Silica-immobilized N-hydroxyphthalimide: An efficient heterogeneous autoxidation catalyst

Despite the potential of N-hydroxyphthalimide as a sustainable organocatalyst for autoxidations, recycling remains a largely unexplored but important topic. In this contribution, impregnation on silica is presented as a successful immobilization strategy. The activity of this catalyst during cyclohexane autoxidation is closely correlated with the support surface structure, as elucidated by 29Si MAS NMR. It is found that sorption of byproducts is the main cause of deactivation such that only silicas with a low number of residual hydroxyl groups result in stable catalytic systems. Nevertheless, even for active catalysts, a slight loss in activity can be observed on a first recycling. Fortunately, the catalytic activity remains steady in subsequent recycling runs, showing only negligible additional byproduct sorption. The reduced hydroperoxide and enhanced ketone yields afforded by this catalyst is explained in terms of the phthalimide-N-oxyl radical chemistry, whereby this nitroxyl radical efficiently converts hydroperoxide to ketone on abstraction of the αH-atom.