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-xy lene 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-al uminium ratio. The results from the silicalite study provide direct ev idence of adsorption site heterogeneity for both the benzene and p-xyl ene systems. Single-component studies of benzene at low loadings sugge st the presence of adsorbed species demonstrating only restricted moti ons in mid-channel sites in both the straight and sinusoidal channels. An increase in sorbate loading causes adsorption sites associated wit h greater sorbate mobility to be accessed; adsorption within the chann el intersections is consistent with these observations. Results for p- xylene adsorption at low loadings reveal that the adsorbed molecules r emain in the straight channels. An increase in the concentration of p- xylene causes the molecules to access a new site in the sinusoidal cha nnels. The deuterium study thus supports the concept of molecular traf fic control within the silicalite framework. In particular, at loading s in excess of one molecule per quarter unit cell (or channel intersec tion), benzene is restricted to 1D (one-dimensional) pathways whereas p-xylene can access the full 3D pore structure. The presence of alumin ium in the MFI framework was found to have a marked affect on the natu re of the adsorption sites and the mobility of the sorbed species.