Reaction Mechanism and Kinetics of Olefin Metathesis by Supported ReOx/Al2O3 Catalysts

The self-metathesis of propylene by heterogeneous supported ReOx/Al2O3 catalysts was investigated with in situ Raman spectroscopy, isotopic switch (D-C-3(=) -> H-C-3(=)), temperature-programmed surface reaction (TPSR) spectroscopy, and steady-state kinetic studies. The in situ Raman studies showed that two distinct surface ReO4 sites are present on alumina and that the olefins preferentially interact with surface ReO4 sites anchored at acidic surface sites of alumina (olefin adsorption: C-4(=) > C-3(=) > C-2(=)). The isotopic switch experiments demonstrate that surface Re*=CH3 and Re*=CHCH3 are present during propylene metathesis, with Re* representing activated surface rhenia sites. At low temperatures (<100 degrees C), the rate-determining step is adsorption of propylene on two adjacent surface sites (rate [C-3(=)][Re*](2). At high temperatures (>100 degrees C), the rate-determining step is the recombination of two surface propylene molecules (rate approximate to [C-3(=)](2)[Re*]). To a lesser extent, the recombination of surface Re*=CH3 and Re*=CHCH3 intermediates also contributes to self-metathesis of propylene at elevated reaction temperatures.