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

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.

Adsorption of CO2 and N2 on Coal-Based Activated Carbon

Adsorption equilibrium data for CO2 and N2 on coal-based activated carbon were obtained at 323.15, 348.15, 363.15 K and at partial pressures from 100 Pa to 82425 Pa by a volumetric apparatus. Lower temperature is helpful for the adsorption. It was observed the saturation loadings at 323.15 K are 0.3 mmolg-1, 0.057mmolg-1 for CO2 and N2, respectively. The Langmuir and Sips equations were used to correlate the experimental data. It was found that the Sips equation is more accurate to describe the adsorption behavior of both gases on activated carbon. Henry’s constant was calculated, it proves that activated carbon has a high selectivity on CO2 than N2, the maximum separation index is 5.5.