Hierarchical ZIF-8 Composite Membranes: Enhancing Gas Separation Performance by Exploiting Molecular Dynamics in Hierarchical Hybrid Materials

Mixed matrix membranes (MMM) incorporating metal-organic framework (MOF) fillers have gained increasing attention in addressing environmental and sustainability challenges. Hierarchical materials combining pore sizes of different length scales are expected to facilitate molecular diffusion and mass transfer for the optimization of catalysis and separation processes. Herein, a novel preparation method for hierarchical ZIF-8 (H-ZIF-8) particles is presented for the synthesis of polyimide (PI)-based MMMs with good compatibility between filler and polymer. Gas permeability measurements of polyimide-Matrimid®/H-ZIF-8 MMMs showed 4-fold improvements in permeability of both CO ₂ and CH ₄ coupled with a marked increase in selectivity and plasticization resistance for MMM with 30 wt% H-ZIF-8 loading. Gas transport analysis in these MMMs revealed that the enhanced gas separation performance of the MMMs can be related to the imidazolate modification of the PI structure and the hierarchical structure of H-ZIF-8, as confirmed by N ₂, Ar, mercury porosimetry, SEM, TEM analysis. CO ₂ permeability for all MMMs increases with increasing CO ₂ concentration and by decreasing temperature. The proof of concept, as demonstrated in this study, could be extended for the preparation of other hierarchical ZIFs and related MMMs.