Zeolite formation, process

Liu s description of the kinetics of the zeolite formation process can be formulated in terms of the following equation. 

From the time of the mixing of the starting materials to the moment of zeolite application, seve processes may occur all of which may contribute to the formation of the final product. In the majority of zeolite formation processes, the synthesis involves the formation of a gel phase. Investigations of the precrystalline gel phase have shown that the chemical composition of this phase resembles the composition of the final zeolite structure (Zhdanov 1971). As such, zeolite X could, for instance, be directly synthesized by treatment of the solid, precrystalline phase of the initid reaction mixture (Breck 1974). Formation of the alumino-silica gel thus must play a crucial role in zeolite synthesis. 

Thermal dealumination. The method involves calcination of the ammonium (or hydrogen) form of the zeolite at relatively high temperatures (usually over 500°C) in the presence of steam. This results in the expulsion of tetrahedral aluminum from the framework into non-framework positions, but does not remove the aluminum from the zeolite. The process consists essentially in a high-temperature hydrolysis of Si-O-Al bonds and leads to the formation of neutral and cationic aluminum species (Figure 1A). 

Donahoe, R. J. Liou, J. G. 1985. An experimental study on the process of zeolite formation. Geochi-mica et Cosmochimica Acta, 49, 2349-2360. 

The reaction process was first described quantitatively by Kerr (S). During kinetic investigations on the formation of zeolite A he found that the rate of zeolite formation is always proportional to the amount of zeolite... 

Zeolite-Based Alkylation. Zeolites have the advantage of being noncot-rosive and environmentally benign. The Mobil-Badger vapor-phase ethylbenzene process was ihe lirsl zeolite-based process to achieve commercial success. It is based on a synthetic zeolite catalyst. ZSM-5. and has the desirable characteristics of high activity, low oligomerization, and low coke formation. See also Molecular Sieves. 

The mechanism of formation of zeolites is very complex, stemming from the diversity of chemical reactions, including various polymerization and depolymerization equilibria, nucleation and crystal growth processes. The physical and chemical nature of the reactants, which typically involve a source of aluminum and silicon along with hydroxides and salts determine the formation of zeolites. Physical effects such as aging, stirring, and temperature also play an important role. These effects lead to the complexity of zeolite formation, but are also responsible for the large number of frameworks that can be synthesized and the rich chemistry associated with this area. Cl. 21... 

There are similar opportunities for improved transalkylation catalysts and processes. Heavies make with the current zeolite-based transalkylation catalysts are already very low, however improved catalysts could lead to even lower byproduct makes and yield losses due to residue formation. The majority of the reaction byproducts in the zeolite-catalyzed processes are produced in the transalkylator. There are opportunities to develop more active transalkylation catalysts that allow operation at lower temperatures where byproduct formation can be minimized. 

Particularly attractive method for preparation of synthetic zeolite is recrystallization of natural aluminosilicates, such as kaolinite (halloysite), previously formed for elimination of plastic flow of highly thixotropic, pulverized zeolite. Some additional components of initial mixtures, such as texture modifiers (hard coal, lignite, cellulose, silica, aluminum oxide) are also introduced. They enrich the structure of zeolite adsorbent in transport pores and prevent an excessive compression of the clay material during the formation process. This results in an increase in product efficiency during the crystallization of zeolite phase. 

Synthetic zeolites are used in a variety of catalyst and adsorbent applications. Most zeolites are synthesized in batch processes, but U.S. 6,773,694 (to UOP) describes a continuous crystallization process for zeolite formation. The resulting crystals can be dried and formulated into catalysts, adsorbents, and other products. Estimate the costs of producing zeolite X and Mordenite by this method. 

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 a word, the formation process of zeolites is very complicated. The crystallization models and kinetics equations proposed during early studies are not very meticulous. 

The burial of surface levels through sedimentation processes and the consequent increase of the geothermal gradient are cause of zeolite formation. Generally, the most open (and hydrated) zeolites, e.g., clinoptilolite and hculanditc, prevail in the most recent layers, whereas more compact zeolites (e.g., analcime and laumontite) are found in the ancient and deeper ones. This type of occurrence is typical of several Circum-Pacific countries, such as Japan, New Zealand and the west coast of the United States [39]. 

Zeolite properties are being studied by nearly every type of modern scientific discipline, and they are being utilized in many new chemical engineering processes. Important advances include detailed basic information on cations in zeolites, more understanding of the mechanism of zeolite formation, the formation and character of structural defects and hydroxyl groups, the role of zeolite structure in adsorption and catalysis, and the increasing technology of the use of molecular sieve zeolites in catalysis and adsorption. 

Process of Zeolite Formation in the System Na20—A1203—Si02—H20... 

MFI zeolite upon alkaline treatment (see also Fig. 1) [6]. Following those results, an optimal framework Si/Al ratio of 25-50 for mesopore formation has been established. The fitting of the data in the range Si/Al ratio 50-200 was somehow arbitrary, due to lack of zeolites with a suitable Si/Al ratio. The increased mesopore surface area of 120 m g obtained upon desilication of FeS, coupled to a framework Si/Fe molar ratio of 77, however perfectly correlates with the previously proposed fitting, despite the different nature of the trivalent framework cation (solid circle in Fig. la). Additionally, the newly created mesoporosity centered around 20 nm also agrees well with the mesopore size vs. Si/Al ratio dependency, as shown in Fig. lb. These results provide supplementary convincing evidence of the crucial role of the trivalent metal cation in framework positions on the mesopore formation process, which appears to be independent on the nature of the trivalent metal cation. In addition, this confirms the universality of the pore formation mechanism as previously proposed for the alkaline treatment of MFI zeolites [18]. 

With the help of multinuclear solid state NMR experiments, the promoter (phosphate) induced acceleration of zeolite formation is proved unambiguously. It is proposed that the hydration spheres of the silicate units formed by the hydrolysis of the tetraethyl ortho silicate (TEOS) will be modified as part of the water molecules will be taken away by the promoter. This will speed up the process of the association of Qi, Q2, and Q3 units to form more Q4 units in the gel, leading to the formation of secondary building units at a faster rate. The promoter can also enhance the assembling of the SBU s in the similar way. 

Recently, selective synthesis of / -cymene from toluene and propane or isopropanol over zeolite catalysts has been thoroughly investigated. Zeolite-based processes avoid the disposal of spent catalyst, product contamination by the catalyst, separation of the catalyst from the product and corrosion of the reactor and tubes. The results using various zeolite types were promising. However, the formation of undesired -propyl toluene is observed particularly in the presence of MFI type zeolites, whereas large pore zeolites yield significant amounts of m-and o-cymene besides the desired / -cymene. However, low conversion and relatively low selectivity are the drawbacks of these investigations. 

---