Isotopic exchange studies and selective oxidation of propene on mixed tin-antimony oxides

Abstract

The propene–D₂O and isobutene–D₂O exchange reactions have been investigated to clarify the initial hydrogen-abstraction stage of the catalytic oxidation of propene to acrolein on a range of mixed tin–antimony oxides. The propene–D₂O exchange at 473 K was stepwise, and i.r. and microwave analysis of the exchange products indicated the formation on propene pretreated catalysts of a symmetrical allyl intermediate similar to that reported to occur in the oxidation process. A carbonium ion species occurred in the absence of pretreatment. The formation of either intermediate is consistent with the observation that the exchange was limited to five of the propene hydrogen atoms. Stepwise exchange of all eight of the isobutene hydrogen atoms was found at 323 K, and probably involved a carbonium ion species.

The oxygen-addition stage of the oxidation has been examined by use of isotopic oxygen of mass 18 either in the gas or catalyst phase. The percentage of C₃H¹⁶₄O in the total acrolein formed at 573 K in the propene–¹⁸O₂ reaction was very high initially and decreased as the reaction proceeded, indicating that the oxygen atom in the acrolein is provided by the catalyst lattice which is subsequently re-oxidised by gas phase oxygen. The ¹⁶O content of the carbon dioxide formed also decreased as the reaction proceeded. Catalysts pre-exchanged with ¹⁸O₂ before use with propene–¹⁶O₂ mixtures gave much less C₃H₄¹⁸O than corresponded to the calculated ¹⁸O occupancy of the catalyst surface anion sites. This may have been due either to diffusion of ¹⁸O into the interior of the lattice at the high temperatures necessary to effect the exchange or to the low catalytic activity of exchanged anions.

The near equality found for the rate of disappearance of [²H₀]propene in the propene–D₂O exchange and in the oxidation reaction at the same temperature suggests that the formation of the allyl intermediate may be the rate determining step in both processes. The rate of the oxygen exchange at the oxidation temperatures used was extremely low, and hence the dissociative chemisorption of oxygen necessary for the exchange does not directly provide oxygen atoms for the oxidation.