Synthesis Of Microstructure Controlled MFI-tvpe Zeolite Films And Their Related Application

Zeolite membranes/films have molecular recognition at sub-nanometer scale,high temperature resistance,high mechanical strength and strong chemical stability,which make them have unparalleled advantages in separation,membrane catalysis and anti-corrosion materials.The MFI-type zeolite has three-dimensional channel network with pore openings near the sizes of many important molecules,making it widely used in the field of separation by "molecular sieve".The separation performance of different oriented membranes are quite different due to their pore structures,(a-direction)sinusoidal channels of circular cross section are interconnected with(b-direction)straight channels of elliptical cross section;the hydrophobic or hydrophobicity property of the MFI-type zeolite membranes make them very attractive for the pervaporation(PV)separation of close-boiling liquid mixtures;the un-calcined pure silica MFI zeolite coating can be used in corrosion resistant,because it can block all gases,sustain strong acid(except HF)and so on.Therefore,the development of MFI-type zeolite membrane/film has been extensively studied.The relationship between the microstructure and properties of zeolite membrane/film is very important.For example,the random zeolite membrane is prone to produce non-selective defects,which reduces the"molecular sieve" selectivity of the membrane layer,while oriented MFI membranes can reduce the formation of non-selective defects during calcination due to similar crystal plane expansion coefficients.At present,the preparation of MFI membranes/films with controllable microstructure is a great challenge.For example,some preparation conditions are demanding,the cost of special templating agent synthesis is high;some synthetic solutions are corrosive,and the support requirements are relatively high.Therefore,some new methods are put forward by microstructural optimization and green preparation,which have important academic significance and practical application value.This thesis mainly focuses on the microstructure design and green preparation of MFI zeolite films/membranes,and the main content mainly includes the following four aspects:(1)A b-oriented MFI films/membrane was prepared by using the crystallization regulator.The crystallization regulator "ammonium salt" was added to the synthetic solution(TEOS-TPAOH-H2O)to effectively inhibit the formation of twins in secondary growth.The effects of the concentration and type of ammonium salts((NH4)2SO4 and NH4HCO3)on the morphology of b-oriented MFI zeolite films were compared and analyzed.The results showed that the twin crystal could be completely eliminated when n(NH4+)/n(TEOS)?0.15 in secondary growth,and the anionic species have little effect on the microstructure of MFI zeolite films.By prolonging the crystallization time in the secondary growth process,the thickness of the b-oriented MFI film could be effectively increased while maintaining good orientation.The mechanism study showed that the ammonium salts not only could effectively lower the pH of the synthetic solution,but also complex the silicate ions to reduce the effective concentration of the silicon source in the synthetic solution,avoiding the crystallization of the synthetic solution itself and promoting the epitaxial growth of the seed layer.Thereby a highly b-oriented MFI zeolite film was obtained.Subsequently,the optimized composition of synthetic solution was used to prepare an oriented MFI zeolite membrane on a porous alumina tube by a seed transfer technique,which is a potential path to effectively solve the limitation of the oriented membrane preparation related to the properties of the tube support surface.(2)A method for preparing oriented MFI-type zeolite films by using seeded secondary growth in the ultra-dilute synthesis solution was put forward.The n(TPAOH)/n(TEOS)ratio in the synthetic solution was maintained,and the water amount was used to control the growth path "epitaxial growth" of zeolite to prepare highly b-oriented MFI films.In the TEOS-TPAOH-H2O system,when n(H2O)/n(TEOS)?1500,only dissolved monomeric species(Si(OH)4,Si(OH)3O-)and oligomers from TEOS were in the solution,which contributed to b-oriented MFI film formation over a large temperature and time range.That was,no particles(nuclei and crystals)exist in the solution.Through the exploration of a large number of experiments,the relation map of the orientation with the composition was drawn which offers greater operating window of the synthesis composition.In order to ensure the effective hydrolysis of the silicon source in the synthetic solution and the appropriate crystallization time in the preparation of the b-oriented MFI zeolite film,we concluded that the OH-concentration of 5.6 mmol L-1 was an optimal value.In other words,the synthesis composition of 1TEOS:0.2TPAOH:2000H2O was a better choice.(3)A new method yet effective technique to prepare continuous and dense b-oriented MFI zeolite film on various supports,namely a solvent-free secondary growth method,was put forward.It used synthetic powder of ground anhydrous starting raw solids in the presence of NH4F.The raw solids was composed of silica gel,tetrapropylammonium bromide and ammonium fluoride without any treatment.The b-oriented MFI film was synthesized by the solvent-free secondary growth on various seeded substrates.No water was introduced in this system,which suggested a different crystallization mechanism "gas transfer" from conventional hydrothermal synthesis.The effects of several synthesis parameters such as synthetic powder composition,support type,support position and the volume of Teflon-lined stainless steel autoclave were investigated in detail.The composition of the synthetic powder with fluoride as mineralizer was broad,for instance,1SiO2:0.01-0.05TPABr:0.05-0.25NH4F,for successfully preparing highly b-oriented MFI-type zeolite films.The solvent-free route could effectively suppress the twin crystal formation to obtain the multilayer b-oriented MFI-type zeolite films.The solvent-free secondary growth method not only reduces the crystallization time in the synthetic system including F-and decreases the intercrystalline defect,but also makes the process more environmentally friendly,energy efficient and safe.(4)A novel,mild,and effective method to make zeolite coatings was put forward,namely,solid phase secondary growth.The dense continuous MFI zeolite anticorrosive coating was directly prepared by grinding the solid powder without the addition of steam or organic template vapor.The effects of the seed layer(silicalite-1,ZSM-5)and the composition of the synthetic powder on the surface microstructure of the zeolite coatings were discussed.The optimized composition was NaSiO3·9H2o:SiO2:TPABr:NH4Cl=1:1.19:0.58:1.82;the solid phase secondary growth has weak corrosion to the support;ZSM-5 could be used as seeds to prepare more dense zeolite coatings.The zeolite coating was characterized by polarization current test.It showed that the anti-corrosion performance of the MFI zeolite coatings prepared by the solid secondary growth method was comparable to that by the hydrothermal secondary growth method.