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Preparation And Separation Studies On ZSM-5 Zeolite Membrane

Posted on:2002-03-27Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y S LiFull Text:PDF
GTID:1101360032454589Subject:Chemical processes
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Zeolite membranes have attracted much attention in the last decade because of their unique characteristics, such as their thermal, chemical and structural properties compared to polymeric membranes, and their great potential applications in the separation and membrane reactors. Among different Zeolites, more focus has been drawn on the synthesis of MFI (Silicalite-1 or ZSM-5) membranes, because of the pore system of this zeolite (straight, circular pores (0.54×0.56nm) running perpendicular to sinusoidal, elliptical pores (0.51×0.54nm)), suitable for separation of several industrial important organic molecules and the relatively large amount of information available for its synthesis. However, there are still some challenges on the zeolite membrane preparation. They are as follows: Most membranes are prepared on flat supports with small membrane area and low permeances, which limits their application on a large scale.It is necessary for supports to have small and unique pores in the zeolite membrane preparation. While the pores of domestic supports are large, and the surface of them are coarse, which result in more difficulties in the preparation of high-quality zeolite membranes.It is difficult for researchers to control the synthesis of zeolite membranes due to the formation mechanism is very comprehensive.In view of these challenges in the field of zeolite membranes, the main works of this thesis are: (1) Effects of some important factors on the preparation of zeolite membranes are thoroughly studied. High-permeance ZSM-5 membranes were synthesized on a-Al2O3 support with pores of 3-5um. So far, the largest membranes in the world were successfully prepared on tubular supports. (2) Two-stage varying-temperature synthesis was firstly used in the preparation of ZSM-5 zeolite membranes. The membranes prepared by this procedure have high permeances and good selectivities. (3) The mechanism of formation of zeolite membranes was discussed. It was pointed out that repeated nucleation and re-growth of the already crystals surfaces was responsible for the formation of dense membrane.1.ZSM-5 zeolite membranes were synthesized on a-Al2O3 tubular supports with 3-5um pores using cheap NBA as structural directing agent. The influence of crystallinity and size of crystals in the membrane, and the seeds introduction methods were studied thoroughly. It was reported for the first time that high-permeance ZSM-5 membrane was directly prepared on porous support. The permeance of Hj is a magnitude order higher than that of the literature. The largest area zeolite membrane was prepared on a-Al2O3 tubular support with 460mm-length and 20mm-outer diameter. The permeance of H2 of it is up to 8.6×10-7 mol/m2-s-Pa, and the ideal selectivity of Hi over nC4H10 is 49. This made it easier in industrial application.2.Two-stage varying-temperature synthesis was introduced to the synthesis of zeolite membranes. The influence of varying-temperature on zeolite and zeolite membranes was studied. Thin and dense zeolite membrane was first achieved by varying-temperature synthesis. The membrane not only has higher permeances, but also could keep higher selectivity for nC4H10/iC4Hio over the temperature range of 273K and 473K.3.The formation course of zeolite membranes was discussed, and the formation mechanisms were developed. During the repeated synthesis under constant temperature, the already crystals are not grow continuously, but dissolve first and induce more nuclei formation. At last, all the nuclei grow together into integral zeolite membrane. For varying-temperature, the mechanism is different from that of constant temperature. Crystals on the support grow into large one by repeated nucleation and re-growth on the crystal surface, so that the final crystal lost their typical shape and the intercrystal area was eliminated, and the dense membrane was formed.4.The model of gas permeation through: zeolite membranes was developed. It covers surface diffusion, gas transtational diffusion and configurational diffusion T...
Keywords/Search Tags:Preparation
PDF Full Text Request
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