Molecular dynamics simulations of longer n-alkanes in silicalite: state-of-the-art models achieving close agreement with experiment

Abstract

The diffusion of longer n-alkanes (n-C8-n-C16) in silicalite was studied using molecular dynamics (MD) simulations at a temperature range of 300-400 K, with loadings appropriate for direct comparison with previously carried out quasielastic neutron scattering (QENS) studies. The calculated diffusion coefficients were in close agreement with experimental values, significantly closer than those calculated using more primitive framework and hydrocarbon models, and in the case of the longer alkanes, closer agreement than those calculated by MD studies using the same model, but not using experimental loadings. The calculated activation energies of diffusion agreed with experiment to within 1.5 kJ mol(-1) for shorter alkanes of the range, but with a larger difference for tetra and hexadecane, due to factors which cannot be reproduced using periodic boundary conditions. Channel switching between the straight and sinusoidal channel system was found for octane at higher temperatures, where more than one octane molecule was located in the channel, which was attributed to the molecular size of octane, and the repulsion caused by the presence of the extra octane molecules in the channel system, allowing the potential barrier of channel switching at the junctions to be breached.

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

“…In particular, the potential for combined experimental and theoretical studies of these systems using complementary techniques such as quasielastic neutron scattering (QENS) and molecular dynamics (MD) simulations has been reported in a number of studies. 27 Recent work has shown how state-of-the-art MD simulations can obtain excellent agreement with experimentally measured diffusivities with QENS and neutron spin-echo techniques, and provide qualitative insight such as preferred siting of linear 28,29 and spherical sorbates in zeolites. 30 Though an understanding of molecular diffusion as a function of zeolite topology is crucial for optimisation of a catalytic process, the composition such as the presence and concentration of counterions in the zeolite structure will also have a significant effect on molecular transport.…”

Investigation of the Dynamics of 1-Octene Adsorption at 293 K in a ZSM-5 Catalyst by Inelastic and Quasielastic Neutron Scattering

“…apparent that the Lorentzian function only contributes significantly to the overall scattering function at higher Q values: the same pattern is also observed for the spectra at 273 and 323 K. Since Q is inversely related to distance, 36 this suggests that the movement of the octene is restricted to localised motions and it does not exhibit the long-range translations reported for linear alkanes in ZSM-5. 15,37 Localised motions in a QENS spectrum may be characterised by deriving the Elastic Incoherent Structure Factor (EISF, A0(Q)) which is the fraction of the total scattering intensity that is elastic, given by:…”

Shape-Selective Diffusion of Olefins in 8-Ring Solid Acid Microporous Zeolites

“…This explicit derivation of diffusivities has been extensively studied in literature, e.g. by Awati et al 33 , Krishna et al 34 , Smit and Maessen et al 26 , Garcia and Dubbeldam et al 35 , O'Malley et al 32 , Kiel et al [36][37][38][39] in various materials, for various hydrocarbons. For processes where diffusion barriers are too high, MD runs may be too slow to efficiently generate particle trajectories over the time-scale of the diffusion process.…”