ZIF/Polyimide Asymmetric Mixed-Matrix Hollow Fiber Membranes With Significantly Enhanced Hydrocarbon Separation Performance

ZIF/Polyimide Asymmetric Mixed-Matrix Hollow Fiber Membranes with Significantly Enhanced Hydrocarbon Separation Performance

Chen Zhang, William J. Koros

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology

Asymmetric hollow fibers are the most industrially practical geometry for gas separation membranes in terms of productivity and membrane packing efficiency. However, studies on polymer-based mixed-matrix membranes in the past two decades have been mostly limited to the geometry of films with only a few extended to the geometry of asymmetric hollow fibers due to many technical challenges. For those few examples^1-3 in which selectivity enhancements were observed in mixed-matrix hollow fiber membranes, defect-sealing post-treatments (e.g. PDMS coating) were required in most cases to realize the enhanced selectivity.

This study reports successful formation of high performance asymmetric mixed-matrix hollow fiber membranes using a zeolitic imidazolate framework (ZIF) and a high T_g (glass transition temperature) polyimide, in which a significant propylene/propane selectivity enhancement was realized without any defect-sealing post-treatments. Spinning dope compositions and spinning parameters were optimized to form mixed-matrix hollow fiber membranes with desirable properties using the dry-jet/wet-quench technique. Propylene/propane gas mixture permeation showed that adding ZIF molecular sieve particles (~17 wt%) in the hollow fiber sheath layer significantly enhanced propylene/propane selectivity over pure polyimide dense film membrane by over 70% while maintaining attractive propylene permeance. The intrinsic compatibility of hydrophobic ZIF particles with the polyimide matrix as well as the optimized spinning dope compositions were keys for success of this study.

To our best knowledge, this is the first study in the open literature that reports successful spinning of asymmetric mixed-matrix hollow fiber membranes showing remarkably enhanced gas separation selectivity without any defect-sealing post-treatments. The current study, therefore, has significant impacts on the development of polymer-based gas separation membranes. An application of ZIF/polyimide mixed-matrix hollow fiber membrane described in this study is to pre-purify the feed stream of C₃ splitters in olefin plants to reduce the energy cost for producing polymer-grade propylene.

References:

(1) Husain, S.; Koros, W. J.: Mixed matrix hollow fiber membranes made with modified HSSZ-13 zeolite in polyetherimide polymer matrix for gas separation. Journal of Membrane Science 2007, 288, 195-207.

(2) Dai, Y.; Johnson, J. R.; Karvan, O.; Sholl, D. S.; Koros, W. J.: Ultem®/ZIF-8 mixed matrix hollow fiber membranes for CO2/N2 separations. J. Membr. Sci. 2012, 401¨C402, 76-82.

(3) Ekiner, O. M.; Kulkarni, S. S.: Process for Making Hollow Fiber Mixed Matrix Membranes. US Patent 6663805, 2003.