Random Distribution of EFG Parameters in 27Al MAS NMR Spectra of AlOx/SiO2 Catalysts and Related Systems

Abstract

²⁷Al magic angle spinning (MAS) nuclear amgnetic resonance (NMR) spectroscopy is one of the primary tools in the structural characterization of a variety of alumina-based catalysts. However, the controversy still exists on the proper interpretation of ²⁷Al NMR spectra due to difficulties associated with distribution of NMR parameters and by the experimental limitations. In this work, we present analysis of the ²⁷Al MAS NMR spectra containing signals from Al(4), Al(5) and Al(6) species on the surface of AlO_x/SiO₂ catalysts and demonstrate several characteristic tell-tale properties of these signals caused by the random distribution of the EFG tensor parameters and by the partial excitation of NMR spectra.