The dynamics of benzene in silicalite at low loading was investigated using transition-state theory. Benzene was found to diffuse by infrequent hops between preferred adsorption sites. Potential energy minima and 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) approach was used to construct the diffusion paths in six dimensions. The IRC equations are presented for the motion of a rigid body (benzene) through a static potential field (silicalite). A rate constant for each transition between minima was calculated using a harmonic approximation to the potential energy function. From the rate constants, the self-diffusivity was computed with a dynamic Monte Carlo simulation. An activation 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 discrepancy are the harmonic approximation invoked in calculating the rate constants and our neglect of zeolite flexibility in the calculations. The predicted time scales for local motions within the channel intersections agree well with spectroscopic results. Many of these motions correspond to rotations of the benzene molecule about its C6 axis.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry