Effective Diffusivities in Zeolites .3. Effects of Polarity, Cation Density, and Site Occupancy in ZSM-5

Garcia, S.F.; Weisz, Paul B.

doi:10.1006/jcat.1993.1317

Cited by 5 publications

(3 citation statements)

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“…One pathway (zeolite pores) is selective for p -xylene and exhibits a significant lag time. Catalytic and chromatographic diffusion studies of ZSM-5 have shown that the diffusivity of p -xylene through MFI is up to 2 orders of magnitude larger than those of the other xylene isomers. , The surface coverage of o -xylene is also smaller than that of p -xylene, so the o -xylene flux through zeolite pores should be small.…”

Section: Discussionmentioning

confidence: 99%

“…Catalytic and chromatographic diffusion studies of ZSM-5 have shown that the diffusivity of p-xylene through MFI is up to 2 orders of magnitude larger than those of the other xylene isomers. 53,54 The surface coverage of o-xylene is also smaller than that of pxylene, 38 so the o-xylene flux through zeolite pores should be small. The second pathway (nonzeolite pores) selectively permeates o-xylene and responds quickly to the introduction of the feed gases.…”

Section: Discussionmentioning

confidence: 99%

“…As a single gas, the flux of o-xylene is below the detection limit of the gas chromatograph for the first 3.5 h of the experiment, whereas p-xylene breaks through the membrane after 2 h. This is expected because the o-xylene diffusivity is lower than the p-xylene diffusivity in MFI. 53,54 The presence of p-xylene in the framework distorts the pores and increases the permeation rate for o-xylene. As a result, the o-xylene breaks through earlier in the mixture than as a single gas and reaches a higher steady-state flux.…”

Section: Discussionmentioning

confidence: 99%

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Aromatic Permeation through Crystalline Molecular Sieve Membranes

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Ind. Eng. Chem. Res.

104

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The fluxes of aromatic molecules (p-xylene, o-xylene, and benzene) were measured as a function of temperature and feed partial pressure through several molecular sieve membranes (SAPO-5, SAPO-11, and mordenite) and three types of MFI membranes (silicalite-1, ZSM-5, and boron-substituted ZSM-5). Single-file diffusion appeared to control transport through the SAPO and mordenite membranes. Hence, those membranes showed ideal selectivities greater than 1 for benzene over the xylene isomers but no separation selectivities for the mixtures. Surface diffusion and activated gaseous transport were the controlling mechanisms for the MFI membranes. The highest p-xylene/o-xylene selectivities were obtained for a boron-substituted ZSM-5 membrane. At feed partial pressures of 2.1 kPa and at a temperature of 425 K, the best selectivities were 130 (ideal) and 60 (separation). Zeolite pores preferentially permeated p-xylene and took as long as 8 h to reach steady state. Nonzeolite pores preferentially permeated o-xylene after much shorter breakthrough times. Higher pressures of p-xylene distorted the membrane framework, resulting in increased o-xylene permeation and reduced selectivity. After reaching steady state, the flux of p-xylene through zeolite pores was stable for at least 10 h. The flux of o-xylene through nonzeolite pores was similarly stable at 373 K but continuously decreased for at least 12 h at 405 K.

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Section: Discussionmentioning

confidence: 99%