Probing the Basic Character of Alkali-Modified Zeolites by CO2 Adsorption Microcalorimetry, Butene Isomerization, and Toluene Alkylation with Ethylene

Abstract

Two types of alkali-modified zeolites X and Y were studied, namely, zeolites containing occluded cesium oxides and zeolites containing occluded alkali metals (Na, Cs). Zeolites with occluded CsOX, obtained via impregnation and decomposition of cesium acetate, exhibited higher CO2 adsorption capacities and higher heats of adsorption than the corresponding ion-exchanged zeolites. However, the heats of adsorption were significantly lower on the zeolite-supported oxides (∼85 kJ mol−1) compared to bulk cesium oxide (∼270 kJ mol−1). The CO2 adsorption capacities of the zeolites containing occluded CsOX increased linearly with the amount of occluded cesium (1 CO2 per 4 occluded Cs atoms), and the catalytic activity for 1-butene isomerization was commensurate with the CO2 uptake on the materials. Stronger base sites were created in zeolites by decomposition of impregnated alkali azides (Na, Cs), due to the formation of alkali metal species. These materials were active catalysts for the side-chain alkylation of toluene with ethylene, whereas zeolites containing occluded CsOX were inactive for the reaction. Dioxygen adsorption followed by thermal treatment of zeolites containing occluded alkali metal resulted in the elimination of their activity for toluene alkylation whereas their activity for butene isomerization was retained. Alkali oxide- and alkali metal-containing microporous carbon materials exhibited significantly different adsorption properties and catalytic activities from their zeolite counterparts, possibly due to the different nature of the occluded species and/or steric constraints in the amorphous carbon.