Colors of Ag⁺-Exchanged Zeolite A

Abstract

UV/vis spectra of hydrated and activated Ag+xNa+12-xA, Ag+xCa2+6-0.5xA, and Ag+9.5ZK-4 materials were studied. Any absorption band or color observed in silver zeolite A materials is due to the presence of silver ions. The marked site preference of ions in Ag+xCa2+6-0.5xA was found to offer the unique possibility of investigating different coordination sites of Ag+ ions in zeolite A. Observations we made lead to the result that Ag+ coordinated to 6- and 8-ring oxygens gives rise to electronic transitions in the near-UV region. Only Ag+ coordinated to 4-ring oxygens leads to the 22 000 cm-1 absorption responsible for the typical deep yellow color. The red color which is caused by a strong absorption band at 19 000 cm-1 is observed if a 4-ring coordinated Ag+ has a second Ag+ as a neighbor. We assume that the second Ag+ is at a 6-ring site. Yellow and red colored materials turn colorless again when exposed to humidity. The reversibility is complete for the yellow samples while a broad but weak absorption in the near-UV remains after rehydration of the red samples. Molecular orbital calculations were carried out on a unit structure cell consisting of 1296 atoms. The occupied frontier orbital region consists mainly of two bunches of levels: the HOMO region which spreads from about −11 to − 12.6 eV and the next lower lying levels below −13.6 eV. The HOMO region consists of oxygen lone pairs O(n). We find that 6-ring coordinated Ag+ gives rise to near-UV electronic transitions and 4-ring coordinated Ag+ causes a band in the visible. These transitions can be interpreted as charge transfer from zeolite oxygen lone pairs to Ag+. We therefore denote them as Ag+(5s) ← O(n) LMCT.