RELIABILITY OF SMALL CLUSTER-MODELS FOR CU-EXCHANGED ZEOLITES

The predictions of a previously developed small cluster model for boun d Cu ions and their interactions with CO and NO in Cu-exchanged zeolit es [Schneider, W.F.; Hass, K.C,; Ramprasad, R; Adams, J.B. J. Phys. Ch em. 1996, 100, 6032] are compared to those of a series of larger, more realistic cluster models. The hypothetical case of Cu-n+ (n = 0, 1, 2 ) coordinated to a single framework oxygen atom, described in the simp lest model as CuOH2n+, is first examined in detail. The larger cluster s considered include explicit Si or Al tetrahedral sites and are termi nated by either H or OH. For nominal Cu(0) and Cu(I) oxidation states, the qualitative predictions of the original ''water-ligand'' model ar e found to be extremely robust, as are quantitative aspects of CO and NO binding. More pronounced differences between models are observed fo r a nominal Cu(II) oxidation state, due primarily to the tendency of t he zeolitic portions of the larger clusters to reduce the Cu. The coor dination of Cu-n+ (n = 1, 2) to a more realistic 4-fold coordination s ite model is similarly examined and compared to the results for Cu(OH2 )(4)(n+). Excellent agreement is obtained between the water-ligand and cluster models for both Cu(I) and Cu(II) in this case. The results in dicate that proper treatment of the local coordination environment of a Cu ion exchanged into a zeolite is of greater importance for describ ing chemistry on the Cu site than is use of a ''realistic'' model for an individual bridge oxygen site. The implications of these results fo r future modeling studies of Cu zeolites are discussed.