Hydrogenation of olefins. Part 3.—Reaction of ethylene and of propylene with deuterium over alumina-supported palladium and rhodium

Abstract

The following reactions have been studied: I, C₂H₄+D₂ over Pd/A1₂O₃(–36 to 67°); II, C₃H₆+D₂ over Pd/A1₂O₃(19°C); III, C₂H₄+D₂ over Rh/Al₂O₃(–18 to 110°); and IV, C₃H₆+D₂ over Rh/Al₂O₃(88°C). Products obtained at low conversions have been separated by preparative G.L.C., and deutero-olefin and deuteroparaffin distributions derived by mass-spectrometric analysis. When the initial D₂/olefin ratio is unity, the near-initial value of F(=∑(deutero-olefin)/(paraffins)) is 0.8 for reaction I at –16° and 2.7 for reaction III at 76°; F decreases with increasing initial D₂/olefin ratio at these temperatures, and increases with increasing temperature within the ranges quoted above. Ethanes initially formed in reaction I (D₂/olefin = 1, –16°) contain an average of about one D atom: virtually no HD is formed. Ethanes initially formed in reaction III (D₂/olefin = 1, 76°) contain an average of 1.5 D atoms: considerable HD formation occurs. Reactions II and IV give product distributions and values of F similar to those given by reactions I and III under the same conditions.

Observed product distributions from reactions I and III have been compared with those computed from a steady-state scheme applicable to the C₂H₄+D₂ system; the parameters giving optimum agreement indicate that ethylene desorption and reversion of ethyl radicals to adsorbed ethylene are both easy over Rh and Pd. Reaction mechanisms are discussed, and a ready “direct” atom transfer between ethyl radicals and ethylene molecules is indicated over Pd.