An alternative approach for ab initio fitted potentials: The n-pentane/silicalite-1 system

Quantum chemical calculations at the second-order Møller-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 composition is O[10]Si[10]H[20]. The energies of these configurations were fitted to analytical functions. Our goal was to reproduce both the experimental heat of adsorption (Qst) as well as the self-diffusion coefficient (Ds). The MP2/6-31+G(d,p) was applied and the collision constants were introduced into the ab initio fitted potential function. The extrapolated Qst from the newly developed function of 52.28 kJ/mol is 9.39% lower than that of the experiment and the dependence of the Ds as a function of temperatures is in good agreement with that observed experimentally. The obtained function was applied for a series of molecular dynamics simulations by varying temperatures and the concentration of n-pentane. The obtained structural data were interpreted in terms of residence distributions. Their changes were, then, described in terms of a competition between potential fields exerted by the zeolite and molecular movement. The attractive potential fields dominate the molecular movement at low temperature and this dominance decreases when the temperature increases.