The H/D Exchange and Hydrogenolysis of Butane on an Alumina‐Supported Nickel Catalyst

Abstract

AbstractThe exchange of butane with deuterium on a nickel/alumina catalyst is studied over a wide range of temperatures (50–230°) and for three hydrocarbon/deuterium ratios. The initial distribution is used to propose a reaction scheme by considering different adsorbed species such as: alkyl‐(α), alkene‐(α, β) and allyl‐(α,β,γ) species, where α, β, and γ are different C‐atoms. It is assumed that each adsorbed species can either desorb directly while exchanging one, two, and three H‐atoms or undergo multiple exchange before desorption. It is shown that by reducing the activity of the catalyst a back‐exchange of the fully deuterated species sets in a lower temperature; a systematic deviation of all model calculations for the estimation of the concentrations of the C4HD9 and C4D10 is, thus, explained. The exchange of CH3CD2CD2CH3 and CD3CH2CH2CD3 with H2 and D2 is also studied, and similar results are obtained as with exchanges of CH3CD2CH3 and CD3CH2CD3. With the help of very‐high‐resolution mass spectra, it is shown that the initial degree of deuteration, i. e. the degree of deuteration after one adsorption step, is 84% for the methylene and 57% for the methyl group respectively. The hydrogenolysis reactions of butane is studied in the same system between 180° and 230°. A wide range of conversion is covered, and the product distributions are fitted to kinetic equations in order to obtain the initial rate constants. On the nickel/alumina catalyst one or several C–C bonds are broken before the desorption of the species. Under the conditions used in these experiments, the surface cracking is the rate‐limiting step. An isotope effect is observed for the decomposition in D2, the production of propane being favored.