Calculation of29Si and27Al MAS NMR Chemical Shifts in Zeolite-β Using Density Functional Theory: Correlation with Lattice Structure

Abstract

29Si and 27Al NMR chemical shifts have been evaluated from the NMR shielding tensors obtained using the SOS-DFPT technique. The 29Si NMR chemical shifts have been calculated for the nine crystallographically distinct Si sites of the zeolite-β lattice. The calculations were carried out on cluster models Si(OSiH3)4 and R3SiOSiR3 (R = OSiH3) representing one (1T) and two (2T) sites in the zeolite lattice, respectively. Using the 1T models, the nine signals of the experimental spectrum have been assigned with a relative error of less than 1 ppm, the absolute error being estimated at about 5 ppm. The use of a “fragment-averaging” technique based on the results obtained with the 2T models leads to calculated absolute NMR shifts with an accuracy of ±1 ppm. The 27Al NMR shifts of AlH(OSiH3)4 models have also been calculated, after relaxation of the local geometry. It was found that the relative positions of the 27Al chemical shifts are in agreement with the experimental spectrum.