Mass Transfer Limitations During Hydrogenation Revealed by CS2 Titration of Active Sites

Abstract

We suggest a new method to test for mass transfer effects in catalytic reactions which accomplishes the same results as the Madon-Boudart test but does not require the preparation of two catalysts with different loadings and the same percent dispersion. This method works for liquid phase hydrogenations and depends on the linear poisoning of addition by CS2. CS2 can be used to titrate active sites on Pt and Pd on which the decrease in turnover frequency for addition is linear with CS2 molecules unless mass transfer occurs. This is true for the hydrogenation of (-)-apopinene and cyclohexene. During mass transfer limitations the decrease in turnover frequency as a function of CS2 molecules is slower during diffusion control and faster after the rate has been reduced below the diffusion limitation. Extrapolation back to zero CS2 of the faster decrease (non-diffusion-limited) reveals the true rate. Moreover, if it is assumed that one CS2 poisons one active site, the true turnover frequency can be determined from the true rate and the end point for the titration. The method allows a simple test of all mass transfer effects for the liquid 158phase hydrogenation reaction. It may be adaptable to other experimental situations.