BEHAVIOR OF BENZENE IN SILICEOUS FAUJASITE - A COMPARATIVE-STUDY BY H-2 NMR AND MOLECULAR-DYNAMICS

The diffusion coefficients and activation energies of benzene in siliceous faujasite (D298 = (4.5 +/- 3.3) x 10(-10) m2 s-1, E(a) = 10.2 +/- 0.8 kJ mol-1) and Na-Y (D298 = (3.6 +/- 1.4) X 10(-12) M2 s-1, E(a) = 23.5 +/- 0.9 kJ mol-1) have been determined from H-2 NMR spin-lattice relaxation experiments. Unlike Na-Y, where benzene binds to SII cations, no specific adsorption site is observed in the siliceous analogue; this is reflected in significantly higher diffusion coefficients and a lower activation energy. Molecular dynamics simulations of benzene adsorbed in siliceous Y zeolite (D298 = (2.0 +/- 0.3) X 10(-9) M2 S-1) are in good agreement with the experimental result, and from the trajectories it can be seen that the sorbate maintains close interactions with the walls of the supercage. On the 25-ps time scale of the MD simulation, the behavior of benzene is largely dominated by intracavity rather than intercavity motion. For the slower diffusion of benzene in Na-Y, trajectories over a period of much greater than 1 ns would be needed to observe intercavity motion.