Preparation of zeolites

Kouwenhoven FI W and de Kroes B 1991 Preparation of zeolitic catalysts Stud. Surf. Sol. Catal. 58 497-529... 

Preparation of Pillared Clay Catalysts. PAG products are used for the preparation of zeolite-like catalysts by intercalation, the insertion of Al polycations molecules between the alurninosiHcate sheets of clay (3,33). Aqueous clay suspensions are slowly added to vigorously stirred PAG solutions, and the reaction mixture is aged for several hours. The clay is separated from the PAG solution and washed free of chloride ion. The treated clay is first dried at low temperature and then calcined in air at 450—500°G, producing a high surface area material having a regular-sized pore opening of about 0.6 to... 

The zeolite nanocrystals have attracted the considerable attention of many researchers [1-5]. The syntheses of several types of zeolites with different nanometer sizes, such as silicalite-1, ZSM-5, A-type and Y-type, have been reported. Recently, micellar solutions or surfactant-containing solutions have been used for the preparation of zeolite nanoerystals [4,5], We have also successMIy prepared silicalite nanoerystals via hydrothermal synthesis using surfactants. In this study, we demonstrate a method for preparing mono-dispersed silicalite nanoerystals in a solution consisting of surfiictants, organic solvents and water. 

In the late 1940s zeolites were synthesized according to the procedure shown in Fig. 3.24. First an amorphous alumino-silicate gel is formed. This process is completely analogous to the production of alumina and silica gels described before. Subsequently this gel is crystallized into zeolite. The preparation of zeolites has drawn tremendous attention of the scientific and industrial community. A wide variety of zeolites have been synthesized, and reproducible synthesis procedures have been reported (often in the patent literature). Natural zeolites also exist massive deposits have been discovered in many places in the world. 

Pradhan, A.R., Macnaughtan, M.A., and Raftery, D. (2002) Preparation of zeolites supported on optical microfibers. Chem. Mater., 14, 3022-3027. 

Kittur, A.A., Kariduraganavar, M.Y., Kulkarni, S.S., and Aralaguppi, M.I. (2005) Preparation of zeolite-incorporated poly(dimethyl siloxane) membranes for the pervaporation separation of isopropyl alcohol/water mixmres. /. Appl. Poly. Sci., 96 (4), 1377-1387. 

A third possibility for the synthesis of nanomaterials in constrained volumes is the use of molds (Figure 3.1c). Advantages of this method include its simplicity, versatility, and precise control over the shape of the solid, even with intricate forms. An elegant example of this strategy is the preparation of zeolites which precisely replicate the complex microstructure of wood. To do this, Dong et al. [43] infiltrated a zeolite synthesis solution into a wood sample. After the necessary hydrothermal treatment, and subsequent calcination to remove the template as well as the wood, a zeolitic structure was obtained that reproduced with full detail and fidelity the wooden sample used as a mold. 

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. 

It is concluded from the characterization studies, that the extraction technique which we have employed for the preparation of zeolite-supported iron catalysts results in the formation of highly dispersed, small particle-sized Y-Fe2°3 on the support surface and, in addition, a small amount ( 1%) of iron present as a spe-... 

An alternative method of preparation of zeolite-carbon adsorbents is the treatment of mixtures clay mineral with hard coal and waste carbon deposits. The treatment consists of several physicochemical processes i.e. formation, carbonization, activation and crystallization, presented in this paper. The adsorbents prepared with this procedure are not a simple mixture of two components but strongly dispersed material resulting fi om thermochemical transformation, thus fecilitating the surface structure. 

From our earlier experiences, we have learned that the zeolite seed hydrothermal approach is rather difficult to reproduce. Consequently, a dvee-step-synthesis procedure involving the preparation of zeolite nanoprecursors (NPs) by a short hydrothermal step, the flocculation of these NPs using a sui ctant, and the steaming of the NPs/surfactant composite to produce the final material was developed. We have recently demonstrated that aggregates of less than 30 nm silicalite nanocrystals can be prepared from this procedure. We further discovered that the nature of the as-collected NPs was very much dependent on the stirring time of NPs/CTAMeBr flocculants. Under identical steaming condition, the 3 h-stirred NPs were converted into nanocrystals of silicalite-1, whereas. 

Recent research on the elementary steps during preparation of zeolite-supported metals has helped to understand the genesis of these particles in considerable detail. For the best studied systems the goal of preparing catalysts by design has been achieved (69-77). 

In the preparation of zeolite membranes, in situ crystallization in an autoclave is used to deposit a zeolite layer on a macroporous metallic membrane substrate. For example, Geus [96] reported the preparation of ZSM-5 zeolite on a macroporous metal substrate, which can be used in separations at high temperatures. 

In spite of all these hurdles, there are already industrial-scale applications of zeolite membranes for solvent dehydration [106] by pervaporation plants using tubular zeolite A membranes with 0.0275 m of permeation area each (see Section 10.2.3). Li et al. [280] have prepared large area (0.0260 m ) ZSM-5 membranes on tubular a-alumina supports. This work is also interesting from the industrial point of view because the authors used inexpensive n-butylamine as template. Indeed, the cost required for industrial modules, on a general basis, is still far from clear. However, it must be noted that most of the costs can be ascribed to the module, and only 10%-20% to the membrane itself [3]. This underlines again the importance of preparation of zeolite membranes on cheaper, alternative supports that can also pack more area per unit volume. 

Nevertheless, the availability of procedures allows the preparation of zeolite membranes and layers with sufficient quality, reproducibility, and reliability only up to a few hundred square centimeters in surface, delaying the industrial implementation of zeolite membrane-based technology. To be realistic, the lack of module reliability under extreme temperature cycling or harsh environment and the necessary raw material cost reductions (supports and chemicals) are two of the main challenges toward which strong efforts must be targeted. 

Erdem-Senatalar A, Tatlier M, and Urgen M. Preparation of zeolite coatings by direct heating of the substrates. Micropor Mesopor Mater 1999 32 331-343. 

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