Calorimetric and spectroscopic study of the coordinative unsaturation of copper(I) and silver(I) cations in ZSM-5 zeolite: Room temperature adsorption of NH 3

Abstract

The room temperature (RT) adsorption of NH 3 was used to probe the coordinative unsaturation of Cu(I) and Ag(I) cations highly dispersed in ZSM-5 zeolites. Adsorption microcalorimetry allowed to characterise both quantitatively and energetically the amino complexes formed at the cationic sites, as revealed by IR spectroscopy. Copper(I) sites were found to form tetra-amino [Cu(NH 3) 4] + adducts, whereas silver(I) ones do form only di-amino [Ag(NH 3) 2] + species. Both results are in agreement with the homogeneous chemistry of Cu(I) and Ag(I) cations. The heat of formation of the different amino-complexes was found to be comprised in the 130–50 kJ/mol interval for both kind of Me(I) sites, according to the number of ligands progressively bound and in spite of the different stoichiometry of the species formed. The adducts were found to be only partially reversible upon outgassing in the adopted conditions (303 K and p≈10 −5 Torr). Two distinct, nearly equally populated families of sites were revealed in both Me(I)-ZSM-5 samples, corresponding to different coordinative unsaturations of the metal cations. EXAFS data, used to check the local environment of noble metal cations, confirmed the proposed model. In the case of Cu(I) sites, the formation of mixed amino-carbonyl complexes was also studied. The thermodynamic stability of amino- and carbonyl-like species was found not to be the same.