Study On Synthesis Of MFI-type Zeolite Crystals And Membranes By Solvent-free Method

MFI type zeolite has uniform pore structure,high specific surface area and good thermal stability,therefore,it has been widely studied in shape-selective catalysis,gas adsorption and gas separation.Pure silica MFI type zeolite has a strong hydrophobic property because there are no aluminum atoms in its framework.The conventional synthesis methods of MFI type zeolite include hydrothermal synthesis,microwave method,dry gel conversion method,etc.These methods require a large amount of water,producing a large amount of waste liquid and consuming high energy,which is not conducive to industrial production.Recently,a solvent-free synthesis method of zeolite has been proposed.The solid raw materials were ground and then crystallized at a high temperature to obtain zeolite crystals in this method.The solvent-free system containing ammonium fluoride does not require sodium silicate sodium nonahydrate,which has a great significance for the industrial production of zeolite.However,it is difficult to synthesize MFI type zeolite crystals just according to the molar composition provided in the literature,therefore,we propose that some unknown factors affect the crystallization.The pure silica MFI type zeolite membrane exhibits good performance in alcohol/water separation.For the first time,b-oriented MFI type zeolite films on dense and smooth support surfaces were prepared by solvent-free method in our group.This method is universal and is expected to be used in preparation of b-oriented MFI type zeolite membranes on porous supports.In this paper,the influencing factors in the preparation of MFI type zeolite crystals by solvent-free synthesis method in the presence of NH4F were investigated and the preparation of b-oriented MFI type zeolite membranes on porous supports by solvent-free synthesis method was challenged.The main research contents and results are as follows:(1)Control of crystallization rate and morphology of zeolite silicalite-1 in solvent-free synthesis.In this study,the effects of amounts of synthesis powder,water and sodium hydroxide on the crystallization rate and morphology of zeolite crystals are investigated.The samples are characterized by XRD,SEM and N2 adsorption/desorption isotherm measurement.It is found that the crystallization rate increases with the increase of the amount of synthetic powder and also increases with the increase of water content in the synthetic powder.When a little amount of water(H2O/SiO2=1.04)is added into the synthetic powder(SiO2:0.15NH4F:0.035TPABr),the crystallinity almost reaches 100%after crystallization at 175 C for 9 h.The addition of a little amount of sodium hydroxide(NaOH/SiO2=0.09)can decrease the crystal size and the addition of higher amount of NaOH(NaOH/SiO2=0.19)can promote the growth along a-axis,resulting in spherical silicalite-1 crystals(i.e.,twins).Moreover,the addition of NaOH may incorporate mesopores into silicalite-1 zeolite crystals.(2)Study on preparation of b-oriented MFI type zeolite membranes on porous supports by solvent-free method.In this study,a porous support is modified by nano-silica particles,and then a b-oriented seeds layer is prepared by a series of operations including coating polymer layer on the support,manual assembly of seeds and removal of the polymer(PVA)layer.The method of coating nano-silica layer and the effect of solvent in wet-rubbing is explored.A flat nano-silica layer without cracks is prepared by wet-rubbing using n-propanol or isopropanol as wetting agent.It is found that the thicker the PVA layer,the easier it is to produce a large area of high quality oriented seed layer.When the PVA aqueous solution concentration is 5 wt%and the spin coating speed is 1000 r/min,an oriented seed layer with a large area can be prepared on the porous silica support modified with nano-silica particles using n-propanol as wetting agent.A b-axis oriented MFI zeolite membrane on a porous support is prepared by solvent-free method for the first time,and the effects of different synthesis conditions on the property of membrane are studied.The synthetic powder is prepared by mixing 1SiO2:xNH4F:0.035TPABr,wherein x was 0.05-0.5,and a b-axis oriented MFI zeolite membrane on a silica disk support is synthesized at 175? for 12 h.When x=0.15 and 0.25,the membrane layer is relatively dense and the orientation is good.The membrane layer prepared by twice growth at x=0.15 still maintains a good b-orientation.The b-oriented membranes on the silica disks are not able to obtain the separation property,mainly because the defects on the surface of the supports were not completely eliminated.