Influence Of Membrane Formation Process On The Microstructure And Gas Separation Performance Of Carbon Membranes

Carbon membranes (CM) are mainly prepared by pyrolysis of polymeric membranes,which can effectively separate gas mixture depending on molecular sieving mechanism.Compared with polymeric membranes, CM possesses not only outstanding gas selectivityand permeability, but also outstanding thermal resistance and chemical inertness, etc. CMcan be used in various fields, such as nitrogen enrichment, hydrogen recovery and acid gasremoval from natural gas, and so on. However, the industrial development of carbonmembranes is still limited by the vital restriction of high-cost and brittlement.In this thesis, high performance carbon membranes were fabricated by investigatingand optimizing process factors for membrane formation, including precursor type,hybriding, curing, preoxidation, pyrolysis, etc. Carbon membranes were prepared byadopting two novel polyimides as precursors, i.e., soluble ODPA-ODA bearing withflexible C-O moieties and6FAPB-PMDA bearing with rigid-CF3moieties in theirmolecular structures. Hybrid membranes were prepared by using ferrocene, zeolite ZSM-5,and carbon nanotube and graphene as dopant, respectively. A large variety of advancedtechniques, including thermogravimetry, Fourier transformed infrared spectroscopy,scanning electron microscopy, transmission electron microscopy, x-ray diffraction,nitrogen adsorption and permeation, etc., were used to analysis the thermal stability ofprecursor, evolution of functional groups, morphology, pore structure, carbon structure andgas separation performance of the samples. The preoxidation mechanism of ODPA-ODAtype polyimide was studied on the effects of preoxidation temperature. Some factors, i.e.,dopant type, dosage, permeation temperature, permeation pressure, pyrolysis temperature,etc., were also investigated on the thermal stability, porous structure, carbon structure andgas separation performance.Results have shown that:(1) ODPA-ODA type polyimide is one kind of idealprecursor candidate to prepare carbon membranes. Preoxidation treatment is an importantstep to make carbon membrane with good separation performance, which can drastically improve the thermal stability and char yield of percursor. When ODPA-ODA-based carbonmembranes were prepared under preoxidation temperature at480℃and pyrolysis at650℃,the gas permeabilities reach to1592.34Brarrer,(1Barrer=1×10-10cm3(STP) cm/cm2scmHg=7.5×10-5cm3(STP) cm/cm2s kPa),150.00Brarrer,84.48Brarrer and8.38Brarrercorrespondingly for H2, CO2, O2and N2, along with the selectivities of189.9(H2/N2),17.9(CO2/N2) and10.1(O2/N2). With increasing the pyrolysis temperature, the gas selectivitesfor H2/N2, CO2/N2and O2/N2of carbon membranes tend to reducing. Also, thepermeabilities for H2, O2and N2first decrease then increase, together with thepermearbility reducing for CO2. In addition, the addition of ZSM-5into precursor canenhance the thermal stability, and the selectivities of ZSM-5hybrid carbon membrane are85.8,10.0and4.2, respectively.(2) The permearbilities of6FAPB-PMDA typepolyimide-based carbon membrane are14.48times (H2),1.24times (CO2),3.38times (O2)and6.35times (N2) higher than those of ODPA-ODA type-polyimide-based carbonmembrane, owing for the highly rigid-CF3groups in the molecular chains of6FAPB-PMDA type polyimide. In the case of ferrocene doped precursor, its thermalstability diminishes to a certain degree. The selectivities of pure carbon membranes(CM-6F) are16times (H2/N2),1.6times (CO2/N2) and0.96times (O2/N2) higher thanthose of ferrocene hybrid carbon membrane (CFe-4), together with24.75-fold,2.53-fold1.48-fold and1.55-fold higher in the respective permeabilities for H2, CO2, O2and N2.(3)The thermal stability of the precursor could be improved as the consequence of introducingwith good compatible carbon additives (i.e., ordered mesoporous carbon, grapheme andcarbon nanotube). Among them, ordered mesoporous carbon hybrid membranes presentthe highest permeability and selectivity, e.g., O2/N2selectivity of12.9. Meanwhile, the gasseparation performance of carbon nanotube hybrid carbon membranes is inferior to theothers. Grapheme hybrid carbon membranes have somewhat higher H2/N2and CO2/N2selectivity, achieving to135.4and82.6, respectively.