Zeolite synthesis crystallite size

During the last decade large progresses have been performed in the so much difficult art of zeolites synthesis. As a consequence, the amounts of structural defects and chemical impurities have been reduced in zeolite samples (crystallites of larger sizes and well-defined morphology have been synthesized ). At the same time, the zeolite sorption capacities increase. Such an observation is well illustrated by the sorption... 

ZSM-5 (20 - 22) and ZSM-12 (19, 23). The reverse is known for the synthesis of low silica zeolites Due to an increased silica content in the synthesis gel, the viscosity of the mixture is increased, which results in a decreased crystallization rate. As a possible explanation of these observations it is assumed that incorporation of aluminium into the framework of ZSM-23 is a difficult process, like it is for ZSM-5 (20). This is underlined by the decrease of the crystallite size of ZS1F23 with increasing aluminium content of the gel For SiOg/A f =110, the crystallites are about 2 pm in length (cf. Figure 3) whereas it diminishes to less than 1 pm for SiOg/... 

The efficiency and selectivity of a supported metal catalyst is closely related to the dispersion and particle size of the metal component and to the nature of the interaction between the metal and the support. For a particular metal, catalytic activity may be varied by changing the metal dispersion and the support thus, the method of synthesis and any pre-treatment of the catalyst is important in the overall process of catalyst evaluation. Supported metal catalysts have traditionally been prepared by impregnation techniques that involve treatment of a support with an aqueous solution of a metal salt followed by calcination (4). In the Fe/ZSM-5 system, the decomposition of the iron nitrate during calcination produces a-Fe2(>3 of relatively large crystallite size (>100 X). This study was initiated in an attempt to produce highly-dispersed, thermally stable supported metal catalysts that are effective for synthesis gas conversion. The carbonyl Fe3(CO) was used as the source of iron the supports used were the acidic zeolites ZSM-5 and mordenite and the non-acidic, larger pore zeolite, 13X. 

The method of zeolite synthesis, the Si/Al ratio in the synthesis gel, and the crystallite size of Beta zeolite are factors to be considered for optimum alkylation performance [123,128,129[. For both USY and Beta zeolites, the amount and type of extraframework Al species (EFAL) remaining in the zeolite after calcination and/or post-synthesis treatments also affected the catalyst performance [130,131], For both zeolites, elimination of highly dispersed cationic-type EFAL formed under mild (hydro)thermal treatments decreased the alkylation... 

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]. 

Alkali ions (salts) influence the formation of the precursor gel for most of the synthetic zeolites (3,34,39,40). Na+ ions were shown to enhance in various ways the nucleation process (structure-directing role) (40-42), the subsequent precipitation and crystallization of the zeolite (salting-out effect) (JO and the final size and morphology of the crystallites (34,43). Informations on the various roles played by the inorganic (alkali) cations in synthesis of ZSM-5, such as reported in some recent publications (7,8,10,14,17,29,30,44,45) remain fragmentary, sometines contradictory and essentially qualitative. 

The choice among the variety of different types of zeolites and related materials in a practical situation will depend on the characteristics of the reacting system and the types of selectivity effects to be expected. The pore size, the deactivation behavior and the chemical and thermal stability of the zeolite material determine whether or not a particular catalyst is attractive. The necessary condition for shape-selectivity effects to occur is that the pore size has to meet the dimensions of the reacting molecules. The radius of the crystallites as well as the strength and the number of the acid sites may then be adapted to the actual requirements during synthesis. 

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