Effect of catalyst preparation conditions on the performance of eggshell cobalt/SiO2 catalysts for FischeraTropsch synthesis

• Published in: Applied catalysis. A, General General, 2012-12, Vol.447-448, p.151-163
• Main Authors: Gardezi, Syed Ali, Wolan, John T, Joseph, Babu
• Format: Article
• Language: English
• Subjects: Calcination; Catalysts; Chemisorption; Cobalt; Dispersions; Selectivity; Solvents; Surface chemistry
• ISSN: 0926-860X

In this article, we examine the impact of solvent and calcination conditions, during catalyst preparation, on the performance of eggshell catalysts. Eggshell profile of silica supported cobalt catalysts provides a means to control selectivity in FischeraTropsch synthesis reactions. Solvents such as water and alcohol attach to the silanol groups on the silica gel surface and compete with metal salts during ion exchange and adsorption. The solution properties impact the metal dispersion and interaction with the metal support. The calcination conditions (static versus dynamic, oxidizing versus reducing atmosphere) also have an impact on metal dispersion and support interaction. Ethanol proved to be a better solvent for enhancing the dispersion due to its surface wetting pattern while direct reduction in dynamic hydrogen provided gradual decomposition of the cobalt precursor thus reducing agglomeration. The use of water as a solvent and a static air environment during calcination led to lower dispersion. Nitrogen physisorption experiments showed that under favorable conditions, synthesized eggshell catalyst retains most of the original support surface area. Surface elemental composition using XPS indicate that the effect of solvent was stronger than the calcination environment in determining catalyst dispersion. This result was also confirmed by hydrogen chemisorption studies. TPR results suggested that back reaction of calcination product (especially H2O) and the agglomeration were competing phenomena in a static oxidizing environment. However, hydrogen chemisorption indicated that metal agglomeration was far more significant than the back reaction in static air atmosphere. Catalyst activity and selectivity were tested in a fixed bed (FBR) reactor fitted with GC and FTIR analyzers. The eggshell catalyst demonstrated high activity as expected; however, selectivity of CO2 in our fixed bed catalytic reactor was higher than usual. The distribution of liquid hydrocarbon was in the narrow range of diesel and aviation fuel.