Effect of Al/Si Substitutions and Silanol Nests on the Local Geometry of Si and Al Framework Sites in Silicone-Rich Zeolites: A Combined High Resolution 27Al and 29Si NMR and Density Functional Theory/Molecular Mechanics Study

We employed 29Si and 27Al (3Q) magic-angle spinning (MAS) NMR spectroscopy and density functional theory/molecular mechanics (DFT/MM) calculations to investigate the effect of Al/Si substitutions and the presence of silanol nests on the 29Si and 27Al NMR parameters as well as the local geometry of SiO4and AlO4−tetrahedra of the nearest and next-nearest neighboring Si and Al atoms. The silicon-rich zeolite of the chabazite structure (Si/Al 38) was chosen for this study as a representative model of silicon-rich zeolites since it exhibits a low number of distinguishable T sites. Our computational results show the following: (I) Al atoms can occupy three different crystallographic T sites in the framework of chabazite (Si/Al 38). This result is in agreement with two observed 27Al NMR resonances. (II) An Al/Si substitution causes a downshift of the 29Si chemical shift of the nearest neighboring Siatoms (Al−O−Si) by 4−11 ppm. (III) The effect of a more distant Al/Si substitution (Al−O−Si−O−Si) is significantly less pronounced (downshift up to 2 ppm). (IV) An Al/Si substitution (Al−O−Si−O−Al) leads to larger 27Al isotropic chemical shifts of the next-nearest neighboring Alatoms by up to 3 ppm. (V) The presence of a silanol “nest” (vacant T site) as a nearest (H−O−Si) and next-nearest (H−O−Si−O−Si) neighbor is responsible for a systematic downshift of the 29Si chemical shift of Si by 11−16 ppm and by 0−1 ppm, respectively. (VI) There is no systematic effect of a silanol “nest” as a next-nearest neighbor (H−O−Si−O−Al) on the 27Al isotropic chemical shift of Al as its values are smaller for some H−O−Si−O−Alsequences (up to −3.6 ppm) and greater for others (up to +2.9 ppm). (VII) Al atoms present in hypothetical Al−O−Al sequences would have their 27Al isotropic chemical shifts larger by 7−9 ppm than single Al atoms. Our predictions of NMR parameters using the quantum mechanics/interatomic potential functions approach and our bare zeolite framework model are in good agreement with the available experiments.