Modeling molecular diffusion in channel networks via displacements between the channel segments

Schüring, Andreas; Fritzsche, Siegfried; Haberlandt, R.; Vasenkov, Sergey; Kärger, Jörg

doi:10.1039/b316174a

Cited by 16 publications

(6 citation statements)

References 24 publications

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“…In the confined space of the MFI structure, it was demonstrated in the literature that propene, having a kinetic diameter of about 4 Å diffuses easily through the internal porosity. , In addition, it was mentioned that at a low surface coverage, n -alkanes ≤ n C 6 or aromatic molecules such as toluene are first adsorbed on intersections between straight and zigzag channels, then into zigzag channels by increasing the hydrocarbon loading. Hence, under our conditions, with propene molecules diffusing faster than toluene, they will be first adsorbed on the acid sites located into zigzag channels, limiting the access to toluene molecules.…”

Section: Resultsmentioning

confidence: 99%

Impact of the Zeolite Structure and Acidity on the Adsorption of Unburnt Hydrocarbons Relevant to Cold Start Conditions

Westermann

Azambre

2016

J. Phys. Chem. C

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Adsorption behavior of commercial zeolites were investigated for the trapping of a hydrocarbon mixture (propene, toluene and decane) mimicking Diesel cold start exhausts under dry conditions. The studied commercial zeolites, in most cases under protonated form, (with Si/Al ratio in the range 2−15) present different structures: FAU, *BEA, MOR, MFI, FER, and LTA. Textural and structural characterizations were carried out using N 2 adsorption at −196 °C and X-ray diffraction. In situ FTIR spectroscopy of adsorbed pyridine was used to probe quantitatively the acid sites. The methodology used in this study consisted of an adsorption phase at 35 °C followed by temperature-programmed desorption (TPD) under He at 10 °C/min, and three different adsorption mechanisms were highlighted according to the pore size and the host framework. MOR and MFI structures appear to be the less selective for any type of hydrocarbon and are thus the most suitable for cold start applications. Except for these types of zeolites presenting a "single-file diffusion" mechanism, propene often desorbs at low temperatures or is adsorbed very little. By contrast, heavier hydrocarbons such as toluene and decane desorb at temperatures that match with threeway catalyst (TWC) or Diesel oxidation catalyst (DOC) light-off temperatures.

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Section: Resultsmentioning

confidence: 99%

Impact of the Zeolite Structure and Acidity on the Adsorption of Unburnt Hydrocarbons Relevant to Cold Start Conditions

Westermann

Azambre

2016

J. Phys. Chem. C

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“…, by three principal values and the orientation of the tensor main axes, rather than by a single diffusivity. Since the positions of the atoms forming the host lattice of such materials are known from X-ray diffraction [9,10,11] and since there exist well established approaches for modeling the force field exerted on the guest molecules by both the host lattice and other guest molecules [12,13,14,15], the exploration of diffusion anisotropy has become a popular area for molecular dynamics simulations, especially for zeolites of structure types MFI [16,17,18,19,20,21,22,23] and CHA [24,25]. …”

Section: Introductionmentioning

confidence: 99%

“…Also here, corroboration from molecular dynamics simulations turned out to be most valuable for determining the conditions under which the resulting correlation rules are applicable. For example, in such simulations, the correlation rules were found to hold for small hydrocarbons in MFI [16,20], including even for multicomponent diffusion [21], while for water in chabazite [24,25] or long-chain paraffins in MFI [23], due to specific host-guest interactions, deviations from these simple rules are to be expected.…”

Section: Introductionmentioning

confidence: 99%

Micro-Imaging by Interference Microscopy: A Case Study of Orientation-Dependent Guest Diffusion in MFI-Type Zeolite Host Crystals

et al. 2012

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Because of the small particle size, orientation-dependent diffusion measurements in microporous materials remains a challenging task. We highlight here the potential of micro-imaging by interference microscopy in a case study with MFI-type crystals in which, although with different accuracies, transient concentration profiles in all three directions can be observed. The measurements, which were performed with “rounded-boat” shaped crystals, reproduce the evolution patterns of the guest profiles recorded in previous studies with the more common “coffin-shaped” MFI crystals. The uptake and release patterns through the four principal faces (which in the coffin-shaped crystals extend in the longitudinal direction) are essentially coincident and there is no perceptible mass transfer in the direction of the long axis. The surface resistances of the four crystal faces through which mass transfer occurs are relatively small and have only a minor effect on the mass transfer rate. As a result of the pore structure, diffusion in the crystallographic c direction (which corresponds to the direction of the long axis) is expected to be much slower than in the transverse directions. This could explain the very low rate of mass transfer observed in the direction of the long axis, but it is also possible that the small end faces of the crystal may have high surface resistance. It is not possible to distinguish unequivocally between these two possibilities. All guest molecules studied (methyl-butane, benzene and 4-methyl-2-pentyne) show the same orientation dependence of mass transfer. The long 4-methyl-2-pentyne molecules would be expected to propagate at very different rates through the straight and sinusoidal channels. The coinciding patterns for uptake through the mutually perpendicular crystal faces therefore provide clear evidence that both the coffin shaped crystals and the rounded-boat-shaped crystals considered in this study, must be intergrowths rather than pure single crystals.

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“…74, 75 While for a number of host-guest systems, this phenomenon of structure-directed diffusion anisotropy could be confirmed in both PFG-NMR diffusion studies 76 and MD simulations, 77 there are also well-documented cases where deviations from these predictions provided valuable additional information on microdynamics and microstructure. 78 Diffusion anisotropy degenerates to one-dimensional diffusion if, as with the zeolite structure types AFI, MTT, MTW and TON, the zeolite is traversed by nonintersecting, parallel channel pores. 79 As soon as the guest molecules are too bulky to pass each other within the channels, molecular propagation does not obey equation (9) of normal diffusion anymore.…”

Section: Diffusion In Microporous Crystallites (Zeolites)mentioning

confidence: 99%

Diffusion in Porous Media

Kärger

Valiullin

2011

Encyclopedia of Magnetic Resonance

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Modeling molecular diffusion in channel networks via displacements between the channel segments

Cited by 16 publications

References 24 publications

Impact of the Zeolite Structure and Acidity on the Adsorption of Unburnt Hydrocarbons Relevant to Cold Start Conditions

Impact of the Zeolite Structure and Acidity on the Adsorption of Unburnt Hydrocarbons Relevant to Cold Start Conditions

Micro-Imaging by Interference Microscopy: A Case Study of Orientation-Dependent Guest Diffusion in MFI-Type Zeolite Host Crystals

Diffusion in Porous Media

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