Typical Zeolite Syntheses

A number of reagents containing oxide components are used in zeolite manufacture [19]. Silica is provided by addihon of sodium or other alkali silicate solutions, precipitated, colloidal, or fumed silica, or tetraalkylorthosihcate (alkyl = methyl, ethyl) and certain mineral silicates such as clays and kaolin. Alumina is provided as sodium aluminate, aluminum sulfate soluhon, hydrous aluminum oxides such as pseudo boehmite, aluminum nitrate, or aluminum alkoxides. Additional alkali is added as hydroxide or as halide salts, while organic amines and/or 

The heats of reaction/solution of some reagents as hydrolysis/dissolution takes place can cause substantial elevation in slurry/solution temperatures, particularly at a large scale where heat transfer and radiative cooUng are not nearly as efficient as it is in small laboratory vessels. Other reagents, such as certain sodium alumi-nates and particularly reagents that are not freshly prepared, may need elevated temperatures for full dissolution in water. These hot or very warm solutions can adversely affect early nucleation conditions in some zeoHte syntheses. Hot reagent solutions and mixtures are sometimes cooled prior to their addition to other reagents to better control the early reactions and speciation of aluminosilicate and silicate precursors. 

In a typical aluminosilicate zeolite synthesis, sodium aluminate is dissolved in water along with some fraction of the additional sodium hydroxide that is needed in the reactant mixture. Separately, sodium siHcate is mixed with the remainder of the sodium hydroxide. The two solutions are combined using the required mix order and agitation level, resulting in the initial zeolite gel. In some cases this initial gel is aged at an intermediate temperature for a time to allow evolution of 

A number of zeolite syntheses result in incomplete conversion of all of the gel components to solid zeolite. In many cases the reactant Si/Al ratio is different than that of the product (usually higher), resulting in a silicate solution remaining behind. At the industrial scale this silicate solution is often recycled to minimize waste and raw material cost [34—36]. 

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A typical zeolite synthesis involves mixing together silicate and aluminate solutions or sols to form an aluminosilicate gel, usually instantaneously, which is then treated hydrothermally to give the crystalline product. The composition and structure of the aluminosilicate gel are of considerable interest and characterization of the aluminosilicate species present would give insight into the crystallization process. 

In a typical zeolite synthesis, the first definite evidence for a successful reaction is the appearance of crystals of the product. As noted above (section 6.1), this signal for the end of the induction period is dependent upon the method of detection most commonly a combination of visual inspection or microscopy with X-ray diffraction. Thereafter, crystal growth can be monitored by the same techniques and the resulting S-shaped growth curve of bulk crystallinity against time is by far the most commonly reported measurement of zeolite crystallisation kinetics (fig- 2). 

Typical zeolite synthesis conditions are strongly alkaline, enabling silica and alumina to be dissolved at concentrations that permit a sufficient supply of species to nuclei and growing crystals. At these high pHs the alumina exists as tetrahedral aluminate Al(OH)4 species, whereas silica exists as a variety of oligomeric species in solution (Section 5.3.3). The hydroxide ion is also acting as a mineraliser under these conditions, i.e. it increases solubility and catalyses the subsequent formation of Si-O-Si bonds. Fluoride ions can also perform this function, even at neutral pH, and this is described further in Section 5.4.3. 

Blasco et al. (12,13) developed a novel method for the synthesis of Al-free Ti-beta zeolite in a fluoride medium. The Ti-beta zeolite thus obtained (Ti-beta(F)) was free of connectivity defects and was hydrophobic. The typical unseeded synthesis of Al-free Ti-beta zeolite (Ti-beta(F)) involves hydrolysis of TEOS in aqueous solutions of TEAOH (35%) and H202, followed by hydrolysis of TEOT and evaporation of ethanol and water. The water lost in the evaporation and... 

Of course, to call the above procedure a typical synthesis is an oversimplification, since many approaches and variations to zeolite synthesis have been developed over the years, including at the manufacturing scale. 

