Preparation And Properties Of Separation And Catalysis Of Beta Zeolite Membranes

Owing to their unique physical and chemical characteristic and similar molecular-sized channel systems, zeolite membranes can not only work at high-temperature, chemical and biological corrosive environments but also have potential applications for separation of different substances at molecular levels and for integration of catalysis and separation processes. In the last decades, zeolite membrane materials drew the growing interests as the cutting age research field in the membrane community. Beta zeolite is a high-silica zeolite possessing a three-dimensional interconnected channel system of 12-O ring large pore and has good heat stability, adsorption capability and proper acidity, so if the beta zeolite membranes can be prepared they will have potential application in separations, even in separation and catalytic membrane reactors due to its unique pore structure and catalytic properties. The research on beta zeolite membrane is very scare, thus it is significant for the beta zeolite membrane to make many efforts on reseach of its preparation and then model application.The key for the research is how to prepare the beta zeolite membranes with better performance under the conditions of lower cost. The membrane preparation was firstly investigated using conventional template method and the membrane preparation condition was optimized. This includes the synthesis condition optimization of small crystal-size beta zeolite as the seeds, the work on how to improve the quality of seeded layer on support tubes with 3～5μm pore diameter and the optimization of preparation conditions of beta zeolite membranes. On the other hand, a novel method of beta zeolite structure-directing agent (ZSDA) was developed for the preparation of beta zeolite membrane. First, the synthesis of beta ZSDA was investigated. Then, the optimal preparation conditions of beta zeolite prepared by ZSDA method were investigated from the following aspects respectively: content of water, crystallization time and crystallization temperature. By this ZSDA method, organic templates were substituted with beta ZSDA, as a result, a lot of expensive organic templates were saved compared with the conventional method. Then the membranes prepared by both template method and ZSDA method were applied for the pervaporation separation of MeOH/MTBE mixture. The effects of permeation side pressure, feed composition, feed flow rate and feed temperature on the flux and separation factor were investigated, At last to exploit the application of beta zeolite membranes the beta-NaA bi-founctional zeolite membranes were prepared on theα-Al₂O₃ support tubes and applied as membrane reactor for the esterification of acetic acid with ethanol. The main results achieved are as follows:(1) For the synthesis of small crystal beta zeolite, the results indicated that when crystallization temperature and crystallization time were 150℃and 72h beta zeoltes with small crystal-size in the 200～300 nm range could be prepared according to the molar composition of SiO₂: 0.02Al₂O₃: 0.45 (TEA)₂O: 0.061Na_aO: 0.8 (NH₄)20: 20H₂O. Silicate gel was used as silicon source and TEAOH-TEABr was used as template.(2) As for the optimal preparation conditions of beta zeolite membranes prepared by conventional template method, it is found that the optimal molar composition of precursor solution is of SiO₂: Al₂O₃: Na₂O: (TEA)₂O: H₂O=1: 0.02: 0.06: 0.35: 35. When the crystallization temperature was 150^C and crystallization time was 72h a continuous and dense membrane with 10μm in thickness, was obtained after 6 synthesis times.Templates were removed by the mode of calcinations in N2 and air orderly. At last beta zeolite membranes were grown on the porous carbon support tubes and porous steel support tubes respectively according to the optimal membranes preparation conditions above obtained. By comparing among the three supports used, it can be known thatα-Al₂O₃ support tubes were the most proper.(3) It is found that beta ZSDA can be synthesized by using the silica gel as the silicon source that is cheap and easy operational. Moreover, the feed order and content of water had important effects on the synthesis of ZSDA. And the optimal preparation conditions were confirmed as follows: Silicon-aluminum sol was prepared by the molar composition of 1SiO₂:0.033Al₂O₃:0.317Na₂O:28H₂O; Precursor solution was confected by adding the ZSDA into the sol and the content of ZSDA added was 1/8 of the sol in .volume; Crystallization temperature was 150℃and crystallization time was 60h; The organic agent was removed by the mode of calcinations in N₂ and air orderly; A continuous and dense membrane with 9μm in thickness was obtained after 6 synthesis times. Compared with the conventional template method it can be concluded that by ZSDA method not only 90% of costly organic agent was saved but also the quality of membrane was improved due to the less defect was formed when the less organic agent needed to be removed. This ZSDA method can be extended to zeolite membrane preparation of other types in which organic templates are necessary.(4) The membranes prepared by ZSDA method had better performance in pervaporation separation of MeOH/MTBE mixture. The maximum separation factor reached 9600.(5) To compare with bi-founctional membranes, single catalytic beta zeolite membranes grown on theα-Al₂O₃ support tubes were also applied in the esterification of acetic acid with. ethanol under the same reaction conditions with that of bi-founcitonal membranes. It can be known from the comparison that conversion rate corresponding to bi-founctional membranes was much higher than that of the single catalytic beta zeolite membranes. This is because the bi-founctionalmembranes had the high performace of removing water, one of the reaction product, so the reaction equribrium was broken and conversion rate was increased.