High resolution 29Si n.m.r. of dealuminated Y-zeolites. 2. Silicon, aluminium ordering in the tetrahedral zeolite lattice

The Influence of Si/Al Ratio on the Distribution of OH Groups in Zeolites with MWW Topology

“…This could serve as the additional evidence that such OH groups are indeed located in the environment from which the desorption of pyridine is difficult. For most of the zeolites, the Si/Al ratio determined on the basis of the 29 Si MAS-NMR and IR spectroscopy match very well. In the case of MCM-22 zeolites, however, chemical shift of the Si signals depends strongly on the geometric environment of each Si site (Fig.…”

“…All Si/Al values determined on this basis are close to the ones derived from the XRF analysis of non-calcined samples. This shows that two steps of the thermal treatment-calcination in order to remove template and further activation transforming ammonium form of the zeolite into protonic one (both at 500°C) are not causing significant dealumination of the samples, which will be further confirmed by the 29 Al MAS-NMR. The number of Lewis acid sites for all MCM-22 samples was around 7% and for MCM-49 ca.…”

“…Such a band is usually present in high silica zeolites at the lowest region of Si/Al or ultrastabilized zeolites [28]. Usually, the presence of such extraframework Al atoms is not connected with the occurrence of the 0 ppm signal in the 27 Al MAS-NMR spectrum [29] because such species cannot form symmetric [Al(H 2 O) 6 ] 3? complexes due to the presence of strongly bonded OH group.…”

“…For the 29 Si MAS-NMR spectra a 3 ls rf pulse (p/2 flipping angle) was used, 4 kHz spinning speed, and 256 scans with the delay of 40 s were acquired. The 27 Al spectra were recorded using the 2 ls rf pulse (p/6 flipping angle), 8 kHz spinning speed, and 1,000 scans with acquisition delay 1 s.…”

“…This property is slowly emerging as a more general trend for zeolites [14]. Another rarity presented by the MCM-22 is the unusually large 29 Si MAS-NMR shift range (ca. 15 ppm), pointing to large distribution of crystallographic environments present in this structure.…”