The mechanism of conversion of hydrocarbons on sulfated metal oxides .2. Reaction of benzene on sulfated zirconia

The conversion of benzene on sulfated zirconia was studied in batch reactor, under mild conditions. The interaction of benzene with the catalyst is a complex process which is initiated by a one-electron oxidation, followed either by trapping of the generated cation-radicals to form sulfite esters on the surface, or by the reaction of the cation-radicals with the excess of benzene, followed by a cascade of coupling and cleavage reactions. The surface esters liberate phenol upon hydrolysis at the end of the reaction. Thus, benzene is retained on sulfated zirconia not because of protonation to benzenium ion, but because of formation of non-volatile products. Together with our previous results on the reaction of adamantane on sulfated zirconia, the present work elucidates the mechanism of conversion of aliphatic hydrocarbons on sulfated metal oxides. The first interaction is a one-electron oxidation of the alkane, leading to an ion-radical pair, followed by recombination and rearrangement to generate sulfite esters on the surface, which are the active intermediates in the mechanism. The high activity of sulfated metal oxides in alkane conversion is due, therefore, to their one-electron oxidizing ability, leading to ion-radicals and then to surface esters. The latter either ionize generating carbocations, or eliminate forming olefins. Both these species can carry on carbocationic reactions with no requirement of superacidity, which these catalysts do not possess. The oxidative mechanism predicts the existence of an induction period in the alkane conversions and a rapid deactivation of the catalyst, both in agreement with the experimental observations.