Preparation Of KAUST-8 Membranes And Their Separation Performance For Low Carbon Alcohol/water System

In recent years,the problem of energy shortage and environmental pollution becomes more and more serious.The development and utilization of new high-quality energy is very important for the future sustainable development.Methanol and ethanol have been widely concerned as liquid fuels with high performance.In industry,distillation is usually used to purify methanol and ethanol,but the energy consumption is high in this process.Ethanol and water will form an azeotropic substance when the mass fraction of ethanol is 95.6%?,which is difficult to be separated by conventional distillation.Therefore,it is particularly important to develop a low energy consumption and environmentally friendly separation method.Membrane separation process,as a new separation technology with low energy consumption and no pollution,is especially suitable for the separation of azeotropes.In this thesis,the particle size of KAUST-8 crystals was adjusted,and nano-sized KAUST-8 crystals were prepared.Usingα-Al₂O₃ as the support,KAUST-8 membranes were grown from KAUST-8 crystal seeds as synthesized by the secondary growth method.The effect of particle size（10μm,300 nm）of KAUST-8 seeds and the kind of medium（pure water,SDBS aqueous solution）on the quality of seed layers and membranes was investigated.The analytical methods including X-ray diffraction（XRD）,thermal field emission scanning electron microscopy（SEM）,thermogravimetric analysis（TG）,particle size and Zeta potential were used to characterize crystals and membranes.Finally,the pervaporation separation performance of KAUST-8 membranes for methanol/water system and ethanol/water system was investigated.The specific research contents are as follows:（1）The particle size of KAUST-8 crystals was controlled by adjusting the crystallization time,the concentration of the synthetic solution,the crystallization method and the kind of capping agent to prepare the high-quality seed layers.The results show:shortening the crystallization time（24 h～18 h）,increasing the concentration of the synthetic solution(the molar ratio of each component of the synthetic solution:n(Ni²⁺):n(Al³⁺):n（Pyr）:n（HF）:n（H₂O）=1:1:8:12:298 and 1:1:8:12:159),and using two-staged crystallization under different temperature（nucleation at 100℃for 6 h and growth at 70℃for 18 h）,the crystal size is slightly reduced compared with that reported in the literatures,but still in the micron scale（5μm）.The size of KAUST-8 crystals can be effectively controlled by adding alcohol（ethanol or n-butanol）to the synthetic solution as capping agent.KAUST-8 crystals with 500nm size is obtained in the synthetic solution with molar ratio of n(Ni²⁺):n(Al³⁺):n（Pyr）:n（HF）:n（H₂O）:n（ethanol）=1:1:8:29:63:209,at 85℃and with the crystallization time of 24 h.If ethanol is replaced with n-butanol,the particle size of KAUST-8 crystals is further reduced to about 300 nm.（2）With pure water as the medium and KAUST-8 crystals of 10μm as seeds,the seed layer was produce,which are discontinuous.The KAUST-8 membrane grown from the seed layer is not dense and has some pinholes.The N₂ single-gas permeance on the membrane is 1.9×10^-6mol/（m²·Pa·s）.The seeds almost cover the support in the seed layer prepared by using 300 nm KAUST crystal as seeds,and the KAUST membrane grown from which is continuous and dense.The N₂ single-gas permeance on the membrane is 8.32×10^-8 mol/（m²·Pa·s）.The SDBS aqueous solution as the medium was used to produce the seed layer,with 300 nm KAUST-8 crystals as seeds.The seeds covered the support completely.The KAUST-8 membrane obtained from this seed layer is the densest one in the membranes we synthesized.The N₂ single-gas permeance is 4.45×10^-8 mol/（m²·Pa·s）.（3）On the KAUST-8 membrane prepared with pure water as the medium,with the increase of operating temperature from 25℃to 60℃,the total flux,water flux and methanol flux increases simultaneously,and the separation factor decreases on for the methanol/water system with water content of 5 wt.%.At 25℃,the separation factor is 15.9,and the total flux is 404.3g/（m²·h）.For the ethanol/water system with water content of 4.4 wt.%,the change trend of the flux and separation factor is similar to that of the methanol/water system,as the operating temperature increases from 25℃to 75℃.At 25℃,the separation factor is 17.2,and the total flux is 281.44 g/（m²·h）.When the water content in the feed side increases from 5 wt.%to20 wt.%,at 25℃,for methanol/water system,the total flux increases from 404.3 g/（m²·h）to509.8 g/（m²·h）and the separation factor increases from 15.9 to 22.3.For ethanol/water system,the total flux increases from 281.44 g/（m²·h）to 362.2 g/（m²·h）and the separation factor increases from 17.2 to 30.5 when the water content in the feed side increases from 4.4 wt.%to20 wt.%.（4）On the KAUST-8 membrane prepared with SDBS aqueous solution as the medium,when the operating temperature increases from 25℃to 60℃,the total flux,water flux and methanol flux increase simultaneously and the separation factor decreases for the methanol/water system with water content of 5 wt.%.At 25℃,the separation factor is 25.1,and the total flux is 365.26 g/（m²·h）.For the ethanol/water system with water content of 4.4wt.%,the change trend of the flux and separation factor is similar to that of the methanol/water system as the operating temperature increases from 25℃to 75℃.At 25℃,the separation factor is 31.2,and the total flux is 208.38g/（m²·h）.For methanol/water system,at 25℃,the total flux increases from 365.26 g/（m²·h）to 457.3 g/（m²·h）and the separation factor increases from 25.1 to 39.3 with the increase of water content from 4.4 wt.%to 20 wt.%in the feed side.For ethanol/water system,the total flux increases from 208.38 g/（m²·h）to 298.7 g/（m²·h）and the separation factor increases from 31.2 to 50.2 when the water content in the feed side increases from 4.4 wt.%to 20 wt.%.