Free volume regulation and CO2 separation in dual-ligand CAU-PBH/Pebax mixed matrix membranes

Metal organic frameworks (MOF) are widely incorporated into mixed matrix membranes (MMMs) for CO 2 separation. However, the relationship between MOF-induced free volume characteristics and transport behavior remains insufficiently understood. In this work, Pebax MH 1657-based MMMs were prepared using a dual ligand aluminum-based MOF (CAU-PBH), derived from CAU-10-H through partial substitution of isophthalic acid by pyridine-3,5-dicarboxylic acid, as a filler to investigate its effect on the free volume and CO 2 transport properties of the Pebax membranes. It exhibits a microporous structure and a polar chemical environment containing carboxylate and N-heterocyclic groups, which are favorable for CO 2 adsorption and polymer-filler compatibility. The optimized MMM containing 5 wt% CAU-PBH showed a CO 2 permeability of 95.10 Barrer and a CO 2 /N 2 selectivity of 66.64 at 0.2 MPa and 35 °C, approaching the 2008 Robeson upper bound. Quantitative positron annihilation lifetime spectroscopy (PALS) analysis revealed that CAU-PBH incorporation modified the free volume characteristics of Pebax through changes in polymer chain packing and crystallization behavior. The MOF-induced changes in free volume characteristics were closely associated with changes in gas diffusivity, while CO 2 solubility increased markedly. The regulated free volume characteristics, together with the intrinsic micropores and CO 2 affinity sites of CAU-PBH, enhanced CO 2 sorption and contributed to selective transport pathways, thereby improving CO 2 separation performance. These results indicate that the separation enhancement cannot be attributed to a simple change in free volume alone, establishing a PALS-based structure-transport correlation between MOF-induced free volume characteristics and CO 2 transport behavior in Pebax MMMs.