Cross-linked Polymers of Intrinsic Microporosity (PIMs)-based Thin-film Composite Hollow Fiber Membrane for Organic Solvent Nanofiltration

Abstract

The increasing concerns over environmental pollution and energy issues have highlighted the need for alternative separation processes to replace traditional, solvent- and energy-intensive methods such as distillation, crystallization, adsorption, and extraction. As a result, over the past few decades, membrane-based separation technologies have gained significant attention and have been widely researched and applied in both academia and industry. These technologies offer numerous advantages, including reduced solvent and energy consumption, lower carbon emissions, minimal secondary pollutant emissions, and seamless integration with other processes. Recently, among membrane technologies, organic solvent nanofiltration (OSN) has gained significant attention due to its ability to separate solutes ranging from 200 to 2000 g/mol from organic solutions. This technology holds great potential for separation and purification in the fine chemical industry, including applications in pharmaceuticals, dyes, adhesives, and resource and solvent recovery in organic wastewater treatment. However, conventional membrane materials for water treatment or gas separation exhibit low separation performance when applied to OSN processes, primarily due to swelling and dissolution caused by organic solvents. As a result, researchers have developed membranes using organic solvent-resistant polymers, such as cross-linked polyimide, polybenzimidazole, and polyetheretherketone. Despite their enhanced organic solvent resistance, these materials have limited applications due to their low permeability. Hollow fiber (HF) membranes have been actively employed in water treatment and gas separation due to their advantages, such as high packing density, large surface area, and easy scale-up. However, despite these benefits, most membranes developed and commercialized for OSN have been flat-sheet membranes. The development of HF membranes specifically for OSN remains limited. In particular, thin-film composite (TFC) HF membranes can tailor their support and selective layers to the filtration environment, but these membranes have been rarely studied. In this dissertation, we aim to overcome the low separation performance and permeability issues associated with conventional membrane materials by developing a TFC HF membrane based on polymers of intrinsic microporosity (PIMs) for OSN. PIMs have micropores due to their contorted structure in polymer molecules and are being actively studied as membrane materials due to their high heat resistance, mechanical strength, solution processability, and excellent pore properties. Chapter 1 introduces the motivation and organization of the research focused on developing a TFC HF membrane based on PIMs for OSN, and Chapter 2 provides a comprehensive overview of OSN and fundamental knowledge of membrane materials. In Chapter 3, PIMs were synthesized and functionalized to develop membranes suitable for OSN. TFC HF membranes were prepared by dip-coating the functionalized PIMs onto cross-linked polyimide HF supports. Subsequently, a functionalized PIMs selective layer was cross-linked to create intermolecular cross-linked PIMs-based TFC HF membranes with enhanced organic solvent stability. The OSN performance of the developed membranes was then evaluated. In Chapter 4, four cross-linkers of varying lengths were employed to cross-link the functionalized PIMs selective layer developed in Chapter 3, to tune the pore size of the cross-linked PIMs selective layer. The OSN performance of the resulting membrane was evaluated, and the membrane was utilized for recycling noble metal-based homogeneous catalysts through the OSN process. In Chapter 5, a semi-interpenetrating polymer network (semi-IPN) was formed within the PIMs to enhance the solvent resistance of the PIMs selective layer and to tighten the pore size. A liquid phase cross-linking method was employed to create the semi-IPN in the PIMs selective layer, simplifying the membrane fabrication process and facilitating the preparation of TFC HF membranes. The properties and filtration performance of the developed membranes were investigated, and these membranes were further utilized for homogeneous photocatalyst recycling through the OSN process. In this study, we successfully developed cross-linked PIMs-based TFC HF membranes with enhanced organic solvent stability for the OSN. The evaluation of the OSN performance confirmed that the developed membranes possess high utilization potential for separation and purification in the fine chemical industry, as well as for resource and solvent recycling in organic wastewater treatment.