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 re actor, under mild conditions. The interaction of benzene with the cata lyst is a complex process which is initiated by a one-electron oxidati on, followed either by trapping of the generated cation-radicals to fo rm sulfite esters on the surface, or by the reaction of the cation-rad icals with the excess of benzene, followed by a cascade of coupling an d cleavage reactions. The surface esters liberate phenol upon hydrolys is at the end of the reaction. Thus, benzene is retained on sulfated z irconia not because of protonation to benzenium ion, but because of fo rmation of non-volatile products. Together with our previous results o n the reaction of adamantane on sulfated zirconia, the present work el ucidates the mechanism of conversion of aliphatic hydrocarbons on sulf ated metal oxides. The first interaction is a one-electron oxidation o f the alkane, leading to an ion-radical pair, followed by recombinatio n and rearrangement to generate sulfite esters on the surface, which a re the active intermediates in the mechanism. The high activity of sul fated metal oxides in alkane conversion is due, therefore, to their on e-electron oxidizing ability, leading to ion-radicals and then to surf ace esters. The latter either ionize generating carbocations, or elimi nate forming olefins. Both these species can carry on carbocationic re actions with no requirement of superacidity, which these catalysts do not possess. The oxidative mechanism predicts the existence of an indu ction period in the alkane conversions and a rapid deactivation of the catalyst, both in agreement with the experimental observations.