Pure versus mixed gas permeation and diffusion in polymers of intrinsic microporosity

Short Introduction The industrial use of gas separation membrane processes is drastically growing since they are replacing traditional separation processes. However, its widespread in new sectors necessitates the development of novel membrane materials with enhanced permeability and selectivity. Novel materials require also novel apparatus for their characterization. Herein, we analyse the permeation transient after the exposure to single gas and gas mixture of three membranes prepared from PIM-1, PIM-EA-TB and amine-PIM-1. The mixed gas performances are carried out by a novel gas permeation instrument, based on mass spectrometric analysis of the permeate gas composition. Material and Methods The PIM-1, amine-PIM-1 and PIM-EA-TB synthesis and membrane preparations were reported previously (Figure 1, Carta et al. 2013 and Mason et al. 2014). Since PIMs are known to undergo strong physical aging, well-aged samples were used during the permeation tests to minimize the effect of the variable time on the performance. Pure gas permeation tests were carried out on a fixed volume/pressure increase instrument. Instead the mixed gas analysis was performed on a novel home-made apparatus equipped with a quadrupole mass spectrometer that allows the monitoring of the permeate composition in time. Results and Discussion Single gas permeation test in PIM-1 and PIM-EA-TB provided fairly reliable results when they are compared with mixed gas, giving similar permeability and selectivity. Instead, amine-PIM-1 shows a concentration-dependent behaviour, as well as competitive sorption effects. For instance, the introduction of amino groups into PIM-1 was found to reduce the CO2/CH4 ideal selectivity nearly fourfold from ca. 20 to ca. 5 (Mason et al. 2014), whereas the mixed gas selectivity was found to increase more than 50%, to over 30 (Figure 2). This can be due to a strong interaction of the CO2 with the amino groups, and a consequent hindering of the CH4 permeation. Molecular modelling supports the experimental evidences since selective CO2/amine-PIM-1 noncovalent interaction are computed by Quantum Mechanics. Molecular dynamics analysis of the radial distribution function shows that the local probability density of finding CO2 atoms close by amine-functional groups is very high. PIM-1Amine-PIM-1PIM-EA-TB Figure 1. Chemical structures of the polymers of intrinsic microporosity used in the present work. Figure 2. Mixed gas permeability (left) and selectivity (right) of a 52/48 vol% CO2/CH4 mixture in Amine-PIM-1, showing much higher selectivity than the ideal selectivity reported in the literature (Mason et al. 2014). Conclusions On-line mass spectrometer can be used to analyse the permeate composition of a membrane exposed to mixture and the rapid sampling time allows the study of the transient behaviour, i.e. gas diffusion, of the different gases concurrently. The gas transport in PIMs analysis shows that pure gas permeation data and ideal selectivities do not always represent the real gas separation performance of membranes, and may in some cases even lead to completely wrong conclusions. References Carta M., Malpass-Evans R., Croad M., Rogan Y., Jansen J.C., Bernardo P., Bazzarelli F., McKeown N.B., An Efficient Polymer Molecular Sieve for Membrane Gas Separations,Science 18 (2013) 303-307. Mason C.R., Maynard-Atem L., Heard K.W.J., Satilmis B., Budd P.M., Friess K., Lanc M., Bernardo P., Clarizia G., Jansen J.C., Enhancement of CO2 affinity in a polymer of intrinsic microporosity by amine modification, Macromolecules. 47 (2014) 1021-1029.