Low-temperature CO and 15N2 adsorption and co-adsorption on alkali cation exchanged EMT zeolites:: an FTIR study

The low-temperature adsorption and co-adsorption of CO and N-15(2) on a series of Me-EMT zeolites (Me=Li, Na, K, Rb and Cs) have been studied by FTIR spectroscopy. Adsorption of N-15(2) on Na-EMT at 85 K results in the formation of Na+ (N-15(2))(2) geminal species (IR band at 2255 cm(-1)), which are converted, at low equilibrium pressures, into Na+-N-15(2) linear complexes (IR band at 2258.5 cm(-1)). Both species are removed by evacuation. CO adsorption at 85 K results in the formation of dicarbonyls at high pressures (2164 cm(-1)), which are converted into linear carbonyls at low pressures (2175 cm(-1)). Mixed Na+(N-15(2))(CO) complexes are produced when N-15(2) and CO are co-adsorbed. The latter easily lose one N-15(2) ligand, thus being converted into linear carbonyls. On Li-EMT only Li+-N-15(2) species were detected, characterized by a band at 2263 cm(-1), and only linear Li+ carbonyls were produced after CO adsorption (band at 2183 cm(-1)). In addition, a weak band at 2199 cm(-1) (tentatively assigned to the Al3+-CO species) was detected. The spectra of N-15(2) and CO adsorbed on K-EMT, Rb-EMT and Cs-EMT display some similarities. In all cases geminal and linear dinitrogen complexes are detected; however, the spectral differences between them are very small. Upon CO adsorption two principal bands (2160 and 2147 cm(-1) for K-EMT; 2159 and 2151 cm(-1) for Rb-EMT; and 2157 and 2144 cm(-1) for Cs-EMT) and two weak features (2275 and 2120 cm(-1) for K-EMT; 2261 and 2119 cm(-1) for Rb-EMT; and 2242 and 2123 cm(-1) for Cs-EMT) are detected. Analysis of the spectra allows assignment of the bands around 2120 cm(-1) to O-bonded CO, the bands in the 2185-2155 cm(-1) region to CO polarized by metal cations, and the band at 2151-2144 cm(-1) to mixed Me+(OC)(CO) species. The reasons for the formation of different complexes are discussed.