Formation Process Of Organic Template-free CHA Zeolite Membrane And Its Pervaporation Performance Of Water-rich Mixtures

Zeolite membranes are commonly used for dehydration separation of organic/water mixtures because of their developed and diverse pore structures,good sieving effect and excellent hydrothermal stability.Chabazite（CHA-type）zeolite with the framework structure Si/Al ratio of 2～∞has a three-dimensional eight-membered ring pore structure,and its effective pore diameter is 0.38 nm.CHA-type zeolite membrane with a low Si/Al ratio possesses excellent hydrophilicity and outstanding hydrothermal stability,Which has a great application prospect in the field of dehydration of organic mixtures in water-rich environment.However,the formation process of CHA-type zeolite membrane is not clear,and how to obtain defect-free and high-performance CHA-type zeolite membranes remains a challenge.In this thesis,the growth process of CHA-type zeolite membranes was studied in detail,and the membranes with the best performance were applied to the pervaporation dehydration process of water-rich mixtures.Based on this,the main research content of this paper is mainly divided into the following two aspects:（1）The growth process of CHA-type zeolite membrane prepared under an organic template-free and fluoride-containing synthesis gel:CHA-type zeolites synthesized by interzeolite conversion were used as crystal seeds,and CHA-type zeolite membranes were synthesized by secondary hydrothermal synthesis in an organic template-free and fluoride-containing synthesis gel by changing crystallization times.Combined with XRD,FE-SEM,FT-IR and UV-Raman,the formation process of CHA seed to CHA-type zeolite membrane was studied,and the growth mechanism of CHA-type zeolite membrane in a fluoride-containing and organic template-free gel was proposed.The experimental results showed that the seeds coated on the surface of stainless steel support dissolved into amorphous phase at the initial stage of synthesis,the secondary building units（D6Rs）of CHA framework were completely destroyed except for the presence of a small number of 4Rs.While the amorphous contained many primary building units（TO₄）as the nucleation sites in the second process of CHA crystal reconstruction,which nucleated rapidly again under the promotion of fluorine anion.With the further prolongation of crystallization time,the regenerated secondary structural units connected together to reconstruct the framework structure of CHA zeolite,and gradually grew into a continuous and dense membrane layer with the cross-linking growth of CHA zeolite.The synthesized CHA-type zeolite membrane under the optimal crystallization time（36 h）showed good pervaporation performance for the dehydration separation of ethanol/water（90 wt.%/10 wt.%）mixture at 348 K,the permeation flux was 5.90 kg·m^-2·h^-1 and the separation factor was 3000.（2）Pervaporation performance of CHA-type zeolite membrane for dehydration of a water-rich organic solution:the pervaporation performance and hydrothermal stability in a water-rich organic solution of CHA-type zeolite membrane were studied in detail.The effects of feed types,feed concentration and operation temperature on the pervaporation performance of CHA-type zeolite membrane were investigated,and the reproducibility of the membrane was discussed.The prepared CHA-type zeolite membrane was used in the mixtures of 50 wt.%methanol/water,50 wt.%ethanol/water and 50 wt.%isopropanol/water for a long time.The permeation fluxes were maintained at 7.40 kg·m^-2·h^-1,13.50 kg·m^-2·h^-1and 14.80 kg·m^-2·h^-1,respectively.And the water content on the permeation side was 99.56 wt.%,99.59 wt.%and 99.61 wt.%,respectively.Finally,the results of XRD and SEM showed that the morphology and crystallinity of the membranes remained almost unchanged after dehydration and separation in the water-rich environment for a long time,which indicated that the membranes were not destroyed and the skeleton did not collapse in the environment with high water content,and thus the CHA-type zeolite membranes showed high permeation flux and excellent long-term hydrothermal stability.