Free volume regulation in poly(ionic liquid)-modified MOF-808/pebax MMMs for enhanced CO2 separation: insights from positron annihilation lifetime spectroscopy

Carbon capture is an important strategy for mitigating climate change, and membrane-based CO 2 separation offers advantages such as low energy consumption, operational simplicity, and continuous operation. Incorporating metal-organic frameworks (MOF), improving the interface between MOF and polymers, and introducing sites with CO 2 affinity are effective approaches for enhancing mixed matrix membranes (MMMs) performance. In this study, poly(ionic liquid) (PIL)-modified MOF-808 fillers were synthesized through in situ polymerization and incorporated into Pebax membranes. The PIL modification adjusted the pore environment of MOF-808, introduced imidazolium and -COOH groups, and strengthened interactions between the filler and Pebax, thereby restricting Pebax chain mobility. The optimized MMM exhibited a CO 2 permeability of 124.57 Barrer at 0.1 MPa and 35 °C, with CO 2 /N 2 selectivity surpassing the 2008 Robeson upper bound and CO 2 /CH 4 selectivity also substantially improved. Positron annihilation lifetime spectroscopy (PALS) revealed a reduction in fractional free volume compared with pristine Pebax, while time-lag analysis showed a corresponding decrease in gas diffusivity. In contrast, the CO 2 solubility coefficient increased markedly because of the CO 2 affinity introduced by PIL. The increase in CO 2 solubility was larger than the loss in diffusivity, leading to enhanced CO 2 permeability and selectivity despite reduced free volume. By combining PALS with time-lag analysis, nanoscale free volume evolution was connected with diffusion and solubility contributions. These results show that PIL-modified MOF-808 changes the balance between diffusivity and solubility in MOF/Pebax MMMs and clarify why CO 2 transport can be improved even when free volume and diffusivity decrease.