Poly(ionic liquids) for CO2 capture
In an attempt to halt the drastic accumulation of carbon dioxide (CO2) in the atmosphere, many countries have agreed to implement industrial solutions for CO2 capture, storage and utilisation. The primary objective is to separate CO2 from water vapour and nitrogen exhaust streams of combustion industries by using established techniques, such as chemical absorption (amine scrubbing), physical adsorption, and cryogenic distillation. While these techniques are already very well established on the market for other applications, their disadvantages in terms of energy and reagent consumption has promoted the use of alternative technical solutions, like membrane based CO2 capture.
Poly(ionic liquids) (PILs) represent a group of innovative polymers with high affinity for CO2 molecules that may exhibit facilitated transport properties. They have been successfully employed in membrane contactors as substitutes for CO2 sorbents like amine solutions and ionic liquids (IL). PILs are mostly synthesised from IL monomers to overcome the restrictions of ILs liquid state of matter by polymerisation. This method allows the production of tailor made PILs with desirable properties. However, their mechanical properties are often impaired by low molecular chain length of the polymeric backbone.
Alternatives suggest to incorporate the ILs in the form of IL pendants in the polymeric chain of already available polymers with well established polymeric structure and known mechanical properties. Although the synthetic changes in the polymer chains might affect their mechanical parameters, it seems to be more realistic to find a compromise between improved separation performance of PILs and process-ability of commercial polymeric precursors.
This work extends the family of polymer derived PILs by exploring various synthetic possibilities for IL pendant incorporation. PIL synthesis were conducted on a variety of commercially available polymers, e.g. cellulose acetate, poly(vinylbenzyl) chloride, poly(diallyldimethyl chloride), used as parent materials. This polymer choice allowed the investigation of the polymeric backbone influence on the properties of the derived PILs. Also, the synthetic composition of ILs was diversified to produce pure and mixed PILs that might intensify the interactions between the pendants on intra- and intermolecular level within the polymeric matrix. The IL pendants contained functional groups that could exhibit hydrogen bonding interactions and affect the CO2 transport mechanism. These aspects are thought to affect positively the solubility of CO2 molecules in the PIL matrix and contribute to their preferential transport across the selective layer.
As the gas transport also depends on the diffusion of CO2 molecules, the polymer matrix structure plays a crucial role in the membrane separation performance. Since PILs are polymers in solid-like state, the gas diffusion is restricted by their molecular dynamics. This restriction may be circumvented by decreasing the glass transition temperature of PILs, and therefore affecting the viscoelastic properties of the material. To do so, PILs were diluted with ILs having a lower viscosity by physical blending. Additionally, the effect of metal salt additives was studied to further facilitate interactions between the PIL matrix and permeating CO2 molecules.
The synthesised PILs and PIL/additive blends underwent a thorough characterisation defining their properties from bulk polymer phase to thin-film composite (TFC) membranes. This multifaceted investigation established the link between the intrinsic properties of PILs with their separation performance by combining the results obtained in gas sorption, time-lag, and mixed-gas permeation tests. The latter, conducted in sweep mode with humidified feed, allowed the assessment of industrially relevant conditions for flue gas CO2 capture.
Upon incorporation of IL pendants into the polymer backbone, the PIL separation performance improved in the majority of cases studied. The PIL CO2/N2 selectivity improved most when the IL pendant exhibited ability for hydrogen bonding, as the solubility of CO2 was affected. In addition, all PIL-based membranes showed enhanced CO2 permeances in mixed-gas tests with humidified feed, confirming their viability for industrially relevant applications.
In general, the PIL-based TFC membranes fully exposed their potential as CO2 selective materials. Their versatility and ease of processing ensure the possibility for commercialisation once the optimisation of the large-scale employment is investigated. Initial assessment of their commercial value provides essential prospects for PILs to become established as a new generation of commercial polymer-based materials for CO2 capture.