The role of sodium ions as adsorption sites in silicalite-1

Abstract

In order to elucidate the role of sodium ions as adsorption sites in silicalite-1, adsorption of six-membered ring compounds on sodium-modified and -unmodified silicalite-1 has been investigated by temperature-programmed desorption (TPD). The presence of sodium ions in silicalite-1 stabilizes adsorption of pyridine, phenol, toluene, benzene and cyclohexene, whereas adsorption of cyclohexane is destabilized. The result suggests that the stabilization is due to the interaction between a sodium ion and the portion of the adsorbate where a high electron density, e.g. lone electron pair or π-bonds, is present; this is supported by ab initio molecular orbital calculations, in which a sodium ion interacts with a six-membered ring compound. On the basis of detailed calculations for the adsorption of ammonia or carbon dioxide, a sodium ion is considered not to work as a Lewis acid. Thus, the stabilization is probably caused by an electrostatic interaction between the sodium ions and the adsorbate.