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Catalytic Reaction Sequences: TON and TOF

Catalytic Reaction Sequences: TON and TOF. Whereas KCL was the key variable in chain reaction sequences, turn-over number (TON) and turn-over frequency (TOF) are widely cited in catalytic reaction literature. Beware of differing definitions, especially amongst chemists and biochemists

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Catalytic Reaction Sequences: TON and TOF

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  1. Catalytic Reaction Sequences: TON and TOF • Whereas KCL was the key variable in chain reaction sequences, turn-over number (TON) and turn-over frequency (TOF) are widely cited in catalytic reaction literature. • Beware of differing definitions, especially amongst chemists and biochemists • TON varies from infinity (ideally) to small values depending on the rate of catalyst deactivation • TOF is a simple measure of kinetic reactivity, but is a function of reagent concentrations

  2. Catalytic 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.

  3. Catalytic Olefin Isomerization - Product Distribution 4-centred planar transition state

  4. Catalytic Olefin Isomerization • Based on this mechanism and set of simplifying assumptions, the rate of 1-pentene consumption should abide by the following rate expression. • The turn-over frequency of this process is given by: • What is the turn-over number?

  5. Catalytic Hydrogenation of High-vinyl Polybutadiene

  6. Catalytic Hydrogenation of High-vinyl Polybutadiene

  7. Catalytic Olefin Hydrogenation Catalyzed by RhCl(PPh3)3

  8. 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

  9. 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:

  10. 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:

  11. Food for thought… • Given the derived rate expression of the general form: • Can you estimate the equation constants from the following extracted data for BR hydrogenation? rHydrg'n [Rh]T [C=C]o [PPh3] [H2] mole/m3.s mole/m3 mole/m3 mole/m3 mole/m3 0.040 3.9 850 5.8 0.25 0.105 3.9 850 5.8 1.30 0.135 3.9 850 5.8 3.14 0.380 3.9 850 0.0 3.14 0.068 3.9 850 15.2 3.14 0.045 3.9 250 5.8 3.14 0.185 3.9 1350 5.8 3.14 0.061 1.9 850 5.8 3.14 0.183 5.3 850 5.8 3.14

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