Abstract
The solid-state ion exchange (SSIE) of H-ZSM-5 by CuCl vapor was investigated with the objective of establishing the effects of the temperature at which exchange is carried out on the level of proton exchange for Cu+, the position of the Cu+ cations relative to the zeolite lattice, and the extent of CuCl occlusion in the zeolite pores. After SSIE, the resulting Cu-ZSM-5 was characterized by XRD, 27Al MAS NMR, infrared spectroscopy, H2-TPR, and Cu and Cl K-edge X-ray absorption spectroscopy. Evidence was found for clusters of CuCl, in addition to Cu+ cations present at zeolite cation-exchange positions (ZCu). It was also observed that a part of the occluded CuCl is hydrolyzed to Cu2O during SSIE. The distribution of ZCu, CuCl, and Cu2O was a strong function of the temperature at which cation exchange was carried out. Virtually complete exchange of all Brønsted acid protons for Cu+ cations was achieved at a SSIE temperature of 1023 K. No loss of framework Al was observed at this temperature, and only 10% of the copper introduced into the zeolite was occluded as CuCl. The Cu+ cations exchanged for Brønsted acid protons are both doubly and triply coordinated to O atoms, with an average Cu−O bond distance of 1.98 Å. Cu EXAFS and Cl XANES indicated that CuCl is occluded in the form of (CuCl)n (n = 2, 3, 4) clusters in the channels and intersections of the zeolite. With a decrease in the amount of occluded CuCl, the amount of Cu2O formed by hydrolysis during SSIE decreases substantially.
Published Version
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