Krypton as a Probe of Electrostatic Fields in Y Zeolites

Abstract

The isosteric heat of adsorption qat of krypton on an 82% Ca-exchanged Y zeolite was obtained at a surface coverage corresponding to about one krypton per 64 supercages. The value of q.,t, calculated from adsorption isotherms taken at pressures between 5 and 180 torr and temperatures between 0°and 150°C ., was found to be 5.3 kcal. per mole after prior evacuation of the adsorbent at 410°C. and 4.6 kcal. per mole when the sample was heated above 700°C. in vacuo before adsorption. These values must be compared with a calculated heat of adsorption of about 20 kcal. per mole, largely due to polarization energy if krypton were adsorbed at Ca2+ ion sites at the window between supercage and sodalite cage. It is concluded that such sites, if present at all, must represent a fraction less than about 10~7 of all the sites available to krypton. Thus, the large electrostatic fields associated with these bivalent cation sites play an insignificant role in the adsorption of krypton. Similar conclusions have been reached by measuring q,( on Y samples containing Mg2+, Sr2+, Ba2+, and Fe2+. It is proposed that krypton is adsorbed at effectively univalent sites consisting of a single M2+ associated with OHor of two M2+ ions bridged by a O2ion, depending on the degree of dehydroxylation of the sample.

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