Modulation of Nanoscale Sinuosity in Asymmetric Nano-Channel for High-Resolution Separation of Trace Xylene Isomer Impurities

Abstract

The purification of para-xylene (pX) to ultra-high levels is critical for producing high-performance polyethylene terephthalate, yet trace-level quantification of its isomeric impurities, meta-xylene (mX) and ortho-xylene (oX), remains a formidable challenge due to their nearly identical boiling points and molecular dimensions. This study presented an aluminum-based metal-organic framework, Al-TCPB-Me₂, featuring asymmetric one-dimensional channels with a high sinuosity ratio that coupled molecular sieving and shape matching to achieve the baseline separation of xylene isomers. Structural characterization confirmed a sinuous channel with an entrance size of ~6.8 Å, which excluded oX while enabling selective recognition of pX over mX through asymmetric binding sites. Density functional theory calculations revealed that four methyl groups in the sinuous channel formed hydrophobic interactions with two methyl groups in pX molecules, while only three methyl groups interacted with methyl groups in mX molecules, resulting in stronger binding of pX than mX. The Al-TCPB-Me₂ column achieved high separation resolutions (24.9 for mX/pX and 31.1 for oX/pX) and successfully qualified the impurities as low as 200 ppm in the real high-quality xylene sample, surpassing all conventional columns. Comparative tests with a straight-channel Al-TCPB variant emphasized the sinuous geometry’s role in enabling tandem separation mechanisms. This work establishes asymmetric sinuous channels as an effective design principle for integrating tandem molecular sieving and shape matching, offering a powerful strategy for the quantification of trace-level structurally similar impurities in real sample analysis.