Adsorption microcalorimetry and catalytic activity of alkali-modified oxides and zeolites

Alkali-modified oxides and zeolites are basic materials with demonstrated catalytic activity in many reactions, including double-bond isomerization of olefins, side-chain alkylation of aromatics, dehydrogenation of alcohols and Knoevenagel condensation of aldehydes. The alkali-support interaction has a pronounced effect on the catalytic activities of these alkali-modified oxides. In this work, the acid-base properties of rubidium- and strontium-modified metal oxides (alumina, titania and silica) and carbon were studied by adsorption microcalorimetry of ammonia and carbon dioxide. The modified metal oxides and carbon were synthesized by impregnation and decomposition of acetate precursors of the alkali and alkaline earth metals. Results from microcalorimetry studies confirmed that the incorporation of Rb and Sr altered the acid-base properties of the supports. However, the extent of these modifications depended on the compositions of both the basic additive and the support. Addition of Rb and Sr to alumina and titania neutralized a part of the surface acidity and increased the surface basicity. Rubidium was more effective than strontium at altering the acid-base character of all the supports studied. The fewest base sites were found on modified silica, presumably due to the formation of surface silicate phases.; Alkali-modified zeolites were also characterized by adsorption microcalorimetry of carbon dioxide. In addition, the catalytic activities of these zeolites were examined in the isomerization of 1-butene, the alkylation of toluene with ethylene, and the reaction of ethylene oxide with carbon dioxide to make ethylene carbonate. Three types of alkali-modified zeolites were studied, namely, alkali ion-exchanged zeolites, zeolites containing occluded alkali oxides and zeolites containing occluded alkali metals. Zeolites containing 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. These materials also showed increased activity over ion-exchanged zeolites for the reaction of ethylene oxide with carbon dioxide to make ethylene carbonate. The CO 2 adsorption capacities of the zeolites containing occluded CsO x increased linearly with the amount of occluded cesium and their activity for 1-butene isomerization was commensurate with their carbon dioxide uptakes. Stronger base sites were created in zeolites through decomposition of impregnated alkali azides, due to the formation of alkali metal species. These materials were active for the side-chain alkylation of toluene with ethylene, whereas zeolites containing occluded CsOx were inactive for the reaction. Dioxygen adsorption on zeolites containing occluded alkali metal resulted in a reduction in their CO2 uptakes and catalytic activities for 1-butene isomerization and toluene alkylation. Alkali oxide- and alkali metal-containing microporous carbon materials exhibited significantly different adsorption and catalytic behavior from their zeolite counterparts, possibly due to a different nature of the occluded oxide and/or steric constraints.; This work demonstrated that adsorption microcalorimetry and appropriate catalytic probe reactions can be used to elucidate the basic nature of supported alkali metals and oxides.