Influence of the Acid Sites on the Intracrystalline Diffusion of Hexanes and Their Mixtures within MFI-Zeolites

Abstract

The intracrystalline diffusivities and component loadings of mixtures of n-hexane and 2-methylpentane within silicalite-1 (crystal size 150 µm) and HZSM-5 (140 µm) have been measured at 433 K as a function of the n-hexane/2-methylpentane ratio in the gas phase (at constant total hydrocarbon pressure of 6.6 kPa) using the Positron Emission Profiling technique. Strong preferential adsorption of the linear alkane over the branched one has been observed in HZSM-5, while in silicalite only a slightly higher adsorption of n-hexane was observed. The diffusivities of both components in the mixtures decrease with increasing 2-methylpentane content and have been found to be approximately two times lower in HZSM-5 than those in silicalite. When the concentration of the branched hexane in the adsorbed phase becomes close to 3 molecules per unit cell, the influence of the acid sites on the diffusion of linear hexane diminishes compared to the influence of the blockage of the zeolite network with the slowly moving 2-methylpentane. The apparent activation energies of the diffusion for n-hexane and 2-methylpentane have been measured at temperatures between 393 K and 533 K under similar conditions. For n-hexane, these values were found to be very close in silicalite and ZSM-5: 18.5 ( 1.5 kJ/mol and 22 ( 2 kJ/mol, respectively. For 2-methylpentane the activation energies were found to be very high: 66 ( 6 kJ/mol and 72 ( 3 kJ/mol, respectively.

Diffusion in porous crystalline materials

“…For analogous reasons, the branched alkanes 2-methylpentane (2MP) and 3-methylpentane (3MP) cause the reduction in the self-diffusivity of the n-hexane (nC6) in nC6-2MP, and nC6-3MP mixtures. 41 Intersection blocking by branched and cyclic hydrocarbons is a plausible explanation for the observed inhibition of the cracking of n-octane using an MFI catalyst. 42 Traffic junction effects are of vital importance in modeling reactors for alkylation of benzene with ethene using the MFI zeolite catalyst (in the acidic form H-ZSM-5) to produce ethylbenzene.…”

Influence of adsorption thermodynamics on guest diffusivities in nanoporous crystalline materials

“…The explanation is again to be found in the hindering of CH 4 diffusion due to blocking of the intersections by benzene. 74 Koriabkina et al 75 have reported experimental data for the self-diffusivities of both nC6 and 2MP in C6-2MP mixtures. Their data, which were measured at 433 K, show that both self-diffusivities are significantly reduced with increasing loading of 2MP in the mixture.…”

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

“…A significant amount of work both theoretical and experimental has been carried out studying the diffusion of hydrocarbons in the MFI zeolite structure, [2][3][4][5][6][7][8][9][10][11][12][13][14][15] the framework type of industrially important ZSM-5 and its siliceous analogue silicalite. The diffusion of n-alkanes longer than octane in this framework is of considerable interest and has been studied by quasi-elastic neutron scattering (QENS) techniques in both Na-ZSM-5 16 and silicalite.…”