FT-Raman studies of single-component and binary adsorption in silicalite-1

Abstract

FT-Raman spectroscopy has been used to probe single-component and binary-component adsorption of benzene, p-xylene and cyclohexane in silicalite-1. It was shown that FT-Raman is not only able to probe the sorbate–framework interaction related to the phase transitions commonly observed when organic species are sorbed within silicalite-1, but also provides a sensitive and direct probe of sorbate–sorbate interactions within the zeolite framework. Single-component adsorption of benzene, p-xylene and cyclohexane in silicalite-1 has been considered at various loadings. In the case of benzene adsorption, FT-Raman spectroscopy was shown to detect the transformation in sorbate interactions associated with the transformation in crystal symmetry from monoclinic P2₁/n.1.1 to orthorhombic Pnma at a sorbate loading of 4 molecules per unit cell. We also confirmed the results of Huang (J. Am. Chem. Soc., 1996, 118, 7233) in detecting the sorbate-induced crystal-phase transition from orthorhombic Pnma to orthorhombic P2₁2₁2₁ for the case of p-xylene adsorption at loadings in excess of 4 molecules per unit cell. No evidence of a crystal-phase transition as a function of sorbate loading was observed for the case of cyclohexane adsorption, consistent with earlier studies. It was shown that, in the case of benzene–p-xylene co-adsorption, benzene and p-xylene access the sites most favoured during the respective single-component adsorption processes. In contrast, cyclohexane when co-adsorbed with either benzene or p-xylene was seen to compete for the same sites, forcing benzene and p-xylene into less favoured adsorption sites.