Modification And Application Of Silicalite-1Membranes On Porous α-Al₂O₃Substrates

Due to uniform pore networks, superior stabilities under harshly chemical andhigh-temperature environments and high hydrophobicity, silicalite-1membrane had agood prospect for recovery of low-concentration aqueous ethanol solution. Hydroxylgroups on the membrane surface, defects and Al leaching all led to the decrease of themembrane surface hydrophobicity, which affected ethanol separation performance. Inthis article, the silicalite-1membrane was prepared on the porous α-Al2O3substrateby the secondary growth method. Treatment with stearic acid and DMS enhanced thehydrophobicity of the membrane surface. The silicalite-1membranes were applied inethanol/water mixture with low ethanol concentration.Silicalite-1crystals with a size of400nm were chosen as the seeds to prepare theseed layer by spin-coating. An h0h oriented,6μm thick silicalite-1membrane wasprepared after the second-time seeded growth for16h in the low water contentsynthesis solution. After the removal of templates at450oC, FT-IR and pore sizedistribution results showed that high temperature calcinations could remove templatesfrom the pores. The pore size distribution experiment showed that a number of defectswould be formed during the templates removal step.The outer surfaces of silicalite-1membranes were modified with stearic acid andDMS to enhance the hydrophobicity. FT-IR measurements revealed that modifyingagents were presented firmly at the membrane surface via chemical bonds. Theoptimum mass fraction of DMS was10%, and the optimum pretreatment temperaturewas150oC. Moreover, the relationship between the wetting property and thermalstability was also investigated, the results showed DMS possessed goodhydrophobicity up to250oC and the stearic acid up to200oC.For the silicalite-1membrane that had a small amount of smaller than5nmdefect pores, the modification not only enhanced hydrophobicity, but also repairedpart of defect pores to improve separation performance of ethanol remarkablely. Theconcentration of ethanol in permeates increased from17.7%to53.2%, and ethanolseparation factor was up to21.6which was four times more than that beforemodification. But once the defect size was larger than5nm or the proportion ofdefects was very big, the strongly hydrophobic membrane surface did not enhance ethanol separation factor. Hydrophobicity and defect both affected ethanol separationperformance. In addition, due to the determination of optimum modificationconditions and strict control of the water content during the process of modification,the problem of the serious reduction of flux after the modification was solved in theliterature.