Effects of Si-O-Si agglomerations on CO₂ transport and separation properties of sol-derived nanohybrid membranes

Nanohybrid membranes comprising organic and inorganic components with high CO₂ affinity are ideal substitutes for traditional high footprint gas separation technologies. The CO₂ permeability of these membranes resembles those of polydimethylsiloxane (PDMS), the most permeable rubbery material, while possessing CO₂/H₂ separation factors that supersede PDMS. Such membranes are synthesized using a simple acid-catalyzed sol-gel process. In this work, we investigate the relationship between the CO₂ permeation properties of these nanohybrid membranes and membrane and siloxane network morphology by attuning the reaction kinetics of the sol-gel process. The CO₂ permeability of these nanohybrid membranes can reach 1810 barrer, an improvement of 7.5 folds; while H₂ permeability increase by 5.7 fold, from 30 to 170 barrer. The mechanism behind gas transport enhancements observed in these nanohybrid membranes is elucidated using positron annihilation lifetime spectroscopy and sorption measurements. Relative fractional free volume (FFV) content and CO₂ sorption behaviors in these membranes are augmented as a function of inorganic phase morphology. The CO₂ sorption behavior of the inorganic phase is regulated by the organic/inorganic ratio and water/silicon ratio in the sol-gel synthesis process. Harnessing the advantages of a unique combination of organic and inorganic materials, these nanohybrid membranes outperform most other CO ₂-philic polymeric membranes.