Preparation, Pyrolysis Mechanism And Structure Modification Of Polyimide Based Carbon Membrane

Carbon molecular sieving membrane, referred to carbon membrane, is a novel carbon-based membrane material prepared by pyrolysis of polymer membrane. Carbon membrane has a great application potential in the gas separation field, especially the small molecular gas separation, and is pronounced "the most promising large-area industrial production of inorganic membrane with high gas separation performance" due to the excellent gas separation performance, good thermal and chemical stability. However, the carbon membrane has not been achieved large-scaled industrial application so far, because of its poor mechanical strength and low gas permeability.In order to solve these issues, the current thesis began with the design for the structure of the polymer precursor, the evolvement from polymer to carbon and the forming mechanism of the porous structure in carbon membrane have been studied, and the reason of low gas permeability in pure carbon membrane has been discussed. On this basis, the method of enhancing the gas permeability of carbon membrane was proposed. To increase the mechanical strength of pure carbon membrane, the supported carbon membranes have been prepared with the inexpensive coal-based carbon plate as support. The porous structure of the support surface was modified by loading of a mesoporous carbon middle layer; and an improved gas permeability of the supported carbon membrane was achieved thereafter.The results are listed as belows:(1) The chemical changing of Kapton typed polyamic acid, which was applied as the precursor of carbon membrane, during the pyrolysis process was studied. The reason of low permeability was discussed. The results show that the carbonization process includes two steps1. Low-temperature pyrolysis, and2. High-temperature carbonization:imidization and structure reform forming the cross-linking net structure, which is the prototype of carbon structure, occurred during the low-temperature pyrolysis process; accompanied by the severe thermal decomposition and thermal polycondensation reactions, the forming cross-linking structure transformed to the polycyclic strcuture with hexagonal carbon ring through removal of hetero atom group, cyclization and aromatization, further formed into the planar carbon net and constructed the carbon micro-crystals, which accumulated to the disordered porous structure of the carbon membrane ("worm"-like pore structure), occurred at the second step. The chemical structure of polyimide has a great effect on the micro structure and performance of the related carbon membrane. The carbon membrane with loose structure, beneficial to the gas molecular diffusion, was produced by the pyrolysis of the polyimide with larger free-volume, higher rigidity and more hetero-atoms. However, the gas permeation routes in the carbon membrane is long and winding due to the "worms"-like pore structure, which is the reason of the low gas permeability in the pure carbon membrane. Therefore, breaking the wormlike porous structure should be the key issue for the preparation the carbon membrane with both high gas permeability and high selectivity.(2) It is a promising method about introduction of inorganic nano-particles into the carbon matrix to break the "worms"-like porous structure of pure carbon membrane. The hybridized carbon membrane was prepared by embedding the nano-zeolites into carbon matrix. The effects of the properties of zeolites, such as type, granularity, channel structure on the gas separation performance were discussed; and the preparation parameters such as involving the content of zeolites and carbonization temperature on the structure and gas separation performance of hybridized carbon membrane were also studied. Results show that the gas molecular diffusion resistance was reduced and the gas separation performance was enhanced in the zeolite/carbon hybrid membrane by breaking the"worms"-like pore structure of carbon membrane in according to the interfacial gaps between carbon matrix and zeolite was diversified the pore system. The property of zeolites and the parameters in membrane preparation affected on the structure and gas separation performance of the zeolite/carbon hybrid membrane greatly, meaning the restructure of ultramicropore and the modification of the porous structure of carbon membrane could be achieved. And the carbon membrane with both high gas permeability and high selectivity could be fabricated with the O2/N2selectivity of15.6, O2and H2permeability of501Barrer and2280Barrer, respectively.(3) Supporting carbon membranes on porous substrates is most effective way to improve their mechanism strength. Using the inexpensive bituminous coal-based carbon plate as support and Kapton as the precursor, the supported carbon membrane with high gas separation performance was produced. The gas permeation rate and selectivity were further improved by modifying the surface of support with a mesoporous middle layer. In this supported composite carbon membrane, O2/N2selectivity of11.03and O2permeation rate of0.70mol·m-2·s-1·Pa-1×10-8. Moreover, the gas separation performance of the low cost supported carbon membrane could be improved by enlarging the porosity of support and improving the affinity of separation layer also.