Reactions of hydrocarbons over Ru/SiO2: exchange with deuterium and the onset of hydrogenolysis
Exchange reactions with deuterium of a number of hydrocarbons, chosen to provide evidence about the nature of the intermediates, have been followed mass spectrometrically over a ruthenium–silica catalyst. The selected hydrocarbons were methane, propane, 2-methylpropane (2MP), 2,2-dimethylpropane (DMP), and 2,2,3,3-tetramethylbutane (TMB). Products from the reaction of propane and 2MP were analysed by deuterium NMR spectroscopy. The extent of multiple exchange increased with temperature and mechanisms involving αα- or αδ-adsorbed intermediates were favoured.
The hydrogenolyses of DMP and TMB were followed at higher temperature with emphasis on the selectivity of the reaction and the change of the pattern of products with contact time. Subsidiary experiments to learn more about the course of the reactions with TMB were carried out with 2,2,3-trimethylbutane (TriMB) and 2,3-dimethylbutane (23DMB).
There was evidence for a strong hydrocarbon–ruthenium interaction leading to a sequence of events with increase of temperature, relatively easy exchange but with self-poisoning, followed by moderately easy hydrogenolysis, again subject to self-poisoning. The sequence suggested the formation in turn of reversibly adsorbed hydrocarbon intermediates, more strongly adsorbed species leading to hydrogenolysis and finally carbonaceous residues.