| Compared with traditional separation technology,membrane separation technology has the advantages of good separation selectivity,low energy consumption,and environmental friendliness which has always been a research hotspot for researchers.In the field of gas separation,ZIF-8 membrane and NaA zeolite membrane are the most representative ones of ZIFs membrane and molecular sieve membrane respectively.Because they all have excellent pore size?3.4?and 4.0?,respectively?,and regular pore structure and they have been widely applied to the separation of small molecular gases such as H2,N2,CO,CO2 and CH4.At present,ZIF-8 membranes and NaA molecular sieve membranes have made good progress in the field of gas separation through the efforts of many scientific researchers.However,there are still some problems in the face of industrialization.It is difficult to effectively form a dense and continuous film layer because that the surface of the support morphology is not uniform and the bonding force between the film layer and the carrier is weak and the thermal expansion coefficient is largely different.Therefore,in this study,an?-Al2O3 ceramic tube with a length of 17cm,a diameter of 0.8cm and an outer diameter of 1.2cm is used as the carrier.And the in-situ growth method is used to grow a uniform layer of inorganic transition layer on the inner surface of the support for improving and optimizing the physical and chemical properties of the support surface,which promote synthesiz a dense,continuous,and well separated ZIF-8 membrane and NaA zeolite membrane.The main experiment contents and results are as follows:?1?In-situ growth of sheet-like ZnO transition layers on the inner surface of the support tubeIn order to improve and modify the properties of the support surface,a sheet-like ZnO transition layer was grown on the inner surface of the support tube by in-situ hydrothermal synthesis.The flake ZnO transition layer is introduced into the ceramic tube inner surface by in-situ hydrothermal synthesis.By investigated the effects of oxalic acid,hexamethylenetetramine?HMT?,concentration of synthetic liquid and synthesis temperature on the structure and morphology of ZnO transition layer,the optimal synthesis condition of the flake ZnO was found:The molar ratio of synthetic solution is 1 Zn?NO3?2·6H2O:2HMT:0.1 H2C2O3:275 H2O,the synthesis temperature is 80°C,and the synthesis time is 6h.With this condition,the network structure layer of ZnO nanosheet can be grown on the inner surface of the support tube.When ZIF-8 film is synthesized,this structure is favorable for the adsorption of seed crystals and the growth of crystals.?2?Synthesis of ZIF-8 membrane on the inner surface of support modified with ZnO transition layersUsing a ceramic tube modified with a ZnO transition layer as a support,which a ZIF-8 film was synthesized on the inner surface by a solvothermal synthesis method.By investigated the effects of sodium formate,2-methylimidazole,the concentration of the synthetic solution and the number of synthesis times on the membrane formation and gas permeation of ZIF-8,the optimal conditions for synthesizing the ZIF-8 film on the ZnO transition layer are:The molar composition of the synthesis solution was 1 ZnCl2:2 Hmim:0.85 NaCOOH:300 CH3OH.The synthesis was performed at 100°C for 6 h and repeated three times.The ideal selectivity to H2/N2 is 5.84,the permeance of H2 is2.72×10-7 mol·m-2·s-1·Pa-1,and the permeance of N2 is 4.66×10-8 mol·m-2·s-1·Pa-1.Comparative experiments show that ZIF-8 films can hardly be synthesized on ceramic tubes that have not been modified with ZnO transition layers.Therefore,the sheet-like ZnO transition layer can not only connect the film layer and the carrier well,but also provide more active sites,and dissolve and transform,and it can well solve the difficulties of heterogeneous nucleation between ZIF-8 and the support.?3?In-situ growth of flake-shaped Mg-Al hydrotalcite transition layers on the inner surface of the ceramic tubeThe in-situ synthesis of a network structure transition layer formed of a flaky Mg-Al hydrotalcite on the inner surface of a ceramic tube.By investigated the effects of synthesis time,synthesis temperature,alkali content and Mg-Al ratio on the structure and morphology of Mg-Al hydrotalcite,the optimal synthesis conditions for synthesis of monolithic Mg-Al hydrotalcite were obtained:6 MgCl2·6H2O:2 Al?NO3?3·9H2O:20 NaOH:1 Na2CO3:666 H2O,synthesis 24h at 90°C.?4?Synthesis of NaA zeolite membrane by microwave synthesis on the inner surface of ceramic tube modified by lamellar Mg-Al hydrotalcite transition layerThe NaA zeolite membrane was synthesized by microwave synthesis.By investigating the effects of silica content,alkali content,water content and synthesis temperature on the structure,morphology,separation selectivity and gas permeation flux of zeolite membrane,the optimal conditions for microwave synthesis of NaA zeolite membrane were determined:The molar composition of synthetic solution was3Na2O:2SiO2:Al2O3:200H2O,and it was aged at room temperature for 24 hours before synthesis.The microwave synthesis condition was that the temperature was increased to 90°C in 5 minutes and kept for 30 minutes.The ideal selectivity of NaA zeolite membrane to H2/N2 is 4.75,the permeance of H2 is 4.69×10-6 mol·m-2·s-1·Pa-1,and the permeance of N2 is 9.87×10-7 mol·m-2·s-1·Pa-1. |
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