Molecular Dynamics Simulations of Mass Transfer Resistance in Grain Boundaries of Twinned Zeolite Membranes David A. Newsome and David S. Sholl* Department of Chemical Engineering, Carnegie-Mellon UniVersity, Pittsburgh, PennsylVania 15213 ReceiVed: May 28, 2006; In Final Form: August 30, 2006 The net mass transfer resistance for gas molecules permeating through zeolite membranes includes contributions from intracrystalline diffusion and contributions from interfacial effects. These interfacial effects can arise either from gas-zeolite interfaces or from interfaces that exist within zeolite crystals due to grain boundaries. We present the first atomically detailed simulations that examine interfacial mass transfer resistance due to internal grain boundaries in zeolites that are relevant for membrane applications. Our calculations examine twinned silicalite crystals in crystallographic configurations that have been identified in previous experiments. We used the dual control volume grand canonical molecular dynamics method to simulate the permeance of CH 4 and CF 4 through thin twinned silicalite crystals. The magnitudes of the grain boundary resistances are quite substantial, at least for the thin crystals that are accessible in our simulations. 1. Introduction The permeation of gases through membranes made from nanoporous inorganic materials such as zeolites is controlled by a combination of adsorption of molecules into the membrane material and the mass transfer resistance associated with molecules moving through the membrane. The most obvious source of this mass transfer resistance is associated with intracrystalline diffusion through the membrane material. There is a large amount of literature dealing with atomically detailed models of intracrystalline diffusion in crystalline nanoporous materials, 1-6 including examples that compare theoretical predictions with experimental observations of membranes. 7-10 Treating intracrystalline diffusion as the only source of mass transfer resistance is appropriate for thick single crystals of nanoporous materials. Real zeolitic membranes differ from this simple limit in at least two important respects. First, it is desirable in practical applications to minimize the thickness of the zeolite layer. 11 Second, real membranes are almost always polycrystalline films grown on macroporous supports. 12-17 These conditions mean that the mass transfer resistance associated with entering and leaving the pores of a zeolite crystal should also be included if a complete description of permeation through real membranes is desired. 18 Atomically detailed simulations have been applied to this issue, primarily using dual control volume grand canonical molecular dynamics (DCV GCMD) to directly observe these so-called surface barriers in ultrathin membranes. 19-23 We have recently introduced a complementary simulation method that can be used to rapidly assess whether surface barriers are significant for a wide range of process conditions once an atomically detailed model of the membrane material and permeating molecules is available. 24-26 A further source of mass transfer resistance in zeolitic membranes can occur due to imperfections in the zeolite crystal structure within an individual crystal. A number of experimental observations of silicalite crystals have reported the existence of twinned crystals in which grain boundaries separate multiple orientationally distinct silicalite single crystals. 27-30 These experiments have assigned the crystal orientations and shapes that form these twinned crystals, which can be thought of as pyramids of silicalite in contact with one another. 27-30 Because of the disruption in the structures between the pyramids, grain boundaries are created that are suspected to pose mass transfer barriers. Unusual adsorption and desorption behavior has been experimentally observed in the twinned zeolite samples. 27-30 For example, Geus et al. experimentally observed that water had hindered diffusion through the crystals, which they attributed to the internal interfaces. 29 Geier et al. observed unusual transient concentration profiles of isobutene, noting that some sections had faster rates of uptake. 30 They could only explain this because of the presence of the interfaces. Kocirik et al. performed mutual diffusion experiments to color the crystals using numerous chemical pairs such as iodine-benzene, iodine-toluene, and iodine-decahydronaphthalene. 27 In this paper, we describe the first atomically detailed calculations to examine the properties of molecular transport through internal grain boundaries in a zeolite relevant for membrane applications. We have focused on twinned silicalite because of the structural characterization for this example from the experiments just mentioned. We used the DCV GCMD approach to model gas permeation through twinned silicalite crystals. Our paper is organized as follows. In section 2 we describe the method used to construct the interfaces between twinned grains of silicalite at an atomic scale. Section 3 presents our DCV GCMD simulations of CH 4 and CF 4 permeation through twinned silicalite crystals in terms of the mass transfer resistances of the twinned zeolite membrane with and without the grain boundaries. We summarize our conclusions and discuss our results in section 4. 2. Grain Boundary Structure The grain boundary structure of twinned silicalite crystals that has been observed experimentally is illustrated in Figure 1. 29,30 The rectangular silicalite crystal illustrated in Figure 1a is comprised of six pyramidal crystals with different crystal- lographic orientations. In all, six crystals, the crystallographic * Corresponding author phone: 412-268-4207; fax: 412-268-7139; e-mail sholl@andrew.cmu.edu. 22681 J. Phys. Chem. B 2006, 110, 22681-22689 10.1021/jp063287g CCC: $33.50 © 2006 American Chemical Society Published on Web 10/25/2006