Catalytic reactions on titanium dioxide. Part 2.—Exchange reactions of ketones with deuterium oxide and the isomerization of n-butenes

Abstract

Exchange of the enolizable hydrogen atoms in ketones with heavy water is catalyzed by rutile at 298 K. Stepwise replacement occurs and all the exchangeable atoms react at the same rate. The mechanism probably involves the reversible formation of a carbanion with the surface acting first as a base and then as an acid. No exchange of acetone occurs with deuterium at 373 K but larger molecules are formed by condensation of the ketone.

The isomerization of the n-butenes is catalyzed by rutile at 373 K and above. All six possible interconnecting reactions occur but the kinetically favoured processes are the formation of cis-but-2-ene from but-1-ene and double-bond migration with cis-but-2-ene. Experiments on isomerization in the presence of deuterium or heavy water and the co-isomerization of mixtures of light cis-but-2-ene and heavy cis-but-2-ene-d₈ were also carried out. The mechanisms of the reactions were thought to involve ionic intermediates and, in keeping with the amphoteric nature of the oxide, there was evidence for both base-catalyzed formation of carbanions and, to a lesser extent, acid-catalyzed processes involving carbonium ions. Selective hydrogenation of butadiene to butene, with preferential formation of but-1-ene, occurs on rutile at 623–693 K.