Thermally induced transformation of ZIF-8 to ZnF2 in CNT/PVDF composite membranes for tunable porosity and gas transport

Abstract

Gas separation membranes offer an energy-efficient alternative to conventional separation technologies. However, achieving high permeability and selectivity simultaneously remains a challenge. In this work, we fabricate thin composite membranes composed of carbon nanotubes (CNTs), polyvinylidene fluoride (PVDF), and zeolitic imidazolate framework-8 (ZIF-8), and investigate the effect of heat treatment on their structural and transport properties. Thermal analysis, XRD, and XPS reveal that PVDF decomposition releases HF, which reacts with ZIF-8 to form ZnF2, inducing pore formation and altering the polymer structure. Gas permeation measurements indicate that ZIF-8 incorporation enhances overall permeability, while heat treatment induces a transition in transport mechanism from solution-diffusion to a mixed regime consistent with molecular sieving and Knudsen diffusion. For example, the H2 permeability of CNT/PVDF membrane increased by more than an order of magnitude, from 0.16 to 32.6 Barrer for CPZ73(330) while also increasing in selectivity against larger molecules. Unlike conventional mixed-matrix membranes where fillers remain inert, ZIF-8 acts as a reactive component that changes the polymer microstructure. Overall, this work demonstrates a scalable, single-step approach for generating tunable gas transport pathways and offers a general framework for thermally reactive polymer–MOF membranes for industrial separations.