High Performance Gas Separation Membrane From a Polymer of Intrinsic Microporosity by Photochemical Surface Modification

The concept of design in next-generation chemical separation membrane is to develop porous materials by tuning the pore size approximate to the kinetic diameter of molecules (sub-nanometer scale) to achieve highly selective separation with high rates of permeation. Among many microporous organic polymers (MOPs) materials, solution-processable polymers of intrinsic microporosity (PIMs) have attracted significant interest in recent years, showing high gas permeability and reasonable selectivity as gas separation membrane.

In this study, we demonstrate a method of surface modification of a microporous polymer (PIM-1) membrane by combination of ultraviolet light irradiation and ozone (UVO) treatment to enhance the gas separation performance. The ultraviolet irradiation in the presence of atmospheric oxygen produced ozone and atomic oxygen which transformed the polymer chains and generated a nanoporous asymmetric thin layer over the pristine membrane. The chemical changes, structure and morphologies of the membranes exposure to UVO treatment were examined by various techniques. We found significant sensitivity of gas permeation properties with respect to the operation conditions, such as the atmosphere of UV irradiation and exposure time.

The pure PIM-1 membrane presents a CO₂ permeability of ~5600 Barrer and CO₂/N₂ selectivity of 20, and CO₂/CH₄ selectivity at ~13, which are in agreement with the literature data of PIM-1 membrane. Control experiments of UV irradiation in pure N₂ showed that the permeability and selectivity were approximate to that of neat polymer. Upon exposure to UV irradiation in air or controlled O₂/N₂ atmosphere, the permeability of CO₂ showed slight increase (up to 7000 Barrer) after short exposure for 5-10 min, but then decreased with extended exposure for 20-60 min, while the selectivity evidently increased. Particularly, the selectivity of H₂ over N₂ and CH₄ increased evidently (up to ~60) with moderate permeability of H₂ (~2000 Barrer). The CO₂ permeability reached ~2000 Barrer and both selectivity of CO₂/N₂ and CO₂/CH₄ increased to approximately 30 after exposure to UV/ozone for 30 min. Most of these data surpassed the Robeson's 2008 upper bound.

The mixed gases permeation properties of PIM-1 membranes with UV/ozone treatment were also investigated using certified gas mixtures of CO₂/N₂ and CO₂/CH₄ with feed pressure up to 30 bar. The surface modification by UV/ozone treatment could enhance the selectivities of mixed gases, which are higher and more stable than that of unmodified membrane, particularly for CO₂/CH₄ separation, at slight expense of CO₂ permeability.

In sum, the photochemical surface modification via the UVO treatment could enhance the selectivity of the composite-layered membrane while the permeability was still maintained at reasonable high level, showing high potential for CO₂ capture, natural gas separation and hydrogen purification. The concept of this process could offer a direction on improving the separation performance of various microporous polymer membrane materials.

Acknowledgements

We acknowledge the NPRP grant from the Qatar National Research Fund (QNRF), the Engineering and Physical Sciences Research Council (EPSRC, UK), and the China Scholarship Council.

Notes

Corresponding author: es10009@cam.ac.uk (Dr. Easan Sivaniah)

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