H-2 NMR-STUDIES OF SINGLE-COMPONENT ADSORPTION IN SILICALITE - A COMPARATIVE-STUDY OF BENZENE AND P-XYLENE

A H-2 NMR study has been conducted on the sorption of benzene and p-xylene in an MFI framework with a high silica content. A wide variety of sorbate loadings have been examined and the results are compared with adsorption in an isostructural ZSM-5 framework of lower silicon-to-aluminium ratio. The results from the silicalite study provide direct evidence of adsorption site heterogeneity for both the benzene and p-xylene systems. Single-component studies of benzene at low loadings suggest the presence of adsorbed species demonstrating only restricted motions in mid-channel sites in both the straight and sinusoidal channels. An increase in sorbate loading causes adsorption sites associated with greater sorbate mobility to be accessed; adsorption within the channel intersections is consistent with these observations. Results for p-xylene adsorption at low loadings reveal that the adsorbed molecules remain in the straight channels. An increase in the concentration of p-xylene causes the molecules to access a new site in the sinusoidal channels. The deuterium study thus supports the concept of molecular traffic control within the silicalite framework. In particular, at loadings in excess of one molecule per quarter unit cell (or channel intersection), benzene is restricted to 1D (one-dimensional) pathways whereas p-xylene can access the full 3D pore structure. The presence of aluminium in the MFI framework was found to have a marked affect on the nature of the adsorption sites and the mobility of the sorbed species.