CHARACTERIZATION OF CUY ZEOLITES AFTER DEHYDRATION, OXIDATION AND REDUCTION WITH CARBON-MONOXIDE - AN ADSORPTION AND XE-129 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY STUDY

The adsorption isotherms and Xe-129 nuclear magnetic resonance (NMR) chemical shifts of xenon and the adsorption isotherms of carbon monoxide of Cu(II)- and Cu(I)-exchanged zeolites NaY were measured. The former zeolites of 53, 75, and 95% exchange degrees were investigated after various pretreatment steps comprising dehydration, oxidation and reduction with CO at 420-degrees-C as well as long-term CO reduction at 470-degrees-C. The Cu(I)Y zeolite of 70% exchange degree was prepared via a solid-state exchange procedure with CuCl and subjected to dehydration at 420-degrees-C. In all cases, except the dehydrated zeolites, almost linear xenon adsorption isotherms and linear Xe-129 NMR chemical shift versus xenon concentration curves running parallel to each other are obtained. In contrast, the chemical shift curves for the dehydrated zeolites are non-linear at low xenon concentrations turning towards negative chemical shift values at very low pressures. The whole body of the experimental xenon data can be explained quantitatively with a unifying approach on the basis of a site adsorption model where the sites are (i) two types of cuprous ions of much different adsorption strength and Xe-129 chemical shift, (ii) Na+ cations, (iii) Lewis acid sites generated through autoreduction and reduction of Cu2+ by CO, and (iv) framework sites free of cations. These five types of sites are each characterized by Langmuir adsorption isotherm constants and local Xe-129 NMR chemical shifts. The adsorption site concentrations in the various zeolites are evaluated. The supercage Cu(I) concentration values are in nice agreement with the results deduced from the CO adsorption isotherm measurements.