Mobility of n-Butane in ZSM-5 Zeolite Studied by 2H NMR

Abstract

The mobility of linear alkane, n-butane, adsorbed in zeolite ZSM-5 was investigated by deuterium solid-state NMR (2H NMR). Analysis of the 2H NMR spectra line shape and spin−lattice relaxation time permitted characterizing the influence of pore confinement on the dynamics of the molecule over a broad temperature range (123−413 K). It has been found, that at a loading of two molecules per unit cell, n-butane molecules are essentially located in both straight and zigzag channels and not the intersections. The adsorbed molecules are involved in fast intramolecular motions and reorientation as a whole. Intramolecular motions are methyl group rotation and conformational exchange occurring at τ ≈ 4−8 × 10−11 s (at 300 K) with an activation barrier of Eci ≈ 11−12 kJ mol−1. The reorientation of the molecule as whole is a complex anisotropic motion, which consists of jump exchange between neighboring channels, occurring at τD ≈ 3.7 × 10−10 s (at 300 K) with an activation barrier of ED ≈ 5 kJ mol−1 and of other rot...