Journal of Physical Chemistry B vol:112 issue:32 pages:9630-9640
Literature [Denayer et al. Microporous Mesoporous Mater. 2007, 103, 1 and Denayer et al. Microporous Mesoporous Mater. 2007, 103, 11] shows that zeolite NaX exchanged with Call and Co2+ ions is able to remove cyclopentadiene (CPD) impurities from a I-octene feed with high selectivity. In the present work, the adsorption of dicyclopentadiene (DCPD), CPD, I-octene, and n-octane on zeolite X, exchanged with Ca2+ and/or Co2+ ions, has been investigated via H-1 magic-angle spinning (MAS) NMR spectroscopy. The liquid adsorbate was dosed under inert atmosphere in an MAS rotor filled with dry adsorbent, at a pore filling degree of 70%. Next, the evolution in time was recorded of the 1H MAS NMR spectrum and the 1H spin-lattice and spin-spin relaxation times of the adsorbed components. For the various adsorbate-adsorbent systems, a plot is made of the signal intensity versus the square root of the contact time. It is found that, over the considered time interval, Fickian diffusion takes place. On the basis of the change in time of the spin-lattice relaxation time, a transport diffusion coefficient ranging between I and 2 x 10(-15) m(2).s(-1) is calculated. Moreover, there appear to be two sorption regimes, with different diffusivities. A comparison is made between the 1H spin-lattice relaxation behavior of DCPD, 1-octene, and n-octane, indicating that 1-octene and n-octane are located closer to the paramagnetic ions than DCPD. The average distance between the adsorbate molecules and the paramagnetic ions is derived from relaxometric data. By analyzing the chemical shifts of the resonance lines, it is found that the pi-interaction of CPD and 1-octene is stronger than that of DCPD.