| Zeolite ZSM-5 membranes were prepared on porous {dollar}alpha{dollar}-{dollar}rm Alsb2Osb3{dollar} disks by in-situ crystallization using a clear solution of optimized composition 100(TPA){dollar}rmsb2O: 400Nasb2O:Alsb2Osb3:1200SiOsb2:114200Hsb2O.{dollar} During the synthesis, the disk was fixed horizontally at the air-liquid interface inside the autoclave and a continuous polycrystalline zeolite film of about 10 {dollar}mu{dollar}m thickness formed on the bottom surface of disk. Extensive experimentation was carried out to find the optimal composition. Pure gas permeation measurements of the most successful preparation yielded hydrogen:isobutane and n-butane:isobutane ratios of 151 and 18 at room temperature and 54 and 31 at 185{dollar}spcirc{dollar}C, respectively.; Electron probe microanalysis of the cross section of a membrane prepared on a bare alumina disk revealed a layer of siliceous material extending 80 {dollar}mu{dollar}m inside the pores of the support. It is believed that this internal layer adds resistance to permeation and degrades selectivity. To limit the excessive penetration of siliceous species into the support pores, a carbon-silica composite diffusion barrier was introduced into the pores of the support prior to zeolite crystallization and removed after the crystallization is complete. Membranes synthesized using barriers have n-butane flux and n-butane:isobutane selectivity {dollar}rm 2.7times10sp{lcub}-3{rcub} mol/msp2{dollar}-s and 45 at 185{dollar}spcirc{dollar}C which are, respectively, about 1.6 and 4 times as large as those of membranes prepared without using barriers.; The n-butane:isobutane selectivity of ZSM-5 membranes was substantially improved (e.g. 322 vs. 45 at 185{dollar}spcirc{dollar}C) by selectively eliminating micro-defects with a post- synthetic coking treatment which was accomplished by impregnating the membranes with liquid 1,3,5-triisopropylbenzene at room temperature and then calcining them in air at 500{dollar}spcirc{dollar}C for 2 hours. Calcination at 500{dollar}spcirc{dollar}C for up to 30 hours does not destroy the high n-butane:isobutane selectivity.; A model of surface-induced nucleation, crystal growth, and crystal adhesion was proposed for the aforementioned heterogeneous hydrothermal synthesis system. An inactive gel was identified. For zeolite membrane formation on a surface of certain area, the location and orientation of the surface as well as the amount of synthesis liquid accessible to the surface are critical for the quality of the zeolite membrane. |
