(544ep) Enhanced Stability of a Chromium Oxide on Alumina for Propane Dehydrogenation By Introduction of Cobalt

Propane dehydrogenation has become an increasingly essential reaction for the present day. The contrasting nature between the ever-increasing applications of propylene in the polymer industry and the continuously waning supply of petroleum deposits, it becomes clear why this reaction has become such an important one. In fact, the worth of this reaction is measured by the difference in the costs of propane and propylene. Upwards of 5 million tonnes of propylene is produced by propane dehydrogenation. And this number is going to notably increase in the near future as the scope of technologies improve. Propane dehydrogenation is a highly endothermic reaction, due to which it has to often be carried out at very high temperatures, usually upwards of 500^o Celsius. The drawbacks of this reaction being carried out at such a high temperature have been largely agreed to be these – side reactions such as cracking and hydrogenolysis, which often lead to the formation of coke deposition, and the sintering of the active metal particles at this high temperature. It has also been generally agreed upon that smaller the particle size on the metal support, the better dispersion and furthermore improved yield is obtained. Commercially, Catofin and Oleflex processes have been scaled up. The Catofin process makes use of an environmentally harmful chromium oxide catalyst supported on alumina promoted by sodium or potassium, to posion the acidic sites on alumina. Whereas the Oleflex process employs a Platinum and tin bimetallic catalyst on alumina, also doped with either sodium or potassium.

Chromium catalysts have been known to show great activity for catalytic non-oxidative propane dehydrogenation, with an agonizing drawback, very low stability due to metal sintering as well as coke formation, to poison the active sites. Here we aim to show increased stability for a chromium alumina catalyst by doping the support with a minimum amount of cobalt. In our experiments, chromium on γ-alumina deactivated completely in the second half of the second hour whereas the cobalt doped support showed improved metal support interactions and reasonably stable conversion for over 4 hours. The yield is comparable in the first hour (36% yield for cobalt doped alumina, 32% yield for γ-alumina), but as the reaction progresses, but it drops sharply for γ-alumina whereas the cobalt doped alumina showed more lasting conversion for the same (26% yield for cobalt doped alumina, 2% yield for γ-alumina). The reaction conditions were as follows. It is carried out at a temperature of 600^o Celsius. The catalyst was reduced from 200-400^o prior to the reaction. From the XRD plot we can see clear peak broadening, which implies that smaller sized particles of the catalyst were formed. There is general consensus that the smaller the size, the better dispersion hence the better activity. We also saw the formation of the more stable spinel phase of cobalt (CoAl₂O₄) which might also be one of the reasons for improved stability. From the Temperature Programmed Reduction of the Chromium oxide catalyst, we see improved and stronger metal support interactions when the support is slightly doped with cobalt as compared to regular γ-alumina.

In conclusion, we prepared a chromium oxide catalyst by wetness impregnation method on two supports, namely γ-alumina and cobalt doped alumina. We compared the activities for both the catalysts for non-oxidative propane dehydrogenation and found the cobalt doped alumina to show much greater stability in its performance in comparison with the γ-alumina.