Aluminosilicate Synthesis

4 SYNTHESIS OF MICROPOROUS CRYSTALLINE MATERIALS 3.4.1 Aluminosilicate Synthesis 

The process is affected by several factors that can be modified during the synthesis procedure, that is, presence of cations in the reaction mixture, OH, concentration, Si02/Al203 ratio, H20, content, temperature, pH, time, aging, stirring of the reaction mixture, order of mixing, and other factors [52,53], 

The first hypothesis, proposed by Breck and Flanigen [52,55], to account for the crystallization of aluminosilicate zeolites affirms that it proceeds through the formation of the aluminosilicate gel or reaction mixture, and the nucleation and growth of zeolite crystals from the reaction mixture. This initial model has been almost abandoned, and replaced by the hypothesis of Barrer and others [53,55], In the framework of this hypothesis, it is assumed that the formation of zeolite crystals occurs in solution. Accordingly, in this model, the nucleation and growth of crystalline nuclei are a consequence of condensation reactions between soluble species, where the gel plays a limited role as a reservoir of matter. 

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Fig. 14. Mordenite crystals prepared from mixtures of composition 100 Si02 5.26Al203 22.74 Na20 1545 H2O at 175 °C. Pre-treatment of the silicic acid (silica source) in air for 20 h at a un-treated b 300 °C c 550 °C d 850 °C. Reprinted with permission from Zeolites, vol. 16, Warzywoda J, Dixon AG, Thompson RW, Sacco A, Suib L, The role of the dissolution of silicic acid powders in aluminosilicate synthesis mixtures in the crystallization of large mordenite crystals, (1996),pp. 125-37, Elsevier Science Inc.

Guth J-L and Kessler H 1999 Synthesis of aluminosilicate zeolites and related silica-based materials Catalysis and Zeolites, Fundamentals and Applications ed J Weitkamp and L Puppe (Berlin Springer) pp 1-52... 

J. A. Martens, Synthesis of High-Silica Aluminosilicate Zeolites, Elsevier, Amsterdam, 1987, 390 pp. M. L. Occelli and H. E. Robson (eds.). Zeolite Synthesis, ACS Symposium Series No. 398, 1989, 664 pp. J. Klinowski and P. J. Barrie (eds.) Recent Advances in Zeolite Science, Elsevier, Amsterdam, 1990, 310 pp. G. V. Tsitsishvili, T. G. Andronikashvili, G. M. Kirov and L. D. Filizova, Natural Zeolites, Ellis Horwood, Chichester, 1990, 274 pp. 

Volume 33 Synthesis of High-silica Aluminosilicate Zeolites edited by PA Jacobs and JAMartens... 

Synthesis of multiwall carbon nanotubes by using mesoporous aluminosilicates... 

The name of zeolites, which originates from the Greek words zeo (to boil) and lithos (stone), was given some 250 years ago to a family of minerals (hydrated aluminosilicates) that exhibited intumescence when heated in a flame. However, the history of zeolites really began 60 years ago with the development of synthesis methods. Commercial applications in three main fields—ion exchange, adsorption, and catalysis—were rapidly developed, the corresponding processes being more environmentally friendly than their predecessors. 

The method developed by Milton in the late 1940s, involves the hydrothermal crystallization of reactive alkali metal aluminosilicate gels at high pH and low temperatures and pressures, typically 100°C and ambient pressure. Milton, Breck and coworkers synthesis work led to over 20 zeolitic materials with low to intermediate Si/Al ratios (1-5) [86]. Chapter 3 and references [1] and [25] provide more detailed discussion of synthesis. 

Jacobs, P.A. and Martens, G.A. (1987) Synthesis in High-Silica Aluminosilicate Zeolites, Elsevier, Amsterdam, p. 321. 

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... 

Flanigen monitored the changes in the IR spectra that occur during the synthesis of NaX zeolite from a sodium aluminosilicate gel. The appearance of absorption bands due to the formation of structural units in the zeoUte as the crystallization of NaX proceeded were observed [93]. In particular, the growth of a band around 575 cm indicated the formation of double six-rings which is one of the structural sub-units of X zeolite. 

Synthesis and structural characterization of the aluminosilicate LZ-135, a zeolite related to ZSM-10. /. Phys. 

The discrepancy in numbers between natural and synthetic varieties is an expression of the usefulness of zeolitic materials in industry, a reflection of their unique physicochemical properties. The crystal chemistry of these aluminosilicates provides selective absorbtion and exchange of a remarkably wide range of molecules. Some zeolites have been called molecular sieves. This property is exploited in the purification and separation of various chemicals, such as in obtaining gasoline from crude petroleum, pollution control, or radioactive waste disposal (Mumpton, 1978). The synthesis of zeolites with a particular crystal structure, and thus specific absorbtion characteristics, has become very competitive (Fox, 1985). Small, often barely detectable, changes in composition and structure are now covered by patents. A brief review of the crystal chemistry of this mineral group illustrates their potential and introduces those that occur as fibers. 

P.A. Jacobs and J.A. Martens, Synthesis of High-Silica Aluminosilicate Zeolites (Stud. Surf. Sci. Catal., 33), Elsevier, Amsterdam, 1987. 

In zeolite synthesis, large cations such as tetramethylammonium (NMe4" ) and tetrapropylammonium (N(C3H7)4" ) can be used as a template around which the aluminosilicate framework crystallizes with large cavities to accommodate the ion. On subsequent heating the cation is pyrolysed, but the structure retains the cavities. Such structures formed around a single molecule template, with pore sizes between 200 and 2000 pm, are known as microporous. 

The formation of novel silicon-rich synthetic zeolites has been facilitated by the use of templates, such as large quaternary ammonium cations instead of Na+. For instance, the tetramethylammonium cation, [(CH3)4N], is used in the synthesis of ZK-4. The aluminosilicate framework condenses around this large cation, which can subsequently be removed by chemical or thermal decomposition. ZSM-5 is produced in a similar way using the tetra-.n-propyl ammonium ion. Only a limited number of large cations can fit into the zeolite framework, and this severely reduces the number of [AIO4] tetrahedra that can be present, producing a silicon-rich structure. 

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