Radiolabelling Measurements of Free Radicals Derived from Aromatic Volatile Organic Compounds Adsorbed in Zeolite Nanomaterials to and above Saturation Loadings

Abstract

Benzene, para-xylene and mesitylene were adsorbed into silicalite, Na - morde- nite, Na - ZSM5 and Li - X zeolites, and studied using the longitudinal field muon spin relaxation (LF-μSRx) technique. The zeolites ZSM5/silicalite and mordenite contain microporous channels while zeolite X has only supercages. For cyclohex- adienyl 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 5kJ mol−1; however, in all cases there appeared a second fraction with an activation energy of ca 12kJmol’. [In Li-X only a single fraction was observed with Ea = 8.1kJ mol−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−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 A diameter) and we may ascribe therefore, the ca 5 kJ mol−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 12kJ mol−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 -1 processes involve molecular motion within clusters of benzene molecules with properties similar to the bulk phase (6.6 kJ mol -1 was measured in pure benzene), while those characterised at ca 12kJ mol−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.