Ion Exchange as a Route of Cu-Substituted ZSM-5 Synthesis: Theory and Applications
Статья (Full article),
||Ion exchange: Theory and applications
Nova Science Publishers, Inc. 2017.
319 c. ISBN 9781536123517.
||Cu-substituted zeolite, Cu-ZSM-5, Ion exchange, Isotherm, Sorption kinetics
Boreskov Institute of Catalysis SB RAS, Novosibirsk, Russian Federation
Institute of Coal Chemistry and Materials Science, Kemerovo, Russian Federation
The interest of researchers to Cu-substituted zeolites (ZSM-5, SSZ- 13, etc.) is caused by their unique properties in DeNOx processes discovered by Iwamoto (Chem. Lett. 2 (1989) 213) and Held (SAE Technical Paper Series. 900496 (1990) 13). At present, Cu-substituted zeolites are increasingly considered as promising catalysts for other redox reactions, specifically for selective methane oxidation to oxygenates, catalytic wet peroxide oxidation of organic contaminants, and others. Ion exchange remains the most widely used method of Cu-ZSM-5 synthesis; this is caused by the simplicity of its technical implementation, reproducibility of the catalyst properties, and higher catalytic activity of the ion-exchanged catalysts. Numerous ion exchange techniques have been reported. However, the regularities of ion exchange in dependence on its main parameters are discussed quite rarely, despite the fact that they play a key role in the formation of catalytically active sites and catalytic activity. For example, the DeNOx activity of Cu-substituted ZSM-5 catalysts was found to depend nonlinearly on the copper content. Sometimes the catalytic activity of the ion-exchanged Cu-ZSM-5 with the same copper loading may differ substantially. This chapter will summarize the main regularities of ion exchange of H-, Na- and NH4-forms of ZSM-5 zeolite with aqueous and waterammonia solutions of copper acetate, copper chloride, copper nitrate, and other copper salts. It will be demonstrated that the sorption of Cu(II) ions by H-, Na- and NH4-ZSM-5 during the ion exchange with aqueous and water-ammonia solutions of copper salts is well described within the Langmuir monomolecular adsorption and second order kinetics. We will discuss regularities of the main chemical reactions taking place during the ion exchange with copper salt solutions, which have different parameters (nature of a salt, pH, concentration) and result in the formation of various copper electronic states, such as 1) isolated copper Cu2+ ions, 2) the structures of copper ions with extra-lattice oxygen (dimer, chain-like, and square-planar coordination of oxygen ligands) located in zeolite channels, 3) the oxide clusters, and 4) CuO nanoparticles dispersed on the zeolite crystallite surface. For example, adding of ammonia to aqueous solutions of copper salts leads to a nonlinear effect on the copper content in the Cu- ZSM-5, which is due to side reactions in the solution – hydrolysis, polycondensation, and complexation. The control of the copper electronic states detected in the ion-exchanged Cu-ZSM-5 by ESR, UV-Vis DR and TPR-H2 will be discussed.