Benzene, para-xylene and mesitylene were adsorbed into silicalite, Na -mordenite, Na -ZSM5 and Li -X zeolites, and studied using the longitudinal field muon spin relaxation (LF-mSRx) technique. The zeolites ZSM5=silicalite and mordenite contain microporous channels while zeolite X has only supercages. For cyclohexadienyl radicals=benzene adsorbed in Na -ZSM5, silicalite and Na -mordenite, a fraction was detected with a common reorientational activation energy in the region of ca 5 kJ mol À 1 ; however, in all cases there appeared a second fraction with an activation energy of ca 12 kJ mol À 1 . [In Li -X only a single fraction was observed with E a ¼ 8:1 kJ mol À 1 , from molecules adsorbed in supercages]. Given that high loadings of benzene, beyond the saturation capacities of the zeolites were employed, we believe the two distinct motional distributions represent the channel intersection and channel=external-surface niche locations in ZSM5=silicalite, the latter having the higher activation energy. An extraneous fraction may also be present (possibly as a thin film coating the zeolite grains), which probably also contributes to the detected ca 5 kJ mol À 1 component in which, as at the channel-intersections, the motion tends toward bulk behaviour. Mordenite has only a single accessible channel structure and is hence devoid of the relatively unrestricting channel intersecting pores that are present in ZSM5=silicalite. However, the channels are wider (ca 7 Å diameter) and we may ascribe therefore, the ca 5 kJ mol À 1 fraction to the formation of benzene clusters within these channels that exhibit bulk-type behaviour, while we assign, in analogy with the results for ZSM5=silicalite, the ca 12 kJ mol À 1 fraction to molecules adsorbed in niches on the external surface. It is thought that the essential difference between the two activation energies is that the ca 5 kJ mol À 1 processes involve molecular motion within clusters of benzene molecules with properties similar to the bulk phase (6.6 kJ mol À 1 was measured in pure benzene), while those characterised at ca 12 kJ mol À 1 reflect single benzene molecules or small, ordered molecular clusters interacting with a zeolite surface. The results for dimethylcyclohexadienyl radicals (derived from p-xylene) are rather similar, but indicate slightly reduced activation energies. The larger mesitylene molecules can penetrate zeolite X and (more slowly) mordenite, but are excluded from the internal pores of ZSM5=silicalite.
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