129Xe NMR study of the dehydroxylation of zeolite HY

Abstract

Zeolite HY was dehydroxylated invacuo at temperatures in the range 420–800 °C. The prepared samples were investigated with the probe molecules carbon monoxide, xenon and nitrogen using adsorption techniques and ¹²⁹Xe NMR spectroscopy. The concentration of ‘true’ Lewis acid sites (LAS), which are generated by the thermal treatment, was evaluated quantitatively from the adsorption isotherms of CO. This concentration depends strongly on the temperature of dehydroxylation with a maximum of ca. 0.1 LAS per supercage at 600 °C. The adsorption and NMR experiments with Xe reveal specific adsorption of this probe at the LAS as well, but reflect more sensitively framework modification and destruction which become apparent in changes of the micropore volume determined from the saturation capacities of nitrogen at low temperatures.