Diffusion of Methanol in Zeolite NaY:  A Molecular Dynamics Study

Abstract

Molecular dynamics simulations were performed in order to obtain a detailed understanding of the self-diffusion mechanisms of methanol in the zeolite NaY system. We derived a new force-field term to describe the interactions between the methanol molecules and the extraframework cations. From the simulations, we show that diffusive behavior in the high-temperature range consists of a combination of both short- and long-range motions at low and intermediate loadings. This type of motion is characterized by an activation energy that decreases as the loading increases. At low loadings, we also observe short-range diffusive behavior based on a surface-mediated mechanism. The short-range behavior corresponds to motion only on the length scale of an FAU supercage, whereas the long-range behavior involves intercage diffusion. For the saturation loading corresponding to 96 methanol molecules per unit cell, only short-range motions within the same supercage predominate. Finally, the preferential arrangement of the adsorbate molecules around the extraframework cations are examined and compared with those previously deduced from experimental data.

Mechanistic Insight into the Framework Methylation of H-ZSM-5 for Varying Methanol Loadings and Si/Al Ratios Using First-Principles Molecular Dynamics Simulations

“…For example, for methanol in a NaY system, theoretical investigations showed that methanol facilitates migration of the Na + cation from the vicinity of the active site to the center of the pore, which influences the stability of the methanol cluster. 65 The stability of the solvated cation in the center of the pore, surrounded by methanol molecules, may be due to a favorable electrostatic environment as well as the distance of the methanol cluster from the active site hindering the retransfer of the Brønsted proton from the methanol. Theoretical studies of gas-phase methanol reported that the energy to form a methanol cluster, protonated or neutral, reaches a plateau after adding five methanol molecules.…”

Influence of Topology and Brønsted Acid Site Presence on Methanol Diffusion in Zeolites Beta and MFI

“…The charges for methanol were obtained by a Mulliken population analysis of ab initio DFT calculations by Blanco and Auerbach [52]. These parameters were scaled by Plant and associates [53] to attain a more polarized methyl group corresponding to methanol at acidic sites in zeolite frameworks. Methanol intramolecular harmonic potentials for covalent bonds, angles, and dihedrals were determined using ab initio DFT calculation at the quadratic configuration interaction calculation with single and double excitations (QCISD) level of theory to attain the ground state energy and electron density distribution [52].…”

Molecular dynamics investigation of the self-diffusion of binary mixture diffusion in the metal-organic framework Zn(tbip) accounting for framework flexibility

“…(2). Both bonded and non-bonded potential parameters were taken from the work of Plant et al [14] for methanol diffusion in the NaY zeolite as listed in Tables 1 and 2. The validity of these force field parameters was confirmed by an agreement of the diffusion coefficient of methanol obtained in the liquid phase by simulation to be 1:365 Â 10 À9 m 2 s À1 at an ambient temperature with the corresponding experimental results of 1:3 Â 10 À9 m 2 s À1 [15].…”