The catalytic performance of the Wells-Dawson-type polyoxotungstate, K6P2W18O62, in the oxydehydrogenation of isobutane to isobutene has been studied, and compared with the activity of the corresponding Keggin compound, K3PW12O40 The Wells-Dawson compound is characterized by a lower activity than the Keggin salt, but by a remarkably higher selectivity to the olefin. The activity of the Wells-Dawson catalyst is strongly dependent on the hydrocarbon content in the feed, and increases with increased partial pressure of isobutane. This unexpected autocatalytic behavior is explained by a proposed mechanism where the olefin product induces a modification of the catalyst surface, with the creation of more active sites. In addition, at the highest hydrocarbon content in the feed, the contribution of heterogeneously-initiated, homogeneous gas-phase reactions become important, thus favoring high selectivity to isobutene.
