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 silic eous 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 catio ns, 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 a dsorbed 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 traje ctories it can be seen that the sorbate maintains close interactions w ith the walls of the supercage. On the 25-ps time scale of the MD simu lation, the behavior of benzene is largely dominated by intracavity ra ther than intercavity motion. For the slower diffusion of benzene in N a-Y, trajectories over a period of much greater than 1 ns would be nee ded to observe intercavity motion.