Diffusion of pentane isomers in silicalite-1 by molecular dynamic simulation

Abstract

Quantum chemical calculations at the second order Moller-Plesset Perturbation (MP2) levels were performed to evaluate n-pentane/n-pentane and n-pentane/silicalite-1 interactions where several hundred configurations of the pair were generated. The silicalite-1 crystal structure was represented by a 10-T ring, in which the chemical compositions is O[10]Si[10]H[20]. The energies and corresponding coordinates of these configurations were fitted to analytical functions. Several attempts were made to get the best functions which reproduce the experimental heat of adsorption(Qst) as well as the self-diffusion coefficient (Ds). In the final form, the MP2/6-31+G(d,p) was applied and the collision constants were introduced into the ab initio fitted potential. With this newly developed approach, ratio of the atomic pairs in the molecular pair potential was nicely partitioned. The obtained function was applied for a series of molecular dynamic simulations by varying temperatures and loadings of n-pentane. The obtained structural data was interpreted in term of residence distributions where their changes were, then, described by a competition between potential fields exerted by zeolite and molecular movement. The potential fields dominate the molecular movement at low temperature and decreases when the temperature increases. In addition, the extrapolated Qst from the newly developed function of 52.28 kJ/mol is 9.39% lower than that of the experiment and change of the Ds as a function of temperatures is in good agreement with that observed experimentally.