Molecular path control in zeolite membranes

Abstract

We report molecular simulations of diffusion in confinement showing a phenomenon that we denote as molecular path control (MPC); depending on loading, molecules follow a preferred pathway. MPC raises the important question to which extent the loading may affect the molecular trajectories in nanoporous materials. Through MPC one is able to manually adjust the ratio of the diffusivities through different types of pores, and as an application one can direct the flow of diffusing particles in membranes forward or sideward by simply adjusting the pressure, without the need for mechanical parts like valves. We show that the key ingredient of MPC is the anisotropic nature of the nanoporous material that results in a complex interplay between different diffusion paths as a function of loading. These paths may be controlled by changing the loading, either through a change in pressure or temperature.anomalous diffusion ͉ nanoporous materials ͉ statistical physics A mong other emerging membrane technologies like polymerinorganic composites, carbon films, and micro-and mesoporous silica films, zeolite membranes offer outstanding potential for molecular recognition at the subnanometer level and the ability to operate at high temperatures (1, 2). Zeolites are crystalline structures made up of ''T-atoms,'' where T is an aluminum or silicon atom, which are tetrahedrally bonded to each other with oxygen bridges. Because of the regularity of the crystalline structure and the pores with angstrom-size dimensions, these crystals, when grown together to form a membrane, can operate as separation devices for gas and liquid mixtures. From a scientific point of view zeolites are ideal systems to study the effect of confinement on the properties of the adsorbed molecules.Transport of adsorbates in nanoporous adsorbents such as zeolites is determined by a complex interplay between adsorbent-adsorbate and adsorbate-adsorbate interactions. Molecules diffuse through the pores via various diffusion mechanisms (3). Although interesting effects like single-file diffusion (4), incommensurate diffusion (5, 6), and levitation effects (7) are well known, most of the effects of confinement on diffusion remain poorly understood. This is particularly true for loading effects in materials with different channels and͞or cages in the x, y, and z direction. Anisotropic single-component diffusion in silicalite has been known for a long time (8-12). In general, the diffusion coefficients in the different directions can have different dependencies on temperature and loading. A limited number of studies deal with nonzero loading. Bussai et al. (13) found little change in anisotropy for water in silicalite as a function of loading. In this article, we report a reversal of anisotropy, i.e., at low loading the diffusivity in the z direction is two times faster than in the xy direction for both the self and collective diffusivity, whereas for higher loadings this changes into a z diffusivity that is more than two times slower. This behavior is due to a complete ...

Understanding cage effects in the n-alkane conversion on zeolites

“…2). At sufficiently low loading, the diffusion rate through the ERI-type pores is not a function of loading [62,81]. Furthermore, the intrinsic rate constant, k (1/s), is not a strong function of n-alkane length [19,20].…”

“…Such a phenomenon was already known as "incommensurate diffusion" in physics [56], but was renamed "resonance diffusion" in catalysis [51]. In catalysis the existence of incommensurate diffusion remains controversial, highlighting the experimental difficulties in obtaining consistent diffusion rates [54,[57][58][59][60][61][62]. In this paper we show that incommensurate diffusion in ERI-type zeolites is a prerequisite to understanding these zeolites' remarkable reactant shape selectivity in n-alkane hydroconversion.…”

Dynamically Corrected Transition State Theory Calculations of Self-Diffusion in Anisotropic Nanoporous Materials

“…32 We note that MPC is different from molecular traffic control, 33 which is caused by mutual correlations in the movement of a multicomponent fluid through two types of pores. As a specific MPC example, we study the mechanism behind tunable anisotropy of ethane in ERI-type zeolite membranes.…”

On the Effects of the External Surface on the Equilibrium Transport in Zeolite Crystals

“…Because of different chemical properties, however, the surface can be tuned such that it forms chemical barriers to one component that is unwanted in the interior of the micropores. In the context of molecular path control 42 which may be seen as a "degree of freedom to membrane design purposes", the surface tayloring described above provides a new and independent "design degree of freedom".…”