(355f) Design of ZIF/Polyimide Mixed Matrix Membrane for Olefin/Paraffin Separations

C₂~C₄ olefins are consumed as feedstocks
to produce a variety of significant chemicals. Membrane separations have been
emerging as more energy-efficient alternatives to fractional distillation to
enrich olefin from olefin/paraffin mixtures. The development of
high-performance membrane materials has been the key to promote membrane
separations as practical approaches for large-scale industrial gas separations.

Mixed
matrix membranes, formed by dispersing molecular sieve particles/platelets in
polymer matrices, are promising approaches for gas/vapor separations that combine
the ease of processing polymers with the superior separation performance of
molecular sieves. The selection of molecular sieving phase and polymer phase is
crucial to fabrication of mixed matrix membranes with intact polymer-sieve
adhesion and favorable gas transport properties. Zeolitic imidazolate
frameworks (ZIFs) are microporous crystalline solids with zeolite or
zeolite-like topologies. The favorable adsorption and molecular sieving
properties of ZIFs enable the possibility of being used as selective adsorbents
as well as membrane materials.

Our
study suggests that with appropriate choice of ZIF and polymer, enhanced and
industrially attractive permeability and permselectivity can be simultaneously
obtained in the ZIF/polymer mixed matrix membrane. ZIF-8/6FDA-DAM mixed matrix dense
films were fabricated with hydrophobic ZIF-8 as the molecular sieving phase and
a high performance polyimide 6FDA-DAM as the polymer phase.  Permeation
properties of the neat 6FDA-DAM dense film and ZIF-8/6FDA-DAM mixed matrix
dense films were studied with pure propylene (C₃H₆) and
propane (C₃H₈) feeds as well as a 50/50 (vol %) C₃H₆/C₃H₈
mixture feed. Pure gas permeation showed significantly enhanced mixed matrix
ZIF-8/6FDA-DAM membrane C₃H₆/C₃H₈
separation performance compared to the pure 6FDA-DAM membrane. A C₃H₆
permeability of 56 Barrers and C₃H₆/C₃H₈
ideal selectivity of 31 was found in ZIF-8/6FDA-DAM mixed matrix membrane with
48 wt % ZIF-8 loading. These results are 258 % and 150 % higher than the pure
6FDA-DAM membrane, respectively for permeability and selectivity. Mixed gas
permeation also verified that selectivity enhancements were achievable in mixed
gas environment by ZIF-8.