Mobility of <i>tert-</i>Butyl Alcohol in MFI Framework Type Studied by Deuterium NMR

Abstract

The molecular mobility of deuterated <i>tert-</i>butyl alcohol (TBA) adsorbed in MFI framework type (silicalite-1 and ZSM-5 zeolite) has been studied by use of ²H NMR spectroscopy in the range of 106–453 K. In H-ZSM-5, the reorientation of the molecule as a whole is strongly restricted (τ_C ≪ <i>Q</i>₀^–1 ≈ 10^–6 s) by hydrogen bonding to Brønsted acid sites (BAS). Being adsorbed to BAS, the motion of TBA molecules is described by intramolecular rotations around two successive <i>C</i>₃ and <i>C</i>₃′ axes (CD₃–C and C–O bonds). The activation energy for the methyl groups rotation around the C–O bond (<i>E</i>_a = 8.0 ± 1.6 kJ mol^–1) is two times lower compared to that in solid TBA (<i>J. Phys. Chem. A</i> <b>2011</b>, <i>115</i>, 7428). This shows that the motion of the butyl fragment of TBA in MFI framework is less restricted compared to the case of solid TBA. In silicalite-1, the TBA molecule is additionally involved into reorientational motions as a whole: one of the motions represents an exchange among orientations provided by the directions of four framework channels driven by the translational jump diffusion. The activation barriers for the jump diffusion between two zigzag channels (<i>E</i>_a = 7 ± 2 kJ mol^–1) and between zigzag and straight channels (<i>E</i>_a = 5 ± 1 kJ mol^–1) are of the similar values. The other motion represents a large-amplitude wobbling of the TBA molecule localized at a channel intersection site cavity. This motion is described by a fast restricted wobbling of the molecular axis in a sphere sector. The wobbling boundaries become gradually broader as temperature increases, allowing thus a larger accessible space for the TBA molecule to explore. The anisotropy of this motion persists even at 453 K, revealing the presence of a strong intracavity barrier that blocks the TBA from free rotational diffusion inside the cavity. This study demonstrates that the ²H NMR analysis of both line shape and spin–lattice relaxation represents a powerful tool to investigate the particular pore confinement effect on the molecular mobility of TBA adsorbed in the MFI framework