Design, Preparation And Gas Separation Performance Of The Functional Carbon Membranes

Membrane separation technology is a new and interdisciplinary separation technology developed in the 20th century, which has been widely investigated and used in the world. Especially, membrane-based gas separation technology has attracted much attention in various fields such as the developing and utilization of clean energy (H₂), the greenhouse gas carbon dioxide separation and recovery, the natural gas purification and oxygen/nitrogen separation. Carbon molecular sieving membrane (Carbon membrane) is a novel inorganic membrane material with molecular sieve ability, and it has attached more attention owing to its high selectivity, thermal stability and good chemical stability. However, the strong trade-off relationship between the gas permeability and selectivity make it impossible to satisfy the industry requirement. How to solve the trade-off bottleneck problem and to prepare high performance carbon membranes is a key point for its industrialization.One important approach to solve the challenging task mentioned above is to design the structure of the precursor by incorporating functional groups and to tune the ultramicropores distribution using the "interface effect" formed between the membrane matrix and the functional groups.In this dissertation, in order to prepare high performance carbon membranes with high gas permeability as well as high permselectiviy, valuable explorations have been carried out on designing the structure of the precursor, developing new synthetic strategies and investigating the gas separation mechanism of the as-synthesized membranes. The main results are summarized as below:A novel nano-oxide/polyimide organic precursor is designed and prepared based on the "sol-gel" technique, which is used to produce nano-oxide / carbon composite membrane (Chapter 3). The interfacial gaps between inorganic particles and the carbon phase are believed to help to increase the gas diffusion ability and to improve the gas permeability. As such the gas permeability of the as-synthesized composite membrane is significantly improved. For single gas test, the composite membrane has an intrinsic O₂/N₂ selectivity of 8.4 with O₂ permeability of 520.30 Barrers. The fabrication of this kind of membrane material provides a new impetus to developing new generation of "inorganic-inorganic" composite membranes. In order to improve the gas permeability without decrease the selectivity of the synthesized membrane material, novel carbon/zeolite membrane materials are designed and prepared by incorporating zeolites (4A, ZSM-5, T) into the membrane precursors (Chapter 4). The micropores of the zeolite and the interfacial pores between zeolite and carbon will help the gas diffusion and increase the gas permeability. The results show that the membrane separation performance is affected by the zeolite type, zeolite loadings, the particle size and the pyrolysis parameters. The permeability of O₂ in ZSM-5/carbon membrane (10wt. %, 700℃) is 671.23 Barrer with the O₂/N₂ selectivities of 11.4. This kind of functional membrane can be able synthesized by tuning the zeolite loadings and the pyrolysis temperatures.In order to separate CO₂/CH₄ gas pairs, zeolite T/carbon membranes were prepared by incorporating zeolite T into the carbon matrix. The gas selectivity (CO₂/CH₄) of the membranes for both single gas and mixed-gas (CO₂/CH₄: 50/50 mol. %) can be controlled in a wide range by changing the zeolite T particle size and morphology without altering the final pyrolysis temperatures and zeolite loadings.In chapter 5, mesoporous material/carbon membranes are designed and prepared by incorporating SBA-15 and MCM-48 into the polyamic acid. The results show that the as-prepared membranes show excellent gas separation performance compared to pure carbon membranes, which indicated that the wide pore size of the mesoporous materials help improve gas diffusion rate in the membranes. The MCM-48/carbon membrane shows higher gas permeability than that of SBA-15/carbon membrane. The permeabilities of pure gas H₂, CO₂, O₂ in MCM-48/carbon membrane are 3838, 2508, 527 Barrer, and the selectivity of CO₂/CH₄, CO₂/N₂, O₂/N₂ are 100.3, 39.2, 8.2, respectively. The main gas separation mechanism of the functional membrane is "Knudsen Diffusion assisted molecular sieving mechanism". These kinds of membrane materials are expected to bring new opportunities for preparation of unique "mesoporous - microporous" composite membranes.Carbon nanotubes (CNTs) and ordered mesoporous carbon are novel carbon materials; however, they cannot separate gas molecules effectively. In chapter 6, multi-walled carbon nanotubes (MCNTs) and ordered mesoporous carbon CMK-3 are chosen as fillers to prepare MCNTs/C and CMK-3/C membranes. The results show that the loading of CMK-3 and the holding time at final pyrolysis temperature significantly affect the gas separation performance of the CMK-3/C membranes. The gas permeabilities of the CMK-3/C functional membranes increase with the CMK-3 loading increase as well as decrease the holding time. This research is expected to broaden the research field of carbon membranes and it is helpful for controlled synthesis of high performance carbon membrane materials.