Microdynamics of methane, ethane and propane in ZSM-5 type zeolites

Abstract

The n.m.r. pulsed field-gradient technique has been used to study systematically the intracrystalline self-diffusion of methane, ethane and propane in ZSM-5. In conjunction with the information obtained from nuclear magnetic relaxation studies the elementary steps of diffusion are found to be activated jumps between the channel intersections. Only for sorbate concentrations > 2.5 molecules per intersection is a decrease in the jump lengths observed. The results are compared with alkane self-diffusion measurements in zeolites A and X, as well as with the self-diffusion of water in ZSM-5 and zeolites A and X.

Loading Dependence of the Diffusion Coefficient of Methane in Nanoporous Materials

“…Figure 27 shows our simulation results for the diffusion of methane in MFI-type silica as a function of the adsorbate loading, 33 together with results obtained by several other groups, through simulation [13][14][15][16][17] and experiment. [29][30][31]93,94 The experimental data (black symbols) are self-diffusion coefficients, except those obtained by single-crystal membrane (SCM) measurements, which should be corrected for transport diffusivities. The results have been plotted against the loading as reported in the original papers, wherever possible.…”

“…3,[19][20][21][22][23][24][25][26][27][28] However, these studies were often limited to the infinite dilution limit, whereas most experimental values were obtained at nonzero loading. [29][30][31] Only recently, a dynamically corrected transition-state theory method (dcTST) has been developed that can be applied to study diffusion in confined systems beyond the infinite dilution limit. 32 This method can provide a molecular explanation for diffusion behavior in terms of free energy.…”

Diffusion in Confinement: Agreement between Experiments Better than Expected

“…15 together with results obtained by several other groups through simulation 2-6 and experiment. 1,[7][8][9][10] Data were obtained at 300 K, except for the results of Jobic et al, which were obtained at 250 K.…”

“…Figure 2 shows our recent simulation results for the diffusion of methane in MFI-type silica as a function of the adsorbate loading, 15 together with results obtained by several other groups through simulation 2-6 and experiment. 1,[7][8][9][10] Note that the potential parameters used in the simulations have been fitted to adsorption data, not diffusion data. 19 The experimental data (black symbols) are self-diffusion coefficients, except those obtained by single-crystal membrane (SCM) measurements, which should be corrected diffusivities.…”

“…Although there are over 130 different known zeolite topologies, most groups focus on the understanding of the diffusion in silicalite (MFI) because this material is very well characterized and the MFI structure is used in many industrial applications. [1][2][3][4][5][6][7][8][9][10][11][12] Talu et al 1 made a comparison of the various techniques used to measure diffusion in confinement. Figure 1 is based on their results.…”