Isomerization of n-buteneses on titanium dioxide and other oxide catalysts

The work in this thesis involved the study of the n-butene isomerization reactions over titanium dioxide (rutile), the oxides of the other metals in the first transition series, magnesium oxide and tin oxide. Over rutile, it was found that double-bond migration was more predominant than cis-trans isomerization. The isomerization reactions of but-l-ene were unaffected by the presence of deuterium, but, in the presence of heavy water, the reactions were poisoned and deuterium atoms were incorporated into the product butene molecules by an exchange process. Evidence was found that suggested that double-bond migration took place on basic sites (probably produced by the removal of water during the outgassing process) via a carbanion mechanism, whilst cis-trans isomerization occurred by means of a carbonium ion mechanism on a smaller number of acidic sites (probably associated with residual hydroxyl groups on the surface of the oxide). An activity pattern was found over the oxides of the metals in the first transition series which resembled those found for H₂-D₂ exchange, ethylene hydrogenation and the reactions of cyclohexene. Maximum activity was found for Cr₂O₃, Cr₂O₃O₄ and ZnO, whilst MnO and CuO were the least active catalysts. The results were discussed in terms of the d-electron and crystal field theories of metal oxide catalysis. A correlation was found between the acidity and basicity of the metal oxides and the nature of the isomerization reactions. Magnesium oxide was found to be the most active of all the oxides studied. An almost complete conversion of but-l-ene to cis-but-2-ene occurred within a few minutes over 0.01 g samples at -68°C. It was suggested that double-bond migration occurred on basic sites via a carbanion mechanism, and there was a possibility that a base-catalysed chain reaction was present. Cis-trans isomerization was slower and was thought to occur by a carbonium ion mechanism on acidic sites. It was found that but-l-ene and cis-but-2-ene underwent very different reactions over tin oxide. The reactions of but-l-ene involved rapid exchange with hydrogen from the catalyst, oxidative dehydrogenation to butadiene and irreversible isomerization to the but-2-enes. It was suggested that the 1-methylallyl radical might be a common intermediate in these processes. Cis-but-2-ene, on the other hand, reacted to give trans-but-2-ene as the only product, and did not exchange with hydrogen from the catalyst. A carbonium ion mechanism was put forward to explain the reaction of this butene.