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Black plate (260,1) is thermally stable at 473K under dry conditions, it undergoes decomposition in the presence of moisture. Theoretical studies using DFT and cluster models of the zeolite acid site affirmed that both reaction steps are exothermic and predicted chemical shifts that were in excellent agreement with their experimental values123. A portion of the silicon may be removed from a zeolite by treatment with alkaline solutions as a simple way to obtain mesoporous ZSM-5 zeolites in which the integrity of the essential structure is preserved. The process of framework silicon extraction is controlled by framework aluminium and makes desilication selective towards the creation of intracrystalline mesopores. The process is optimal at a range of SiyAl ratios of 25–50 while at higher SiyAl ratios the loss of framework silicon becomes excessive and unselective; at lower ratios comparatively little extraction of silicon occurs. The presence of non-framework aluminium species, as are formed by steam treatment (as in USY), is found to inhibit the formation of mesopores on treatment with alkaline solutions because these species become reinserted into the zeolite frame- work. It is thought that mesoporous zeolites should induce a more efficient use of the zeolite crystal since the accessibility is improved along with more facile transport to and from the active sites124. The technique of dry-gel conversion involves treating the pre-dried gel powder with water vapour at elevated temperature and pressure, and though water vapour is known to be involved in the process the mechanism for this is not clear. In one study, 17O-labelled water vapour was used in the crystal- lization of AlPO4-11 and the process was monitored using 17O MAS NMR spectroscopy. The results show that water from the vapour phase initially exchanges slowly with water molecules that are strongly bound to the AlPO intermediates and that gradually 17O atoms become incorporated in both P2O2H and P2O2Al units in the layered intermediate125. The effect of particle size in the range of 0.16–2.01 microns on the hydrothermal stability of H–Y zeolite under hydrothermal treatment conditions similar to those used in Fluidised Catalytic Cracking (FCC) processes has been investigated. It was found that the optimum particle size for zeolite H–Y to retain a high percentage of crystallinity upon hydrothermal treatment is 0.45 microns. The relative crystallinity (CyCo) could be correlated with the reciprocal of the square root of the zeolite particle size (do) and hence provides an accurate prediction of the hydro- thermal stability of zeolite Y in the small to medium particle size 260 Christopher J. RhodesPDF Image | Properties and applications of zeolites
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