Pure versus mixed gas permeability of novel polymers of intrinsic microporosity

From their first introduction in 2004 as potential membrane materials,1 the research on polymers of intrinsic microporosity (PIMs) has undergone an explosive growth because of a number of unique properties. A typical feature of PIMs is their extremely rigid and highly contorted polymer backbone, which prevents efficient packing. Therefore, most PIMs have a high BET surface area and a high free volume, arranged as continuous interconnected channels in the polymer matrix. The large free volume fraction and the interconnected void structure, to which they owe the classification of "intrinsically microporous" polymers, provides PIMs with a very high permeability and a high diffusion coefficient for most gases. Besides fast diffusion, PIMs also exhibit very high solubility of easily condensable gases like CO2, and of organic vapours. This combination of a strong size sieving character because of the polymer stiffness, and the high sorption due to the high surface area and microporosity, is quite unique for PIMs and provides them with exceptional performance.2 Most studies on novel PIM-based membrane materials focus on pure gas permeation, and relatively few on mixed gases. Depending on their structure, either the size sieving behaviour or the strong sorption selectivity will dominate and this may cause significant differences between pure and mixed gas permeability. It was previously found that properties may even be opposite to what can be expected intuitively, for instance, when the affinity of amino groups3 for CO2 yields a decrease rather than the expected increase in CO2 permeability. The present paper will study in detail the difference between pure gas permeation and mixed gas separation for a number of different polymers. The effect of mixture composition and other operational conditions (temperature, pressure) will be analysed. From one side the pure gas permeation measurements, especially in the time lag mode, offer valuable information on the basic transport phenomena, such as permeability, diffusivity and solubility. From the other side, only mixed gas permeation experiments can show the behaviour of the membranes under representative operating conditions. The attention will be focused on a number of separations of great industrial interest, such as CO2/N2 (CO2 capture from flue gas), CO2/CH4 (biogas upgrading) or O2/N2 (nitrogen production or oxygen enrichment) Acknowledgements: Part of the research leading to these results received funding from the European Union's Seventh Framework Program (FP7/2007-2013) under grant agreement n° 608490, project M4CO2. References (1) Budd, P. M.; Elabas, E. S.; Ghanem, B. S.; Makhseed, S.; McKeown, N. B.; Msayib, K. J.; Tattershall, C. E.; Wang, D. Adv. Mater. 2004, 16, 456-459. (2) Carta M., Malpass-Evans R., Croad M., Rogan Y., Jansen J.C., Bernardo P., Bazzarelli F., M. N. B. Science 2013, 339, 303-308. (3) Mason, C. R.; Maynard-Atem, L.; Heard, K. W. J.; Satilmis, B.; Budd, P. M.; Friess, K.; Lancì, M.; Bernardo, P.; Clarizia, G.; Jansen, J. C. Macromolecules 2014, 47, 1021-1029.