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Molecular cages, in which host molecules cling to the outer surfaces of the cages rather than entering an internal cavity, could reduce the environmental impact of separating blends of industrial chemicals, KAUST research suggests.
Large-scale molecular separations performed by the chemical industry collectively account for up to 15% of global energy consumption. One of the most energy-intensive separations involves benzene derivatives, called xylenes, which are produced as a mixture of three isomeric forms that must be separated for their various industrial uses. The most valuable isomer, para-xylene, is a key ingredient in the production of polyester and polyethylene terephthalate (PET) polymers.
“Conventionally, these isomers are separated by energy-requiring methods, such as fractional crystallization,” says Basem Moosa, a researcher in Niveen Khashab’s lab. “Alternative techniques that require less heat would reduce the carbon footprint and overall pollution of xylene separation,” he adds.
Khashab and his team investigated the possibility of separating xylene isomers using cage-like materials, which selectively absorb a xylene isomer in the mixture, as an energy-efficient alternative separation technique. Previous research has focused on porous inorganic materials called zeolites, but the processing challenges and limited selectivity of zeolites have somewhat limited their adoption by industry.
In their latest work, KAUST researchers turned to stable, easy-to-make organic cage materials that incorporated nitrogen-based azo groups into their structure. The materials captured the para-xylene isomer with high selectively. “Compared to other organic materials, it showed one of the highest adsorbents for xylene separations,” says Aliyah Fakim, Ph.D. student on Khashab’s team. Surprisingly, however, the adsorption of para-xylene did not involve the isomer entering the azo cage. Instead, the isomer attached itself to the outside of the cage, forming crystals in which each para-xylene molecule was surrounded by four cage molecules.
The team plans to refine the performance of non-porous organic cages by lowering the activation temperature and reducing the time it takes to absorb and then release the para-xylene once it is extracted from the mixture.
However, the concept of separation using non-porous organic cages could be adopted for many industrial-scale chemical separations, reducing the energy demand of these important industrial processes, Khashab notes. “We believe these facilities will represent disruptive next-generation technology for many energy-intensive chemical separations,” he says. “Organic cages are cheap to scale compared to other organic materials and, more interestingly, they can be easily tuned for selective separations, unlike their inorganic zeolite counterparts.”
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