New advances in the understanding of host-guest zeolite materials through NMR experimental and theoretical approaches: the TPA-F-Silicalite-1 case

We undertook a detailed NMR characterization of the prototypical host-guest zeolite TPA-F-Silicalite-1, made of tetrapropylammonium cations (TPA), fluoride and the MFI silica framework. The use of a high magnetic field (B0 = 20 T) and very fast MAS frequency (MAS = 100 kHz) greatly improves the 1 H resolution in the related 1D spectrum and 2D 1 H-1 H and 1 H-13 C correlation spectra, enabling eight distinct 1 H peaks to be identified, whereas TPA shows only three peaks in solution NMR. We also analyzed a 2D 1 H-29 Si NMR correlation spectrum on a 29 Si labelled sample allowing to decipher the location of the propyl (Pr) arms in MFI channels. In parallel, we conducted complementary Density Functional Theory calculations using two approaches: geometry optimization and ab initio molecular dynamics (AIMD) based on Born-Oppenheimer molecular dynamics (BOMD). On the basis of the highly resolved experimental NMR spectra together with the BOMD calculated NMR parameters, new sets of 1 H and 13 C NMR peaks were assigned. The BOMD simulation (5.2 ps trajectory length) allows further analysis of the structure and dynamics of the TPA guest, confined in the Silicalite-1 host. From the obtained results, there is no evidence of any major restriction of H motions (C-H stretching, H-C-H bending, methyl rotation) due to sterically constrained Pr arms located in MFI zigzag channels. We found however that Pr arms in zigzag channels remain in trans conformation, while the conformation of Pr arms in straight channels alternates between trans and gauche.