The Preparation Of Zeolite Membranes On Porous Ceramic Supports And Their Gas Separation Properties

Most industrial chemists spend their days separating the components of large quantities of chemical mix tures into pure or purer form s.The processes involved,such as distillation,account for 10–15% of the world's energy consumption.Compared with the traditional separation technology,such as distillation,membrane-based separation would use 90% less energy.Meanwhile,membrane-based separation has the advantages of simple operation,environment friendly,no phaxe change,easy amplification,good selectivity and so on.Membrane-based separation technology has been listed as the major supporting project b y our country from the 6th Five-year plan to 973 plan and then to the 13 th Five-year plan.With the further research,membrane-based separation technology has made great progress and been widely used in food processing,biological environmental protection,medicine,chemical industry,electronic bionics,energy chemistry and other fields.Zeolite membranes,which have uniform pore sizes at the molecular level,excellent thermal stability,high chemical inertness,high mechanical strength,and other excellent properties,are emerging as attractive candidates for membrane separation.In 1987,continuous zeolite membrane was prepared for the first time and then various methods and technologys have been developed to synthesis dense and compact zeolite membrane.Although with great advancements in zeolite membrane separation technology,especially in a lcoholwater separation,the formation of compact and thin films for gas separation are still left to chance.The cost of the support occupies 70% of the whole zeolite membrane preparation proces s,indicating the importance of the support in the preparation of zeolite membrane.Generally,there is a lot of defects in the support in bussineess application,the surface of which is rough and the pore size distribution is uneven.Therefore,how to obt ain the high quality zaolite membrane with no defect in those supports still bothers the researchers.On the other hand,organic template should be avoided in the process of synthesizing of zeolite membrane without affecting the quality of zaolite membrane.However,up to now,it is difficult to avoid defects caused b y the support or baking template agent in the existing synthesis technology.In order to improve the membrane integrity,mainly two strategies were adopted b y researchers recently.One strateg y involved the cr ystal transformation,which was expected to synthesis the zeolite with the same pore as the original zeolite crystal.However,this strategy is often used in the synthesis zeolite cr ystal(such as RUB-13,ECB-50),but few reports have been made on the preparation of zeolite membrane.Another strategy involved the employment of ph ysical external force,which was expected to promote the anchoring and adhesion of the zeolite cr ystals on support surface under the action of ph ysical external for ce.While various external forces were used to prepare zeolite membrane,a method using external force,which is easy and convenient to operat,is still to be detailed studied.Zeolite membrane with high silicon content or treated with hydrophobic substances should be considered for the separation components containing H 2O or other strong adsorption impurities.Modifiying the zeolite membrane b y silane is an impo rtant method to increase the h ydrophobicity of the zeolite membrane surface.Although the hydrop hobicity of the zeolite membrane surface was increased,super h ydrophobic still could not be reached and the flux permeation and selectivity of th e zeolite membrane were affected b y the coverage of the large amount of silane.Therefore,it is still a chall enge to choose a suitable method to prepare h ydrophobic surface without affecting the permeability of the zeolite membrane.This article focuses on the analyzing the technical difficulties of synthesizing zeolite membrane and proposing the correspong solut ions.The specific contents can be divided into the following four parts: 1.Synthesis of SOD zeolite membrane b y the induction of MCM-22 zeoliteThe s ynthesis of SOD zeolite membrane by the induction of flaky zeolite MCM-22 was s ystematically investigate d.The results show ed that MCM-22 zeolite la yer could significantly improve the distribution of the SOD zeolite on the support and thus affect ed the sensity of the SOD zeolite.Synthetic alkalinity,reaction temperature and reaction time had great influence on the synthesis of SOD zeolite membra ne,while the effect of Si/Al was small in a certain range.The optimum conditions of synthesis SOD zeolite membrane by the induction of flaky z eolite MCM-22 were as follows: The s ynthesis solution was 50Na2O:5Si O2:1Al2O3:1005H2O and hydrothermal synthesis ed at 140 ? for 5h.At this time,the permselevtivity of H 2 over N2 could reach to 6.07,being larger than the corresponding Knudsen diffusion,while the permeance of H 2 could be as high as 0.85×10-7 mol·s-1·m-2·Pa-1.The mechanism research showed that a large number of twelve membrane ring cup cavity,which is unstable under alkaline condition,were exposed to the surface of MCM-22 zeolite cr ystal