Diffusion mechanisms of normal alkanes in faujasite zeolites

Connecting theory and simulation with experiment for the study of diffusion in nanoporous solids

Nanoporous solids are ubiquitous in chemical, energy, and environmental processes, where controlled transport of molecules through the pores plays a crucial role. They are used as sorbents, chromatographic or membrane materials for separations, and as catalysts and catalyst supports. Defined as materials where confinement effects lead to substantial deviations from bulk diffusion, nanoporous materials include crystalline microporous zeotypes and metal–organic frameworks (MOFs), and a number of semi-crystalline and amorphous mesoporous solids, as well as hierarchically structured materials, containing both nanopores and wider meso- or macropores to facilitate transport over macroscopic distances. The ranges of pore sizes, shapes, and topologies spanned by these materials represent a considerable challenge for predicting molecular diffusivities, but fundamental understanding also provides an opportunity to guide the design of new nanoporous materials to increase the performance of transport limited processes. Remarkable progress in synthesis increasingly allows these designs to be put into practice. Molecular simulation techniques have been used in conjunction with experimental measurements to examine in detail the fundamental diffusion processes within nanoporous solids, to provide insight into the free energy landscape navigated by adsorbates, and to better understand nano-confinement effects. Pore network models, discrete particle models and synthesis-mimicking atomistic models allow to tackle diffusion in mesoporous and hierarchically structured porous materials, where multiscale approaches benefit from ever cheaper parallel computing and higher resolution imaging. Here, we discuss synergistic combinations of simulation and experiment to showcase theoretical progress and computational techniques that have been successful in predicting guest diffusion and providing insights. We also outline where new fundamental developments and experimental techniques are needed to enable more accurate predictions for complex systems.

Methanol diffusion in zeolite HY: a combined quasielastic neutron scattering and molecular dynamics simulation study

The diffusion of methanol in zeolite HY is studied using tandem quasielastic neutron scattering (QENS) experiments and molecular dynamics (MD) simulations at 300–400 K.

Translational and rotational diffusion of SF6 in zeolite NaY

Temperature dependence of equilibrium as well as dynamical properties of SF(6) in zeolite NaY have been investigated by molecular dynamics simulation. By about 200 K, SF(6) begins to have increased mobility. Strong orientational preference is exhibited by SF(6) during its passage through the 12-ring window, the bottleneck for diffusion. The preference is for orientation with C(3) followed by C(2) and then C(4) molecular symmetry axis perpendicular to the window plane. Translational motion is diffusive with an activation energy of 5.5 kJ/mol. Rotational-diffusion coefficient has an activation energy of 2.83 kJ/mol. Rotational motion is facile within the alpha-cage. Translational motion is hindered during passage through the 12-ring window when C(4) is perpendicular to the window plane. Orientational correlation functions P(1) and P(2) around C(2), C(3) and C(4) are reported. Only the long time decay of C(4) shows oscillations. This is attributed to the hindered rotation during intercage migration while passing through the 12-ring window.

n-Pentane and isopentane in one-dimensional channels

Molecular dynamics studies of n-pentane and isopentane in one-dimensional channels of AlPO(4)-5 and a carbon nanotube are reported. Variation of the structure and energetics in AlPO(4)-5 along the channel axis of isopentane is similar to what has been found for other rigid molecular systems. In n-pentane, these properties exhibit more frequent undulations along the channel due to flexibility. The end-to-end distance of n-pentane is a function of its position along the channel in AlPO(4)-5, suggesting that n-pentane has to alternately stretch in the narrow part and destretch or coil in the broader part of the channel. n-Pentane lies flat instead of upright on the inner surface of the carbon nanotube. Both of the species exhibit diffusive motion in AlPO(4)-5, and the self-diffusivity is higher than that in bulk. Isopentane has a higher diffusivity than does n-pentane. This is attributed to the higher cross section of isopentane, which is closer to the void cross section. Further, the coupling of the translational motion with the slower dihedral angle reorientation in the case of n-pentane decreases its mobility. Superdiffusive motion is seen for both species in the carbon nanotube. These results can be understood in terms of the levitation effect.

Diffusion anomaly as a function of molecular length of linear molecules: levitation effect

Previous work on monatomic spherical sorbates has shown the existence of an anomalous peak in self-diffusivity (D) when plotted as a function of size of the diffusant. Molecular dynamics studies on linear molecules of different lengths l in zeolite NaY at 140 and 200 K are reported. It is seen that there is a peak in D as a function of l, suggesting that the levitation effect exists for linear molecules, the simplest member of polyatomics. This is confirmed by the lowering of the activation energy for the molecule whose length l exhibits highest D. Related quantities of interest such as the guest-host interaction energy and preexponential factor are discussed.

Reaction Rate Enhancement in Nanoporous Materials with Single-File Behaviour

In the concept of ´´molecular traffic control´´, Derouane and Gabelica postulated an enhancement of the effective conversion rate in catalytic reactions if the reactant and product molecules avoid each other by preferentially choosing different channel systems on their diffusion path into and out of the catalyst particles. After two decades of controversial discussions, the feasibility of this concept has recently been demonstrated by considering a network of intersecting single-file channels, whose accessibility by the reactant and product molecules was mutually excluded. The present communication analyses the dependence of molecular distribution over the different elements of the channel system on the intrinsic reactivity and the pore filling factor. The obtained patterns are compared with those of a reference system where all channels are equally accessible by the reactant and product molecules. Special attention is given to the effect of particle self-blockages under molecular traffic control, which occurs for large intrinsic reactivities and pore filling factors. It is demonstrated that under these conditions the simulations end up in an immobilised state which is a function of the particular simulation run. Such a behaviour is not observable in the reference system.