Dynamic Properties of Cyclohexane Guest Molecules Constrained within the Zeolite H-ZSM-5 Host Structure: A Wide-Line Solid State 2H NMR Investigation

Abstract

Dynamic properties of perdeuterated cyclohexane (C6D12) molecules included within the zeolite H-ZSM-5 host structure have been studied by variable-temperature wide-line solid-state 2H NMR spectroscopy. Three types of rapid motion have been identified for the C6D12 molecules in the temperature range 93−233 K as follows: motion A, rapid reorientation about the C3 symmetry axis of the C6D12 molecule; motion B, rapid restricted wobbling of the C3 symmetry axis, described as precession of the C3 symmetry axis on the surface of a cone (the half angle of which increases with increasing temperature); motion C, a rapid four-site jump motion, the geometry of which is dictated by the orientations of the tunnel axes at the intersection between the zigzag and straight tunnels of the H-ZSM-5 host structure. The temperature dependence of the quadrupole echo 2H NMR line shape in the temperature range 263−368 K is interpreted in terms of cyclohexane ring inversion occurring in the intermediate motion regime, in addition to motions A−C, which are in the rapid motion regime. From the 2H NMR line shape analysis, the activation energy for the cyclohexane ring inversion process is estimated to be ca. 48 kJ mol-1. The dynamic properties are compared with those of cyclohexane in other solid host structures, and the constraints imposed by the H-ZSM-5 host structure on the dynamic properties of the cyclohexane guest molecules are assessed.