Structure/Function Relationships for Basic Zeolite Catalysts
Containing Occluded Alkali
Robert J. Davis
Department of Chemical Engineering, University of Virginia
Charlottesville, VA 22903, USA
Incorporation of occluded alkali species in zeolite cages beyond the ion-exchange capacity increases the base strength of these materials. However, the nature of the occluded alkali has been elusive. In this work, the basicity and reactivity of alkali-modified zeolites were investigated in order to elucidate the role of occluded species on catalytic activity. To synthesize intrazeolite oxide or metal species, cesium acetate or cesium azide was impregnated into the pores of zeolites and decomposed in situ. Differences in the Cs LIII edge spectra of zeolites loaded with cesium oxide and bulk cesium compounds indicated that cesium was less coordinated to oxygen in the supported samples than in the bulk materials. To characterize the basicity of the samples, we used adsorption of iodine and carbon dioxide. Blue shifts in the visible spectra of adsorbed iodine were found to increase with increasing electropositivity of the exchangeable cation, indicating greater donor strengths of the zeolite frameworks. For a series of X zeolites with different loadings of occluded cesium oxide, the CO2 uptake increased linearly with the amount of occluded Cs. A majority of the base sites in the CsOx/CsX samples exhibited a heat of CO2 adsorption around 85 kJ mol-1, which is much lower than the 270 kJ mol-1 seen on bulk cesium oxide. After thermal pretreatment, Na(azide) and Cs(azide) modified X zeolites catalyzed the side chain alkylation of toluene with ethylene, whereas, CsOx/CsX was inactive for the reaction. However, the catalytic activities of the CsOx/CsX samples for the isomerization of 1-butene and dehydrogenation of 2-propanol increased linearly with the amount of excess cesium, which is consistent with the results from the adsorption microcalorimetry of CO2.
Opportunities for Collaboration (R. Davis)
(1) Structure and Reactivity of Basic Sites on Solid Surfaces
Nanophase alkali oxide clusters supported on a variety of carriers are interesting strong base catalysts. We have focused recently on the incorporation of cesium oxides in the cages of zeolite hosts. However, the microstructure and chemical state of the supported oxide clusters remain elusive. Therefore, we have developed an experimental program to discover fundamental relationships between the physical state of an alkali oxide nanocluster and its chemical activity for base-catalyzed reactions. These studies have been expanded to include supported alkali metal clusters. Collaborations in the areas of strong base catalysis and characterization of nanophase alkali species (metals and oxides) are possible.
(2) Metal-Support Interactions in Basic Catalysts
When transition metal clusters are in contact with a basic support, the electronic structure and catalytic activity of the metallic phase can be greatly affected. Fundamental investigations of the metal-support interface are being pursued with the goal of developing new ammonia synthesis catalysts. In this work, supported nanoclusters of ruthenium metal are promoted with alkaline earth oxides in order to enhance ammonia synthesis activity. Our past work has focused on zeolite-supported Ru, but collaborations could involve novel supports and/or promoters for Ru-based ammonia synthesis catalysts.
(3) Synthesis and Characterization of Strong Solid Acid Catalysts
Over the last several years, we examined the structure and reactivity of a variety of solid acids, including Ti-Si mixed oxides, sulfated zirconias and heteropolyacids. Our current efforts are focused on the effect of water on the acid character of supported heteropolyacids. Our program includes both theoretical (quantum chemical calculations in collaboration with M. Neurock) and experimental (probe reactions and sorption microcalorimetry) components. Collaborations may include the utilization of novel characterization methods and/or new test reactions for supported heteropolyacids.
(4) Selective Oxidation of Hydrocarbons over Supported Vanadia Catalysts
We initiated last year a fundamental study of selective oxidation catalysts based on supported vanadia, with the target reaction being the oxidative dehydrogenation of propane to propene. Kinetic investigations and some initial characterization studies have been performed. Since this is a relatively new effort in our laboratory, collaborations will add significantly to the progress of this research.