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Olefin Isomerization

Olefin Isomerization. RuHCl(PPh 3 ) 3 will hydrogenate olefins in the presence of H 2 , but it also isomerizes a -olefins to internal olefins through reactions of the Ru-H bond. Olefin Isomerization - Product Distribution. 4-centred planar transition state.

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Olefin Isomerization

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  1. Olefin Isomerization • RuHCl(PPh3)3 will hydrogenate olefins in the presence of H2, but it also isomerizes a-olefins to internal olefins through reactions of the Ru-H bond. J.S. Parent

  2. Olefin Isomerization - Product Distribution 4-centred planar transition state J.S. Parent

  3. Olefin Hydrogenation Catalyzed by RhCl(PPh3)3 J.S. Parent

  4. Modeling - Not Just for Beautiful People • Using our proposed catalytic mechanism, we can derive a design equation which expresses the hydrogenation rate as a function of process conditions. • This expression can be used to test the mechanism against experimental data • Fitting the expression to the data can yield a model of the reaction for use in process design and control • We will apply the “hydride” pathway as opposed to the “olefin” pathway. 1 K1 K2 K3 2 3 5 r.d.s. k4 irreversible 4 J.S. Parent

  5. Modeling RhCl(PPh3)3 Catalyzed Olefin Hydrogenation • The rate of hydrogenation, as defined by the mechanism, is that of the rate determining step, r4: • Therefore, the reaction rate is: • However, this is not a useful design equation, given that the concentration of RhClH2(C=C)(PPh3)2 cannot be measured. • Treating the mechanism as a sequence of elementary reactions, we can express the reaction rate as: J.S. Parent

  6. Modeling RhCl(PPh3)3 Catalyzed Olefin Hydrogenation • We now have an equation that represents the reaction rate as a function of the process conditions. A simplified form is consistent with experimental data: • If the reaction is run under constant pressure and kinetic control (as opposed to mass transfer limited) we expect the rate of olefin hydrogenation to be: • where: J.S. Parent

  7. Hydrogenation of High-vinyl Polybutadiene • As predicted, the hydrogenation rate is linear with respect to the olefin concentration • the reaction is therefore first-order with respect to [C=C], meaning hydrogenation rates are relatively fast at low olefin conversions, but become quite slow as [C=C] decreases. J.S. Parent

  8. Effect of [Rh]T on the Hydrogenation Rate • We have little or no knowledge of the quantity of each rhodium complex in solution. • We only know the total amount of catalyst precursor charged to the system • The observed reaction rate for PBD hydrogenation is linear with respect to [Rh]T, indicating that the process is first-order with respect to total rhodium concentration. J.S. Parent

  9. Effect of [H2] on the Hydrogenation Rate • The influence of H2 pressure on the reaction rate is complex • At low pressure, the reaction is first-order with respect to [H2] • As the pressure increases, the reaction order diminishes towards zero-order behaviour. • This results from a saturation of the catalytic system with hydrogen. • Once reaction 2 is displaced towards the dihydride, the influence of further H2 is reduced. J.S. Parent

  10. Effect of [PPh3] on the Hydrogenation Rate • PPh3 should be added to the reaction mixture to limit the extent of Rh dimerization (a side reaction shown in Slide 2). • Our kinetic model predicts that additional phosphine will inhibit the hydrogenation rate by displacing reaction 1 towards the tris phosphine complex. • This is demonstrated quite clearly in the hydrogenation of PBD. J.S. Parent

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