Dynamics of linear n-C6-n-C22 alkanes inside 5A zeolite studied by 2H NMR

The dynamic behavior of deuterated analogues of linear alkanes, n-C-6-n-C-22, adsorbed in zeolite 5A has been studied by deuterium solid-state NMR (H-2 NMR). Temperature dependences of spin-lattice (T-1) and spin-spin (T-2) relaxation times of the deuterium located in the CD3 groups of the adsorbed n-alkanes were rationalized on the basis of a model derived for the motion of n-alkanes located in the pores of the zeolite. The model implies that the adsorbed molecules consist of two ensembles: diffusing (or stretched) and temporarily blocked from diffusion (or coiled). The possible intramolecular motions for the alkane chains were taken into account based on both the finite size of the zeolite cage and the allowable hydrocarbon chain conformations. The coiled molecules are involved in two modes of motion: isotropic reorientation and intramolecular conformational isomerization, whereas the stretched molecules are additionally involved in a diffusion process. Dynamics parameters for different modes of motion and a proportion of the blocked and stretched molecules were derived from the analysis of relaxation data. The estimated proportion of the diffusing molecules correlates with the alkanes diffusivities earlier obtained by neutron spin echo measurements.