The role of promoters in the oxidehydrogenation of ethane over structurally stable Keggin-type heteropolyoxomolybdates

摘要

Potassium salts of 12-molybdophosphoric acid (Keggin-type heteropolycompounds) were tested as catalysts for the oxidehydrogenation of ethane. The catalyst composition comprised at least one atom of antimony per Keggin formula, which induced a considerable structural stability in the primary structure of the heteropolycompound. The base composition was doped with variable amounts of cerium, chromium and iron ions. Each ion showed a peculiar effect in enhancing the catalytic performance: cerium and chromium mainly affected the catalytic activity, so counteracting the deactivating effect induced by antimony addition, while iron improved the selectivity. A further improvement of the selectivity was achieved by adding a slight excess of phosphorus in the composition, and by partial replacement of molybdenum with tungsten. All catalysts showed an initial unstable catalytic performance; the attainment of a stable performance was also a function of the composition. The best catalytic composition was finally obtained, corresponding to K_2P_(1.2)Mo_(10)W_1Sb_1Fe_1Cr_(0.5)Ce_(0.75)O_n; the catalyst exhibited a stable catalytic performance for 100 h at least; the spent compound was characterized by an XRD pattern exhibiting only the features of the cubic heteropolycompound.