| Membrane based separation technology is one of the general technology to solve the problems involving environmental pollution,energy saving,emission reduction,and livelihood security.As the key to membrane technology,the membrane materials with high gas separation performance are beneficial and indispensable to solve the problems,such as water resources,energy,environmental problems and traditional industrial technology upgrading.Carbon molecular sieving membrane,referred to carbon membrane,is a novel carbon-based membrane material which is prepared by pyrolysis of the polymer membrane.Carbon membrane has not only presented high efficiency and energy saving,which are as the same as simple membrane materials,but also possesses the structural characteristics as developed ultra-micro pore structure,good thermal stability and chemical resistance of carbon materials.Carbon membrane has shown an excellent gas separation performance and potential application prospect.However,it is difficult to make a thin and dense carbon membrane because of its crisp and weak mechanical strength.Furthermore,although carbon membrane has an excellent intrinsic gas selectivity,the gas permeate flux of carbon membrane still can't satisfy the demand of the commercial application.Hence,increasing the gas permeate flux of carbon membrane is the key to realizing the commercial application of carbon membrane.In this thesis,based on the design of the pore structure of membrane,carbon membranes with asymmetric structure were prepared by reducing the thickness of the surface separation layer to enhance the gas permeate rate of carbon membrane.Polymer membrane,thermal cross-linking membrane and carbon membrane with asymmetric structure were prepared.The formation mechanism of pore structure during the phase inversion process,and the effects of polymer membrane preparing conditions,thermal cross-linking and carbonization processes on the asymmetric structure and properties of the obtained membranes were investigated by the modern instruments and analysis technologies.The pore structures and gas separation properties of polymer membranes,thermal cross-linking membranes and carbon membranes were tuned by optimizing the preparation process.Furthermore,the permeating mechanism of carbon membrane on CO2 was also investigated.(1)Polyacrylonitrile(PAN)based asymmetric polymer membrane was prepared by dry-wet phrase inversion process.Optical microscopy and light transmission experiment were used to investigate the formation mechanism of pore structure during phase inversion process.The results show that during the phase inversion process,the dense layer on the top of membrane was firstly formed due to the exchange of solvent and nonsolvent.Then,the transition structure of sponge-like porous structure was formed.The finger-like porous structure grew gradually with the exchange of solvent and nonsolvent until all polymer solidified.The morphology of pore structure was controlled by the growth rate of polymer poor phase and the gel rate of polymer rich phase.When the growth rate of polymer poor phase was faster than the gel rate of polymer rich phase,it was easier for the finger-like porous structure to be formed,whereas the sponge-like porous structure easily tend to be formed.The regulation of the pore structure of the asymmetric membrane was easier to realize by tuning the growth rate of polymer poor phase(the rate of nonsolvent diffusion into the casting solution and the diffusion rate of nonsolvent in the casting solution).(2)PAN asymmetric carbon membranes were prepared after thermal cross-linking and carbonizing.The effects of thermal cross-linking and carbonization process on the structure and properties of PAN asymmetric cross-linking membrane and carbon membrane were studied.And the thermal cross-linking mechanism of PAN asymmetric membrane was proposed.Results show that the cross-linking of PAN asymmetric membrane was mainly intramolecular cyclization,and intermolecular cross-linking occurred simultaneously.During the process of intramolecular cyclization,the dehydrogenation reaction of the a-H in PAN molecular chain occurred to form-C=C,while the ?-C in PAN chains was oxidized to carbonyl,and-C?N was cyclized to form-C=N,and three intramolecular cross-linking structures were formed.For the process of intermolecular cross-linking,the cross-linking reaction toke place between the ?-C of two PAN molecular chains that combined with 0 and formed-C-O-C structure.The thermal cross-linking conditions had great influences on the structure,permeation and separation performance,thermal stability and chemical resistance of the asymmetric cross-linked membranes.With the increase of cross-linking temperature,the structure of PAN amorphous region was reorganized and changed into ordered,and the quasicrystal structure gradually grew.As the cross-linking temperature above 180?,the original quasicrystal system was damaged by heating and gradually transformed into a more stable ladderlike molecular structure.With the increase of cross-linking temperature and enhancement of soaking time,the gas permeance of the cross-linking membrane increased,and the fluxes of PEG and BSA decreased while the rejection rates increased.Meanwhile,the cross-linking membrane exhibited an excellent thermal and chemical stability.The obtained asymmetric cross-linked membranes could be used to prepare ultrafiltration membrane for water treatment and as a substrate for nanofiltration or gas separation in an aggressive and demanding environment.Asymmetric PAN carbon membranes prepared by the carbonization of cross-linking membrane still maintained the asymmetric membrane structure.The microstructure was transformed into amorphous carbon structure,and formed a mesopore and micropore structure.The gas permeance of the carbon membrane increased with the enhancement of carbonization temperature,while the gas permeance increased firstly and then decreased with the enhancement of soaking time.Under the optimized carbonization conditions,the O2 permeance of PAN asymmetric carbon membrane reached 2905 GPU with the O2/N2 selectivity of 1.1.The O2 permeance of the obtained PAN asymmetric carbon membrane was an order of magnitude higher than the literature data.(3)The carbon membrane with asymmetric structure and high gas permeance had been successfully prepared by PAN,but the gas selectivity of these carbon membranes was low.The asymmetric carbon membrane with both high gas permeation and selectivity had not been obtained.Therefore,gas separation carbon membranes with asymmetric structure were prepared by PMDA-ODA typed polyamic acid.Furthermore,the effects of phase inversion and carbonization parameters on the structure and properties of carbon membranes were also studied.The gas permeation mechanism of carbon membrane with asymmetric structure was also discussed.Results showed that carbon membranes with high gas permeation and selectivity could be prepared by using PMDA-ODA polyamic acid as the precursor.The structure and properties of the asymmetric membrane could be influenced by changing the phase transformation process.By increasing the viscosity of the casting solution and the thickness of the dense layer of the primary membrane,a sponge-like structure could be obtained,which reduced the permeance of the carbon membrane.By increasing the carbonization temperature of the asymmetric carbon membrane,the gas permeance of the carbon membrane firstly increased and then decreased,while the selectivity increased.With the enhancement of the soaking time,the gas permeance of asymmetric carbon membrane increased and the selectivity slightly decreased.The asymmetric carbon membrane with a thin dense surface layer without pin holes or defects,and a supporting layer with abundant sponge-like and finger-like pores,was successfully prepared by finely controlling and optimizing the phase inversion and carbonization process conditions,which exhibited the excellent gas permeation and separation performance.The O2 permeance of the asymmetric carbon membrane was 49.3 GPU,and O2/N2 selectivity was 4.5.The O2 permeance of the prepared PMDA-ODA polyimide asymmetric carbon membrane was 60%higher than the literature data with the similar O2/N2 selectivity.According to the fitting of CO2 adsorption isotherms in PI carbon membrane obtained at different carbonization temperatures,with the increase of carbonization temperature,the adsorption coefficient of CO2 in carbon membrane firstly increased and then decreased,and the permeability coefficient and diffusion coefficient of CO2 gradually decreased,indicating that the diffusion rate of CO2 in carbon membrane was the controlling step for determining the permeate property of CO2.Additionally,the Sips equation had the best fitting effect on the gas adsorption performance of carbon membrane,indicating that the micro pore structure and size of carbon membrane was non-uniform. |
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