Dealuminated ZSM

Observations regarding activity and selectivity of steam treated ZSM-5 in this work could thus apply to dealuminated ZSM-5. 

The incorporation of TiIV in the crystal lattice of silicalite has been attempted by the reaction of TiCl4 with dealuminated ZSM-5 (Kraushaar et al., 1988) or deborated borosilicalite (Carati et al., 1990). The same reaction has been used in the attempt to incorporate titanium in the crystal lattice of zeolite beta, morde-nite or zeolite Y. In many cases catalytic properties have resulted, but the way in which the incorporation takes place has been questioned. Because of its molecular dimensions, TiCl4 cannot enter or leave the pore system of ZSM-5. It has been shown that 89% of the OH groups present in the preformed zeolite as SiOH remain unreacted after treatment at 573 K with TiCl4. The incorporation of titanium must therefore be limited to the outer part of the crystals or proceed through a severe chemical attack with removal of silicon and formation of a secondary pore system (De Ruiter et al., 1993). Deposits of Ti02 on the outer part of the crystal treated with TiCl4 have indeed been observed (Carati et al., 1990), as has abnormal behavior in the oxidation of phenol (Section V.C.3.c). 

Matsuda, Yogo, Mogi and Kikuchi.27 ZSM-5 and ZSM-5 dealuminated with (NH SiF were used as catalysts in these reactions. ZSM-5 gave rise to several DMN isomers in contrast to selective formation of the 2,6 and 2,7 DMN isomers over dealuminated ZSM-5 as shown in Figure 1 After 27).27... 

Figure 1. Dimethylnaphthylene Isomers Produced over ZSM-5 and Dealuminated ZSM-5.

Table I. Conditions of hydrothermal treatment of dealuminated ZSM-5 (samples 1-2) and silicalite (samples 3-9), chemical shift (CS) and full-width-at-half-maximum (FWHM) of thenBMASNMR signals...

Silicalite (samples 3-9) and dealuminated ZSM-5 (samples 1-2) were subjected to hydrothermal treatment under mild alkaline conditions at various boron-to-zeolite ratios, temperatures and times of treatment (see Table I). To consider the question of the status of boron in the boronated samples we have used nB MAS NMR, a technique capable of providing direct information on the type of short-range environment of the nucleus. 

In the first case the secondary synthesis is performed on a sample of dealuminated ZSM-5 and the substitution presumably takes place on the sites made vacant from the removal of aluminum. In the second case the synthesis is performed on a ZSM-5 not dealuminated and the treatment with TiCl does not reduce the aluminum content in this case, if titanium replace some lattice aluminum, extraframework aluminum oxide must be formed in the zeolite crystals. In both the above mentioned works, samples treated with TiCl have an IR absorbance band at 960 cm . Ferrini observed the appearance of such a band also on Ti-contalning Beta and Y samples prepared by secondary synthesis. 

A series of dealuminated ZSM-5 zeolites with various framework Si/Al ratios were prepared by different methods (HCl, ammonium hexafluorosilicate, steaming). The number of acid sites that correspond to the high-temperature desorption peak of the ammonia-TPD spectra of all the dealuminated samples is in 1 1 mole analogy to the framework Al (FAl), irrespective the degree and the type of dealumination method. The catalytic activity of the H-ZSM-5 zeolites for isopropanol dehydration is linearly related to the number of acid sites that correspond to the FAl-content (Bronsted acidity). The Si-Al amorphous phase that is formed in the high-temperature steamed samples affects activity and induces different product selectivity for propene and diisopropyl ether. 

Nayak and Choudhary [149] studied hydrothermally dealuminated ZSM-5 zeolites and found that the decrease in catalytic activity of cumene conversion and the decrease in the amount of strong acid sites (which adsorb pyridine irreversibly at 400 °C) followed the same trend. They reported a hnear relation between cumene conversion and the concentration of strong acid sites. 

---