Benchmark CO₂ separation achieved by highly fluorinated nanoporous molecular sieve membranes from nonporous precursor via in situ cross-linking

Molecular sieve membranes with rigid micropores and CO₂-philic functionalities within the architectures are promising candidates in CO₂ separation. However, the development of effective approaches for their fabrication still remains a significant challenge. Herein, an in situ cross-linking strategy is developed for the preparation of nanoporous fluorinated molecular sieve membranes using commercially available dense and non-porous polystyrene (M_PS) as a precursor template. Based on the dehydrative Friedel-Crafts reactions with highly fluorinated benzylic alcohols, M_PS membranes are cross-linked in situ upon exposure to Brønsted acid (CF₃SO₃H), affording fluorinated microporous polymeric membranes with surface areas up to 523 m² g⁻¹ and the presence of micropores centered at 1.1–1.3 nm as well as ultra-micropores (~0.6 nm). The obtained modified membranes exhibit good ideal CO₂ permeability of 797 barrer and CO₂/N₂ selectivity of 28.5. In addition, high fluorine content (up to 28.5 wt%) and good thermal stability made the cross-linked membranes promising candidates to produce fluorinated carbon molecular sieve membranes with improved textural properties, exhibiting surface areas up to 1020 m² g⁻¹ and ultra-micropores of ~0.4 nm. These membranes achieve superior CO₂/N₂ separation performances exceeding the Robeson upper bound limit (2008).