| Chemical separation is one of the most important operating units in industrial production processes, and it also occupies a high proportion of equipment investment and energy cost. Traditional separation technologies,including distillation, absorption, extraction, crystallization and so on, either rely heavily on energy consumption or may be harmful to the environment. As one type of new technologies, membrane separation has drawn more and more attention because of its high energy efficiency and good environment compatibility. The development of membrane separation technology to a large extent depends on uncovering high-performance membrane materials. In light of the indivisually remarkable merits of novel porous materials and polymers,incorporating the former into the latter to prepare mixed matrix membranes(MMMs) is currently an important research direction.In recent years, due to the unique advantages in the modulation of pore structures and chemical properties, metal-organic frameworks (MOFs) have shown great application potential in the field of separation. On this foundation,the study of MMMs based on MOFs has been paid more and more attention for the purpose of preparing high-performance separation membranes. In this thesis, a series of new MMMs in connection with different separation systems were prepared by using the structural advantages of MOFs. The underlying intensification mechanisms of polymer films by the incorporation of fillers were further analyzed from different aspects such as pore structures,adsorption properties, morphologies and surface properties. The main results are outlined as follows:1. Towards CO2/CH4 separation, a CO2-selective MOF material,NH2-MIL-125(Ti), was chosen to prepare MMMs with PSF. Compared with pure PSF membrane, the MMM with 20 wt% MOF loading shows an enhancement of 208% in CO2 permeability, under the conditions of 30 0C and 3 bar, together with a slight improvement in CO2/CH4 separation factor. The reason is that the incorporation of NH2-MIL-125(Ti) provides extra transport channels for gas molecules, and there are strong interactions between CO2 molecules and -OH and -NH2 groups on the pore surface, both of which can facilitate the transport of CO2 by surface diffusion in the pores. Moreover, the MMMs prepared in this work also exhibit a good resistance upon pressures,which will be beneficial for the industrial application of natural gas purification.2. According to the Lewis acid-base theory, there are strong interactions between the O atoms of CO2 and the coordinatively unsaturated Cr(III) of MIL-101(Cr), providing this material potential advantages in the preparation of MMMs for CO2 separation. As a proof-of-concept demonstration, this MOF was selected as the filler to prepare MMMs using three different kinds of polymers. The results indicate that when the polymer with low permeability and selectivity (PSF) is used, both the CO2 permeability and separation factor of the prepared MMMs were significantly enhanced. When the polymer with high selectivity and permeability (6FDA-Durene) is used, more gas molecules would pass through the pore channels of MIL-101 (Cr), leading to the great improvement on the permeability of the MMMs while a slight decrease of the separation factor. When using the polymer with high chain mobility (Pebax 2533, the surface pores could be blocked by the polymer chains, resulting in an obvious decline in the CO2 permeability of MMMs. This work may provide guidance for the screening and matching of filler and polymer materials during MMMs preparation.3. Cu-TCPP nanosheets were chosen as a representative candidate to explore the potential advantages of two-dimensional (2D) lamellar materials in the preparation of MMMs. The orientation and distribution of the 2D nanosheets in MMMs were intutively characterized through SEM, and the effects of incorporating Cu-TCPP nanosheets was analyzed by combining pure gas permeation results. The results demonstrate that Cu-TCPP nanosheets in MMMs are orientated in parallel to the membrane surface, and its appropriate loading is much lower than that of ordinary granulate fillers because of the low density. It can be inferred that 2D lamellar materials can be considered as promising candidates for the preparation of ultra-thin MMMs.4. On the basis of previous work, g-C3N4 nanosheets with ultra-high aspect ratio were selected as filler and were added into Pebax 2533 to prepare ultra-thin MMMs. The effects of the incorporation of g-C3N4 nanosheets on CO2/N2 separation were further analyzed. Due to the restriction of the pore aperture (3.11 A) of g-C3N4 nanosheets, gas permeability of the MMMs decreases with the increase of filler loading. However, there are large amounts of N-containing basic groups with an affinity for CO2 molecules, as well as some large-size (3.1-3.4 A) defects in the structure of g-C3N4, the CO-2 permeability is compensated by the incorporation of g-C3N4 to some extent.As a result, the CO2/N2 separation factor of the prepared MMMs can be improved obviously.5. Preparation of thin film nanocomposite (TFN) membranes by incorporating nanoparticles into the polyamide layers of traditional polyamide composite membranes is an effective way to enhance the process of organic solvent nanofiltration. However, it still remains a great challenge to prepare defect-free TFN membranes due to the poor dispersibility of filler particles in organic phases (such as n-hexane) as well as the compatible issue between fillers and polymers. Aiming at these difficulties, UiO-66-NH2 was selected as the studied MOF material and the surface of its nanoparticles were modified with long alkyl chains before preparing TFN membranes. Benefited from the improved particle dispersibility in n-hexane, defect-free TFN membranes with ultrathin MOF/polyamide layer were successfully prepared through interfacial polymerization. Significant enhancement was found in methanol permeance after nanoparticle incorporation, without comprising the tetracycline rejection evidently. |
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