The synthesis of a series of sulfated zirconia catalysts was optimized using the isomerization of n-butane as a reaction probe. The normality of the H2SO4 solution used in the sulfation step was found to be the most important variable. A systematic change in the concentration of the H2SO4 solution showed that the optimum acid concentration was 0.25 N. When a catalyst prepared with this acid concentration was used, the conversion of n-butane at 200 °C was 35% at 5 min t-o-s. This was close to the thermodynamic equilibrium value of 56% conversion. This maximum was coincident with a catalyst with the highest specific surface area. An increase in the concentration of the H2SO4 solution above 0.25 N resulted in a decrease in both surface area and zirconia crystallinity. XPS studies showed a linear relationship between the H2SO4 solution concentration and the surface sulfur concentration. Bulk concentrations were determined by elemental analysis. The surface area increased to a maximum for a H2SO4 concentration of 0.25 N, while the concentration of bulk sulfur continued to increase when the acid concentration was progressively increased to 2.00 N. The use of a mordenite trap in the reactant stream resulted in an increase in n-butane conversion and a decrease in the rate of catalyst deactivation. XPS studies showed that the sulfur was present as sulfate species and that the oxidation state was not affected by the reaction.
Reprints available from U. Diebold (diebold).
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