Polar Triptycene-Based Nonmetal Organic Frameworks Show Enhanced Hydrogen Adsorption

Area of Science:

• Materials Science
• Chemistry
• Nanotechnology

Background:

• Porous nonmetal-organic frameworks (N-MOFs) are a promising class of materials with applications in gas separation, storage, and catalysis.
• Polymorphism, the ability of a solid material to exist in multiple crystalline forms, is common in many materials but has not been experimentally observed in porous N-MOFs until now.

Purpose of the Study:

• To present the first experimental evidence of polymorphism in porous N-MOFs.
• To characterize the structural and gas sorption properties of different polymorphic phases.

Main Methods:

• Synthesis and single-crystal X-ray diffraction were used to identify and determine the structures of two polymorphic phases (T.Cl-α and T.Cl-β) of a triptycene framework.
• Gas sorption isotherms (CO2, N2, H2) were measured at cryogenic temperatures to evaluate the porosity and gas uptake capabilities of the N-MOF polymorphs.

Main Results:

• Two stable porous phases of a triptycene framework, T.Cl-α and T.Cl-β, were identified, providing the first experimental evidence of N-MOF polymorphism.
• Single-crystal structures of two isostructural porous N-MOFs, T.Cl-α and T.Br-α, were determined.
• All identified polymorphs demonstrated porosity to CO2 and N2.
• The T.Cl-α phase exhibited a remarkable hydrogen uptake of 7.2 mmol g⁻¹ at 77 K and 1 bar, significantly exceeding that of most other porous crystals and comparable to many metal-organic frameworks (MOFs).

Conclusions:

• The discovery of polymorphism in porous N-MOFs opens new avenues for material design and property tuning.
• The exceptional hydrogen uptake in T.Cl-α, attributed to London-dispersion interactions and geometric confinement, highlights the potential of N-MOFs for advanced gas storage applications.
• Further research into N-MOF polymorphism could lead to tailored materials for specific industrial applications such as gas separation and catalysis.