Selective Defect Reparation And Pore Size Distribution Of MFI Zeolite Membranes

Zeolite membranes have been widely appiled as separation membranes,catalytic membranes,and chenmical sensors due to their molecular level pore size,high hydrothermal and mechanical stabilities,and specific hydrophilicity/hydrophobicity.Silicalite-1 membranes with high hydrophobicity show potential application in separation of the low concentration ethanol and water mixture.Imperfect synthetic technology and leaking of Al account for the non-zeolitic defects,declining surface hydrophobicity,which decrease the separation performance.Therefore,the reparation of defects and the improvement of hydrophbicity are crucial for practical applications.Various post-treatment methods including multiple synthesis,surface coating,coke deposition,chemical vapor or liquid deposition have been investigated in order to reduce these defects.In this paper,we use chemical liquid deposition for selectively healing non-zeolitic pores,which is more effective and maneuverable than currently available blocking methods.MTMS with a kinetic diameter larger than MFI zeolitic pore size goes into non-zeolitic pores.The high hydrophobicity of silicalite-1 crystals is helpful to obtain low water permeation flux through the zeolite membrane.Especially,the amount of water molecules diffusing through zeolitic pores should be minial compared with that through defects.As a result,hydrolysis-condensation reaction could preferentially occur in the intercrystalline channels.To repair defects selectively,MTMS molecule has a hydrophobic methyl group.Therefore,using MTMS can eliminate defects and enhance the surface hydrophobicity.Silicalite-1 membranes with nonselective pores were succecssfully repaired and modified by counter-diffusion at the organic-aqueous interface.The hybrid material was only deposited at the pore-mouth of defects,SEM results showed the appearance of spherical particles.After modification,the pore size was mainly distributed between 0.4 and 0.8 nm.The amount of micropore and mesopore defects could be reduced to 11.67% and 0.17%,respectively.Furthermore,almost no defects larger than 3.08 nm could be detected by permporometry test.The permeation of larger molecules such as o-xylene and TIPB by pervaporation was in excellent agreement with permporometry test.FT-IR and contact angle results confirmed that the hydrophobicity of silicalite-1 membranes was drastically enhanced.The separation factor of 5 wt% ethanol/water mixture through the membrane could be increased to 17.25,while the total flux decreased to 0.67 kg.m^-2.h^-1,corresponding to a deduction of ca.32%.