Self-diffusion of various cations in natural mordenite
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Abstract:
The self-diffusion of various cations in hydrated natural mordenite has been studied. From the variation of the self-diffusion coefficient D* with temperature, the energy of activation E and the pre-exponential factor D0 for the self-diffusion process have been calculated from the Arrhenius equation D*=D0 exp (E/RT). The values of D*, D0 and E have been found to exhibit peculiarities which were not so evident in studies with analcite1 and chabazite,2 two zeolites which have simpler channel structures. Only limited exchange of the divalent ions Ca2+, Sr2+ and Ba2+ with the sodium form of the exchanger could be achieved. These divalent-ion self-diffusion studies exhibited unusual features and the interpretation of the results was complicated by the unknown effect of the sodium ions present in the lattice. These studies have shown, however, that much insight of the motion of ions in these zeolite channels can be obtained from kinetic measurements.Keywords:
Mordenite
Chabazite
Divalent
Self-diffusion
Chabazite
Mordenite
Clinoptilolite
Sieve (category theory)
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Chabazite
Mordenite
Clinoptilolite
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Zeolites are crystalline microporous alumosilicates whose unique pore and channel systems are the reason for their important role in catalysis, separation, and ion exchange. This work focuses on the morphology and structure of a natural zeolite, mordenite. Our samples were collected at Lengyendi-galya (Gyökeres-tető) in the Mátra Mountains (NE-Hungary). Zeolite samples were investigated by X-ray powder diffraction, scanning, and transmission electron microscopy. Mordenite has typically fibrous appearance and occurs in association with other zeolites such as heulandite and chabazite. Based on intense streaks and superlattice reflections in selected area electron diffraction patterns, we identified planar faults in the structure. A single fault produces a dachiardite-type structural slab within the mordenite lattice, reducing its channel size.
Mordenite
Chabazite
Selected area diffraction
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Using classical molecular dynamics (MD) simulations, we have studied some structural and diffusive properties of water molecules adsorbed in chabazite. In particular, we have investigated the variation of the self-diffusion coefficient of the water molecules as a function of their concentration and the nature of the hydration shells of the extraframework Ca++ ions with varying concentrations of water. Our study indicates that the well-defined and stable hydration shells of this ion play an important role in the diffusion process. The diffusion anisotropy is computed at T = 600 K. It is compared with theoretical results based on jump models and qualitatively compared with pulsed field gradient nuclear magnetic resonance (PFG NMR) experiments of a single chabazite crystal at 293 K and with tracer diffusion studies.
Chabazite
Pulsed field gradient
Self-diffusion
Molecular diffusion
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Chabazite
Mordenite
Clinoptilolite
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Chabazite
Mordenite
Reactivity
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Chabazite
Clinoptilolite
Mordenite
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Chabazite
Mordenite
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Chabazite
Mordenite
Ferrierite
Silanol
ZSM-5
Magic angle spinning
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This paper reviews advances in zeolite ion exchange which have taken place since the Second International Conference and focuses attention on some remaining problem areas. Ion exchange in zeolites A, X, Y, mordenite and chabazite is covered. The review is in three sections: thermodynamic aspects, equilibria and kinetics.
Chabazite
Mordenite
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