Molecular dynamics of n-octane inside zeolite ZSM-5 as studied by deuterium solid-state NMR and quasi-elastic neutron scattering

The dynamics of a linear alkane, n-octane, adsorbed in zeolite ZSM-5 was st udied using deuterium solid-state NMR (H-2 NMR) and quasi-elastic neutron s cattering (QENS). It has been found that at the loading of 1.8 molecules pe r unit cell, adsorbed n-octane molecules are essentially located in the str aight channels and diffuse along the direction of the straight channels wit h a diffusion coefficient D = 12.0 x 10(-11) m(2)/s at 300 K. In the course of translational movement along the straight channels, some coupled rotati onal motions of all CHn- (n = 2, 3), groups of the hydrocarbon skeleton of the molecule take place. They are reflected in the H-2 NMR spectrum of deut erated n-octane-d(18), in the temperature range 253-373 K, as fast rotation s of the separate methylene and methyl groups simultaneously around two and three C-C bonds of the molecule with a characteristic time tau(c) approxim ate to 10(-11) s and an activation energy E-R approximate to 10-12 kJ/mol. These internal motions may correspond to fast interconversion between trans and gauche conformations in the adsorbed alkane molecule while the molecul e moves along the straight channels. Upon heating at 373 K for 1 h, n-octan e molecules, formerly located in the straight channels, become redistribute d over straight and zigzag channels. Subsequent translational motion of n-o ctane consists of two independent modes of motion. One of them represents t he movement along the tortuous zigzag channels. The other one represents th e movement along the straight channels, disturbed by collisions with the ot her 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 H-2 NMR spec trum. This line shape corresponds to isotropically reorienting,a n-octane m olecules, changing the direction of their translational motion (from straig ht to zigzag channels) under collision with the other molecules at the chan nel intersections.