Global fresh water shortage has continued to put water desalination on top of the list of water treatment technology. Reverse osmosis is the most popular water desalination method owing to its relatively low energy cost. The effectiveness of a reverse osmosis system is contingent upon the type of membrane used, which is defined by easy preparation, high salt selectivity, and high water flux. Polymeric membranes have been used in reverse osmosis for a long time now, but they suffer from abrasion, fouling, and oxidation.
In order to improve the effectiveness of reverse osmosis, developing new membranes with perfect salt rejection and excellent water flux is invaluable. Zeolitic and carbonaceous membranes are traditional membranes that have been tested for water desalination. Zeolitic membranes are designed with high water flux, excellent salt rejection, and high thermal stability. Unfortunately, they are fragile and can’t be easily processed.
Porous organic cages are a new class of materials with discrete molecular building blocks, which can assemble into crystals with extrinsic as well as intrinsic cavities. As opposed to zeolitic, porous organic compounds are dissolvable in typical organic solvents, making them easily processable and can spin coat onto porous substrates forming membranes.
Professor Jianwen Jiang and Dr. Xian Kong at the National University of Singapore envisioned that porous organic cages could be attractive membrane materials for liquid phase separation. Therefore, they reported the first proof-of-concept simulation study to investigate crystalline porous organic cages as reverse osmosis membranes for water desalination. Their research work is published and featured as a cover in peer-reviewed journal, Physical Chemistry Chemical Physics.
The authors examined five porous organic cages (CC1, CC2, CC3, CC16, and CC17). The five cages shared a similar tetrahedral cage. This cage contained four triangular windows as well as six peripheries. By changing the periphery groups, various structures could be formed.
The researchers observed that irrespective of their similar chemical structure, the pore geometries and desalination capabilities of the cages varied significantly. They observed that CC1 was impermeable to water in the absence of interconnected pores and was not ideal for desalination. CC2, on the other side, had straight pores with the radius ranging from 3 to 4 Å, thus leading to high water permeability as well as complete salt rejection. CC17 was reported to have high water permeability, but compromised salt rejection owing to wide-open pore network. CC3 and CC16 had intermediate water permeabilities as a result of an interconnected tetrahedral pore network. In general, CC22 was identified as the best among the five cages for water desalination and outperformed other membranes.
The results of Jianwen Jiang and Xian Kong demonstrated that porous organic cages are attractive membrane materials for water desalination. It should be pointed out that the cages in their simulation study were assumed to be stable. Although porous organic cages have been found to be stable in water for a particular period, it is important to further test their long-term stability.
Xian Kong and Jianwen Jiang. Porous organic cage membranes for water desalination: a simulation exploration. Phys. Chem. Chem. Phys. Volume 19 (2017), 18178.
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