Abstract
Oxygen isotopic exchange between CO/sub 2/ and Na-Y, Na-X, and the germanic homologue of Na-X was studied at 25 to 500/sup 0/C by mass-spectrometric analysis of the change in the /sup 18/O//sup 16/O ratio in the gas circulated across the zeolite. Infrared spectra of CO/sub 2/ adsorption and reaction with the X zeolites were obtained under similar experimental conditions. Isotopic exchange is measurable in all of the zeolites at room temperature. However, exchange in Na-X and NaGe-X was much more rapid under comparable conditions than exchange in Na-Y. This difference in behavior must be attributed to the effects associated with the substitution of Al/sup 3 +/ and Ge/sup 4 +/ for Si/sup 4 +/ in the tetrahedral sites of X zeolites. In particular, the infrared results demonstrate that various chemisorbed carbonate species can form on the X varieties. Isotopic exchange is promoted by some of these species, whereas others can evidently block active exchange sites on the zeolite surface. The absence of chemisorbed CO/sub 2/ species on Na-Y accounts for the relatively low rate of exchange in this molecular sieve.
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