Datensatz

Zusammenfassung

We have shown that the initial St production rate over unpromoted hematite is high. Deactivation occurs within roughly one hour and only the final state with the lowest rate has been studied in previous experiments. The initial rate is almost an order of magnitude higher than that at steady state and thus similar to that of potassium-promoted catalysts.[2,3] Deactivation occurs by reduction of the hematite catalyst to magnetite and by coke deposition. Water prevents further reduction of magnetite and reduces carbon accumulation. The deactivation can largely be suppressed by adding even low concentrations of oxygen which prevent reduction of hematite to magnetite and inhibit accumulation of carbon deposits. The absolute rates (per cm-2) obtained for the flat model catalysts are about two orders of magnitude higher than for the porous powder pellets. Using a single surface of only 0.5 cm2, the model studies achieved about 2% of the technical conversion for which several meters of catalyst bed height are required. This shows that the performance of the technical styrene synthesis process is not limited by elementary surface processes but is limited by mass and energy transport. An essential improvement of the overall performance is thus not to be expected from a variation of the chemical composition of the catalyst but from an optimization of the process engineering.