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Removing Water Pollutants With Water-Stable Porous Organic Cages

Water-stable hydrazone-linked porous organic cages demonstrates the ability to remove more than one kind of water contaminant.

Porous organic cages are a rising class of self-assembling, porous materials with unique properties and have found applications in gas storage, separations, sensing, and catalysis. However, many of these porous organic cages are linked by dynamic and reversible imine (C=N) and B–O bonds, which are susceptible to hydrolysis and skeleton collapse when exposed to water, hence limiting their applications in such environments. As current strategies to create water-stable porous organic cages are costly, inefficient, and tedious, there is a need to develop a more direct procedure that is both efficient and cost-effective.

Calix[4]resorcinarene (C4RA) is a macrocyclic molecule with an intrinsic cavity and eight polar upper-rim phenolic groups. It can be used to assemble cage compounds with rich cavity shapes and sizes that can encapsulate small gases and large molecules.

The study led by Professor Yuan Daqiang from the Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences built upon the C4RA structure and developed seven new water-robust porous organic cages linked by hydrazone bonds from the same concave-shaped tetraformyl-functionalized C4RA (C4RACHO).

Through X-ray crystallography, the group determined the structures of the seven new water-robust porous organic cages and showed how their window diameters and cavity volumes could be fine-tuned within a particular range.

The group also validated the molecules’ water stability by measuring proton nuclear magnetic resonance spectroscopy of porous organic cage samples that were soaked in water for a week, and uncharged signals suggest the robustness of the cages in the water.

From the results, the scientists attributed the stable nature of the porous organic cages to the adjacent nitrogen atom of C=N bond of hydrazone, which can form a charge-separated resonance structure that reduces the electrophilicity of carbon atom and thereby makes the hydrazone strong enough against hydrolysis.

Furthermore, the porosity of the cages was estimated by carbon dioxide adsorption and the group found that these hydrazone-linked porous organic cages can adsorb a relatively high volume of carbon dioxide.

Seeing that human activities have led to various contaminants, such as radionuclide waste and organic micropollutants, in our water sources, there is a dire need to develop new materials for effective removal of these pollutants from contaminated water. Given their water-soluble and porous nature, abundant multidentate N/O chelating sites, and π-rich cavities within their structures, the group looked into the application of their constructs in water to remove pollutants. They found that these hydrazone-linked porous organic cages are stable in water and could remove radionuclide from contaminated water. When tested with other contaminants, the group’s novel cages could also eliminate other organic micropollutants like bisphenol A and bisphenol S with an efficiency of 96 per cent.

The findings of this study have revealed how C4RACHO may be utilised to construct versatile and robust porous organic cages and presented us with the first example of a porous organic cage adsorbent that can remove many kinds of water pollutants, spelling great promise for the systemic removal of water contaminants. [APBN]

Source: Yang et al. (2021). Water-stable hydrazone-linked porous organic cages. Chemical science, 12(40), 13307-13315.