The hydrothermal method has been employed in recent years to synthesize a variety of solids that include aluminium phosphates (ALPOs) and other microporous transition-metal phosphates and transition-metal polychalcogenides (Davis Lobo, 1992 Haushalter Mundi, 1992 Liao Kanatzidis, 1990, 1992). Unlike zeolites, synthesis of ALPOs requires acidic or mildly basic conditions and no alkali metal cations. A typical synthetic mixture for making ALPO consists of alumina, H3PO4, water and an organic material such as a quaternary ammonium salt or an amine. The hydrothermal reaction occurs around 373-573 K. The use of fluoride ions, instead of hydroxide ions as mineralizer, allows synthesis of novel microporous materials under acidic conditions (Estermann et al, 1991 Ferey et ai, 1994). 

Dent Glasser and colleagues found that a correlation exists between the gelation rate and the synthesis of zeolites.[41] They found that a low gelation rate would lead to a higher framework density for the synthesized zeolites. The framework densities for some typical zeolites are summarized in Table 5.9. 

Molecular design and rational synthesis of inorganic microporous crystalline materials are frontier subjects in the fields of zeolites science and molecular engineering. Zeolite synthesis is an active field of research because zeolites with uniform micropores are important in many industrial processes in catalysis, adsorption, and separation, and are finding new applications in electronics, magnetism, chemical sensors, and medicine, etc.12 91 Synthesis of such materials typically involves crystallization from a gel medium under hydrothermal/solvothermal conditions in the presence of organic amines as... 

Zeolites are a subclass of microporous materials in which the crystalline inorganic framework is composed of four-coordinated species interconnected by two-coordinated species. Traditionally these materials are aluminosilicates however, many different compositions have been synthesized. The templates used in the synthesis of microporous materials are typically small ionic or neutral molecular species. The function of the template in the synthesis of microporous materials is little understood, and there are at least four different modes by which an additive can operate in a zeolite synthesis a) It may act as a space filler occupying the voids in the structure, thereby energetically stabilizing less dense inorganic framework b) the additive may control the equilibria in the synthesis mixture, such as solution pH or complexation equilibria c) it may preorganize the solution species to favor the nucleation of a specific structure d) it may act as a true template determining the size and the shape of the voids in the structure. 

A typical exotemplating synthesis procedure consists of an acid catalyzed polymerization of a carbon precursor (e.g., sucrose [10] or furfuryl alcohol [11]) adsorbed in the pore system of the matrix followed by carbonization of the polymerization product at elevated temperatures. In the next synthesis step, the OMC is liberated by removal of the matrix. In the case of zeolite or silica matrices, this can be achieved by treatment with hydrofluoric acid or sodium hydroxide. Finally, the OMC may be modified in a postsynthesis heat-treatment... 

Just like IR spectroscopy, Raman can detect small. X-ray amorphous zeolite particles. Therefore RWan has been used to examine both the liquid and the solid phase of zeolite synthesis mixtures [28], Ex situ methods (with separation of solid and liquid) and in situ methods have been applied. In studying the liquid phase [10-11], one should remember that (i) minimum concentrations for detection of spontaneous Raman from liquids are typic ly 0.05 - 0.1 M, [15-27, 29] (ii) that the cross-section of the Al(OH)4 species is much stronger than e.g. for silicate or aluminosilicate anions [30]. Thus species which are present in low concentration or with variable structures may easily be overlooked in Raman spectra of the synthesis liquors. 

Precursor solutions for the phosphates are much easier to handle, since the pH is not extreme, all components are liquid, and typically no two phase systems which are difficult to mix are formed, as could happen with alkoxysilane precursors. The additional problems encountered when doing zeolite synthesis on miniature scale have been discussed by Newsam et al. [4], and include ... 

Several of these problems can also occur during zeolite synthesis on a larger scale. However, due to the much larger volumes involved, their effect is typically negligible, which is not the case for syntheses carried out on the pi scale. 

Even more strongly related with conventional zeolite synthesis are systems consisting of several individual autoclaves. Also in conventional zeolite synthesis, researchers typically... 

A typical hydrothermal zeolite synthesis can be summarized as follows Amorphous precursors containing silica and alumina are mixed together with a cation source, usually in alkaline media. This aqueous reaction mixture is then... 

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