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

Ag+-ZSM-5 and Cu+-ZSM-5 samples were prepared starting from (NH4)(+)-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 Angstrom and Ag+ surrounded by 2.5 +/- 0.4 oxygen atoms at 2.30 +/- 0.03 Angstrom. 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 v(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. (C) 1999 Elsevier Science B.V. All fights reserved.