Zeolite high-temperature-induced

In order for the SSIE process to proceed, there must be some type of driving force making the cations in the ingoing phase mobile enough to break them apart from the solid matrix where they begin, such that they can travel into the zeolite. In this chapter, we break these processes down into three fundamental types (1) moisture induced, (2) high-temperature induced, and (3) reaction induced. 

The ion-exchange reaction of the synthetic zeolites NaX and NaY with cobalt, zinc and nickel ions is shown to be non-stoichiometric at low bivalent-ion occupancy, the hydrolytic sodium loss being about twice as large for NaX ( 5 ions/unit cell) as for NaY. The effect is more pronounced at high temperatures and disappears at high occupancies. Reversibility tests in NaX toward zinc and cobalt ions, as studied by a temperature-variation method, show the temperature history to be an important factor in the irreversibility characteristics. The low-temperature partial irreversibility, induced by a high-temperature treatment (45°C) is interpreted in terms of a temperature-dependent occupancy of the small-cage sites by divalent cations, which become irreversibly blocked at low temperature (5°C). 

The observed zeolite destruction is almost certainly caused by exposure to high-temperature steam in the regenerator since USY zeolite is thermally stable in dry air to temperatures in excess of 1000°C (10). It is unlikely that the steam-induced collapse of crystalline zeolite in this USY catalyst is much affected by the presence of a low concentration of vanadium... 

Acidic clays are widely applied in the dehydration of alcohols [38]. Although similar to zeolites in their capacity to induce the formation of both alkenes and ethers, selective alkene synthesis is possible. Various layered materials (clays, ion-exchanged montmorillonite, pillared layered clays) are very active and, in general, selective in transforming primary, secondary, and tertiary aliphatic alcohols to 1-alkenes [39-43]. Al -exchanged montmorillonite, however, induces ether formation from primary alcohols and 2-propanol [41]. Substituted 1-phenyl-1-ethanols yield the corresponding styrene derivatives at high temperature (653-673 K) [44]. 

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. 

Recent applications of the dynamic diffraction experimental techniques were mostly in the areas of temperature-induced dehydration processes in natural zeolites, hydrothermal crystallization of aluminosilicate and aluminophosphate microporous materials, thermal decomposition of layer silicate minerals, high-temperature synthesis of advanced ceramics, and hydrothermal ion-exchange and conversion processes in synthetic molecular sieves. The time-resolved powder diffraction patterns relative to the isothermal nucleation and growth process of zeoUte-A by the hydrothermal treatment of activated metakaollnite is shown in Figure 6. 

Fyfe CA, Strobl H, Kokotailo GT, Kennedy GJ, Barlow GE. Ultra-high-resolution Si solid-state MAS NMR investigation of sorbate and temperature-induced changes in the lattice structure of zeolite ZSM-5. 

Oxidations initiated by thermally induced electron transfer in an oxygen-CT complex represent the thermal analog of the Frei photo-oxidation and are properly classified as hybrid type IlAOi-type IIaRH oxidations (Fig, 2), Such reactions require either zeolites with high electrostatic fields or substrates with low oxidation potentials. In addition, elevated temperatures are known to promote the thermally initiated electron-transfer step, although the possible intrusion of a classical free-radical initiation chain oxidation at higher temperatures must be considered. 

---