Probing Disorder in Al-ZSM-5 Zeolites by 14 N NMR Spectroscopy
Résumé
14N solid-state NMR spectroscopy is used to investigate and quantify the nanometer scale disorder promoted by Al/Si substitution in ZSM-5 zeolites. After a preliminary characterization by SEM, XRD and multinuclear (1H, 13C, 19F, 27Al, 29Si) solid-state NMR, the 14N MAS NMR spectra of a series of as-synthesized ZSM-5 zeolites containing various amount of Al are analyzed. The 14N spinning sidebands patterns are shown to evolve with the Si/Al ratio. The modeling of the 14N NMR spectra allows to estimate the local disorder arising from the Al site distribution within the tetrahedral sites of the zeolites, the variations of F locations and the presence of silanol defects. The influence of the zeolite framework modifications due to Al/Si substitution on 14N NMR parameters is discussed on the basis of the results obtained with the Density Functional Theory periodic quantum chemical calculations augmented with an empirical London dispersion term. Analysis of the results highlighted the influence of CNC angles variations on the 14N quadrupole coupling constant distributions.