P-XYLENE is a xylene isomer that is widely used as a feedstock for the manufacture of other industrial chemicals. It is a key starting material for the production of purified terephthalic acid (PTA) and dimethyl terephthalate (DMT), which can be further converted into a variety of polyester products such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT).

Xylene isomers include paraxylene (p-xylene), m-xylene (m-xylene) and o-xylene (o-xylene). The separation of xylene isomers is considered to be one of the most challenging problems in the chemical industry due to their similar boiling points and kinetic diameters.

Currently, most of the separation for p-xylene is done by adsorption separation. It was found that Co(aip)(bpy)0.5 exhibited a significant para-selectivity in liquid-phase competitive adsorption of xylene isomers, and the competitive separation factors reached as high as 30 for p-xylene versus m-xylene and 16 for p-xylene versus o-xylene. Desorption experiments further confirmed the preferential adsorption of p-xylene on the adsorbent. Molecular simulations and calculations revealed that the order of interaction strengths for xylene molecules and the adsorbent framework was p-xylene ? o-xylene ≈ m-xylene, which illustrated the selective adsorption phenomena arising from the mechanism for microscopic interactions. This work broadens the application of pillar-layer MOF materials in the field of xylene isomer adsorption and separation.

In addition, graphene oxide (GO) membranes have also been used for the separation of paraxylene (PX, 0.58 nm) from o-xylene (OX, 0.68 nm). Attributing to the suitable d-spacing size (0.58?nm), the GO membrane allows more PX to pass through than OX, leading to a high PX/OX selectivity. At 110?°C for the separation of equimolar PX/OX mixture, the PX/OX separation factor is 9.4 and PX flux is 145.3?g?m?2h?1, respectively, which is promising for the separation of xylene isomers by pervaporation.