Molecular Dynamics of n-Octane Inside Zeolite ZSM-5 As Studied by Deuterium Solid-State NMR and Quasi-Elastic Neutron Scattering

Abstract

The dynamics of a linear alkane, n-octane, adsorbed in zeolite ZSM-5 was studied using deuterium solid-state NMR (2H NMR) and quasi-elastic neutron scattering (QENS). It has been found that at the loading of 1.8 molecules per unit cell, adsorbed n-octane molecules are essentially located in the straight channels and diffuse along the direction of the straight channels with a diffusion coefficient D = 12.0 × 10-11 m2/s at 300 K. In the course of translational movement along the straight channels, some coupled rotational motions of all CHn− (n = 2, 3) groups of the hydrocarbon skeleton of the molecule take place. They are reflected in the 2H NMR spectrum of deuterated n-octane-d18, in the temperature range 253−373 K, as fast rotations of the separate methylene and methyl groups simultaneously around two and three C−C bonds of the molecule with a characteristic time τC ≈ 10-11 s and an activation energy ER ≈ 10−12 kJ/mol. These internal motions may correspond to fast interconversion between trans and gauche conformations in the adsorbed alkane molecule while the molecule moves along the straight channels. Upon heating at 373 K for 1 h, n-octane molecules, formerly located in the straight channels, become redistributed over straight and zigzag channels. Subsequent translational motion of n-octane consists of two independent modes of motion. One of them represents the movement along the tortuous zigzag channels. The other one represents the movement along the straight channels, disturbed by collisions with the other molecules at the channel intersections. For a loading of 3.5 molecules per unit cell, a liquidlike line shape appears at 253 K in the 2H NMR spectrum. This line shape corresponds to isotropically reorienting n-octane molecules, changing the direction of their translational motion (from straight to zigzag channels) under collision with the other molecules at the channel intersections.