Molecular Dynamics Simulations of Hexadecane/Silicalite Interfaces
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increased computational requirements. There has been no work addressing the equilibrium interface between a molecular liquid and a zeolite. Since nearly all zeolite applications involve molecules entering the zeolite channels to some degree and exiting again, we performed a study of interfacial dynamics in such a system. While some applications are gradient driven separations and are thus well addressed by nonequilibrium simulations, many catalysis operations involve extremely low fluxes. A feed stream may be in contact with a zeolite powder bed for minutes to hours and it is likely that near equilibrium interfaces are established throughout most of the reaction chamber. For this reason we present simulations of the interface between an alkane liquid and a zeolite. The system chosen for this initial study is hexadecane in contact with silicalite, referred to in this work by its structure type code: MFI. Hexadecane was chosen because it is large enough to demonstrate properties of a flexible chain molecule at the temperature we studied. The zeolite MFI has a three dimensional pore structure composed of two distinct types of channels. Channels run in the [100] or [010] direction; the walls of channels running in the [100] direction have well pronounced oscillations so these channels are described as sinusoidal. Channels running in the [010] direction do not undulate and so are called straight. Intersections between straight and sinusoidal channels create pathways for diffusion in the third dimension. In the present work, we only study the (010) surface as this is the surface from which molecules can enter the zeolite most quickly. MODEL AND METHOD The alkane molecules were simulated using an united atom (UA) model in which CH, groups are treated as single particles. The UA model used to describe alkane interactions is due to Siepmann et al. [24] with a torsional potential from Jorgensen et al. [25]. Interactions between hexadecane and the zeolite are represented by a Lennard-Jones (LJ) potential between CH, monomers and the oxygens of the MFI lattice [26]. The cutoff used for all LJ interactions was r, = 10 A. The zeolite lattice was held rigid, a common simplification employed in prior simulations of diffusion in zeolites. The structure for MFI was obtained directly from the zeolite structure database contained in the Solids Builder module of the Insight If software package [27]. As such, the lattice used represents ideal stoichiometry and crystal structure. Simulations presented here were done at T = 600 K and the hexadecane bulk density was the atmospheric pressure value for this temperature, p = 0.5621 gm/cm3 . Temperature was maintained via a stochastic force thermostat with a time constant -r = 1 ps. The equations of motion were integrated using the velocity Verlet algorithm and bond lengths were kept constant using the RATTLE algorithm [28]. A 5 fs time step was used in all simulations. The initial state of the system was formed by placing a slab of hexadecane liquid, periodic in the x and z dimensions, betwe
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