NMR studies of thermally-treated Zeolite Y
The effect of thermal treatments on the lattice of Zeolite Y and on intracrystalline non-lattice material is investigated by solid-state magic-angle-spinning NMR. (^29)Si and (^27)Al NMR show that large amounts of both silicon and aluminium are dislodged from the lattice by calcination and steaming treatments. Non-lattice material is readily observed by (^29)Si CP/MAS; the presence of such material may also be deduced from the single-pulse spectra by means of spectral deconvolution techniques. Non-lattice aluminium atoms occur in tetrahedral and octahedral coordination; a technique is proposed for measuring the ratio of Al in these two environments. Leaching with aqueous (NH(^4))(_2)EDTA removes non-lattice Al but also attacks lattice Si and may re-insert some Al into the lattice. A reliable technique has been developed for acquisition of (^1)H MAS-NMR spectra of totally dehydrated samples of Zeolite HY. Exceptionally well-resolved signals have been observed for silanol groups and Bronsted-acidic protons. Particular attention has focused on a broad signal at 2.3 ppm attributed to OH groups on non-lattice Al atoms, whose intensity depends on the degree of polymerization of the non-lattice alumina. The degree of polymerization rises on prolonged heating under vacuum. If the sample is kept dry for several months, the AlOH concentration rises at the expense of other hydroxylated species present, suggesting a slow reversal of the polymerization process. The highly-condensed material is also capable of reacting with injected water. Cation-containing Zeolite Y behaves quite differently from HY on dehydration. NaY requires severe heating under vacuum to achieve dryness; however, the product is free from non-lattice material. NH(_4)Y releases its intracrystalline water readily but, on decomposition of NH(^4) at around 300ºC, a new broad signal emerges close to 4 ppm; this is tentatively assigned to NH3 trapped in small lattice cages.