Basic Site Heterogeneity and Location in Alkali Cation Exchanged EMT Zeolite. An IR Study Using Adsorbed Pyrrole

Abstract

The basicity of alkali cation exchanged EMT zeolites was investigated for the first time using pyrrole as the IR spectroscopic probe. It was found that the overall Lewis basicity in this material increased as the cation was exchanged from LiEMT to CsEMT in the alkali cation series. The shift in the ν(NH) stretching frequency of adsorbed pyrrole correlated well with the corresponding oxygen charge (calculated for both bulk and local composition) in the respective EMT samples. Computer deconvolution of the main and broad ν(NH) band of pyrrole chemisorbed on basic sites was performed using a band simulation program. This analysis revealed that the broad ν(NH) band is actually composed of four individual ν(NH) component bands in Li, Na, Rb, and CsEMT (and five in the case of KEMT), each one representing pyrrole adsorbed on a distinct basic site. These four (or five) bands evidence the intrinsic heterogeneity of basic sites existing in the EMT zeolite. This heterogeneity is shown to depend on both the nature and the location of the exchanged cations. By considering both their integrated intensities and ν(NH) frequencies, the various component bands could be assigned to the various basic sites located on the adjacent framework oxygens of the alkali cations at sites I‘a, Ia, I‘b, and II in EMT.