Fourier-transform infrared study of the oxidation and oxidative dehydrogenation of n-butenes on the surface of FeCrO3

Abstract

The interaction of the three linear butene isomers and of butadiene with the surface of a well characterized FeCrO₃ oxidative dehydrogenation catalyst has been investigated by IR spectroscopy. Both (Z)- and (E)-but-2-enes produce well characterized molecularly adsorbed species and rapidly form sec-butoxides and ethyl methyl ketone at room temperature. At higher temperatures, acetates are formed by oxidative CC bond breaking. But-1-ene gives at least two different conformeric adsorbed species and exhibits, together with the same oxidation pathway, an allylic activation pathway, with the formation of butadiene and methyl vinyl ketone. Butadiene also forms two different adsorbed species, very weakly held, and slowly transforms into crotonaldehyde, which is further oxidized at higher temperatures. These results provide evidence for different oxidative and dehydrogenative pathways at the surface and are correlated with the known behaviour of FeCrO₃ as an oxidative dehydrogenation catalyst for linear butenes. Its very weak interaction with butadiene explains the high selectivities obtained in oxidative dehydrogenation over this catalyst. In contrast, catalysts that are highly selective for the production of oxygenates like furan and maleic anhydride, have been found to interact very strongly with butadiene.