Publication Catalysis 2011

A. R. Almeida, R. Berger, J. A. Moulijn, G. Mul, Phys. Chem. Chem. Phys. 2011, 13, 1345.

The rate of cyclohexane photo-catalytic oxidn. to cyclohexanone over anatase TiO2 was studied at 23-60 °C by in situ ATR-FTIR spectroscopy, and the kinetic parameters were estd. using a microkinetic model. At low temps., surface cyclohexanone formation is limited by cyclohexane adsorption due to unfavorable desorption of H2O, rather than previously proposed slow desorption of the product cyclohexanone. Up to 50 °C, the activation energy for photocatalytic cyclohexanone formation is zero, while carboxylates are formed with an activation energy of 18.4 ± 3.3 kJ mol-1. Above 50 °C, significant (thermal) oxidn. of cyclohexanone contributes to carboxylate formation.

The irreversibly adsorbed carboxylates lead to deactivation of the catalyst, and are most likely the predominant cause of the non-Arrhenius behavior at relatively high reaction temps., rather than cyclohexane adsorption limitations. The results imply that elevating the reaction temp. of photocatalytic cyclohexane oxidn. reduces selectivity, and is not a means to suppress catalyst deactivation.


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