Coordination chemistry and reactivity of copper in zeolites

摘要

Cu-zeolites are highly promising heterogeneous catalysts for a variety of chemical reactions. Knowledge of the location and coordination of copper in the zeolite lattice is key to fully understand its fascinating catalytic properties. Various methods exist to introduce Cu in zeolites, cation exchange being commonly used. The location of Cu in the zeolite structure has been researched for many years and an overview is presented here for the common zeolite topologies. Besides simple Cu-ion exchange, also ligated copper complexes are included in the pores and cages of zeolites. Irrespective of their introduction method, the zeolite does not merely serve as the inorganic host. As Cu interacts with the zeolite framework, while being located in a unique constraint microenvironment, a distinctly different Cu coordination chemistry often exists in the zeolites. Effects thereof on the activity of various Cu complexes are discussed in relation to oxidation chemistry and catalysis. An interesting case is the (re)activity of Cu, when exchanged in a ZSM-5 zeolite. Cu-ZSM-5 is for instance able to catalytically decompose NO_x in its elements very rapidly and, even more intriguing, recent research shows that some Cu species in Cu-ZSM-5, and by extension also in Cu-MOR, selectively oxidizes methane into methanol after activation with O_2 or N_2O. This review summarizes the latest advances and discussions on the identification of the reactive intermediates and their particular role in the reaction cycle of the methane to methanol conversion. Interestingly, recent spectroscopic and kinetic (isotopically labeled) studies on the Cu-ZSM-5 system have shed new light on the activation of small molecules on Cu-sites. The structural and mechanistic details have already been and will be very useful and instructive to understand the working fundamentals of Cu-containing biomimics and enzymes. Prof. Edward I. Solomon greatly contributed to this pioneering work.