Development of polyimide membranes for gaseous separations and oligomide membranes for reactive liquid separations
Creators: name: BIGHANE, Neha
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400401
subject: Carbon capture engineering (excl. sequestration)
classificationCode: 400409
subject: Separation technologies
classificationCode: 401605
subject: Functional materials

description: A membrane is selective barrier between different sized molecules, ions or particles. Membranes are materials that enable separation by pressure difference and provide low cost alternatives to industrial separation processes. Novel membranes have been developed for gaseous separations and reactive liquid separations. Torlon polymer was doped with Zif8 nanoparticles to provide efficient gas separation membranes. The surfaces of PTFE substrates were chemically modified by covalent molecular assembly to provide functional ultrathin films for reactive liquid separations.<br><br>
Coal combustion power plants produce tons of by-product flue gas emissions. Flue gas mainly comprises a mixture of carbon dioxide and nitrogen. CO2 is a greenhouse gas and its emissions contribute to global warming in the atmosphere. It is important to develop technologies to separate and capture CO2 before it is emitted into the atmosphere. Membranes are robust materials that provide selective permeation of one gas from a mixture under a partial pressure gradient. The separated gas streams can be utilized for various purposes or sequestered. The currently used technology for flue gas capture is reversible absorption in amine solutions but composite membranes (as discovered in this thesis) can provide single stage separation at lower cost and are more environment-friendly. In this thesis, gas separation performance of dense films of polyethersulfone, polysulfone, novel pure Torlon® AI-10 polyamide-imide membrane and four novel mixed matrix membranes of Torlon with Zif-8 nanoparticles is presented. In pure Torlon membranes, the observed pure gas CO2/N2 selectivity is 40 and He/N2 selectivity is 97 at 60psia feed pressure at 25°C. In Torlon-Zif8(10%) mixed matrix membranes, He/N2 selectivity is 192 at 100psia. In mixed gas experiments, 99.8% pure CO2 in permeate was obtained from Torlon-Zif8(20%) mixed matrix membrane dense films. Material characterization studies include DMTA, DSC, TGA, FTIR, SEM, FE-SEM, EDS, XRD and UTM. The developed membranes are suitable for flue gas separation. The membranes can also lower cost and improve energy efficiency in helium extraction process.<br><br>
Generation of combustible biogas from anaerobic digestion of food waste is an interesting and developing alternative for the production of a cheap source of energy. In the present work, Torlon polyamide-imide based membrane films were studied for biogas separation (On-site) since Torlon-Zif8 membranes have the capability to exclude methane based on its larger size. In pure Torlon polymeric membrane, the methane content increased from 62% in feed biogas to 94% methane in retentate. But, there was about 3.7% methane loss into the permeate stream. In a mixed matrix membrane of Torlon-Zif8(20%), the methane content enriched from 62% in feed biogas to 96% methane in retentate, with no loss (0%) of methane into the permeate stream. Thus, this thesis describes an important development in the application of membranes to purify biogas and enrich its energy/calorific value.<br><br>
In the next phase of research, thin films of oligomers were synthesized on PTFE substrates and they were studied for simultaneous chemical reaction and separations. Three different membranes were fabricated using a technique called 'covalent molecular assembly'. The first was oligoimide PMDA-ODA. The second was crosslinked oligoamide TC-TAB. The third was a catalytic membrane Nickel terminated oligoimide Ni-PMDA-ODA. The first two membranes were studied for Friedel-Crafts' alkylation reaction between benzene and methyl chloride in presence of ferric chloride. With 30psia chloromethane pressure, the chemical reaction proceeded to 100% conversion and 99% pure toluene product was obtained in the permeate streams. The third membrane was catalytically active to convert p-nitrophenol to p-aminophenol in presence of sodium borohydride in ethanol. 98.18%pure p-aminophenol was obtained in the permeate stream. All three membranes were thermally stable upto 450°C in nitrogen. The project proved that covalent molecular assembly can be employed to make novel dense microporous membrane films.