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Platinum Metals Rev., 1996, 40, (1), 37

Alkane Oxidation by Encapsulated Ruthenium Complexes

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The oxidation reaction of alkanes by the usual metal complex catalysts eventually results in deactivation or catalyst degradation. Catalysts, such as porphyrin- or phthalocyanine-based complexes, can acquire improved stability by a halogenation treatment. However, these sys-tems still eventually degrade. Encapsulating catalytic materials within other structures is now a routine operation, and has resulted in many interesting properties (see, for example, Platinum Metals Rev., 1994, 38, (2), 59) including enhanced catalytic activity. Therefore, isolating the phthalocyanine catalyst, to remove it from outside attack, by encasing it within zeolite pores was tried as a way to improve its lifetime. However, there are still drawbacks caused by the peroxide and iodosylbenzene oxidants.

Recent work has involved encapsulating an iron phthalocyanine, FePc, catalyst in zeolite within a polymer matrix, and this has been successful in producing a more active and stable catalyst. A perfluorinated FePc complex catalyst, however, was as sensitive to the oxidant as FePc

Based on this researchers at the University of Texas at Dallas, U.S.A., (K. J. Balkus, M. Eissa and R. Levado, J. Am. Chem. Soc., 1995, 117, (43), 10753–10754) prepared a fluorinated ruthenium phthalocyanine catalyst, RuF16Pc, and encapsulated it in zeolite NaX. It was used together with tert -butyl hydroperoxide oxidant for the oxidation of cyclohexane and cyclo-hexanol at room temperature.

Encapsulation prevented dimerisation of the complex and enhanced the activity. Even though the catalyst now contained less metal, it had over ten times the activity of the homogeneous catalyst. Cyclohexane was oxidised primarily to the ketone at a rate of almost 3000 turnovers/day and no deactivation was seen after over 20,000 turnovers. Placing the RuF16Pc into the zeolite increased its selectivity towards cyclohexanone, compared with the homogeneous catalyst. The peroxide efficiency was also improved.

This system may be one of the best low-temperature peroxide-based oxidation catalysts.

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