Speaker
Description
The main challenge in the catalytic partial oxidation of alkanes to valuable products, such as olefins, aldehydes or acids, is to control the reaction pathways to desired and undesired products in complex reaction networks. Propane activation and olefin formation over structurally stable perovskites$^{1}$ and alkali-doped supported vanadium oxide catalysts, which show structural dynamics under reaction conditions, are compared. The activation of oxygen was studied by using $^{18}$O$_{2}$ exchange.$^{2}$
The impact of the chemical composition on the reactivity was investigated by synthesizing a 4x4 matrix of perovskites ABO$_{3}$ with A= La, Pr, Nd, Sm and B=Cr, Mn, Fe, Co. The B element, which is redox active, shows a more pronounced effect on activity and selectivity in propane oxidation than the A element. Changing the chemical potential by adding steam to the feed or depositing silica, alumina or phosphate on the surface by atomic layer deposition leads to suppression of the direct combustion of propane in the reaction network. Reductive or oxidative pretreatment of the material prior to the catalytic measurement affects the concentration of the different surface oxygen species and defects, all of which are interrelated, resulting in a very complex response in terms of catalyst performance. Exchange experiments with $^{18}$O$_{2}$ indicate that mainly surface oxygen is involved in propane oxidation in the applied temperature range,$^{2}$ and in presence of steam a rapid exchange with water is detected.
Significant structural changes were observed in alkali vanadium catalysts on silica supports (alkali = Li, Na, K, Rb and Cs) under the reaction conditions of propane oxidation. Alkali vanadates are present on the surface. In the K-, Rb- and Cs-containing catalysts these phases melt at reaction temperature, which is reflected in a decrease in activity and an increase in selectivity. The molten alkali vanadates are inactive. The activity decreases because the melt covers active surface vanadium oxide species. The selectivity increases because undesired subsequent reactions of intermediates are suppressed.
Consequently, propane oxidation was carried out over inert materials. When a propane-rich feed is used so that spontaneous ignition is possible, the rapid gas-phase reaction over non-specific interfaces, e.g. SiO$_{2}$, has been shown to allow the synthesis of products susceptible to over-oxidation, such as propylene oxide.$^{3}$
References
1. G. Koch, M. Hävecker, D. Teschner, S. J. Carey, Y. Wang, P. Kube, W. Hetaba, T. Lunkenbein, G. Auffermann, O. Timpe, F. Rosowski, R. Schlögl, A. Trunschke, ACS Catalysis, 10, 7007 (2020).
2. Y. Wang, F. Rosowski, R. Schlögl, A. Trunschke, The Journal of Physical Chemistry C, 126, 3443 (2022).
3. P. Kube, J. Dong, N. Sánchez Bastardo, H. Ruland, R. Schlögl, J. T. Markgraf, K. Reuter, and A. Trunschke, Nature Communications, submitted.
| Abstract Number (department-wise) | AC 1.1 |
|---|---|
| Department | AC (Schlögl) |
