Zeolite membranes hydrothermal preparation

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

Different ways have been proposed to prepare zeolite membranes. A layer of a zeolite structure can be synthesized on a porous alumina or Vycor glass support [27, 28]. Another way is to allow zeolite crystals to grow on a support and then to plug the intercrystalline pores with a dense matrix [29], However, these two ways often lead to defects which strongly decrease the performance of the resulting membrane. A different approach consists in the direct synthesis of a thin (but fragile) unsupported monolithic zeolite membrane [30]. Recent papers have reported on the preparation of zeolite composite membranes by hydrothermal synthesis of a zeolite structure in (or on) a porous substrate [31-34]. These membranes can act as molecular sieve separators (Fig. 2), suggesting that dcfcct-frcc materials can be prepared in this way. The control of the thickness of the separative layer seems to be the key for the future of zeolite membranes. 

Hardly any research has been performed on ferrierite in zeolite membrane configurations. Matsukada et al. [50,51] prepared a ferrierite-based membrane by the frequently used Vapour-phase Transport Method. By using ethylenediamine, triethylamine and steam (under hydrothermal conditions), a porous alumina support, covered with the proper aluminosilicate gel, was transformed into a alumina supported (30 pm thick) ferrierite layer. No permeation with 1,3,5-triisopropylbenzene coirld be observed, proving the layer to be defect-free. Fluxes of small gases were found in the order of 10" -10 mol.m. s. Pa and decreased in the order H2>He>CH4>N2>02>C02... 

Supported zeolite membranes have been prepared using numerous procedures [4] such as alignment of crystals in electrical fields, electroplating, self-assembly, growth on organic molecular layers, covalent linkages, hydrothermal synthesis (in situ and ex situ), hydrothermal method microwave heating assisted, dry gel method (vapor-phase transport method and steam-assisted crystallization), synthesis at the interface between two fluid phases, etc. 

A survey of recent literature on zeolite membrane preparation reveals that synthesis processes, even for well-known zeolite structures (i.e., MFl, LTA), are still carried out batchwise, using a hydrothermal route to produce a thin layer from hydrogels or sols containing the corresponding nutrients. As a general rule, the reactant mixture in contact with the support changes in composition with time provoking a reduction of the membrane quality. 

Basically, the hydrothermal synthesis procedures used to prepare zeolite membranes can be classified in two general groups ex situ and in situ methods, that is with and without a previous seeding step, which are briefly discussed below. 

Silicalite-1 membranes, supported on porous alumina ceramic discs, have been prepared by two different routes. In the first the zeolite membrane has been formed by in situ hydrothermal synthesis. Secondly a layer has been formed by controlled filtration of zeolite colloids. To optimise membrane stability, conditions have been established in which penetration of zeolite into the support sublayer occurs. The pore structure of these membranes has been characterised by a combination of SEM and Hg-porosimetry. The permeabilities of several gases have been measured together with gas mbeture separation behaviour. 

Thin zeolite membranes can also be prepared through a spin-coating process of a nanoparticle suspension. Yan and coworkers synthesized silicalite-1 and silicalite-2 in a nanoparticle suspension using a two-stage hydrothermal process.[138,139] First, the precursor... 

Polycrystalline zeolite membranes consist of inter-grown zeolite crystals with no apparent cracks or pinholes (Fig. lA). These films are composed of only zeolite (i.e., there are no non-zeolite components such as amorphous silica or polymer). They are normally supported on a substrate although free-standing films have also been synthesized. Membranes can be prepared on different substrates such as silicon wafer, quartz, porous alumina, carbon, glass, stainless steel (SS), gold, etc. Polycrystalline films are primarily prepared by hydrothermal synthesis methods including in situ crystallization, seeded growth,and vapor transport, " and have potential use in all of the applications discussed in this entry. 

