REMARKABLE DECARBONYLATION OF CHROMIUM, MOLYBDENUM, AND TUNGSTEN HEXACARBONYLS SUPPORTED ON SILICA AND THE NATURE OF THESE MATERIALS AS HETEROGENEOUS CATALYSTS

The temperature-programmed decomposition (TPDE) of the hexacarbonyls supported on SiO2 gives rise to a single, narrow peak which is quite uncharacteristic of high surface area supports. Unlike A12O3, SiO2 does not sufficiently stabilize zerovalent subcarbonyl species during TPDE to allow for their ready isolation. Nonetheless, such species exist during the initial stages of TPDE and at least some of them are reversibly formed as was found for all of the subcarbonyl species identified on alumina. Thus, the hexacarbonyls provide a route to low-valent, supported Cr, Mo, and W which cannot be achieved by the traditional methods of catalyst synthesis. Contrary to published reports, the metals undergo partial oxidation by reaction with the support during TPDE and the complete decompositions are irreversible. The nature of the active sites for the metathesis of propylene is reinterpreted in light of this surface chemistry and it is suggested that the activity is due to both a subcarbonyl species which is formed near 100 °C and mildly oxidized and decarbonylated species which are formed at higher temperatures. It is reported that these materials are also slightly active for propylene hydrogenation, and the surface chemistry and activity of the catalysts are compared to those of the hexacarbonyls supported on A12O3. © 1979, American Chemical Society. All rights reserved.