Effect of Al−Si−Al and Al−Si−Si−Al Pairs in the ZSM-5 Zeolite Framework on the²⁷Al NMR Spectra. A Combined High-Resolution²⁷Al NMR and DFT/MM Study

Abstract

The effect of the presence of Al−O−Si−O−Al and Al−O−Si−O−Si−O−Al sequences in the ZSM-5 zeolite framework on the local geometry of AlO4 − tetrahedra and the 27Al NMR parameters was investigated employing 27Al 3Q MAS NMR spectroscopy and DFT/MM calculations. The presence of an Al atom as a next-nearest (Al−O−Si−O−Al) and next-next-nearest (Al−O−Si−O−Si−O−Al) neighbor can significantly affect both the local geometry of AlO4 − tetrahedra as well as 27Al NMR isotropic chemical shift (up to 4 ppm). There is no systematic contribution of Al in Al−O−Si−O−Al or Al−O−(Si−O)2−Al chains to the 27Al isotropic chemical shift, and not even the direction can be predicted without explicit DFT calculations. Our combined experimental and computational approach employing 27Al (3Q) MAS NMR spectroscopy supported by DFT/MM calculations ( Sklenak S. Dědeček J. Li C. Wichterlová B. Gábová V. Sierka M. Sauer J. Sklenak S. Dědeček J. Li C. Wichterlová B. Gábová V. Sierka M. Sauer J. Angew. Chem., Int. Ed.2007467286.) can be employed only for ZSM-5 samples having a low or negligible concentration of Al−O−(Si−O) n −Al (n = 1 and 2) sequences in the zeolite matrix, otherwise 27Al (3Q) MAS NMR spectroscopy cannot be used to even identify the number of framework T sites occupied by Al.

Distribution of aluminum over different T-sites in ferrierite zeolites studied with aluminum valence to core X-ray emission spectroscopy

“…27 Al MAS NMR is sensitive to the averaged Al-O-Si angle and therefore partially yields aluminum occupation. 2,3 Combined with DFT calculations, 29 Si and 27 Al NMR have been used to study the distribution of aluminum in zeolites ZSM-5 4 (MFI framework type) and ferrierite 5 (FER framework type).…”

Theoretical Chemistry of Zeolite Reactivity

“…The results thus obtained allowed to assign the observed 27 Al resonances to the particular T-sites in the MFI framework [94][95][96]. Although a trend has been detected for smaller 27 Al isotropic chemical shifts with increasing average T-O-T angle [95] that had been previously proposed as a tool for the identification of the Al sitting in zeolites [98], the corresponding correlation was shown to be not suitable for the assignment purposes.…”

“…Recently, a tool based on the complementary use of the high resolution of 27 Al 3Q MAS (magic-angle spinning) NMR spectroscopy and DFT calculations has been proposed for studying the local geometry of framework [AlO 4 ] − units and for the identification of the Al sitting in high-silica zeolites [94][95][96]. The respective experimental technique allows identification and quantification of the 27 Al resonances corresponding to individual T-sites in the zeolite framework.…”

Characterization of Aluminum(III) Complexes in Coal Organic Matter

“…For identification, it is therefore more feasible to use a highly sensitive and high resolution solid state 27 Al MAS NMR spectrometer, have a comparative scale of complexes as reference compounds, and assign complexes in question on the basis of the same or very similar peak shifts (allowable difference of 1 -2 ppm [17]). …”