Cyclohexane oxidation catalyzed by titanium silicalite (TS-1): Overoxidation and comparison with other oxidation systems

At 100 degrees C cyclohexanol and cyclohexanone, obtained in the TS-1 catalyzed oxidation of cyclohexane, are further oxidized in uncatalyzed and TS-1 catalyzed reactions, Cyclohexanol is very selectively oxidized to cyclohexanone inside the porous system of TS-1 and unselectively oxidized to several oxidation products on the external surface. This unselective oxidation can be suppressed by the addition of 2,6-di-tert-butyl-4-methylphenol (BHT), which does not enter the molecular sieve pore system and which efficiently reduces uncatalyzed oxidation, Cyclohexanone oxidation is mostly uncatalyzed, forming predominantly dicarboxylic acids, and is not influenced by BHT, The products of the TS-1 catalyzed cyclohexanone oxidation are partially retained in the porous system, thus explaining the deactivation of the catalyst. Comparison with other systems shows that the turnover frequency of the TS-1 catalyzed cyclohexane oxidation is very similar to that of the radical-chain process, thus suggesting that the rate-determining step of the TS-1 catalyzed reaction is also the homolytic cleavage of a C-H bond. (C) 1995 Academic Press, Inc.