Effects on solid basicity for sodium metal and metal oxide occluded NaX zeolites

Abstract

Zeolite NaX, containing occluded sodium metal and oxide clusters at loadings of 0.25, 1, and 3 sodium species per supercage, were prepared via controlled thermal decomposition of impregnated sodium azide and sodium acetate salts, respectively. In addition, occluded sodium oxide clusters were prepared by oxidation of the supported sodium metal clusters in air at 513 K. Adsorption microcalorimetry of carbon dioxide was used to indicate the strength and number of surface base sites created by incorporation of these sodium species into the NaX supercages. For similar loadings of sodium species, the strength of the CO 2 adsorption sites for zeolites that contained occluded sodium metal clusters was greater than for the zeolites containing occluded sodium oxide clusters prepared by both methods. While the number of CO 2 adsorption sites for both supported sodium oxide catalysts were similar at identical sodium loadings, the heats of adsorption for sites created via the oxidation of supported sodium metal clusters were higher than those prepared via the decomposition of sodium acetate. The decomposition of 2-propanol, used as a probe of the acido-basic character of the surface, indicated that loadings of sodium species of one or greater per supercage produced catalysts with greater than 90% selectivity to the base catalyzed product acetone, compared to 24% selectivity over the unmodified NaX zeolite. At near steady-state conditions for the range of loadings investigated, the rate of acetone production increased linearly with increasing occluded sodium content for all catalysts investigated. A linear relationship between the rate of acetone formation and the total uptake of carbon dioxide was also found. The rate of acid catalyzed propene production reached a minimum for catalysts with loadings near two sodium species per supercage. Higher loadings resulted in an increase in the acid catalyzed propene production, presumably due to the incorporation of acid–base pairs that result from incomplete decomposition of the supported azide.