(149e) Nanocomposite Membrane of a Polymer of Intrinsic Microporosity and Zeolitic Imidazolate Frameworks for Gas Separation

Developing
energy-efficient and environmentally-friendly separation processes has become
an important research topic in dealing with global issues, such as carbon
capture, natural gas production, and water purification. Membrane separation
technology has great potential for solving these challenges as an alternative
to conventional industrial processes.  Nanocomposite membranes, also known as mixed matrix
membranes, are promising materials for gas separation with potentially enhanced
permeability and selectivity. In this study, we report the preparation and gas
separation performance of nanocomposite membranes
combined of zeolitic imidazolate
frameworks (ZIF-8) nanocrystals in a polymer of
intrinsic microporosity (PIMs).

The
PIMs polymers present microporosity due to their
unique contorted and rigid molecular structure, and have shown great potential
for a wide range of applications, such as membrane for gas separation, gas
storage, and catalysis. As membrane for gas separation, PIMs have shown high
gas permeability but have selectivity that need to be enhanced to a
satisfactory level for industrial applications. Zeolitic
imidazolate frameworks (ZIFs), a sub-family of MOFs,
have attracted significant attention with tuneable pore sizes, exceptional chemical
stability, and versatile structures analogous to that of inorganic zeolites.
The ZIFs nanoparticles with sub-nanometer channels
could present synergetic effects as fillers for mixed matrix membranes: (1)
disruption of the packing of polymer chains; and (2) molecular sieving effect.
The PIMs/ZIFs composite membrane is a representative example of combination of amorphous microporous
polymer and crystalline microporous MOFs. Therefore, understanding the physical
and chemical properties of this composite, such as porosity, free volume,
thermal stability, and gas sorption properties, is not only important for
developing high performance composite membrane for gas separation, but also
useful for many other applications, such as adsorbent and catalysis.

In this
study, the as synthesised ZIF-8 nanoparticles, dispersed as colloids in solvent
solution, were incorporated into the PIM-1 polymer matrix forming a nanocomposite membrane, which gives uniform dispersion of
ZIF-8 in the polymer matrix. A series of composite membrane were fabricated
from solution casting of the PIM-1/ZIF-8 solution with loading of ZIF-8 up to
the significantly high values of ~40 wt%. The nanocomposite
membranes were further treated with various techniques, such as methanol
treatment, thermal treatment, and photochemical modification, aiming to improve
the permeability or selectivity. The pure gas permeation properties were
evaluated in a constant-volume variable-pressure apparatus (time lag method).
Mixed gas permeation was tested in a similar membrane cell equipped with a gas
chromatography.

For the
nanocomposite membrane without methanol treatment,
the gas permeability decreased with the loading of ZIF-8 nanoparticles, while
the selectivity was maintained without significant loss. After methanol
treatment, the permeability of nanocomposite membrane
increased with the loading of ZIF-8 nanocrystals and
surpassed the Robeson's upper bound. The effects of annealing temperature
(100-300°C) on the gas permeation performance of pure polymer and nanocomposite membrane were monitored. For the pure polymer
membranes annealed at high temperature (up to 300°C), the gas permeability
slightly decreased while the selectivity was found to be
stable. For the PIM-1/ZIF-8 nanocomposite membranes,
the gas permeability decreased with the annealing temperature, interestingly,
the selectivity could be significantly enhanced. Particularly, for the
composite membrane with 20 wt% loading of ZIF-8 annealed at 300°C, the CO₂
permeability could reach approximately 1000 Barrer
with selectivity of both CO₂/N₂ and CO₂/CH₄
as high as 30. The ideal selectivity of other gas pairs, such as O₂/N₂,
H₂/N₂ and H₂/CH₄, were all enhanced
surpassing the Robeson's upper bound.

Gas
sorption properties of these nanocomposites were also
examined with various types of gases, so their potential as adsorbents or gas
storage materials were also evaluated.

Acknowledgements

We
acknowledge the NPRP grant from the 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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