Commercial xylene is composed of m-xylene (40–65%), p-xylene (~20%) and o-xylene (~20%) 9. m-Xylene can be oxidised to isophthalic acid as PET resin blends, and, more importantly, isomerised into value-added p-xylene 8. For example, p-xylene is used in the manufacture of poly(ethylene terephthalate) (PET), whereas o-xylene is used to produce phthalic anhydride 6, 7. The supply chains for many polymers, plastics, fibres, textiles, solvents and fuel additives rely heavily upon delivery of pure xylene isomers 4, 5. The separation of xylene isomers is regarded as one of the seven world-challenging separations 1. This requires the design of smart functional materials that can discriminate small molecules based upon slight differences in molecular structures and/or physical properties. There is a continual search for alternative separation technologies operating under ambient conditions that can, over time, replace the current processes to reduce energy consumption 3. These processes account for 10–15% of the world’s energy consumption and up to 70% of the running-costs of chemical plants 1, 2. The chemical separation of valuable feedstocks based upon fractional crystallisation or distillation is being carried out on a vast scale worldwide 1. Nature Communications volume 11, Article number: 4280 ( 2020) Refinement of pore size at sub-angstrom precision in robust metal–organic frameworks for separation of xylenes
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