Ultramicroporous polyimides with hierarchical morphology for carbon dioxide separation

Abstract

Microporous organic polyimides are promising materials to be used as adsorbents to remove carbon dioxide from flue gases by physisorptive separation processes. Especially, ultramicroporous systems often exhibit excellent CO₂ selectivities and uptakes. Ultramicroporosity is usually achieved by using small linkers, making conventional synthesis routes challenging due to the stress in the resulting networks. Using toluene as an entraining agent, we were able to synthesise two new microporous organic polyimides – MOPI-6 and MOPI-7 – with very small linker molecules. The composition of the solvent mixture m-cresol/toluene was varied and all polymers were characterised using ¹³C and ¹⁵N CP MAS NMR as well as IR spectroscopy, CHN, PXRD, TGA, SEM and physisorption. Both polymer series exhibit high crosslinking degrees, and the systems prepared in m-cresol/toluene mixtures show ultra- and supermicroporosity with CO₂ uptakes comparable with values for systems reported in the literature. By preferentially adsorbing CO₂, which blocks the accessible pore space for methane, these systems feature outstanding selectivity values up to 72 for CO₂/CH₄ mixtures. Additionally, they exhibit a sponge-like morphology resulting in unusual hierarchical porosity, which we attribute to pronounced boiling of the solvent mixture by adjusting the solvent polarity to the polymer particle surface properties. The hierarchical porosity reduces diffusion paths and thus avoids kinetic hindrances important in dynamic sorption processes used in industrial relevant applications like pressure (PSA), vacuum (VSA) or temperature swing adsorption (TSA). Thus, we consider the use of solvent mixtures with strong boiling point differences as an approach to synthesise hierarchical porous polymers, which is applicable for a wide range of porous polymers.