Activities and stabilities of heterogeneous catalysts in selective liquid phase oxidations: recent developments

Different strategies for the heterogenization of redox-active elements in s olid matrices are reviewed. These include framework-substituted molecular s ieves, amorphous mixed oxides by grafting or sol-gel methods, grafting or t ethering to the inner walls of mesoporous molecular sieves, encapsulation b y ship-in-a-bottle or other techniques and ion exchange in layered double h ydroxides. The different approaches are illustrated by reference to recent developments involving a variety of metal catalysts - titanium, chromium, c obalt, manganese, iron, ruthenium, tungsten, molybdenum, vanadium and tanta lum - in oxidations with O-2, H2O2 and RO2H as primary oxidants. Emphasis i s placed on an evaluation of the stability of the various catalysts under r eaction conditions, a conditio sine qua non for practical utility. Protocol s for establishing heterogeneity are discussed. An analysis of experimental results leads to the conclusion that many of the systems described in the literature, particularly those involving oxometal species, are unstable tow ards leaching or the appropriate rigorous tests for heterogeneity have not been-performed. (C) 2001 Elsevier Science B.V. All rights reserved.