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Record-large pore molecular crystals: A leap toward clean energy storage

In our study in the Journal of the American Chemical Society, our team from The University of Hong Kong and Northwestern University, led by the late Nobel Laureate Professor Fraser Stoddart, developed RP-H200, a hydrogen-bonded organic framework (HOF) with the largest pores in its class.

This innovation unlocks new possibilities for sustainable energy storage, gas separation, and catalysis.

The challenge of designing stable, large-pore molecular crystals

Molecular crystals, such as hydrogen-bonded organic frameworks (HOFs), are ordered structures formed by organic molecules linked through noncovalent interactions, such as hydrogen bonds or π-π stacking. Rock candy, where sucrose molecules form crystals via hydrogen bonds, is a simple example. Endowing molecular crystals with porosity unlocks vast potential for gas separation, sensing, and catalysis.

Yet, a persistent challenge remains: creating molecular crystals with large, stable pores. Large-pore, highly porous crystals often lack stability, while densely packed structures, though robust, have small or no pores. This trade-off has long hindered practical applications.

Our solution

To address this challenge, we designed a mesoporous hydrogen-bonded organic framework (HOF) that combines large pores with exceptional stability. The result is RP-H200, a novel molecular crystal featuring 3.6-nanometer pores—the largest ever reported in its class.

This breakthrough was achieved through a unique noncoplanar assembly strategy using imidazole-annulated triptycene hexaacids. These molecules self-assemble into a double-walled, honeycomb-like structure, creating pores lined with aromatic surfaces.

The material exhibits an impressive surface area of 2313 m²/g—equivalent to one-third of a football field per gram. Remarkably, RP-H200 maintains its structural integrity at temperatures up to 350°C and in solvents such as ethanol, as confirmed by X-ray diffraction.

High performance in gas storage

RP-H200's large pores, vast surface area, and exceptional stability make it ideal for storing clean-energy gases. At 270 K and 100 bar—typical conditions for methane storage—it captures 0.31 grams of methane per gram, surpassing many existing materials. Its aromatic pore surfaces form C-H-π interactions with methane, with an adsorption heat of 12 kJ/mol, enabling efficient storage and release without extreme conditions. RP-H200 also stores 6.7% of its weight in hydrogen at 77 K, advancing hydrogen-powered vehicles.

Through repeated adsorption cycles, we confirmed RP-H200's durability, with no significant methane capacity loss after three cycles. It remains stable in humid air and organic solvents, positioning RP-H200 as a practical solution for real-world energy storage.

Future implications

Our noncoplanar assembly strategy delivers ultra-large pores, robust stability, and solution-processability, enabling cost-effective production. By using organic molecules, RP-H200 embraces sustainable chemistry, and its recyclability minimizes waste.

This material opens exciting possibilities for HOFs in clean energy, gas separation, and catalysis. Imagine lightweight tanks for methane-fueled vehicles, reducing reliance on fossil fuels, or drug delivery systems that use these pores to transport medications. Our approach can be extended to other HOFs, accelerating the development of functional molecular crystals for diverse applications.

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