Zeolites dehydroxylation mechanism

The nature of the surface acidity is dependent on the temperature of activation of the NH4-faujasite. With a series of samples of NH4—Y zeolite calcined at temperatures in the range of 200° to 800°C, Ward 148) observed that pyridine-exposed samples calcined below 450°C displayed a strong infrared band at 1545 cm-1, corresponding to pyridine bound at Brpnsted (protonic) sites. As the temperature of calcination was increased, the intensity of the 1545-cm 1 band decreased and a band appeared at 1450 cm-1, resulting from pyridine adsorbed at Lewis (dehydroxylated) sites. The Brtfnsted acidity increased with calcination temperature up to about 325°C. It then remained constant to 500°C, after which it declined to about 1/10 of its maximum value (Fig. 19). The Lewis acidity was virtually nil until a calcination temperature of 450°C was reached, after which it increased slowly and then rapidly at calcination temperatures above 550°C. This behavior was considered to be a result of the combination of two adjacent hydroxyl groups followed by loss of water to form tricoordinate aluminum atoms (structure I) as suggested by Uytterhoeven et al. 146). Support for the proposed dehydroxylation mechanism was provided by Ward s observations of the relationship of Brpnsted site concentration with respect to Lewis site concentration over a range of calcination tem-... 

When zeolite H-Y obtained by decationation of NH4-Y is heated further, water is irreversibly lost from the framework. The dehydroxylated zeolite Y displays Bronsted and Lewis acid properties. The mechanism for this process... 

The formation of structural hydroxyl groups in the presence of divalent cations has been explained on the basis of a hydrolysis mechanism (148) involving water initially coordinated to the metal ions (210, 214-216). The formation of a nonacidic hydroxyl group on the metal ion and an acidic hydroxyl on the zeolite framework by dissociation of the water molecule is consistent with the observed IR spectra and pyridine adsorption experiments. Further calcination at higher temperatures results in dehydroxylation and formation of Lewis acid sites at tricoordinate aluminum atoms in the zeolite framework (149). 

At the same time, specific properties (primarily the Si/Al ratio) of a zeolite should be taken into account when discussing the mechanism of its dehy-droxylation. It is quite possible that the mechanism typical of H forms of faujasites would be completely improper for high-silica-containing zeolites. Thus, in their studies of dehydroxylated forms of ZSM-5 zeolite by means of IR spectroscopy of molecular hydrogen adsorbed at low temperatures, Kazansky et al. (72, 76) have demonstrated that the Uytterhoeven-Cristner-Hall scheme seems valid in this case. 

XPS of chemisorbed nitrogen containing basic molecules such as pyridine and ammonia have been employed to monitor the strength of acid sites in H and cationic forms of zeolites. One interesting problem is to follow the changes in concentration of the Brbnsted and Lewis acidic centers with the temperature of thermal treatment. Two mechanisms have been proposed for the thermal degradation. Dehydroxylation occurs through a process first described by Uytterhoven et al. [35]... 

The interaction of many hydrocarbons (both aliphatic and aromatic) with zeolites has been investigated. HY zeolite catalyses the conversion of cyclopropane at room temperature to isobutane. The proposed mechanism involves a non-classical protonated cyclopropane ion intermediate. At 200 cyclopropane isomerizes to propene and also forms aromatic species. Adsorption and transformation of but-ene has been widely studied. It is useful to draw a distinction between hydroxylated and dehydroxylated samples. On hydroxylated samples but-ene isomerizes and also oligo-The -OH groups vibrating at 3640 cm" were found to be... 

---