(501c) Speciation of Liquid Ion-Exchanged Cu into SSZ-13, ZSM-5, and Beta Zeolites

Arthur J. Shih¹, Juan M. González^1,2, Ishant Khurana¹, Lucía Pérez Ramírez, Andres Peña L.¹, Aleksey Yezerets³, Rajamani Gounder¹, Aída Luz Villa², and Fabio H. Ribeiro¹

¹Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN

²Chemical Engineering Department, Universidad de Antioquia, Medellín, Colombia

³Cummins Incorporated, Columbus, IN

Cu-zeolites are versatile materials due to their hydrothermal stability and their ability to catalyze a variety of reactions such as the selective catalytic reduction of NO_x, methane to methanol, and NO oxidation, among others. [1-3] Though there are various methods to exchange Cu onto a zeolite support (aqueous ion-exchange, solid-state exchange, incipient wetness impregnation, and one-pot synthesis), it has been shown that these Cu-exchange methods do not always yield the same type and fraction of Cu species. [2-4] Since Cu active sites for the reactions listed above often consist of a subset of the species formed, it is important to understand peculiarities of the Cu-exchange process and how materials with well-defined Cu sites can be synthesized and characterized. In this study, we report the speciation of Cu onto zeolites (SSZ-13, ZSM-5, Beta) during and after aqueous ion-exchange.

Copper speciation from aqueous ion exchange with Cu(NO₃)₂ as a function of zeolite Si:Al ratio, final Cu molarity, and pH was determined from spectroscopy (UV-Visible, XAS) and selective NH₃ titration of Brønsted acid (BA) sites. The following species were observed: (1) monomeric Cu²⁺ ions charged balanced by a framework O and/or an –OH ligand and (2) clustered Cu_xO_y species that are not charge balanced by the framework. The onset of Cu_xO_y species during aqueous ion exchange correlates well with Cu-precipitate formation, but not with the zeolite’s point of zero charge. [5] Immediately after ion-exchange, ionic Cu speciates as either Z₂H/CuOH or ZCuOH (Z indicates an anionic zeolite framework oxygen, H₂O molecules solvated to Cu are not shown for clarity). Upon dehydration or calcination, Z₂H/CuOH condenses into Z₂Cu as evidenced by the stoichiometric loss of BA sites from selective NH₃ titration.

The understanding of Cu speciation has made large strides over the last few decades. This study opens avenues to better understand the nature of the active site for various reactions, and to strategically synthesize Cu-zeolites with different fractions of these active sites.

References:

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