Excavating hidden adsorption sites in porous materials for great enhancement in gas storage

A high-throughput computational screening of an experimental metal-organic framework database has led to the discovery of 13 porous materials with potential for gas storage enhancement upon the introduction of defects. When linker vacancy defects are engineered into these materials, previously inaccessible sites hidden from the gas molecules can be newly accessed for enhancement in gas adsorption.

Article  |  Spring 2018

Can defects or imperfections actually be beneficial for material performance? The presence of flaws may negatively influence the overall performance of a material, causing it to deviate away from the ideal projections of the pristine form. However, in certain cases, the presence of defects can be extremely helpful in enhancing material performance in a desired field of application. For example, defects can provide highly active sites for heterogeneous catalysis in nanocomposite materials.

In the case of a class of porous materials called metal-organic frameworks, or MOFs, Professor Jihan Kim and his research team have used defects to significantly enhance their gas adsorption and storage performance.

MOFs are crystalline materials formed from the coordination bonding of metal clusters and organic linkers, often exhibiting high porosity. The resulting large surface area and pore volume of these materials make them promising candidates for many applications involving the adsorption of gas molecules. Tens of thousands of MOFs have been synthesized from the various combinations of metal clusters and linkers, each with its unique chemical environment and pore geometry. However, majority of the research in the MOF field is focused on a select few frameworks with the most desirable properties. With many others being mostly overlooked, it is worthwhile to revisit the already synthesized structures and seek ways to further improve their performance.

 

In some cases, MOFs can contain secluded pores that are disconnected from the main pore channel, which are ultimately inaccessible for the gas molecules of interest. Upon their excavation, these previously inaccessible pores can be considered as hidden adsorption sites for enhanced gas capture and storage. To achieve this novel expansion of surface area and pore volume, Prof. Kim’s research group developed the following defect engineering scheme: by introducing a small number of linker vacancy defects are into these MOFs, new diffusion channels into the previously inaccessible pores can be created, effectively opening them up for additional adsorption of gas molecules.

In efficiently searching for the presence of inaccessible pores in MOFs, a parallel computing code developed by Prof. Kim was utilized for the high-throughput screening of an experimental MOF database. This code performed a flood-fill algorithm on the low adsorption energy regions within the MOF structure to identify the endlessly connected main channels, which is similar to how Microsoft Paint instantly fills a connected area with the same color. The other energy-low regions that have not been connected to the main channel would then be identified as the inaccessible pores that can be opened up with the introduction of linker vacancy defects.

 

Screening was performed with methane as the target adsorbate of interest, and 13 structures with significant volumes of inaccessible pores were successfully identified. Then, the research team virtually introduced linker vacancy defects into the structures of the candidate frameworks so that the previously inaccessible pores could be opened up and newly accessed by the methane gas molecules. Molecular simulations on the resulting defective frameworks have shown a significant increase in the gas storage capacities for all of the candidates, with the top candidate exhibiting a 55.56% increase in its methane uptake with less than 8.33% of the linkers affected.

This rational defect engineering work was published in Nature Communications (DOI:10.1038/s41467-017-01478-4) on November 16, 2017. The research team believes this rational defect engineering and inaccessible pore engineering scheme can be further utilized for many other applications, such as gas separation and harmful gas capture.