The Mechanism of Conversion of Saturated Hydrocarbons Catalyzed by Sulfated Metal Oxides: Reaction of Adamantane on Sulfated Zirconia

• Published in: Journal of catalysis 1996-01, Vol.158 (1), p.116-127
• Main Authors: Fărcaşiu, Dan, Ghenciu, Anca, Li, Jing Qi
• Format: Article
• Language: English
• Subjects: Catalysis; Catalytic reactions; Chemistry; Exact sciences and technology; General and physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1006/jcat.1996.0013

The high activity of sulfated zirconia (SZ) toward hydrocarbon conversions has been confirmed by the study of the isomerization of methylcyclopentane to cyclohexane. This catalytic activity is generally rationalized by the catalyst having superacidic strength. The reaction of methylcyclopentane 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 which undergoes rearrangement. By contrast, no isohexanes (products of ring 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 observed after reaction at temperatures from 65 to 135°C, indicating that the reaction is an oxidation followed by hydride transfers. Small amounts of diadamantanes were also formed, proving that oxidation to carbocations goes through the free radical stage. At 150°C, additional reaction products were observed, 1-adamantanethiol (larger amount) and 2-adamantanethiol (smaller amount), indicating reduction of sulfate all the way to sulfide, which then traps the adamantyl cation in competition with the oxygen anions or water formed in the redox process. 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 can be ascribed to initiation through a one-electron oxidation of the hydrocarbon by sulfate to a carbocation precursor.