Exchange of oxygen isotopes between carbon dioxide and ion-exchanged zeolites A

Abstract

The reactivity of the framework of zeolite A has been studied through exchange of oxygen isotopes between carbon dioxide and (Na, Ca)-A and (K, Ca)-A zeolites. Special care was taken to exclude effects of residual water by using a clean vacuum system. All zeolites investigated contained a very reactive part, amounting to 2%, on which the reaction rate was too fast to be followed. This part was assigned to amorphous impurities which could not be detected by X-ray diffraction techniques. The Al—O—Si linkage in zeolite A was not so easily broken as in X- and Y-type zeolites. About a half of the framework oxygen atoms were very difficult to exchange, and were assigned to O_III in crystallographic notation. The exchange rate depended sensitively upon the composition of the zeolite. A model for the activated complex, in the exchange reaction, was derived from this composition dependence. This activated complex was a carbonate ion bridged to the framework containing a pair of exchangeable cations supplied from the four- and eight-membered ring sites. It was concluded that O_I is more reactive than O_II, since the former has a larger electron concentration than the latter, and more easily forms a carbonate ion. The rate constant showed an unusual temperature dependence; there was a maximum in Arrhenius plots with zeolite K_1.0Ca_5.5-A, but not with Na₁₂-A. The reason for this is qualitatively discussed.