Characterization of silicon sites in monoclinic zeolite ZSM-5 using ²⁹Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) and molecular modelling

Abstract

An attempt has been made to correlate the experimentally observed ²⁹Si MAS NMR chemical shifts of monoclinic phase of highly siliceous ZSM-5 with their electronic properties. In order to incorporate the influence of next neighbor atoms on the ²⁹Si chemical shielding of central SiO₄, a pentameric cluster model (H₁₂Si₅O₁₆) has been chosen. Each of the 24 crystallographically distinct Si sites, of ZSM-5 framework has been modelled by such cluster models. Based on semi-empirical quantum chemical calculations, a multiple linear regression analysis of the various electronic properties with the ²⁹Si chemical shifts has been attempted. The relative difference in ²⁹Si chemical shifts for the Si sites in ZSM-5 is reasonably accounted, although quantitative prediction may require non-empirical quantum chemical calculations.