Reactions of unsaturated hydrocarbons with deuterium on titanium dioxide

Abstract

The catalytic reactions of ethylene, propylene, isobutene, benzene, toluene, m-xylene and hydrogen with deuterium have been studied on rutile in the temperature range 250–450°C. Some reactions were also carried out using heavy water in place of deuterium. The olefins which can form allyl radicals were exchanged about 100 times more rapidly than ethylene and only five of the hydrogen atoms in propylene were replaced by the faster process. Rates of conversion of the olefins to alkanes were similar to the rates of exchange of the hydrogen atoms in ethylene and the main initial product in each case was the alkane-d₂. All the hydrogen atoms in toluene were exchanged at the same rate but the ring hydrogen atoms of m-xylene reacted slightly more rapidly than those in the side groups.

The exchange reactions involving heavy water occurred at similar rates and with similar activation energies to those with deuterium. The molecules fell into three groups: (a) water, (b) hydrogen, and (c) olefins and alkylbenzenes, in terms of the ease with which they were activated by the catalyst for the exchange processes. The nature of the intermediates taking part in the reactions is discussed and the possibility of mechanisms involving carbanions or carbonium ions is considered. The number of hydroxyl groups on the surface of the catalyst was determined by exchange with deuterium.