Dynamics of long n-alkanes in silicalite: A hierarchical simulation approach

A hierarchical approach for simulating the diffusion of long n-alkanes in the zeolite silicalite is described. The simulation strategy utilizes concepts from Brownian motion theory and transition state theory and can be used to determine diffusivities for chains at least as long as C-20 using modest computational resources. The results indicate that self-diffusivities decline steadily with increasing chain length up until about n-C-8, after which the self-diffusivities plateau and become nearly constant as a function of chain length. Activation energies for diffusion are constant and equal to 5 kJ/mol for short chains but increase to roughly 12 kJ/mol for chains longer than C-8. A microscopic explanation for these trends is proposed.