Studies on polymer anchored and homogeneous catalysis : liquid phase selective oxidation of styrene to benzaldehyde by wilkinson catalyst

Abstract

Wilkinson catalyst [tris(triphenylphosphine)chlororhodium(I)] finds extensive application in liquid-phase organic reactions, but its industrial applications are severely restricted by the impracticality of economically recovering it from homogeneous reaction media. This limitation can be overcome by anchoring the catalyst to an inert organic or inorganic support to heterogenize the homogeneous catalyst. Such anchoring not only offers advantages like energy and raw material conservation and easy catalyst recovery, but in some cases may also enhance catalytic activity and selectivity. Furthermore, the scope of Wilkinson catalyst in oxidation reactions of industrial importance has not been fully explored.

In this study, the kinetics of liquid-phase selective oxidation of styrene to benzaldehyde using Wilkinson catalyst in both homogeneous and anchored forms has been investigated. Wilkinson catalyst and its analog anchored to cross-linked polystyrene beads were prepared in the laboratory. Catalyst characteristics were studied using IR and NMR spectroscopy, as well as SEM (scanning electron microscopy) observations.

Styrene oxidation by molecular oxygen under controlled conditions in a toluene medium in a semi-batch apparatus yielded benzaldehyde and formaldehyde as the exclusive primary products. Prolonged oxidation led to the formation of acids as secondary products. The effects of temperature, catalyst concentration, and solvent/reactant ratio on styrene conversion and product formation were studied. A reaction scheme based on a free radical mechanism gave rise to a pseudo-first-order model, which agreed well with the observed kinetic data. A second-order model was proposed for the co-oxidation of benzaldehyde to benzoic acid, which also matched experimental results.

The objective of the kinetic study with anchored catalyst was to delineate the significance of diffusional limitations on chemical reactions, using a suitable model against which the observed rate data could be tested. The studies were performed in specially designed closed batch apparatus, which allowed monitoring of the reaction course by measuring gaseous oxygen consumption.

The effects of temperature, catalyst loading, and catalyst distribution on the rate of oxygen consumption were analyzed. A study of diffusion effects in liquid-filled pores of the catalyst-anchored polymer support, along with a proposed empirical relationship between fractional surface coverage of the polymer support and catalyst loadings, provided a working model for testing the observed data. Computed values of effectiveness factors, based on experimental data and the theoretical model, conclusively demonstrated that within the range of experimental parameters employed, the chemical reaction control regime was dominant.