and transformed into SOD zeolite under the induction of its six membra ne ring cavity.Meanwhile,the Si/Al active ingredients in the synthesis solution provides the nutrition for the growth of z eolite cr ystal,and thus promoting the formation of the SOD zeolite membrane.2.Pressure-driven s ynesis of Na A zeolite membrane on rough supportNaA zeolite membranes were prepared on 170 mm length rough support by matintaining the pressure difference between the two sides of the support during the hydrothermal synthesis process.The results show ed that pre-degassing of the synthesis s olution could effectivel y remove the bubbles in the s ynthesis solution and between the synthesis solution and the support,which was beneficial to the s ynthesis of dense zeolite membrane.It was found that the bubbles could be removed effectivel y when the degassing time was 30 min.The optimum molar ratio of the synthesis solution for the pressure-driven synthesis of Na A zeolite memebranes was 50Na2O:4.68 Si O2:1Al2O3:1000H2O.When the pressure difference was produced b y controlling temperature,a dense zeoli te membrane could be synthesized by maintaining the pressure difference in 0.047 MPa at 80 ? for 4h.At this time,the permselevtivity of H2 over N2 could reach to 4.39,being larger than the corresponding Knudsen diffusion,while the permeance of H2 could be as high as 0.79×10-6 mol·s-1·m-2·Pa-1.When the pressure difference was produced by vac uuming,the optimum pressure difference was 0.049 MPa,which was similar with the pressure difference produced b y controlling temparture.The permselevtivity of H 2 over N2 could reach to 5.21,while the permeance of H 2 could be as high as 3.61×10-6 mol·s-1·m-2·Pa-1.The mechanism research showed that the pressure difference not only drived numbers of z eolite particles move to the surface of the support and increased the binding force of the zeolite membra ne with support,but also drived fresh s ynthesis solu tion move to the inside surface of the support continuously and provide adequate nutrition for the growth of zeolite membrane.3.Hydrophobic modification of Na A zeolite membrane b y using low surface energy material PFASHydrophobic NaA zeolite membrane wa s prepared b y hydrophobic modification of hydrophilic Na A zeolite membrane.The optimum conditions of hydrophobic modification were as follows: N-hexane was used as solvent,the concentration of PFAS solution was 20 g/L,Na A zeolite membrane soaked at 25 ? for 24 h with the dring time for 24 h.T he contact angle on the surface of zeolite membrane making under the optimum conditions was 160.72° and this hydrophobic propert y could stable at 110 ? for 84 h.The hydrophobic modified layer on the zeolite membrane surface was ver y thin,which didn't change the surface morphology and structure of the zeolite membrane.However,the hydrophobic molecules with long chains could enter the defects,blocking the defect holes,thereb y improving the selectivity of the zeolit e membrane with a slight reducing the flux permeance.The mechanism research showed that the hydrophobicit y of the zeolite membrane was determined by both the low surface energy material PFAS and the surface morphology of zeolite membrane.The surface energy of the surface of the zeolite membrane was obviously reduced b y the low surface energy material PFAS,while the concave and convex structure and pore structure of the zeolite membrane make it difficult for water droplet to contact with the surface of th e zeolite membrane.The s ynergistic effect of both two factors made the surface of the zeolite membrane reach high hydrophobicity.4.Gas separation mechanism of Na A zeolite membraneThe gas separation mechanism of NaA zeolite membrane was systematicall y investigated.The results show ed that the pore diameter of the support in bussineess application was larger than 100 nm and the permeance of H 2 could be as high as above 10-6 mol·s-1·m-2·Pa-1 with the permeselectivity of H 2 over N2 being about 2,indicating that the gases through the support was controlled by molecular diffusion mechanism and the contribution of the support to the pressure drop was ver y small.The Na A zeolite membrane synthesized b y matintaining the pressure difference at 0.049 MPa had better integrit y with the selectivities of H 2 over other gases higher than the corresponding Knudsen diffusion coefficients.The ideal selectivities of H 2 over various other gases were determined as 6.13,5.21,6.49,6.70,11.34 and 13.67 for ?(H2/CO2),?(H2/N2),?(H2/CO),?(H2/CH4)and ?(H2/C3H8),respectively,while the real selectivies were lower than ideal selectivies,being determined as 4.82,3.94,4.31,4.48 and 7.26,respectively.The permeance of H 2 would increase when increased the test temperature or test pressure,while ideal selectivities of H 2 over N2 and CO2 decreased.Among them,test temperature had greater influence on the selectivity of H2 over CO2.The mechanism research showed that the gases through Na A zeolite membrane were mainl y controlled by molecular sieving mechanism,but Knudse n diffusion mechanism also pla yed a role.Meanwhile,the ads orption-desorption of gases would also affect the permeability of the zeolite membrane for highl y adsorptive gases(CO2).