Fabrication And Investigation Of Ceramic Matrix Composite Membranes

Separation technology is a widely used purification technology,which can extract various components of the mixture individually by means of extraction,distillation and membrane separation,with high application value.Among them,membrane separation technology has attracted attention for its advantages such as high efficiency and low energy consumption.In the application of separation membranes,they are divided into two categories,organic and inorganic,according to the different preparation materials,and each of these two categories has its own advantages in practical applications.Organic membranes have the advantages of low production cost and high separation performance,but it is difficult to get rid of the problems of low mechanical strength and long service life;inorganic membranes have good stability,but high preparation cost and difficult to guarantee the separation accuracy.Although the advantages of both membrane materials are obvious,the shortcomings of each limit their application and development.Based on the above problems,in this thesis,we used commercially available flat ceramic membranes as the substrate and coated them with nano-titanium dioxide to prepare ultrafiltration ceramic membranes with flat surfaces;we used two polymers,self-polymerized microporous polymer（PIM-1）and polyimide（PI）,to coat them to produce a solvent-resistant nanofiltration membrane and a gas separation membrane,and investigated the separation performance of these two composite membranes.1)Using ceramic flat microfiltration membrane as the base membrane,titanium dioxide composite ceramic membrane was prepared by dip slurry coating method.Three kinds of titanium dioxide coating solutions with particle sizes of 25 nm,15 nm and 5-10 nm were prepared and coated by dip slurry in order of particle size from large to small,followed by high-temperature sintering to prepare ultrafiltration ceramic substrate membranes.We investigated the effects of solid content and dispersant addition in the coating solution on the membrane structure and properties,and characterized the membrane surface by SEM and AFM.The results showed that the surface of the ceramic membrane was flat and free of defects,and the water flux was 128.6 L·m^-2·h^-1·bar^-1at 0.1 MPa transmembrane pressure,and the retention rate of Sirius red dye(Mw=1373.1 g·mol^-1)was 97.4%.According to the subsequent scale-up test,the preparation process is stable and can be prepared on a large scale,which has certain prospects for industrial application.2)Using the above ultrafiltration ceramic membrane as the support layer and polymer PIM-1 as the separation layer,a composite solvent-resistant nanofiltration membrane was prepared by dynamic spin-coating method.By investigating the effect of the solid content of spin coating solution on the structure and performance of the separation layer using SEM and dead-end separation performance tests,the composite membrane made at 3 wt%had a low film defect rate,and its methanol flux reached4.9 L·m^-2·h^-1·bar^-1at 0.4 MPa transmembrane pressure,and the retention rate of Congo red dye(Mw=696.68 g·mol^-1)was 95.2%.After the heat treatment at 385℃,there was a significant decrease in the molecular weight of the retained material,and the retention rate of methyl orange with a molecular weight of 327.33 g·mol^-1was92.7%.This is because the high temperature heat treatment induced the cross-linking reaction of-CN to form a triazine ring structure,which effectively reduced the MWCO of the membrane and also enhanced the stability of the composite membrane in DMF and acetone.3)We used 2,2-bis（3-amino-4-hydroxyphenyl）hexafluoropropane（APAF）and2,4,6-trimethyl-1,3-isophthalamide（DAM）with 4,4’-（hexafluoroisopropyl）diphthalic anhydride（6FDA）as monomers in a two-step process to prepare polyimide as the separation layer.A composite gas separation membrane was prepared by dynamic spin coating method.At 0.2 MPa transmembrane pressure,the CO₂gas permeability was31.3 GPU and the CO₂/N₂selectivity was 25.08.The composite membrane was heat treated at 430℃,and the high temperature induced the thermal rearrangement reaction between-OH and imine ring to generate an oxazole ring structure,which increased the microporous structure and free volume within the membrane and substantially improved the gas permeability performance of the composite membrane.The permeability of CO₂was increased to 131.7 GPU,and the selectivity of CO₂/N₂was not significantly changed.Meanwhile,the oxazole structure formed by thermal rearrangement also enhanced the physical aging resistance of the composite membrane.