| Energy and environmental issues have become two major problems that thepeople all over the world have to face—it is also an important issue during theeconomic and social development of our country, especially the crisis of globalwarming caused by CO2and other greenhouse gases. So it is imminent that reducingenvironmental pollution, improving energy efficiency and shifting the way ofproduction and life. Compared to the traditional method—cryogenic and pressureswing adsorption process for gas separation, membrane technology has been subjectto widespread attention, owing to no phase change, low energy consumption, simpleequipment, convenient and flexible operation, etc. Gas membrane separationtechnology has been used in air separation for oxygen and nitrogen enrichment,hydrogen recovery, CO2separation and adsorption, natural gas dehydration,olefins-paraffin separation, etc. Commercialized gas separation membrane materialsare silicone rubber, cellulose, polysulfone and polyimide. Compared togeneral-purpose plastics, poly(aryl ether) could be adapted in high temperature andpressure environment and may be used for separation and adsorption CO2, SO2, H2Sand other acidic gases because of its outstanding chemical stability, thermal stabilityand mechanical properties, and resistance to plasticizing properties and resistance toacid.Taking the design idea of this thesis as the starting point, the trifluoromethyl(-CF3) and amino (-NH2) were introduced into polyethersulfone to improve the gasseparation performance. The three series of polyethersulfone containingtrifluoromethyl and amino were gotten by changing the type and proportion of thefluorine-containing bisphenol and amino polyether sulfone. Chemical structure ofpolymers was characterized by IR and NMR. DSC and TGA results indicated that each polymer in three series had a high glass transition temperature and good thermalstability. These polymers exhibited higher tensile strength and Young’s modulus butlower elongation at break. The high electron density of the fluorine atom made thebulk density of polymer chains decreased, so that the fractional of free volume of thepolymer increased. The fractional of free volume decreased with the introducing ofamino group and the decreasing of trifluoromethyl, and then the permeabilitycoefficients of N2and O2decreased. Lewis acid-base interaction between-NH2andCO2molecules in the polymer could improve the solubility coefficient of CO2,thereby increasing the CO2transmission rate through the membrane. The promotionof-NH2to CO2permeation offset the CO2permeation rate decreasing caused by thereduced free volume, so that the permeability coefficient P(CO2) reduced slightly,even increased with increasing of the amino segment ratio. However there was nosuch a interaction between-NH2and O2, N2and other non-polar gases, so the impacton gas separation was limited. Because the ideal separation factor is ratio ofpermeability coefficients of the fast gas and the slow gas, the gas separation factor ofCO2to other gases were improved.Preparation of the mixed matrix membrane (MMM) and the asymmetricmembrane has been an effective way to improve the gas separation performance. Asintroducing of the mixed inorganic particles into the polymer matrix, the MMMpossesses the advantages of both the polymer matrix and the inorganic particles. Dueto the diversity of inorganic particles, we have more choices in improvement of theperformance of gas separation membranes, and the permeability and separation factorcould be improved simultaneously. In this thesis, the titanium dioxide nanoparticlewas used as inorganic dispersed phase to improve the gas separation performance ofmembranes. The interaction between inorganic particles and the polymer chains willreduce the entanglement between polymer chains and hinder the movement of thepolymer segments, so that the bulk density decreases, the diffusion coefficient ofMMM will increase, thus the gas permeation rate will be improved; on the other hand,the interaction between the hydroxyl groups on TiO2nanoparticles and CO2moleculescould promote the transfer of CO2. Scanning electron micrographs of MMM observed that TiO2nanoparticles dispersed even in the polymer matrix, it could be attributed tothe good affinity between hydrophilic TiO2nanoparticles and a water soluble solventNMP. The hydrogen bonds between the hydroxyl groups on TiO2nanoparticles and-NH2groups were beneficial to TiO2dispersion. EDX test results for AmFPES-TiO2mixed matrix membranes obtained a signal of element Ti; three peaks reflectingdifferent TiO2crystal types appeared in XRD test results. Since the introduction ofTiO2nanoparticles to the MMMs led to decreasing of tensile strength and breakingelongation of mixed matrix membranes compared with the dense films. The gasseparation performance analysis showed that the introduction of TiO2nanoparticlesreduced bulk density of the polymer chains and the gas permeation coefficientincreased obviously. The interaction between hydroxyl groups on TiO2nanoparticlesand CO2molecules improved the permeability coefficient while improved CO2separation factor with respect to other gases.We selected Am-6FDA-PES-20and Am-6FDA-PES-60as polymer matrix,acidification modified multi-walled carbon nanotubes as dispersed phase, andprepared AmFPES-MWNT mixed matrix membranes by solution blending method.The acidulated MWCNTs were characterized by IR spectroscopy, and we couldobserve enhancement of carboxyl characteristic absorption peak indicating that thecarboxyl content increased after acid treatment. The microstructure of the mixedmatrix membranes were characterized by scanning electron micrograph. At lowaddition amounts, MWNT-COOH dispersed well in the polymer matrix and contactedclosely with the substrate. When the addition amount of increased to5wt%, a largeamount of carbon nanotube clusters appeared. The gas separation performanceanalysis for AmFPES-MWNT mixed matrix membranes, showed that as theMWNT-COOH was added into the mixed matrix membrane, the gas permeabilitycoefficients improved significantly. This is because carbon nanotubes withsize-controllable one-dimensional hollow structure provid a good channel fordiffusion of gas molecules.Preparation of asymmetric membrane by dry-wet phase inversion is an importantway to improve the gas separation performance of membrane materials. Asymmetric membranes generally consisted of a dense skin and a porous support layer thatproviding the separation performance and supporting respectively. The gaspermeability of the asymmetric membrane could be improved by the porous structureof the support layer. In this thesis, we used3FPEEK to prepare asymmetricmembranes, dichloromethane as solvent and alcohol as additive. The cross-sectionmorphology of asymmetric membranes was characterized by SEM. When the additiveof n-butanol was at a low amount, the support layer was a sponge-like structure. Themacroporous layer appeared at a lower amount when isobutanol, t-butanol andisopropanol were used as non-solvent addition. The surface morphology androughness of asymmetric membranes were characterized by AFM. The porosity ofasymmetric membranes obtained by gravimetric method showed that in the case ofthe same type of alcohol additive, the porosity increases with increasing of additiveamount. Compared with the dense3FPEEK membrane, the tensile strength andYoung’s modulus of asymmetric membranes with a porous support layer decreased,but the elongation at break increased obviously, the maximum could increase from8.13%to49.93%. Mechanical properties of asymmetric membranes prepared in thisthesis could meet the strength requirement in the gas separation process. Since theporous support layer promoted a substantial increase in gas permeation rate ofasymmetric membranes comparing with the dense film and it could be attributed tothe porosity increase. The presence of dense skin of asymmetric membranesmaintained a good separation performance, CO2/O2separation factor significantlyincreased. |
PDF Full Text Request