Some chemical treatments diminish the long-range ordering in the aluminosilicate framework of zeolite X

The long-range ordering in the aluminosilicate framework of zeolite X is sometimes diminished or lost during aqueous ion exchange and/or subsequent dehydration. This observation is based on 46 single-crystal determinations of the structure of zeolite X modified by a wide range of chemical treatments. A relationship is seen between the loss of long-range ordering and acidity (H3O+ or H+) within the zeolite, whether the result of direct H3O+ exchange or the exchange of hydrolyzing cations into the zeolite. This effect is seen more strongly when ion exchange is done at an elevated temperature and when vacuum dehydration is attempted, especially at elevated temperatures. Further increases in acidity and/or temperature lead to the loss of the single-crystal diffraction pattern and ultimately to the physical break-up of the crystal, including dissolution if a solvent is present. All crystals, of initial composition Na92Si100Al92O384 per unit cell exclusive of water molecules, were taken from batches prepared by Petranovskii, and all appear to have had Fd(3) over bar symmetry initially with a high degree of Si/Al ordering. After relatively little crystal damage, the space group appears to become Fd(3) over bar m as would be expected if the long-range Si/Al ordering were lost. Before then, the distinction between the Si and Al positions, both initially relatively purr with Si and Al atoms, respectively, diminishes, and the mean apparent Si-O and Al-O bond lengths converge. It is proposed that these initial effects are caused by the loss of infinitesimal to small amounts of aluminum from the zeolite framework, allowing, by local recrystallization, antidomains with opposite Si/Al ordering to form. (C) 2001 Elsevier Science B.V. All rights reserved.