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

An attempt has been made to correlate the experimentally observed Si-29 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 the 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 Si-29 chemical shifts has been attempted. The relative difference in Si-29 chemical shifts for the Si sites in ZSM-5 is reasonably accounted, although quantitative prediction may require non-empirical quantum chemical calculations. (C) 1998 Elsevier Science B.V. All rights reserved.