Mobility of n-Butane in ZSM-5 Zeolite Studied by H-2 NMR
Résumé
The mobility of linear alkane, n-butane, adsorbed in zeolite ZSM-5 was investigated by deuterium Solid-state NMR (H-2 NMR). Analysis of the H-2 NMR spectra line shape and spin-lattice relaxation time permitted characterizing the influence or pore confinement oil the dynamics of the molecule over I 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 tau approximate to 4-8 x 10(-11) s (at 300 K) with an activation barrier of E-ci approximate to 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 tau(D) approximate to 3.7 x 10(-10) s (at 300 K) with all activation barrier of E-D approximate to 5 kJ mol(-1) and of other rotational modes that may correspond to uniaxial rotation and librations of n-butane in the pores. influence of pore confinement oil the dynamics of the adsorbed alkane is manifested by the complete suppression Of isotropic reorientation.