INVESTIGATION OF THE DYNAMICS OF BENZENE IN SILICALITE USING TRANSITION-STATE THEORY

The dynamics of benzene in silicalite at low loading was investigated using transition-state theory. Benzene was found to diffuse by infrequ ent hops between preferred adsorption sites. Potential energy minima a nd saddle points were located using an atomistic model, and diffusion paths connecting pairs of minima were constructed through each saddle point (transition state). The intrinsic reaction coordinate (IRC) appr oach was used to construct the diffusion paths in six dimensions. The IRC equations are presented for the motion of a rigid body (benzene) t hrough a static potential field (silicalite). A rate constant for each transition between minima was calculated using a harmonic approximati on to the potential energy function. From the rate constants. the self -diffusivity was computed with a dynamic Monte Carlo simulation, An ac tivation energy of 36.7 kJ/mol was calculated. This is larger than the experimental value, and the predicted diffusivities are 1-2 orders of magnitude smaller than experiment. Likely reasons for this discrepanc y are the harmonic approximation invoked in calculating the rate const ants and our neglect of zeolite flexibility in the calculations. The p redicted time scales for local motions within the channel intersection s agree well with spectroscopic results. Many of these motions corresp ond to rotations of the benzene molecule about its C-6 axis.