High-Flux ZIF‑8 Membranes on ZnO-Coated Supports for Propane/Propylene Separation

Zeolitic-imidazolate framework ZIF-8, a subclass of metal–organic frameworks (MOFs) with sodalite topology, has been a promising candidate for gas separation applications. Nevertheless, fabricating a high-flux propylene-selective ZIF-8 membrane is still challenging owing to the intrinsic crystalline nature of ZIF-8. In this study, an effective vapor-phase technique has been used to synthesize thin ZIF-8 membranes. ZnO was coated on anodic alumina oxide (AAO) supports via atomic layer deposition (ALD). The ZnO layer was vapor-phase treated with 2-methyl imidazole, which converted the ZnO layer to ZIF-8. The vapor-phase synthesized ZIF-8 membranes were evaluated for propylene/propane separation. With a ZIF-8 layer of ∼400 nm thickness, a high propylene/propane separation factor of 58 was obtained, concomitant with a high propylene permeance of 3.6 × 10–7 mol m–2 s–1 Pa–1. It was found that the vapor-phase solvent-free synthesis minimized the number of chemicals needed to fabricate a thin, highly permeable, and propylene-selective ZIF-8 membrane.