Alternating Charge Pairs by the Assembly of Covalent Organic Frameworks into Mobil-Type Five Zeolite Membranes for Enhanced Xylene Separation

Mobil-type five (MFI) zeolite membranes are predominantly employed for the exceptional separation of PX from xylene isomers (p-, m-, o-xylene, PX, OX, MX). However, the defect-free fabrication complexity and the limited mechanical flexibility hinder the full exploitation of the size-sieving effect. Here, an approach is proposed involving the alternating intercalation of negatively charged covalent organic frameworks (COF) nanosheets into positively charged MFI layers to mitigate the adverse effects of MFI defects. The alternating charge pairs facilitate the rapid and selective transport of PX, resulting in high separation factors of 29 for PX/OX(1:1) and 24 for PX/MX(1:1) with a PX permeance of 1.46 × 10⁻⁷ mol m⁻²s⁻¹ Pa⁻¹ through organic solvent nanofiltration, and 586 for PX/OX and 566 for PX/MX with a PX permeance of 0.87 × 10⁻⁷ mol m⁻² s⁻¹Pa⁻¹ through pervaporation. Molecular dynamics simulation shows that the altered charge pairs enhance sensitivity to subtle differences in xylene polarity: π-bond electrostatic field shifts of COF induce repulsion of OX and MX relative to PX from crossing MFI defects. The introduction of COF layer reduces the brittleness of MFI membrane, contributing to >2-fold increase in mechanical stability and durability. This study provides a promising strategy for the efficient separation of xylene isomers and other isomers.