THE MECHANISM OF CONVERSION OF SATURATED-HYDROCARBONS CATALYZED BY SULFATED METAL-OXIDES - REACTION OF ADAMANTANE ON SULFATED ZIRCONIA

The high activity of sulfated zirconia (SZ) toward hydrocarbon convers ions has been confirmed by the study of the isomerization of methylcyc lopentane to cyclohexane. This catalytic activity is generally rationa lized by the catalyst having superacidic strength. The reaction of met hylcyclopentane with superacids is initiated, however, by the cleavage of a carbon-carbon bond with the formation of an acyclic carbocation, followed by hydride transfer giving the methylcyclopentyl cation whic h undergoes rearrangement. By contrast, no isohexanes (products of rin g cleavage) were formed in the reaction on SZ, suggesting a different reaction mechanism. The mechanism of interaction of SZ with saturated hydrocarbons was elucidated by a study of adamantane. Small amounts of 1-adamantanol and adamantanone and traces of 2-adamantanol were obser ved after reaction at temperatures from 65 to 135 degrees C, indicatin g that the reaction is an oxidation followed by hydride transfers. Sma ll amounts of diadamantanes were also formed, proving that oxidation t o carbocations goes through the free radical stage. At 150 degrees C, additional reaction products were observed, 1-adamantanethiol (larger amount) and 2-adamantanethiol (smaller amount), indicating reduction o f sulfate all the way to sulfide, which then traps the adamantyl catio n in competition with the oxygen anions or water formed in the redox p rocess. Ring cleavage and disproportionation to form alkyladamantanes and aromatics also occurred. Thus, the increase in activity of SZ over the parent oxide for carbocationic alkane and cycloalkane reactions c an be ascribed to initiation through a one-electron oxidation of the h ydrocarbon by sulfate to a carbocation precursor. (c) 1996 Academic Pr ess, Inc.