Selective liquid phase oxidation of o-xylene with gaseous oxygen by transition metal containing polysiloxane initiator/catalyst systems

The selective liquid phase oxidation of o-xylene over hydrophobic porous Co ²⁺ and Mn ³⁺ containing polysiloxane catalysts showed that both catalysts had higher activity than the homogeneous benchmark system Co naphthenate. The solid catalyst/initiator systems accelerate the radical initiation and the hydroperoxide decomposition. 2-Methylbenzyl hydroperoxide plays a key role in the reaction network as it is the first observed intermediate and is efficiently decomposed by the catalysts. The selectivity of the hydroperoxide decomposition can be controlled to some extent by the choice of transition metal. Co ²⁺ enhances the formation of o-tolualdehyde, while Mn ³⁺ increases the tendency to 2-methylbenzyl alcohol formation. The hydrophobic character, demonstrated by the weak adsorption of water, retards the interaction with the polar reaction products and allows their fast desorption, which in turn is a major cause for the high catalytic activity.