Zeolites hydrogel process

Hydrogel drug carriers, 18 710 Hydrogel formation, PVA and, 25 603 Hydrogel processes, for preparing synthetic zeolites, 16 831-833... 

The manufacture of molecular sieve adsorbents has been reviewed by Breck > and more recently by Roberts. The steps in the process are shown schematically in Figure 1.12. A variety of different starting materials may be used. In the hydrogel process the reagents are added in soluble form as sodium silicate and sodium aluminate, whereas in the clay conversion process the alumina is added as a clay mineral, usually metakaolin. Formation of the desired zeolite depends on maintenance of the correct conditions of pH, temperature, and concentration. Seeding may be used to promote crystalliza-... 

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. 

Another name for solid hydrogel transformation mechanism is solid-phase mechanism, while solution-mediated transport mechanism is also called liquid-phase mechanism. The main difference in explaining the formation process of zeolites by these two mechanisms lies in whether the liquid component is involved during the crystallization of zeolites. The views of these two mechanisms are opposite to each other and have their own experimental supporting evidence. To date, the liquid-phase mechanism has more experimental support than does the solid-phase mechanism. 

In 1968, Breck and Flanigen for the first time proposed the solid-phase mechanism based on their studies of the crystallization of aluminosilicate. They found that the formation and transformation of amorphous aluminosilicate hydrogel always happened during the crystallization process of zeolites and that the composition of the hydrogel was similar to that of the resultant zeolites. 

M. Ogura, Y. Kawazu, H. Takahashi, and T. Okubo, Aluminosilicate Species in the Hydrogel Phase formed during the Aging Process for the Crystallization of FAU Zeolite. Chem. Mater., 2003, 15, 2661-2667. 

On the other hand, it was recognized that the presence of ammonium ions in hydrogels leading to high-silica zeolites, such as ZSM-5 plays an inhibiting role on the nucleation process [19],... 

For both systems the pH value plays an important role in the crystallization process. As a matter of fact, the pure MFI type zeolite formed when the pH value of the initial hydrogel ranged between 9 to 12.5. 

Attrition-resistant microspheroidal catalyst particles for fluidized beds are manufactured by means of spray drying. The catalyst particles, for instance zeolite crystals, are suspended in an aqueous sol or hydrogel of binder particles that, after processing, serve as a mechanically strong and porous matrix. Typical binders are silica-alumina and alumina gels, clays (e.g. kaolin, bentonite), and... 

The process may also be run semicontinuously. In this case, the hydrogel is prepared continuously and zeolite is crystallized batchwise in large reaction vessels [123]. 

The hydrothermal conversion of modified clay mineral, for example, metakaolin, is another manufacturing route to zeolites. Although the raw material costs may be as much as 15% lower than that of the hydrogel route, the major shortcoming of the process is that iron impurities in the raw material may cause an undesirable coloring of the zeolite product [15,18,99-101,104,108,116,122,123,125,126]. 

The aluminosilicate hydrogel-containing solution may then be pumped to a separate crystallization vessel of a size of up to 100 m and above. The crystallization of zeolite A is usually carried out batchwise at temperatures of 90-95°C. It is accomplished after 40-60 min. Discontinuous processes may use the same reactor for precipitation and crystallization [15,99,104,108,110,116, 119,122,125-128,139,140,142]. 

The application of a seeding process may be advantageous. Thus, a slurry of P-type zeolite may be recirculated to the hydrogel precipitation vessel via the stream of recycled filtrate and mother liquor. As a result, a higher product yield can be realized [169,172]. 

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