Session 1A - Gas separation 1

Polymers of Intrinsic Microporosity (PIMs) combine the desirable processability of polymers with microporosity generated from the inefficient packing of their rigid and contorted macromolecular structures. They are attracting attention for a number of applications but especially as membrane materials for gas separations, pervaporation, organic solvent nanofiltration and as separators in batteries. The ultimate aim of our research is the development of PIMs with enhanced permeability and selectivity for these applications. In particular, we focus on PIMs with exceptionally rigid polymeric structures (e.g. PIM-Trip-TB) and those with macromolecular chains of 2-dimensional shape (e.g. PIM-TMN-Trip), which demonstrate permeabilities similar to those of the ultrapermeable polymer PTMSP but with much higher selectivities.¹ Recent work with 2D benzotriptycene based PIMs has resulted in the proposed revision of the CO₂/CH₄ and CO₂/N₂ Robeson upper bounds.² We will describe how further structural and chemical modifications to PIMs results in attractive enhancement of properties for gas selectivity and performance and as membrane separators in redox flow batteries with high cation flux and low cross-over of redox active species.³

Fig.1. The evolution of hydrogen permeability using PIMs.

[1] I. Rose et al., Nature Materials, 2017, 16, 932.

[2] B. Comesaña-Gándara, et al., Energy & Environmental Science, 2019, 12, 2733.

[3] R. Tan et al., Nature Materials, 2019, doi.org/10.1038/s41563-019-0536-8

Gas separation membranes suitable for economically feasible post-combustion CO₂ capture should combine the moderate CO₂/N₂ selectivity (20-60) with a substantial values of CO₂ permeances (>1000 GPU). Most of membranes tailored to have these properties are so-called thin-film composite membranes (TFCM) containing different layers it structure, each having a specific function (porous support, gutter, selective, protective layer). Due to highest resistance towards gas transport selective layers in TFCM are commonly fabricated with thicknesses below 100 nm.

In this work, in order to achieve high CO₂ permeances in TFCM we have fabricated ultra-thin selective layers (2-20 nm) composed of well-known hydrophilic and CO₂-selective block-copolymer (Pebax MH-1657). It was deposited on the O₂-plasma activated surface of much thicker (~400 nm) gutter layer composed of polydimethylsiloxane (PDMS). The structure was subsequently transferred on the polyacrylonitrile (PAN) microporous support to complete the TFCM. We have found that contrary to the theoretical predictions (resistance in series model) for the layered membrane, highly selective CO₂/N₂ separation membrane was achieved in the TFCM with ultra-thin selective layer. Critical role to achieve this selectivity was attributed to the specific interface formed between selective and gutter layers which was controlled by the duration of oxygen plasma treatment (PDMS activation).

Permeances of CO₂ in the developed TFCM were between 1000-3000 GPU and CO₂/N₂ selectivities between 30-100, providing the gas separation parameters that are within optimal range for cost-efficient CO₂ capture in post-combustion processes. Detailed characterization of the interface revealed the chemical structure of the outermost membrane surface suggesting the blending of the ultra-thin Pebax-1657 layer with activated surface of PDMS. This nano-thick blend layer contributed to the overall selectivity of the membrane significantly exceeding the selectivity that can be achieved by Pebax-1657 alone. Formed interface demonstrates stable gas separation with a moderate change of performance over the one year.

Polymers of Intrinsic Microporosity (PIMs) are rapidly emerging as potential materials for gas separation membranes.¹ They exhibit physical aging as a consequence of their nonequilibrium nature, a typical feature for all glassy polymers, particularly those with high fractional free volume. Aging leads to a decrease in permeability, and for many PIMs also to a concurrent increase in selectivity, moving their permselectivity nearly parallel to the Robeson upper bound.³ Herein we report for the first time the analysis of the mechanical properties of polymeric membranes by AFM force spectroscopy measurements, and the relationship between the transport properties and the rigidity of dense membranes during their physical ageing (Figure 1).⁴ A systematic study on different polymers, PIM-1 and ultrapermeable benzotriptycene-PIMs included, provides direct evidence that size selectivity increases with increasing film rigidity, i.e. increasing Young’s modulus. Samples of PIM-DTFM-BTrip were subjected to both normal physical aging and to accelerated aging by thermal treatment under vacuum, to study the possible effect of post-conditioning on the membrane performance. The results show an increase in the rigidity of PIM membranes as a function aging, regardless the specific aging method, with almost no influence on the gas solubility but a strong decrease in their diffusion coefficients. The latter effect is stronger for bulkier molecules, suggesting that size-selectivity is related to the polymer rigidity.

Figure 1. Diffusion coefficients of O₂, CH₄, and CO₂ as a function of Young’s modulus for PIM-DTFM-BTrip samples with different histories, and frequency distribution of Young’s modulus of ultrapermeable PIMs by multiple force spectroscopy measurements.

1) Low, Z. X.; et al. Chemical Reviews. 2018, pp 5871–5911.

2) Huang, Y.; Paul, D. R. Polymer (Guildf). 2004, 45 (25), 8377–8393.

3) Comesaña-Gándara, B. et al. Energy Environ. Sci. 2019, 12 (9), 2733–2740.

4) Longo, M. et al. Ind. Eng. Chem. Res. 2019.

Gas separation with dense membranes follows the solution-diffusion mechanism. In the case of mixtures containing gases with high solubility differences, such as CH₄ and CO₂, solubility plays a predominant role compared to diffusivity. However, solubility of gas mixtures in polymers is seldom characterized, and only measurements for binary mixtures were reported so far. In applications such as natural gas upgrading, the stream contains also additional components, such as C₂H₆, that are highly soluble in the membrane and can cause significant competitive exclusion of the other gases.

To characterize these effects, we measured the sorption isotherms of C₂H₆, CO₂, and CH₄ in PIM-1, and of all their binary and ternary mixtures combinations. The development of a new measurement protocol allowed to measure sorption of ternary gas mixtures for the first time. A pressure decay apparatus was used, working at constant composition and variable pressure of the gas phase.

The results obtained for binary mixtures show that C₂H₆ has a strong exclusion power over CH₄, analogously to the case of CO₂/CH₄ mixtures.^[1] Surprisingly, we observe competitive sorption also in the case of C₂H₆/CO₂ mixtures: even though these gases have very similar condensability and pure-gas sorption in PIM-1, CO₂ is significantly excluded from the membrane in the presence of C₂H₆. For ternary CO₂/C₂H₆/CH₄ mixtures, we observed the superposition of the effects highlighted in binary tests: both CO₂ and CH₄ are excluded due to the presence of C₂H₆, while C₂H₆ solubility is barely altered compared to pure-gas conditions. Overall, the effect of C₂H₆ on CO₂/CH₄ solubility-selectivity is modest compared to the corresponding binary case.

The Non-Equilibrium Lattice Fluid (NELF) model^[2] predictions for binary and ternary sorption, performed using only pure-gas parameters as input, are in good agreement with the data, indicating that the model is a reliable tool to study mixed-gas sorption, even in complex scenarios.

Figure 1. Sorption of (a) CO₂ (blue) and (b) CH₄ (green) in PIM-1 at 35 °C. Circles: pure gases. Triangles: binary 20/80% CO₂/CH₄ mixture.^[1] Diamonds: CO₂/C₂H₆/CH₄ ternary mixture (15/15/70%). Lines represent NELF model predictions of pure- and mixed gas sorption.

^[1] O. Vopička, et al. J. Membr. Sci., 459 (2014) 264–276

^[2] F. Doghieri, G. C. Sarti, Macromolecules, 24 (1996) 7885–7896