Gas transport analysis in the ultrapermeable polymer of intrinsicmicroporosity, PIM-TMN-Trip.

Short Introduction For many years, the ultrapermeable poly(trimethylsilylpropyne) (PTMSP) was the most permeable known polymer, but it was never used in industrial application due to its lack of selectivity. Recently, we have reported the first ultrapermeable polymer of intrinsic microporosity (PIM), namely PIM-TMN-Trip, which has a similar permeability but is much more selective than PTMSP (Rose et al. 2017). The present paper discusses the effect of temperature on the gas transport properties of PIM-TMN-Trip. The activation energy of permeation and diffusion, as well as heat of sorption are discussed in comparison with the values for PIM-1 and PTMSP. This paper reports for the first time the gas diffusion analysis in terms of entropic and energetic selectivity (Koros and Zhang 2017) for PIMs. Material and Methods The synthesis of PIM-TMN-Trip and the membrane preparation was reported previously (Rose et al. 2017). In order to have a stable membrane for the permeation tests, the membrane was washed in methanol, thermally treated at 140°C for 4h under vacuum, and then aged for 100 days. Pure gas permeation tests were performed in the temperature range 25-55 °C at 1 bar feed pressure on a fixed volume/pressure increase instrument in the time-lag mode. Results and Discussion The permeation rate order at 35°C in PIM-TMN-Trip is CO2 > H2 > He ? O2 > CH4 > N2 like for most other PIMs, but at much higher permeability. The data fall in an unfilled region of the Robeson plots for several gas pairs (Figure 1). At increasing temperature, the permeability of the faster gases (i.e. CO2, H2, He, O2) decreases, while the permeability of the bulkier N2 and CH4 increases. This means that the relative contributions of diffusivity coefficients and solubility have a different weight, depending on the penetrant. A temperature increase leads to both an increase in the diffusivity coefficient and to a decrease in solubility and the resulting effect on the permeability coefficient is governed by the most affected term between solubility and diffusion. For instance, the decrease in CH4 solubility with increasing temperature is compensated by a strong increase in diffusivity, resulting in an overall increase in permeability. The activation energy of permeation (Ep) is positive for the bulkier gases (N2 < CH4) and negative for the other four gases (i.e. He > H2 > O2 > CO2). The activation energy of diffusion, Ed, for all six gases in PIM-TMN-Trip are among the lowest values measured for any polymer to date. A low Ed indicates that gas diffusion is weakly influenced by the temperature and it can be associated to the presence of microvoids in the polymer and to its high stiffness. The decoupling of the diffusion selectivity into the energetic and entropic effects, shows that for PIM-TMN-Trip the energetic values of all except CH4 are greater than one. This is due to the bigger effective diameter of the CH4 molecule, requiring a more extended motion-enabled zone for diffusion. The entropic effect is related to the ability of a material to limit the degree of freedom of a molecule with respect to another. The entropic effect falls in the medium-low range for bulkier gases, while it is high for H2 and He. Figure 1. Permeabilities of six gases in PIM-TMN-Trip as function of temperature Conclusions PIM-TMN-Trip is the most selective ultrapermeable polymer, with permselectivity properties that are above the Robeson upper bounds for several gas pairs. Its ultrapermeability originates from both high diffusion coefficients and high solubility. Its selectivity is mainly based on diffusion selectivity, while there is room for improvement of the solubility selectivity. The analysis of diffusion in the rigid microporous PIM-TMN-Trip shows that it acts as a traditional polymer for bulkier gases, while it has a behaviour similar to carbon molecular sieves for He and H2. Acknowledgements: The work leading to these results has received funding from the EU's 7th Framework Program under grant agreement n° 608490, project M4CO2. References Koros W.J. and Zhang C., Materials for next-generation molecularly selective synthetic membranes, Nature Materials 16 (2017) 289-297 Rose I., Bezzu C. G., Carta M., Comesaña-Gándara B., Lasseuguette E., Ferrari M.C., Bernardo P., Clarizia G., Fuoco A., Jansen J.C., Hart K.E., Liyana-Arachchi T. P., Colina C. M. and McKeown N. B., Polymer ultrapermeability from the inefficient packing of 2D chains, Nature Materials 16 (2017) 932-937