The effect of reaction conditions and time on stream on the coke formed during propane dehydrogenation

Pt/Al2O3 and Pt-Sn/Al2O3 catalysts, coked during propane dehydrogenation, have been studied using temperature-programmed oxidation (TPO). Time on stream, temperature, and reaction gas composition have been varied. Three different peaks were identified from the TPO profiles on the Pt-Sn catalyst and attributed to different types of coke; coke on and in the vicinity of the metal, coke on the carrier, and graphitic coke on the carrier The amounts of these types were related to reaction conditions. The formation of the coke belonging to the first two peaks in the TPO profiles increases with temperature and partial pressure of propene. Hydrogen, on the other hand, suppresses the formation. The amount of coke that can be attributed to the third peak increases with temperature and propane partial pressure. A model is discussed where a minor part of the coke deactivates the catalyst. This coke is formed in parallel with the coke that is seen in the first two peaks in the TPO experiments. The graphitic coke formed on the carrier is not formed through this route. The experiments with different time on stream revealed that the first peak reached a constant level after about 15 h, while the second one still increased. Hydrogen was very efficient in preventing coke formation and deactivation but could not remove coke already formed on the catalyst. The hydrogenolysis and cracking mechanisms during the propane dehydrogenation are also discussed. (C) 1996 Academic Press, Inc.