Chapter 28 - The Fascinating Chemistry of Iron- and Copper-Containing Zeolites

Abstract

This chapter collects the available information about the nature of surface oxygen species in iron and copper zeolites and explains the fascinating chemistry of these materials. Transition-metal cations, which are embedded in a zeolite host matrix, do not have the same catalytic properties as the corresponding transition-metal oxides. Iron- and copper-containing zeolites are among the most remarkable examples. They catalyse a number of different reactions, the most fascinating being the selective oxidation of very inert hydrocarbon substrates. The chapter attempts to identify the factors that give some iron and copper zeolites these particular, “enzyme-like” properties. Iron zeolites work best as selective oxidation catalysts at low iron concentrations. Moreover, the active sites for selective oxidation react specifically with N2O, but not with O2. This indicates that site isolation is an important premise for obtaining selective iron zeolite catalysts. All iron zeolites that were identified as selective catalysts for benzene oxidation are high-silica zeolites. Copper zeolites catalyse similar reactions as their iron analogues, but there are some distinct differences: Cu zeolites work best at high copper loadings and their catalytic activity is strongly related to the formation of Cu dimers. In contrast to metal oxide catalysts, the zeolite host structure stabilizes the transition-metal cations in monomeric or dimeric form, allows them to switch easily between two oxidation states and to generate highly reactive extra-lattice oxygen species. These features are responsible for the remarkable catalytic properties of the transition-metal zeolites.