Studies of unusual adsorption and diffusion behaviour of benzene in silicalite-1

Abstract

The adsorption and diffusion properties of benzene in silicalite-1 have been investigated using gravimetric techniques, molecular simulation methods and the frequency response (FR) approach. Two steps at loadings of ca. 4 and 6 molecules per unit cell (m. u.c.⁻¹), respectively, and a hysteresis loop between loadings from 6 to 8 m. u.c.⁻¹ can be found in the isotherms of benzene in silicalite-1. These stepped isotherms could be classified as showing type VI isotherm behaviour but in this system the reasons behind the steps are of a new and novel nature. The anomalous adsorption behaviour has been ascribed to the subtle interplay of increased sorbate–sorbate interactions and decreases in the entropy of sorption due to the energetically heterogeneous surfaces which are present in the adsorbent. The diffusivity of benzene in silicalite-1 also displays an unusual behaviour. At loadings lower than 4 m. u.c.⁻¹, only a pure, single diffusion process can be observed, while at higher loadings, two diffusion processes are found. The former case can be ascribed to the movement of the sorbed benzene molecules mainly down the straight channel direction, whereas the latter case has been associated with diffusion processes involving (a) benzene molecules which are part of clusters and (b) molecules which are not clustered, respectively. It is an entropic driving force which causes the faster diffusivity of the clustered molecules. Both the adsorption and diffusion properties of this system depend on the composition and structure of the silicalite-1 samples.