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Black plate (261,1) ranges at a given temperature. The correlation was found not to hold for the larger particles of Y zeolite (40.45 microns), probably due to the presence of a greater number of structural defects in the zeolite, according to the higher BrønstedyLewis acid ratios which pertain126. There is a controversy about whether the H-form of natrolite actually exists. To cast light on the matter, the thermal decomposition of NH4-exchanged natrolite (as a precursor of the H-form) was studied by gas chromatography, IR spectroscopy and X-ray diffraction, leading to the conclusion that throughout the decomposition, the de- ammoniation is followed rapidly by dehydroxylation, in accord with an intrinsic instability for H-natrolite formed once NH3 has been removed. The final step of the decomposition involves the adsorption of NH3 onto Lewis acid centres which have accumu- lated in the now amorphous framework and this hampers the loss of ammonia. In accord with this interpretation, the activation energy observed for NH3 desorption increases from an initial 117 (+13) kJ mol1 to a final value of 270 (+20) kJ mol1. If the NH4-exchanged natrolite is degassed smoothly under vacuum at 550K, a partial de-ammoniation is observed rather than formation of H-natrolite127. A deep-eutectic solvent (DES) is a mixture of two compounds which together result in a depression in the freezing point of the mixture compared with those for the separate components. In one study, the controlled use is described of DES which are unstable at high temperatures and so the organic template is not added in the normal manner to the reaction mixture but is delivered thus by the decomposition of one of the components in the DES itself128. Weckhuysen et al. have used a multi-technique approach to unravelling the mechanism of formation of zeolites and obtained results that support the inference that molecular organisation takes place before crystallisation occurs. Thus it is molecular recogni- tion that creates the seeds of order around which complex lattices grow129–131, remarkably like the self-assembling systems that are believed to be key to the origin of life on earth132. The presence of phosphorus atoms is found to stabilize the ZSM-5 zeolite structure by preventing the separation of the framework Al atoms during hydrothermal treatment. In addition to the classical frame- work tetracoordinated aluminium, which is a major factor in determining the acidity of the zeolites, a second tetrahedral aluminium (probably in a distorted environment) also seems to be active in n-hexane cracking, in addition to favouring the formation of light olefins during gasoil cracking133. www.scienceprogress.co.uk Properties and applications of zeolites 261PDF Image | Properties and applications of zeolites
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