Composite membranes comprising nanoporous materials for carbon dioxide capture
Date of Issue2018-01-03
School of Chemical and Biomedical Engineering
Membrane-based gas separation provides various advantages over conventional gas separation techniques such as desirable energy efficiency, small footprint and ease of operations. However, conventional polymeric membranes in which mass transport is governed by solution-diffusion have shown the limited performance although their excellent processability is highly desirable for the fabrication of membrane in large scales. In this regard, incorporating microporous materials that can selectively transport the targeted molecules into polymer membranes has been proposed as an attractive option to fabricate membranes with large membrane areas and high separation performance. In this thesis, composite membranes containing various nanoporous materials are reported. The membranes were designed to be applied in CO2 separation which is highly important in clean energy production and greenhouse gas control. At first, the CO2 separation performance of commercial polyimide membrane was enhanced by incorporating engineered Ca-A zeolite. In order to improve filler/polymer adhesion, LTA zeolites with highly roughened surfaces were made by the ion-exchange-induced growth of Mg(OH)2 nanostructures on the zeolite surfaces. The morphology of LTA crystals was tuned by the systematic modification of reaction parameters such as the pH of the reaction medium and the amount of magnesium loaded in the substrates. After converting surface-modified LTA to Ca-A, which has been known as a good candidate for selective CO2 uptake and transport, by replacing extra-framework cations remaining in LTA with Ca2+, a series of mixed-matrix membrane incorporating Matrimid® and Ca-A was fabricated. Owing to improved zeolite/polymer adhesion property along with the selective transport of CO2 by the fillers, mixed-matrix membranes containing surface modified Ca-A showed enhanced CO2/CH4 separation properties, which are measured by a binary mixture permeation testing. In light of that, a dramatically increase in CO2 permeability (ca. 120 %) was observed for Matrimid®/20 wt% Ca-A membrane which also showed an improved CO2/CH4 selectivity. In contrast, untreated Ca-A decreased CO2/CH4 selectivity of Matrimid® membrane due to defects formed at zeolite/polymer interfaces. Secondly, amine-appended hierarchical Ca-A zeolite possessing an outstanding CO2 capture property was synthesized and incorporated into polyethylene oxide and Matrimid®, to design mixed-matrix membranes with high CO2/CH4 selectivity. Binary mixture permeation testing revealed that amine-appended mesoporous Ca-A is highly effective in improving CO2/CH4 selectivity of polymeric membranes. Furthermore, the formation of filler/polymer interfacial defects, which is typically found in glassy polymer-zeolite pairs, was inhibited owing to the interaction between the amine groups on the external surface of zeolites and polymer chains. Thirdly, metal-organic frameworks, an emerging class of nanoporous material, were employed as the filler materials for composite membranes. Among them the Zn(pyrz)2(SiF6) (or SIFSIX-3-Zn) which is known to have strong affinity to CO2 was chosen as a promising candidate. However, crystals harvested from traditional synthesis method are too large to be used in membrane fabrication. In order to utilize this filler, therefore, a facile sonochemical method was employed to synthesize uniform submicron-sized SIFSIX-3-Zn crystals. Then, high-quality mixed-matrix membranes free of filler/polymer interfacial voids were successfully fabricated by employing cross-linked polyethylene oxide (XLPEO) as a polymer matrix. CO2/CH4 and CO2/N2 mixture gas permeation tests revealed that the separation properties of mixed-matrix membranes, especially selectivities, were significantly improved compared to those of pure polymeric membrane owing to the selective CO2 uptake and transport in Zn(pyrz)2(SiF6) crystals. Although the Zn(pyrz)2(SiF6) was proved to be an effective filler for designing CO2-selective membrane, the overall CO2 separation performace was limited the moderate performance of XLPEO, which is also suffering from poor mechanical stability. A theoretical analysis with Maxwell model revealed that 6FDA-TMPDA polyimide is a promising candidate to design high-performance membranes surpassing the upper-bound limit for polymer membranes. In addition, to maximize the effect of the filler, the nanoscrystal form of Zn(pyrz)2(SiF6) was synthesized by a modified sonochemical method. The FESEM and EDX analysis showed that the nanocrystal fillers were unformly distributed in the polymer matrix. Binary CO2/CH4 mixture gas permeation tests revealed that both CO2 permeability and CO2/CH4 selectivity of mixed-matrix membranes, especially for the membrane with 20 wt% filler loading, were significantly improved compared to those of pure polymeric membrane due to the selective CO2 uptake and transport by Zn(pyrz)2(SiF6) crystals. As a result, a performance surpassed the upper bound limt for polymeric membranes was achived.