Zeolite membranes, in particular, show potential for widespread pervaporation-type applications. They are, typically, prepared either by the hydrothermal route (direct or secondary growth, in which a layer of seed crystals are first applied on the support and during... 

Zeolite NaA and FAU crystals were grown under the same hydrothermal condition using the same composition of clear solution. The only difference was the type of seed crystal used. This result suggested that the crystal growth did not occur by the attachment of nano-crystals. Zeolite films/membranes were prepared by the growth of seeded crystals. Based on the SEM and TEM observations and single gas permeation measurements, densification model of film/membrane is presented. 

Silicalite colloidal solutions with approximate diameters of 100 nm were prepared using distilled water, tetrapro-pylammonium hydroxide (TPAOM), and sodium hydroxide. Cylindrical ot-alumina microfiltration membranes (average pore diameter 1 pm, outer diameter 10 mm, iimer diameter 8 mmj were used as substrates for the zeolite membranes. A hydrothermal synthesis was carried out in a solution of distilled water, tetrapropylammo-nium bromide (TPABr), and sodium hydroxide in a molar composition of TPABr/Si02/H20/Na0H = 0.1/1/80/ 0 1 [2,26] Pqj. ZSM-5 membranes, sodium... 

Highly hydrophobic zeolite membranes, such as silicalite-1 (Sano et al. 1994), Ge-ZSM-5 (Li et al. 2001a), and p-type (Tuan et al. 2003), have been used for separating organic compounds from water. Matsuda et al. (2001) developed silicate PV membranes with a high separation factor for an EtOH-water system. The silicate membranes were prepared on porous supports of sintered stainless steel (2 pm) by hydrothermal synthesis. 

Mordenite membranes were prepared by seeded hydrothermal synthesis onto commercial ceramic tubular supports by Casado et al. (2003) and nsed for the PV of alcohol-water mixtures. It was reported by them that selective adsorption of water on zeolite pores and small intercrystalline defects controlled the separation mechanism in the mordenite. 

Several types of zeolite membranes such as A-type, Y-type, silicalite, ZSM-5, etc. have been developed, and have been applied mainly to gas and pervapo-ration separations. Kumakiri et al. [43] prepared A-type zeolite membranes by hydrothermal synthesis with seed growth, and applied these to the reverse osmosis separation of water/ethanol mixtures. The zeolite A membrane showed a rejection of 40% and a permeate flux of 0.06 kg m h for 10 wt% ethanol at a pressure difference of 1.5 MPa, while a permeate flux of 0.8 kgm h and a separation factor of 80 were obtained in PV. 

Nowadays research efforts are mainly directed to the synthesis of microporous and dense inorganic membranes. The multilayer casting method based on sol-gel technology is not the sole approach for the preparation of these membranes. CVD and hydrothermal synthesis are currently used as well as the sol-gel process. CVI (chemical vapor infiltration) for support infiltrated membranes, PE-CVD (plasma enhanced chemical vapor deposition) for surface modification of existing membranes, growing of zeolite membrane layers, pyrolysis of polymeric preciusors have been described as alternative preparation methods... 

Conventional hydrothermal synthesis is the most common method for zeolite membrane preparation. In the in-situ crystallization method, a porous support is immersed in a zeolite synthesis solution. A membrane... 

The in situ membrane growth technique cannot be applied using the zeolite-based ceramic porous membrane as support, under hydrothermal conditions in a solution containing sodium hydroxide. The high pH conditions will cause membrane amorphization and lead to final dissolution. Therefore, we tried to synthesize an aluminophosphate zeolite such as AlP04-5 [105] over a zeolite porous ceramic membrane. For the synthesis of the AlP04-5-zeolite-based porous membrane composite, the in situ membrane growth technique [7,13,22] was chosen. Then, the support, that is, the zeolite-based porous ceramic membrane, was placed in contact with the synthesis mixture and, subsequently, subjected to a hydrothermal synthesis process [18]. The batch preparation was as follows [106] ... 

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