Free-Standing Nanometer-Thick Covalent Organic Framework Films for Separating CO₂ and N₂

Covalent organic frameworks (COF) have been gathering much attention because the shapes, sizes, and chemical functions of their nanostructured pores can be arbitrarily controlled by designing the organic precursors. We fabricated cm-sized free-standing COF films with the thickness of 50–100 nm by an alternating vacuum deposition polymerization method. Precise adjustment of the stoichiometry by digitally controlled deposition was essential for producing the robust free-standing COF films. High-resolution electron microscopy revealed 3 nm pore structures, which correspond to the atomic structure of the COF. Small angle X-ray diffraction shows the existence of a similar periodicity. The CO2/N2 gas phase separation properties were evaluated from 103–105 Pa, and the parameters for each molecular permeation were determined. Based on the detailed analysis, it was found that the selectivity comes from the greater sorption affinity of CO2 to the COF compared to N2, which is consistent with the quantum chemical calculation. Since the vapor phase method can be used to coat various shaped templates, our method provides a new option for fabrication of neat COF membranes with various structures and their applications for the separation membrane.