MFI Membranes Research Articles

Separation of alcohols and alcohols/O 2 mixtures using zeolite MFI membranes

Two different types of supported silicalite membranes were employed for the separation of alcohols and alcohols/O 2 mixtures: in one of them the zeolite material was deposited on the top of the γ-alumina supports, while in the other the zeolitic material was mainly present in the porous structure of the α-alumina supports. While both kinds of membranes were able to separate the above mixtures, the second type of membranes having the zeolite material inside the support performed more efficiently. The maximum selectivity reported in this work is 7415 for the ethanol/O 2 separation in an ethanol/methanol/O 2 mixture. For a better understanding of the separation mechanism, the performance of both zeolite membranes was compared to that of a mesoporous silica membrane. Also, the adsorption of methanol and propanol on silicalite crystals was measured using a microbalance.

Synthesis of a zeolitic thin layer by a vapor-phase transport method: appearance of a preferential orientation of MFI zeolite

Zeolitic membranes were synthesized on a porous alumina support by the vapor-phase transport method using one of two alumina sources: aluminum sulfate and sodium aluminate. By choosing the appropriate period and temperature for drying the aluminosilicate gel on the alumina support, MOR, FER and MFI could be synthesized. MOR was formed when the water content in the dry gel was high. As the water content decreased, FER and MFI were formed sequentially. Preferential formation of MFI was achieved by treating the dry gel at 373 K. The temperature at which the support was dipped in the parent gel also influenced the type of zeolite formed, although the effect of drying conditions was more pronounced. A similar trend was found for both aluminum sulfate and sodium aluminate. When the alumina source was sodium aluminate and the aluminosilicate gel was treated at 373 K, a preferentially oriented MFI membrane could be synthesized, as the ( a00) and (0 b0) crystal faces were oriented in parallel to the surface of porous alumina support. The uniform and dense morphology of the dry gel seems to account for the appearance of the orientation.

Transport properties of alkanes through ceramic thin zeolite MFI membranes

Polycrystalline randomly oriented defect free zeolite layers on porous α-Al 2O 3 supports are prepared with a thickness of less than 5 μm by in situ crystallisation of silicalite-1. The flux of alkanes is a function of the sorption and intracrystalline diffusion. In mixtures of strongly and weakly adsorbing gases and a high loadings of the strongly adsorbing molecule in the zeolite poze, the flux of the weakly adsorbing molecule is suppressed by the sorption and the mobility of the strongly adsorbing molecule resulting in pore-blocking effects. The separation of these mixtures is mainly based on the sorption and completely different from the permselectivity. At low loadings of the strongly adsorbing molecules the separation is based on the sorption and the diffusion and is the same as the permselectivity. Separation factors for the isomers of butane ( n-butane/isobutane) and hexane (hexane/2,2-dimethylbutane) are respectively high (10) and very high (> 2000) at 200°C. These high separation factors are a strong evidence that the membrane shows selectivity by size-exclusion and that transport in pores larger than the zeolite MFI pores (possible defects, etc) can be neglected.

High-temperature stainless steel supported zeolite (MFI) membranes: Preparation, module construction, and permeation experiments

Continuous layers of MFI (silicalite-1; Si-rich ZSM-5) have been prepared on porous, sintered stainless steel supports. Similar metal supported MFI membranes of ∼ 50 μm thickness have been grown within stainless steel membrane modules in order to perform (high-temperature) permeation experiments. As-synthesized layers are found to be gas-tight even for small molecules such as neon. The supported MFI layers remain thermomechanically stable upon calcination at 400°C in air to remove template ions (tetrapropylammonium). Gas permeation experiments have been performed using neon, methane, n-butane, and isobutane according to the Wicke—Kallenbach principle with helium as a purge gas. The sequence of the pure gas permeabilities at room temperature and 0.3 bar partial pressure difference is methane > n-butane > neon å isobutane, demonstrating that the permeation is based on both adsorption and diffusion. The deviating permeation behaviour between the butane isomers is attributed to the bulkiness of isobutane, which is also reflected in the substantially lower adsorption capacity as compared with n-butane. In experiments using binary mixtures of strongly (butane isomers) and weakly (methane) adsorbing species, the permeation rate of the former is hardly affected, whereas for the latter a drop in permeability of some two orders of magnitude is observed. At higher temperatures (up to 350°C) with a constant feed composition, the methane permeation rate increases as a result of the decreased adsorption of n-butane. The MFI layer retains its separation potential after several heating and cooling cycles.