Hybrid organic-inorganic materials: application in oxidative catalysis

Abstract

Oxidative catalysis has been and will be playing an important role in the production of large quantities of intermediates. This technology has a great potential for improvement and this has led to a series of better processes, including the development of a great diversity of suitable catalysts. Within these compounds, metal phosphates and phosphonates possessing layered and pillared structures were found to constitute a very good alternative as catalysts to be used in oxidative catalysis. Metal phosphates and phosphonates can be considered as strong inorganic solid acids and much of their catalytic activity has been attributed to the Brønsted acidity of the interlayered hydroxyl groups and to the Lewis acidity of the metal center. Metal phosphates and phosphonates can be obtained at low temperatures, in aqueous media, using soft chemical routes and their preparation is quite accessible if the correct phosphonic acids and phophates are available. These heterogeneous materials are very interesting from the economical as well as from the environmental points of view because of the high yields and short reaction times, easy recovery from the reaction mixtures and the possibility to perform reactions in solvent-free conditions. Due to their particular physical and chemical properties and high versatility, in the last forty years, this class of organic-inorganic materials has been captivating the attention of many researchers involved in heterogeneous catalysis. The main research effort in the metal phosphate and phosphonate field was initially directed towards compounds with tetravalent metal cations, but a wide variety of divalent and trivalent metals have also been reported. Nowadays, a large number of metal phosphates and phosphonates of the - and γ-type are known. The general interest in the chemistry of metal phosphates and phosphonates is mainly due to their unusual compositional and structural diversity varying from one-dimensional arrangements to three-dimensional microporous frameworks, passing by the most common layered networks. Good results have been obtained with a large diversity of metal phosphates and phosphonates in a variety of organic reactions, and particularly in oxidative catalysis. The importance of these systems has also been recognized in research areas such as electrochemistry, microelectronics, biological membranes and photochemical mechanisms.