Well defined carbonyl complexes in Ag +- and Cu +-exchanged ZSM-5 zeolite: a comparison with homogeneous counterparts

Abstract

Ag +-ZSM-5 and Cu +-ZSM-5 samples were prepared starting from (NH 4) +-ZSM-5 and achieving, in both cases, a nearly total cation exchange. EXAFS studies have shown that the local environments of Ag + and Cu + ions are very similar, being Cu + surrounded by 2.5±0.3 oxygen atoms at 2.00±0.02 Å and Ag + surrounded by 2.5±0.4 oxygen atoms at 2.30±0.03 Å. Low temperature IR spectroscopy has shown formation of M +(CO) n species where n=1,2 for Ag + and n=1,2,3 for Cu +. However, the corresponding ν(C–O) values are quite different: Cu +(CO) species absorb at 2157 cm −1, while Ag +(CO) is associated with a band at 2192 cm −1. Furthermore, Cu +(CO) 2 species were observed at 2178 and 2151 cm −1 (symmetric and asymmetric stretching, respectively) while Ag +(CO) 2 gave corresponding bands at 2190 and 2184 cm −1. Comparison with literature data for Cu + and Ag + carbonyls formed as homogeneous compounds showed that notable analogies, but also significant differences, exist between homogeneous and heterogeneous systems.