Modification of zeolites

Kuhl G H 1999 Modification of zeolites Catalysis and Zeolites, Fundamentals and Applications ed J Weitkamp and L Puppe (Berlin Springer) pp 81-197... 

The most successful application of microwave energy in the preparation of heterogeneous solid catalysts has been the microwave synthesis and modification of zeolites [21, 22], For example, cracking catalysts in the form of uniformly sized Y zeolite crystallites were prepared by microwave irradiation in 10 min, whereas 10-50 h were required by conventional heating techniques. Similarly, ZSM-5 was synthesized in 30 min by use of this technique. The rapid internal heating induced by microwaves not only led to a shorter synthesis time, and high crystallinity, but also enhanced substitution and ion exchange [22]. 

Post-Synthesis Treatment of Zeolites and Modification of Zeolites.105... 

In recent years, modification of zeolites, such as HZSM-5, by phosphoric compounds or metal oxides has been extensively studied, but little information is available on the modification of zeolites by diazomethane, which is an excellent methylating agent for protonic acidic sites. It is capable of entering into the small pores of zeolites because of its small molecular size and linear molecular structure. Yin and Peng (1,2) reported that the acidity and specific surface area of the inorganic oxide supports (AljOs, SiOj) and zeolite catalysts... 

Based on the above results, we conclude that the diazomethane modification of zeolites is an effective method to change selectively the amount and strength of the surface Bronsted acidic sites. Therefore the method could be used to study the role of Bronsted and Lewis acidic sites preferably for low temperature 300 0 catalytic reactions. 

The above-mentioned route to the preparation of quarternary ammonium ions in acidic zeolites is remarkable for the following reasons an immobilization of quaternary ammonium ions in a well-controlled concentration is an approach to modification of zeolite catalysts. Furthermore, the synthesis of isotopically labeled compounds is of importance in organic, pharmaceutical, and agricultural chemistry. This method is an approach to the synthesis of C-labeled (or C, N-labeled) tertiary amines via a thermal decomposition (243,251) of the corresponding quaternary ammonium ions in zeolites. 

Cundy, C.S., Microwave techniques in the synthesis and modification of zeolite catalysts a review, Collect. Czech. Chem. Commun., 1998,63, 1699. 

Direct synthesis of metal-substituted zeolites has long been sought. However, since the post-synthesis modifications can be made under wide-ranging conditions (temperature, solvent, atmosphere, pH, etc.) far from those for the zeolite synthesis, the modifications of zeolites present us with powerful indirect methods for manipulating the properties of zeolites. Therefore, the fine-tuning of the properties of zeolites will continue to be achieved by developing various post-synthesis modification procedures as well as direct synthetic techniques. 

Modification of zeolites, based on chemisorption of silane or diborane and subsequent hydrolysis of the chemisorbed hydride groups can also be applied for encapsulating gas molecules in zeolites. For example, krypton and xenon can be encapsulated in mordenite combining the modification process with a physical adsorption of the noble gases at moderate pressures and temperatures (e.g. 100 kPa, 300 K). The encapsulates are homogeneous and stable towards acids, mechanical grinding and y-irradiation. By controlling the pore size reduction however, the thermal stability can be controlled. 

Modification of zeolites by means of monolayer dispersion of oxides or salts is different from that by ion exchange. By ion exchange, only cations are introduced into the zeolites, but in the case of monolayer dispersion anions disperse along with cations onto the internal surface. Monolayer dispersion of oxides and salts proceeds more readily than ion exchange. It can be done efficiently by both dry and wet methods. 

Modification of zeolite catalysts can be achieved by solid-state reactions between zeolite and other crystal-... 

Unique adsorption selectivities are employed in the separation of 0, aromatic isomers, a classical problem that cannot be easily solved by distillation, crystallization, or solvent extraction (10). Although xylene [106-42-3] can be separated by crystallization, its recovery is limited because of the formation of eutectic with / -xylene [108-58-3]. However, either jfr-xylene, -xylene, 0-xylene [95-47-6]y or ethylbenzene [100A1 -4] can be extracted selectively by suitable modification of Zeolitic adsorbents. 

Several procedures for modification of zeolites have been adopted to date and, among these, various analogs of ferrisilicates were synthesized by isomorphous substition of a few per cent of the central silicon ions with ferric ones in the elementary building blocks to form [Fe04/2] isomorphous substitution of... 

There is little information about the introduction of niobia into the zeolite lattice [5,6]. As niobia salts are very sensitive to moisture, the traditional methods for modification of zeolites, i.e., slurrying in solution, cannot be applied. The present studies were undertaken to obtain information about the possibility of introducing niobia into zeolites via solid state interaction [7]. 

Figure 1 shows the XRD pattern of niobium oxide which indicates that the Nb205 used for the modification of zeolites is the mixture of a and y forms [9]. 

We presented a facile route for the modification of zeolites and for the preparation of bifunctional catalysts possessing both acidic and hydrogenation functions via solid-solid reaction. Branched and higher hydrocarbons were obtained over such modified composite catalysts. Sodium migration from the surface of the iron-based catalyst to the zeolite during the solid-solid reaction accounts for the change of catalytic activity. XRD measurements exhibited evidence for Na migration. 

Other modifications of zeolites can be achieved with, reagents which react with OH groups. The silanation of various mordenites is the only process to be studied in detail. ZSM-5 treated with dimethyl silane has been described as a catalyst for the conversion of methanol to olefins. The use of various other reagents (P, B, and Sb compounds) with ZSM-5 and related zeolites has also been described. The effects observed in catalysis are broadly similar... 

The modification of zeolites mainly relies on secondary synthesis methods. The aim of modification is to reprocess the zeolites using suitable techniques to improve the properties and functions such as (1) acidity, (2) thermal and hydrothermal stability, (3) catalytic performance such as redox catalytic and coordination catalytic properties, etc., (4) channel structures, (5) surface properties and microporous frameworks and charge-balancing ions. Modification is also called secondary synthesis and can lead to new properties that cannot be achieved through direct synthesis. Let us consider the case of faujasite (FAU), the main component of the cracking catalyst, and its catalytic performance (represented by the catalytic activity K/K Std for n-hexane cracking). From Table 6.1 it is seen that the secondary synthesis affects the catalytic performance to a considerable degree. 

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