Size-Selective Ionically Crosslinked Polymer Films for Light Gas Separation: Breaking the Upper Bound in H2 Purification

Size-Selective Ionically Crosslinked Polymer Films for Light Gas Separation: Breaking the Upper Bound in H₂ Purification

Daejin Kim,¹ Ping Tzeng,¹ Kevin J. Barnett,¹ You-Hao Yang,¹ Jaime C. Grunlan^1,2, and Benjamin A. Wilhite,¹

¹Artie McFerrin Department of Chemical Engineering, Texas A&M University, 3122 TAMU, College Station, TX 77843, USA.  ²Department of Mechanical Engineering, Material Science and Engineering Program, Texas A&M University, 3123 TAMU, College Station, TX 77843, USA.

Polymer-based membranes have been broadly applied into petroleum refinery, syngas production, and ammonia plant for hydrogen separation with their advantages of low cost, mass production, and good quality control. However, most of the polymeric membranes suffer from their low selectivity and weak mechanical strength. The general tendency of ‘upper bound limits' for homogeneous polymeric membranes established by Robeson[1] yields a trade-off line between gas permeability and permselectivity. In order to overcome the upper bound limits of polymeric membranes in gas separation, alternative membranes have been diversely studied, such as metallic membranes, zeolite imidazolate framework (ZIF) membranes, metal-organic framework (MOF) membranes, carbon molecular sieve (CMS) membranes, and organic-inorganic mixed matrix membranes (MMM), etc. [2]

Over the last two decades, layer-by-layer (LbL) assembly has been extensively investigated because of its simplicity and versatility for developing multifunctional thin films for antimicrobial, antireflection, flame retardant, fuel cell electrolytes, and gas barrier applications [3]. Application of the LbL technology to gas purification membranes would provide the capability of tuning gas permeability and selectivity by varying the cation-anion materials and the number of bilayers. In this study, a branched polyethylenimine (PEI) / poly(acrylic acid) (PAA) assembly is explored for investigating hydrogen separation through the LbL membranes. When the results obtained from PEI/PAA films were compared in the Robeson's plot for H₂/N₂ and H₂/CO₂ separation, the LbL films exceed the upper bound limits, indicating a superior capability of H₂ separation ( , ) compared to the other homogeneous and heterogeneous polymeric membranes. Remarkably high elastic modulus (23~30 GPa) and hardness (0.5~0.7 GPa) of the PEI/PAA films obtained by nanoindentation suggest that the ionically cross-linked LbL films are denser than any traditional polymer films, making it a promising candidate for hydrogen separation applications.

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References

1.      L.M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci. 62, 165-185 (1991).

2.      N.W. Ockwig, T.M. Nenoff, Membranes for hydrogen separation, Chem. Rev. 107, 4078-4110 (2007).

3.      P. T. Hammond, Form and function in multilayer assembly: New applications at the nanoscale, Adv. Mater. 16, 1271-1293 (2004).