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Porous organic materials: Gassed up

The need for sustainable and clean energy sources has fuelled considerable research activity in materials science, for example to find new materials capable of trapping and storing gases efficiently. The search for gas storage materials is also important for the transport of industrial gases. 

Researchers at Pohang University of Science and Technology (POSTECH) and colleagues at Chonnam National University have synthesised a molecular solid with high gas storage capability¹. The material is highly efficient at storing acetylene, important industrial gas, which is used to synthesise a wide range organic compounds and plastics.

Conventional molecular solids are considered to be unsuitable for gas storage because they are unstable and, their voids (pores) are too small to house molecules.  But in this report, Kimoon Kim and colleagues demonstrate that solids incorporating cucurbit[6]uril (or CB[6]) molecules are stable and have large voids, thus overcoming both these limitations. 

“Since CB[6] has a rigid, highly symmetrical structure with a cavity, and exceptional thermal and chemical stability, we thought it may have a stable crystal structure with a large void useful for gas sorption,” says Kim.

X-ray crystallography revealed that the CB[6] molecules tend to form a hexagonal pattern, leaving a void in the middle. The hexagons then form a full solid by strongly interacting with nearest neighbours, explaining the high stability.

In as synthesised materials, the voids in the honeycomb structure host water and HCl molecules, but these can be removed by activation—that is, by heating the crystals at 100^oC in vacuum for an appropriate time. Tests of the sorption capability of the activated crystals revealed that the solid stored acetylene 11 % of its weight at 196K and still 6.1 % at room temperature (Fig. 1). 

“The acetylene storage density of the CB[6] is 0.087 g/cm³ (at STP), equivalent to the density of acetylene at 8.3 MPa at room temperature, and almost 42 times greater than the compression limit for the safe storage of acetylene (0.2 MPa). These values are far better than those of known organic molecular porous materials and comparable or superior to those of metal-organic frameworks,” says Kim. 

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.

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