Copper and Silver Atoms in the β‐Cage of a Zeolite: Model Calculations

Abstract

The electronic structures ofthe 4-4 SBU, the 6-6 SBU, the8-cage, and the 8-cage with two 4-4 SBUs attached to it have been studied by means of EH-MO calculations. No indication of the formation of a band structure has been found. The HOMO region consists of many closely spaced, localized states, 98.6% of them concentrated on the 0-atoms. Reversible color changes of Cu+' and Ag" zeolites observed upon hydration-dehydration experiments can be understood as charge-transfer transitions from the HOMO concentrated on the zeolite 0-atoms to the metal cations. As soon as the Cu" or Ag" are partially hydrated, the ns* and np* states are shifted to higher energies. The luminescence observed with dehydrated Cu+'-zeolites X is caused by a 4p*+HOMO absorption, followed by spontaneous 4s*t4p* emission. After a detailed study of a Cu+' in the 6-6 SBU, we discuss the electronic structure o f a 8-cage filled with I , 2,4, 8, and 9 Cu". In each case, the B-cage is found to be too small to allow the formation of a band structure. The levels caused by the added copper are distinctly quantized. Calculations on [Ag,(H20)3]3+ in a 8-cage are reported. The direct interaction between the Ag-atoms is significant. As a consequence, the states formed by Ag 5s and 5p atomic orbitals are delocalized over the three Ag-centers. In both thcCu+' and the Ag+' zeolites, the ligand-field picture is found to bc insufficient to explain the electronic structure, when the metal is coordinated to the zcolite oxygen framework.

Enhancing oxidation resistance of Cu(I) by tailoring microenvironment in zeolites for efficient adsorptive desulfurization

“…After VIR, the intensity of band around 800 nm declines slightly. This is caused by the incomplete reduction of Cu(II) and oxygen-to-metal charge transfer of Cu(I) in the 6-6 secondary building unit of zeolite 41 . Meanwhile, two new absorption peaks at 350 and 450 nm readily attributed to the charge transfer transitions (3d 10 → 3d 9 4s 1 ) of Cu(I) in the zeolite matrix can be clearly identified, which is absent in Cu(II)Y 42,43 .…”

Silver Clusters and Chemistry in Zeolites

“…An extended Hiickel quantum-chemical calculation showed that the color change correlated with charge transfer from framework oxygens to silver cations; the lowest energy transition in Ag-A was a charge transfer from an orbital localized mainly on framework oxygens to that of a silver ion in a 6-ring. 40 After nearly all of the water is lost, further changes in color must be attributed to the increasing extent of Ag+ reduction (to the formation of silver clusters). The shortest Ag-0 bonds and the darkest colors occur when little or no water is retained and little or no Ag+ has been reduced.…”

Luminescence experiments on copper(I) zeolites

“…Some aspects of the optical spectrum of Cu+ zeolites have previously been reported.3 MO studies, however, suggest a different interpretation, notably that the first electronic transition is due to a Cu+ t zeolite-oxygen lone-pair LMCT transition. 4 Samples with different Cu2+ exchange levels were prepared by ion exchange at room temperature for 3 h with aqueous solutions containing the desired amount of Cu2+ as Cu(NO&. As starting materials Na-4A (Bayer AG: Baylit T) and Na-13X (Union Carbide) were used.…”