Kinetic Monte Carlo Modelling to Study Diffusion in Zeolite. Understanding the Impact of Dual Site Isotherm on the Loading Dependence of n-Hexane and n-Heptane Diffusivities in MFI Zeolite, as Revealed by QENS Experiments

Abstract

Résumé -Étude de la diffusion dans les zéolites par Modélisation Monte Carlo Cinétique : comprendre l'impact d'une isotherme bi-site sur l'évolution des diffusivités du n-hexane et du n-heptane dans la zéolite MFI en fonction du taux de remplissage, telle que révélée par les expériences QENS -Dans ce travail, nous nous intéressons à la diffusion de molécules en monoconstituant dans des zéolithes, caractérisées par une isotherme représentée par un modèle de Dual Site Langmuir présentant un point d'inflexion. Les systèmes étudiés sont le n-hexane et le n-heptane dans la zéolite MFI à 300 K. Des expériences réalisées par la technique du Quasi-Elastic Neutron Scattering (QENS) ont montré que cette inflexion se reflète dans l'évolution en fonction du remplissage des coefficients de diffusion de transport D t et corrigé D C de ces systèmes. Les résultats de ces expériences sont présentés ici. Une étude par Monte Carlo Cinétique est réalisée montrant de quelle manière les niveaux énergétiques des sites d'adsorption de molécules dans une zéolite affectent les coefficients de diffusion de ces molécules en fonction du remplissage. Abstract -Kinetic Monte Carlo Modelling to Study Diffusion in Zeolite: Understanding the Impact of Dual Site Isotherm on the Loading Dependence of n-Hexane and n-Heptane Diffusivities in MFI Zeolite, as Revealed by QENS Experiments -This study concerns the diffusion of single-component molecules in zeolites, characterised by an isotherm represented by a dual-site Langmuir model with a point of inflection. The systems investigated are n-hexane and n-heptane in

Unraveling Diffusion and Other Shape Selectivity Effects in ZSM5 Usingn-Hexane Hydroconversion Single-Event Microkinetics

“…Equation 20, whether or not complemented with a mean-field approach such as in eq 21, has been applied to numerous microscopic and macroscopic physisorption studies. 7,71,102,111,120,122,123 There remained similar discrepancies between the results obtained using these two groups of techniques as commented above, and as in this work, these discrepancies could not be ascribed to the methodology used to extract the diffusion parameters from the experimental data.…”

“…A similar observation was made by Hansen et al, who ultimately applied corrected diffusion coefficients for all aromatic species involved in benzene alkylation with ethylene, which were more than 3 orders of magnitude lower than those initially simulated from MD, the latter technique agreeing well with results from microscopic measurements. , The reason why the species diffusion coefficients obtained in this and in Hansen’s work correlate better with the reported values specifically obtained from macroscopic measurement techniques is not fully resolved yet. Equation , whether or not complemented with a mean-field approach such as in eq , has been applied to numerous microscopic and macroscopic physisorption studies. ,,,,,, There remained similar discrepancies between the results obtained using these two groups of techniques as commented above, and as in this work, these discrepancies could not be ascribed to the methodology used to extract the diffusion parameters from the experimental data.…”

“…The latter phenomenon is often denoted as "commensurate freezing" 69,70 and could lead to an inflection point in the physisorption isotherm. 40,60,61,71 However, such physisorption behavior was not observed in experimental studies applying high temperatures and low partial pressures such as the ones used in the present work. 32,37,72,73 As commented earlier, a single-site Langmuir isotherm was implicitly assumed in eq 7 in reference to earlier work on non-shape-selective catalysts.…”

“…The peculiar framework structure of ZSM5 consisting of two types of 10-membered pore channels, could induce preferential physisorption of methylpentanes at the channel intersections, and of n -hexane in the sinusoidal channels at relatively high pressures. The latter phenomenon is often denoted as “commensurate freezing” , and could lead to an inflection point in the physisorption isotherm. ,,, However, such physisorption behavior was not observed in experimental studies applying high temperatures and low partial pressures such as the ones used in the present work. ,,, As commented earlier, a single-site Langmuir isotherm was implicitly assumed in eq in reference to earlier work on non-shape-selective catalysts. , Figure shows the comparison between a single-site Langmuir isotherm (eq ) and a dual-site Langmuir isotherm (eq ) for n -hexane and 2-methylpentane physisorption at 503 K over the reactant partial pressure range considered in this work. The single-site Langmuir isotherm parameters were determined by Denayer et al for a ZSM5 catalyst with an identical Si/Al ratio, and the dual-site Langmuir isotherm parameters were reported by Zhu et al for silicalite. …”

Operando Modeling of Zeolite-Catalyzed Reactions Using First-Principles Molecular Dynamics Simulations

“…For example, the current methodology to study phenomena with vastly different times scales is the usage of kinetic Monte Carlo methods, where a series of stochastic trajectories is generated that propagate the system from one state to the other, and afterward the correct dynamics is retrieved by ensemble averaging over all of these trajectories . There are only a few examples of kMC approaches within the field of nanoporous frameworks, and the combination of diffusional, reaction, and adsorption sites has thus far not been performed. ,− As mentioned earlier in this section, kMC methods and also first-principles microkinetic models currently rely on kinetic and thermodynamics information from static-based simulations. However, it may be envisaged that in the future kMC models could be constructed for complex zeolitic systems, where input from first-principles MD simulations is used.…”