Abstract
Addition of steam in the partial oxidation of propene improves selectivity and is frequently mentioned in the patent literature. Despite its use, little has been published on the effect of steam on partial oxidation kinetics or on the mechanism through which steam alters selectivity. This contribution addresses these questions employing results from steady state and step-change measurements of rates of product information, isotopic transient measurements and temperature-programmed desorption studies. Two catalysts were investigated: a Sb/Sn/V oxide, described in the literature as capable of forming acrylic acid, and a Bi/Mo oxide that does not form the acid.
For the Bi/Mo oxide, steam addition reduces the rates of product formation, but does not affect selectivity markedly and leaves the reaction order with respect to reactants unchanged. The role of water is to block reaction sites by competitive adsorption on these sites.
The effect of steam addition on the Sb/Sn/V oxide is complicated. Low levels of steam (< 2 %) sharply alter selectivity by suppressing total oxidation and increasing acrolein and acrylic acid formation rates. Total oxidation products arise solely from a C-C bond scission side-reaction which produces acetaldehyde and acetic acid. Steam levels greater than 5% decrease product formation. Step-change data indicate total oxidation of propene or partial oxidation products must occur and that acrylic acid must form through acrolein. Use of an O-18(2) isotope indicated O2 exchange between steam and the catalyst and suggested rapid formation of weakly adsorbed acrolein on the surface. The Sb/Sn/V oxide as a source of O2 for acrolein formation was indicated. TPD measurements suggested both weak and strong adsorption sites participate in the partial oxidation reactions. Acrolein was associated with the former, while propene and oxygenates where found on the latter sites. Acrylic and acetic acid am strongly adsorbed. These results are consistent with partial oxidation occurring on both weakly and strongly adsorbing surface sites. Acrolein appears to be produced mainly on the former; C-C bond scission and total oxidation occurs primarily on the latter. Acrylic acid is formed on these sites as well but via acrolein. The role of steam seems to be to selectively block the strongly adsorbing sites and enhance re-oxidation of the catalyst surface.