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Black plate (271,1) tionyalkylation processes over zeolites. The selectivity of zeolite catalysts in alkylbenzene transformations are thus evaluated, and the results suggest that the dominant factor controlling the particular reaction pathway is the zeolite geometry, i.e. that ‘‘shape-selec- tivity’’ is at work. The paper provides a convincing demonstration that in situ MAS NMR, and specific isotopic labelling of organic reactants, can provide great insight into the detailed mechanisms of catalytic reactions172. 10. Environmental pollution control using zeolites Zeolites and zeolite-based materials are used as sorbents either pre- emptively to keep toxic materials out of the environment or in remediation strategies to clean-up toxic spillages after the event. Cleaning-up after the Chernobyl nuclear disaster (Figure 12) is a superlative example of the latter in which 500,000 tonnes of zeolites were employed to take-up by cation-exchange radioactive cations, principally 134Cs, 137Cs and 90Sr, that had been spewed out from the broken Unit 4 reactor (by a steam-explosion of sufficient force to throw the 1,000 tonne concrete lid off the top of its containment building), in the form of a cloud that contaminated Ukraine and much of western Europe5,7,8. Radioactivity from Chernobyl was detected as far as the east coast of the USA. Zeolites were baked into bread and biscuits and fed to children in an effort to remove radioactive contamination from their bodies. Cattle were fed zeolites in an effort to minimise the level of radioactive material in their milk. I worked in Russia during the aftermath of Chernobyl, when it was extremely difficult to get any local information about what had happened, and most of what we did know came from our colleagues in the West via the Janet computer network. We were however advised by health physicists not to eat ‘‘anything green’’, but to stick to meat, as this would probably be less contaminated with radioactive isotopes (iodine in particular which concentrates in the thyroid gland) than fresh vegetables would be, given the circum- stances. The words ‘‘sorbent(s)’’, ‘‘sorb’’ and ‘‘sorption’’ are used in the literature, presumably because not all uptake occurs onto a surface per se as in ‘‘adsorb’’ and ‘‘adsorption’’. In the following I have adopted this manner of expression. Surfactant-modified zeolites (SMZ) offer promise in environmental remediation, and Bowman has published a review of the preparation, properties, characterisation and effectiveness of SMZs in this regard. SMZs are interesting materials and have multiple sorbent features so that they www.scienceprogress.co.uk Properties and applications of zeolites 271PDF Image | Properties and applications of zeolites
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