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

Abstract

The low-temperature adsorption and co-adsorption of CO and ¹⁵N₂ on a series of Me-EMT zeolites (Me=Li, Na, K, Rb and Cs) have been studied by FTIR spectroscopy. Adsorption of ¹⁵N₂ on Na-EMT at 85 K results in the formation of Na⁺ (¹⁵N₂)₂ geminal species (IR band at 2255 cm^-1), which are converted, at low equilibrium pressures, into Na⁺–¹⁵N₂ 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⁺(¹⁵N₂)(CO) complexes are produced when ¹⁵N₂ and CO are co-adsorbed. The latter easily lose one ¹⁵N₂ ligand, thus being converted into linear carbonyls. On Li-EMT only Li⁺–¹⁵N₂ 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 ¹⁵N₂ 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.