Influence of isotherm inflection on the loading dependence of the diffusivities of n-hexane and n-heptane in MFI zeolite: quasi-elastic neutron scattering experiments supplemented by molecular simulations

Abstract

Quasi-Elastic Neutron Scattering (QENS) experiments were carried out to determine (a) Fick diffusivity, D (b) self-diffusivity, D_self, and (c) 1/Γ, the inverse of the thermodynamic correction factor, for n-hexane (nC6) and n-heptane (nC7) in MFI zeolite (all silica silicalite-1) at 300 κ for a variety of loadings. These experimental results are compared with configurational-bias Monte Carlo (CBMC) and molecular dynamics (MD) simulations of, respectively, the adsorption isotherms and diffusivities. For n-hexane, the CBMC simulated isotherm shows a slight inflection at a loading ϑ = 4 molecules per unit cell; this inflection manifests, also, in the loading dependence of 1/Γ, obtained from QENS. The trend in the loading dependence of the Fick D and D_self of nC6 obtained from QENS matches the MD simulation results. For nC7 the CBMC simulated isotherm shows a strong inflection at a loading ϑ = 4 molecules per unit cell. At this loading ϑ = 4, 1/Γ tends to zero and there is a very good match between QENS and molecular simulations for the loading dependence of 1/Γ. Both MD simulations and QENS data on the Fick diffusivity shows a sharp maximum at a loading in the region of ϑ = 4. For both nC6 and nC7 the simulated values of diffusivity are about an order of magnitude higher than those determined from QENS.