Preparation,pervaporation Performances And Growth Process Of High Aluminum ZSM-5 Zeolite Membrane

Zeolite membrane has well-developed pore structure,good molecular sieving effect and hydrothermal stability,which is suitable for the separation of organic/water mix system by pervaporation.High aluminum ZSM-5 zeolite membrane has unique advantages in the dehydration application of acidic organic systems for the excellent hydrophilicity and acid stability.However,because of the high crystallization activation energy and crystal repulsion of ZSM-5 crystals in aluminum-rich environment,preparation of high-quality aluminum-rich ZSM-5 zeolite membrane remains a challenge.In addition,growth process and framework cations of the zeolite membrane have an important influence on the growth and separation performance of the membrane.Therefore,this work mainly studies the following two aspects:（1）Growth process of high-aluminum ZSM-5 zeolite membrane.High aluminum ZSM-5 zeolite membranes（Si/Al≈9）are successfully prepared on tubular mullite support by secondary hydrothermal synthesis,the nanometer silicalite-1 zeolite（ca.200nm）is seeded crystals.The formation process of high aluminum ZSM-5 zeolite membranes is characterized by XRD,SEM,FTIR,and a new zeolite membrane growth mechanism in fluoride-containing and organic template-free initial synthesis gel is proposed.The spherical silicalite-1 seeded crystals are dissolved into amorphous by the fluoride-containing precursor synthesis gel,while large number of basic structural units（TO₄）and isolated five-membered ring structure of ZSM-5 zeolites are retained in the amorphous phase,which provide the nucleation sites for high aluminum ZSM-5zeolites.Thereafter,needle high alumina ZSM-5 crystals grow up gradually and form a dense film on the porous support.The as-synthesized high-aluminum ZSM-5 zeolite membrane shows good pervaporation performance in separating high-concentration acetic acid water mixture（75℃,90 wt.%HAc/H₂O）.The permeation flux and separation factor are 0.98 kg·m^-2·h^-1 and 3200,respectively.（2）Influences of alkali metal cation on high aluminum ZSM-5 zeolite membrane.In this work,the effects of alkali metal fluorides（Li F,Na F,KF,Cs F）on the growth and pervaporation performance of high-aluminum ZSM-5 zeolite membranes are investigated by the template-free and fluoride-containing initial synthesis gel.Cations play an important role in the synthesis,morphology,Si/Al ratio and membrane layer density of ZSM-5 zeolite membrane,which are realized by the synergistic reaction of the"bridging"and“salting-out”effects.When the Si/Al ratio of synthetic gel is less than 10,only the Na F or KF containing synthesis gel could prepare a dense and needle aggregate ZSM-5 zeolite layer on the surface of the porous support.When the gel Si/Al ratio is greater than 12,a membrane layer composed of prismatic crystals with Li F,Na F,KF containing synthesis gel.When the Si/Al ratio and fluoride of the initial gel are 9.09 and Na F,the high-aluminum ZSM-5 zeolite membrane has the best pervaporation performance for separating high-concentration acetic acid-water mixtures.