Pentasils, synthesis

One of the earliest direct bonuses of imaging zeolitic catalysts by HRTEM was the discovery (10) that the nominally phase-pure ZSM-5 (structure code MFI) contained sub-unit-cell coherent intergrowths of ZSM-11 (MEL). It soon became apparent (46) that, depending on the mode of synthesis of these and other pentasil (zeolitic) catalysts, some specimens of ZSM-5 contained recurrent (regular) intergrowths of ZSM-11. It also emerged that intergrowths of offretite and erionite are features of both nominally phase-pure erionite and of pure offretite and of many members of the so-called ABC-6 family of zeolites (47). 

Zeolites ZSM-5 and ZSM-11 are the most commercially important end-members of a continuous series of intermediate structures belonging to the so-called pentasil family (4,5). The first preparation of ZSM-5 was described in 1972 (6) and since thbn, a number of elaborate synthesis recipes have been reported in the patent literature. Because of the unique and fascinating activity and (shape) selectivity of this material for a variety of catalytic reactions currently processed in chemical industries, increasing attention has been devoted to a better understanding of the various mechanisms that govern the synthesis of ZSM-5 (7-33). 

The last part of our work consists in extending our investigations of variables influencing the synthesis of type B, by examining the potentiality of various organic compounds to direct the formation of pentasil-type structures. 

Table VIII. Characteristics of some pentasil zeolites obtained from synthesis B using various organic bases or cations (adapted from ref (25), by permission).

Vedrine and coworkers studied vibrational bands for different types of zeoUtes with different particle sizes [95]. They concluded that during the synthesis of ZSM-type zeolite that the presence of vibrational bands at 550 and 450 cm indicate that a ZSM-type zeolite may have formed. Absence of the 550cm" band indicates that such a structure has not formed. The 550cm" band is characteristic of five-member pentasil rings which are a structural unit of ZSM-type zeohtes. 

In comparable reaction conditions as Pd +Cu +Y, Pd + and Cu2+ exchanged pentasil and ferrierite zeolites show a different type of activity [31]. The main products formed by propylene oxidation on these catalysts are acrolein and propionaldehyde below 120°C and 2-propanol above 120 C. Above 150°C consecutive oxidation of 2-propano1 to acetone is observed. The catalytic role of Pd and Cu in the 2-propanol synthesis is proposed to follow the Wacker concept. It is striking that when Pd + and Cu2+ are exchanged in 10-membered ring zeolites, oxidation of a primary carbon atoms seems possible, as acrolein and propionaldehyde are obtained from propylene. 

The first in situ MAS NMR investigation of the synthesis of MTBE on acidic zeolites was performed by Mildner et al. (228) under batch reaction conditions. In this investigation, the temperature-jump MAS NMR technique (stop-and-go experiment, see Section III.A) was applied to characterize the reaction dynamics under non-equilibrium conditions on a boron-modified pentasil zeolite ( si/... 

Synthesis of Pentasil Zeolites With and Without 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.

Amines support the synthesis of aluminium containing pentasils in dependence on their number of carbon atoms. 

A template-free synthesis can be performed in a limited range of the SiC /AKO, ratio. Based on these results a zeolite of pentasil-type is industrially produced by Chemiekombinat Bitterfeld, GDR. 

The frequency of o-type stackings is lower than 0.25, the value determined for the structure of H-BOR-D, in all of the samples obtained in the present investigation. The excess of i-type stackings commonly found in the framework of pentasils, suggests that this type of stacking is favoured with respect to the o-type at least under the conditions usually adopted for the synthesis of these materials. 

In the course of pentasil zeolite ZSM-5 synthesis, the gel mixture obtained from starting aqueous solution is a low density precipitate, which is different from the hydrogelatinous state in ZSM-34 synthesis it can be easily separated from the supernatant fluid by centrifuge. 

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