Probing Disorder in Al-ZSM-5 Zeolites by 14N NMR Spectroscopy

N-14 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 (H-1, C-13, F-19, Al-27, Si-29) solid-state NMR, the N-14 MAS NMR spectra of a series of as-synthesized ZSM-5 zeolites containing various amounts of Al are analyzed. The N-14 spinning sideband patterns are shown to evolve with the Si/Al ratio. The modeling of the NMR spectra allows one 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 N-14 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 angle variations on the N-14 quadrupole coupling constant distributions.