The identification of different active sites on Mo/Al2O3 metathesis catalysts

Mo/Al2O3 catalysts prepared via fixation of Mo(η3-C3H5)4 on Al2O3 or by conventional impregnation (2.2 or 2.9 wt% Mo) have been compared with regard to their catalytic behavior in the metathesis of propene in different temperature ranges (293-323 K, 473 K). Different active sites have been distinguished. A site derived from a Mo(VI) precursor by thermal activation in inert gas exhibits stable activity, with a propene reaction order near 1. Other sites that are derived from a reduced Mo precursor, probably Mo(IV), are of higher activity but unstable with time-on-stream and also at elevated temperatures (>323 K). These sites support the metathesis at a propene reaction order of 0.5 and with activation energies between 10 and 25 kJ/mol depending on unknown structural details. Due to their instability, they cannot contribute to the high-temperature (T > 373 K) metathesis activity observed with Mo/Al2O3 catalysts. The latter is supported by Mo(VI)-derived sites or, at after reduction of catalysts with higher Mo contents, by Mo(IV)-derived sites that are different from those identified in the present study.