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 Resolution27Al and29Si NMR and Density Functional Theory/Molecular Mechanics Study

Abstract

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 SiO4 and 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 Si atoms (Al−O−Si) by 4−11 ppm. (III) The effect of a more distant Al/Si substitution (Al−O−Si−O−Si) is significant...