Green in situ synthesis of ZIF-8 membranes on the inner-surface of PESf hollow fibers and application in hydrogen separation

Abstract

Membrane separation technology is widely recognized as a sustainable option, but achieving green manufacturing for the membranes themselves remains a significant challenge. To ensure sustainable development, it is crucial to prepare membranes in accordance with the “12 principles of green membrane materials and processes”. The preparation of most metal–organic framework (MOF) membranes currently requires the use of certain of toxic organic solvents and appropriate metal sources in the synthetic solution. Developing defect-free internally-supported MOF membranes on polymeric hollow fibers (HFs) via an environmentally friendly green synthetic route represents a significant yet challenging task. In this study, a straightforward continuous flow growth method under organic solvent-free conditions and without external metal sources in the synthetic solution to synthesize zeolitic imidazolate framework-8 (ZIF-8) membranes on the inner surface of PESf(polyethersulfone)-ZnO-HFs was proposed. This approach facilitates the in situ formation of well-intergrown ZIF-8 membranes through the direct coordination of an aqueous solution of 2-methylimidazole (Hmim) with ZnO embedded within PESf-HFs. ZnO particles not only regulate the porosity of PESf-HFs but also serve as both metal sources and nucleation sites for ZIF-8 membrane formation. The recirculating flow process ensures a steady and uniform supply of Hmim aqueous solutions within the HFs, thereby optimizing the regulation of the heterogeneous nucleation rate and crystallization conditions for ZIF-8 crystals across the entire inner surface of the HFs. The resulting ZIF-8 membranes were thin and continuous, with a thickness of approximately 800 nm. The membrane demonstrated outstanding molecular sieving performance, achieving ideal selectivities of 23.1 for H₂/CH₄ and 13.6 for H₂/N₂ mixtures at a H₂ permeance of 3.56 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹. Furthermore, this ZIF-8 membrane exhibited remarkable mechanical, thermal, long-term, and pressure stabilities, as well as excellent reproducibility and scalability. The method developed in this work eliminates the need for metals in the synthetic solution and avoids the formation of ZIF-8 crystals in solution, thereby substantially mitigating the environmental risks and economic costs associated with subsequent separation processes. This contribution paves the way for simple, cost-effective, scalable and environmentally friendly strategies for the design and synthesis of MOF membranes on the inner surface of HFs.