Pentasil zeolite crystallization

Crystallization of Pentasil Zeolite in the Absence of Organic Templates... 

The synthesis of pentasil zeolites is supported by organic cations. Argauer et al. (1) first described syntheses with tetraalkylammonium and tetraalkylphosphonium compounds. The organic cations may not only initiate and sustain a certain crystallization process but also may lead to products of a new structure. This "templating" behaviour is explained by the structure-directing effect of the organic cations in the process of crystallization (2,3). 

Figure 1. Yield of pentasil zeolites (percentage crystallization) as a function of time. Abbreviations are C, for mono-n-butylair ne, CU for mono-n-propylamine, C2 for mono-n-ethylamine, C. for mono-n-methylamine, di-C, for di-n-propylamine, di-C, for di-n-propylamine,+di-C2 for di-n-ethylamine, di-n-C.. for di-n-methylamine, TPA Al-free for the aluminium-free synthesis with the tetrapr pylammonium cation, TPA for tetrapropylammonium cation, TEA Al-free for the+aluminium-free synthesis with the tetraethylammonium cation, TEA for tetraethylammonium cation and tri-C for tri-n-propylamine.

Figure 5. Yield of pentasil zeolites as a function on the SiC /A O ratio for different crystallization times.

On the other hand, in.the case of the nonionic surfactants C-15, NP-15 and 0-15 (the nonionic surfactant/cyclohexane system), mono-dispersed silicalite nanocrystals were obtained as shown in Fig. 1(c), 1(d) and 1(e), respectively. The X-ray diffraction patterns of the samples showed peaks corresponding to pentasile-type zeolite. The average size of the silicalite nanocrystals was approximately 120 nm. These results indicated that the ionicity of the hydrophilic groups in the surfactant molecules played an important role in the formation and crystallization processes of the silicalite nanocrystals. 

The acidic/basic properties of zeolites can be changed by introdnction of B, In, Ga elements into the crystal framework. For example, a coincorporation of alnminnm and boron in the zeolite lattice has revealed weak acidity for boron-associated sites [246] in boron-snbstitnted ZSM5 and ZSMll zeolites. Ammonia adsorption microcalorimetry gave initial heats of adsorption of abont 65 kJ/mol for H-B-ZSMll and showed that B-substituted pentasils have only very weak acidity [247]. Calcination at 800°C increased the heats of NH3 adsorption to about 170 kJ/mol by creation of strong Lewis acid sites as it can be seen in Figure 13.13. The lack of strong Brpnsted acid sites in H-B-ZSMll was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation with methanol. 

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