Effect of Sulfation Using Sulfate Salt Impregnation Method on Acidity of Cobalt Oxide

Abstract

Cobalt is one of the important components in hydrodesulfurization catalysts. The acidity of the support for the active elements is important to increase hydrodesulfurization activity1),2), but few studies have investigated the acid properties of simple cobalt oxides3),4). In contrast, Co ions have been incorporated into many sulfated zirconia and sulfated iron oxides5)~9) to increase catalytic activity as a promoter. The acidity of metal oxides can be increased by the addition of sulfate ion to an oxide surface followed by heat treatment at elevated temperatures10). The acid strength of the metal oxides was greatly increased by such sulfation. A representative example is sulfation of iron oxide, which resulted in a large increase in acidity. Several chemical properties of iron oxide resemble those of cobalt oxide. For example, Co3O4 and Fe3O4 are the more stable s tates of each oxide. Therefore, the acidity of cobalt oxide may be increased by introducing sulfate ions on the oxide surface and heat treatment at a higher temperature. CoO has a promotion effect on sulfated iron oxide9). This study investigated the preparation of sulfated cobalt oxide and the increase in surface acidity. In general, sulfation of zirconia is performed by soaking the metal oxide in dilute sulfuric acid in the equilibrium adsorption method. However, cobalt oxide dissolves in acidic water solution. Therefore, the equilibrium adsorption method cannot be applied to introduce sulfate ions onto the cobalt oxide surface. In this study, sulfate ions were introduced onto the cobalt oxide surface by impregnation of cobalt sulfate6),11). To evaluate the effectiveness of the sulfate salt impregnation method, four types of zirconium oxides were sulfated by the impregnation method. Catalytic activities of the prepared catalysts for pentane isomerization and ethanol dehydration were compared. The order of catalytic activities for several acid-catalyzed reactions and the properties of zirconia gels are known12). The effectiveness of the sulfate salt impregnation method was confirmed by comparing the order of activities in prepared sulfated zirconia samples with reported activities. The ethanol dehydration activity of prepared sulfated cobalt oxide was compared with that of proton-type zeolites and SiO2Al2O3 to estimate its acidity.