Green oxidation catalysis with metal complexes: from bulk to nano recyclable hybrid catalysts

Abstract

Transition metal complexes are efficient homogeneous catalysts for a plethora of liquid-phase reactions, but their low stability, high cost and difficult recovery from the reaction medium limit their implementation in industrial processes. Their immobilization onto solid supports emerged as a potential strategy to overcome these issues, giving rise to recyclable hybrid catalysts which have been conquering new horizons in Green Catalysis. This report reviews the progress on the design and catalytic activity of recyclable hybrid catalysts based on metallosalen, metal acetylacetonate and metalloporphyrin complexes for oxidation reactions, starting from metal complexes supported onto bulk supports – carbon materials, mesoporous silicas, clay based materials and metal-organic frameworks – and progressively scaling down to nanosupports – carbon nanotubes, nanosilicas, magnetic iron oxide nanoparticles and colloidal gold. An overview of the general methodologies developed for metal complex immobilization onto solid supports is presented as a function of the type of interaction between the complex and the support – covalent bonding, non-covalent interactions and encapsulation. For each type of support, the most common immobilization strategies are also delineated. Special emphasis is given to literature examples in which the immobilization strategy and catalytic performance of the hybrid catalysts, including activity and reusability, were analyzed; the influence of the support dimensions on the performance of the hybrid catalysts is also addressed.