Solid state NMR study of $Cu^2+$ exchange on faujasite-type zeolites

Abstract

$^{129}Xe$ NMR and xenon adsorption methods have been used for characterization of NaX zeolite exchanged with $Cu^{2+}$ ion. The chemical shift in $^{129}Xe$ NMR spectrum for the adsorbed xenon appeared to decrease gradually as the ion exchange level of $Cu^{2+}$ was increased, without a significant change in the pressure dependence. This result suggests that the copper ion was located in a site which was inaccessible for xenon collision. Due to a paramagnetic center at the $Cu^{2+}$ ion, the $^{129}Xe$ NMR line width was increased with the Cu loading. Because of two $Na^+$ ions became to substitute for one copper ion, the adsorbed xenon quantity seemed to decrease linearly with increasing the copper ion exchange level. Magic angle spinning (MAS) solid state $^{29}Si$ NMR has also been used for investigation of $Cu^{2+}$ ion exchange site in faujasitetype zeolites. When the $Cu^{2+}$ ion was exchanged as aqueous complex, the paramagnetic $Cu^{2+}$ ion locally affected the line width in the $^{29}Si$ MAS NMR spectrum. This line broadening was very selective and characteristic of the ion exchange level. The site preference for $Cu^{2+}$ ion was $Si(-OAl)_4 > Si(-OAl)_3 > Si(OAl)_2$ etc. On the contrary, if $Cu^{2+}$ ion was exchanged as ammine form, the line width variation was not selective. The result was due to the ion exchange of $Cu(H_2O)^{2+}_4$ at a site which was composed of two $AlO_4$ tetrahedra bridged by a Si atom, and therefore the paramagnetic effect was concentrated at the silicon atom. However, the ammine form seemed to lose the selectivity because it was exchanged as an octahedral complex.