The effect of dehydroxylation of HNaY zeolites on the interaction with ethylene and propylene

Abstract

The extent of dehydroxylation of HNaY zeolites of various degree of decationization was varied by the termal treatment in vacuo over the temperature range 620-890 K. The solid phase was investigated by means of infrared spectroscopy, the gaseous phase by means of mass spectrometry. Lewis acid-base pair centres were formed by the dehydroxylation and these initiated at 310 K the oligomerization of ethylene in zeolite cavities. The reaction occurred via the cationic mechanism. A certain ratio of Lewis centers to OH groups was necessary in order to achieve the maximum oligomerization rate on given type of zeolite. It was showed up in a shift of the maximum to higher activation temperatures with less decationized zeolites, more stable with respect to the dehydroxylation. The zeolite activity increased with increasing decationization. The same trend was found also for the oligomerization related to one Lewis center. The presence of Lewis centers affected the oligomerization rate of propylene, too, though Broensted centers played the dominant role in this case. After the thermal decomposition of the oligomers aliphatic and cyclic hydrocarbons, mostly saturated compounds, were found in the gaseous phase. The isotopic exchange of propylene-d6 at 570K occurred via a multiple mechanism, the value of its rate being of the same order of magnitude as with the hydroxylated forms. In view of the varying amount of slowly exchanging SiOH groups in various decationized zeolites, the dependence of the exchange rate of structural zeolites on the decationization could not be established.The isotopic exchange of ethylene-d4 occurred via a single mechanism with a slightly increased rate in comparison with the hydroxylated forms.