 K and 1 bar.
This hydrogen sorption far exceeds most other porous crystals and
compares favorably with many MOFs under the same condition. The hydrogen
uptake for T.Cl-α is anomalously high for its relatively modest
pore volume (0.198 cm3 g–1 from N2 isotherms at 77 K), which we attribute to London-dispersion
interactions and geometric confinement within subnanometer pores.