Preparation And CO₂ Premselectivity Performance Of Metal Organic Framework (MOF-5) Membrane

With the quickening process of social modernization, massive consumption of fossil fuels causes increasing CO₂ emissions. CO₂ leads to global warming more and more serious. Global warming has become one of the major and urgent issues to solve for international community in the 21st century.Membrane separation technology will be one of the most effective technologies to capture CO₂ emissions from industrial sources. In the membrane separation technology, the development of new membrane materials will be the key point. This dissertation is mainly concerned with study on a novel metal-organic framework (MOF-5) membrane and its permseparation performance for CO₂. This research mainly involves synthesis of the MOF-5 crystals, preparation and pore structure characterization of the MOF-5 membrane, determination of CO₂ adsorption thermodynamics and kinetics on the MOF-5 particles, and measurement of the permeation behaviors and separation factors of several single component gases and CO₂ gas mixture through the MOF-5 membranes. This study belongs to the fields of chemical engineering and membrane science and has scientific research value and practical significance.The effects of synthesis condition for the MOF-5 crystals on its physical properties including the morphology, crystal structure and porous texture were investigated. SEM, XRD and N₂ adsorption/desorption were used to characterize the MOF-5 crystals. The results showed that MOF-5 crystals whose partical sizes ranged from 40 to 60μm and surface area was up to 2500m2/g were obtained under the following conditions: synthesis temperature of 130°C, reaction time of 4 hours, the Zn2+/BDC molar ratio of 2.7 and N, N-dimethylformamide as the solvent.Adsorption equilibrium and kinetics of CO₂ on the MOF-5 particles were studied. The adsorption isotherms and kinetic curves of CO₂ measured by using gravimetric method, and the diffusivity coefficients, isosteric adsorption heat and adsorption activation energy of CO₂ on the MOF-5 particles were estimated. The results showed that the amount adsorbed of CO₂ on the MOF-5 crystals was up to 2.10 mmol/g at ambient temperature and 1 atm, which was obviously higher than the amount adsorbed of CO₂ in activated carbon and 13X zeolite; The diffusivity coefficient of CO₂ within the MOF-5 was about 10-8 cm2/s, which was much higher than that within the 13X zeolite. It suggested that the MOF-5 has an excellent adsorption and transportation properties for CO₂. This is one of new points in this work.The secondary seeded growth method to fabricate MOF-5 membrane onα-Al2O3 substrate was proposed. The effects of fabrication conditions, including coating matter, suspension preparation and secondary growth, on the growth of the MOF-5 membrane were studied. The results showed that a perfect MOF-5 seeds layer of support can be gained by using three-subsequent dip-coating method with short time contacting with suspension; In the process of secondary growth, the addition of EDIA can efficiently prevent seed crystals from dissolving and simultaneously make the MOF-5 crystal membrane grow well. A novel technology for MOF-5 membrane is obtained after a series of experimental conditions were optimized, and thus an ultra thin MOF-5 membrane with the particle size of 2-3μm and the thickness of 6-7μm was successfully fabricated. This is one of the important innovations in this work.A characterization method for distinguishing quality of the MOF-5 membranes was studied. The organic molecule of 2 - Dicyclohexylphosphino - 2' - (N,N-dimethylamino) biphenyl (DCPDMA) was used to characterize quantificationally the quality of MOF-5 membranes with the help of pervaporation experiments. The results showed that the pervaporation flux of DCPDMA was very low through the MOF-5 membrane, which indicated the defects distribution of the MOF-5 membrane was in the range of 8 < p MOF? 5 membrane<14.5?. This characterization method provides a technical reference for the detection of the MOF-5 membrane defects, which is a new point of this work.Permeance diffusion behaviors of single component gas through the MOF-5 membrane were investigated. The results showed that the gas permeation diffusion of H₂, He, CO, N₂ and CO₂ was found to be governed by Knudsen diffusion whereas the diffusion of larger molecule SF6 exhibited activated diffusion in the MOF-5 membrane. The permeability of CO₂ through the MOF-5 membrane estimated by using micropore diffusion model was close to the measured value. This verified that the diffusion behavior of CO₂ through the MOF-5 membrane was controlled by Knudsen diffusion mechanism.Separation selectivity of CO₂/H₂ or CO₂/N₂ mixtures through the MOF-5 membrane was studied. Effects of temperature, mixture composition and feed pressure on the separation factor were discussed. The results showed that the separation factor of the MOF-5 membrane for CO₂/H₂ mixture was increased with temperature and the composition of CO₂ and decreased with feed pressure. For CO₂/N₂ mixture, the separation factor of the MOF-5 membrane was decreased with temperature and increased with the composition of CO₂. The separation factor of the MOF-5 membrane for CO₂/N₂ mixture was up to 64 at 298K and 345kPa, with a CO₂ composition of 88%.