Spirobisindane-Containing Imidazolium Polyimide Ionene: Structural Design and Gas Separation Performance of “Ionic PIMs”

Polymers of intrinsic microporosity (PIMs) have been explored extensively for challenging membrane separation applications. Manipulating microporosity and functionalities of PIM structures has played a major role in the design of membrane materials that can lead to cost-effective CO2 separation processes. Here we have developed a new class of ionenes, in which imidazolium cations are attached (via imides) to spirobisindane groups commonly associated with PIMs yielding a novel ionic-mediated polymer of intrinsic microporosity or “PIM-polyimide ionene” for high-performance CO2 separations. The PIM-polyimide ionene was prepared via Menshutkin reactions between a newly designed spirobisindane-containing bisimidazole monomer and α,α′-dichloro-p-xylene. The resulting thermally and mechanically stable transparent membrane was further studied for its potential utility for CO2 separation. The newly developed PIM-polyimide ionene demonstrated excellent CO2 separation performance relative to other light gases including H2, N2, and CH4. For CO2/CH4 separation, the results surpass the 2008 Robeson upper bound with CO2 permeability of 356.2 barrer and permselectivity of 55. Most notably, the PIM-polyimide ionene membrane outperformed all our previously developed imidazolium ionenes due to the superior CO2 permeability imparted by spirobisindane groups while achieving a comparable performance within the general range of other functionalized PIMs reported in the literature. This unprecedented result of PIM-polyimide ionenes arises from the inclusion of both highly contorted “spiro” centers and the imidazolium cations which precisely alternate in the main chain polymer backbones, resulting in both high diffusivity selectivity and solubility selectivity. It is also noteworthy that PIM-polyimide ionenes exhibited high tolerance against physical aging, as all the gases exhibited very stable permeability and permselectivity values even after a period of 12 months. We hypothesize that the resistance to aging is due to the interactions between ionic groups stabilizing the structure of the polymer matrix, and as such, the ionene microstructure is already close to equilibrium when the membrane is formed. Overall, this is the first example for the integration of PIM structures within the ionene configuration, providing access to potentially numerous novel PIM-ionene materials that can be used in the development of CO2 selective membranes for applications in flue gas, natural gas, and syngas separations.