Catalysis of Organic..

20Supported Re Catalysts for Olefin MetathesisMetathesis activity. A quantitative comparison of metathesis activities was madein the gas phase homometathesis of propylene. The reaction kinetics are readilymonitored since all olefins (propylene, ethylene, cis- and trans-2-butylenes) arepresent in a single phase. Metathesis of 30 Torr propylene was monitored in a batchreactor thermostatted at 0 °C, in the presence of 10 mg catalyst. The disappearanceof propylene over perrhenate/silica-alumina (0.83 wt% Re) activated with SnMe 4 isshown in Figure 2a. The propylene-time profile is pseudo-first-order, with k obs =(1.11 ± 0.04) x 10 -3 s -1 . Subsequent additions of propylene to the catalyst gave aslightly lower rate constant, (0.67 ± 0.02) x 10 -3 s -1 . The pseudo-first-order rateconstants are linearly dependent on Re loading, Figure 3. The slope yields thesecond-order rate constant k = (13.2 ± 0.2) s -1 (g Re) -1 at 0°C.A similar experiment was performed with MeReO 3 supported on silica-alumina.However, prior to deposition of the organometallic catalyst, the support was firsttreated with hexamethyldisilazane (HMDS), eliminating Brønsted acid sites on thesilica-alumina surface by converting them to unreactive silyl ethers (14). This servesto reproduce one of the effects of treating perrhenate/silica-alumina with SnMe 4 ,since the latter is protonated by the hydroxyl groups to generate stannyl ethers andmethane.1.000.95a1.000.95bmole fraction propylene0.900.850.800.750.700.900.850.800.750.650.700.600 20 40 60 80 100 120 140time / min0.650 1020304050607080time / minFigure 2 Kinetics of gas-phase propylene homometathesis at 0°C, catalyzed by (a)perrhenate/silica-alumina activated by SnMe 4 (10 mg, 0.83 wt % Re); and (b)MeReO 3 on HMDS-capped silica-alumina (10 mg, 1.4 wt % Re). Solid lines arecurve-fits to the first-order integrated rate equation. Solid squares: first addition;solid circles: second addition; open circles: third addition of propylene (30 Torr) tothe catalyst.