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J. Kärger,
J. Caro,
P. Cool,
M.-O. Coppens,
D. Jones,
F. Kapteijn,
F. Rodríguez-Reinoso, M. Stöcker,
D. Theodorou,
E. F. Vansant,
J. Weitkamp
Chemical Engineering & Technology 08/2009; 32(10):1494 - 1511. · 1.60 Impact Factor
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P Kortunov,
S Vasenkov,
J Kärger,
R Valiullin,
P Gottschalk,
M Fé Elía,
M Perez, M Stöcker,
B Drescher,
G McElhiney,
C Berger,
R Gläser,
J Weitkamp
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ABSTRACT: PFG NMR has been applied to study intracrystalline diffusion in USY zeolite as well as in the parent ammonium-ion exchanged zeolite Y used to produce the USY by zeolite steaming. The diffusion studies have been performed for a broad range of molecular displacements and with two different types of probe molecules (n-octane and 1,3,5-triisopropylbenzene) having critical molecular diameters smaller and larger than the openings of the zeolite micropores. Our experimental data unambiguously show that, in contrast to what is usually assumed in the literature, the intracrystalline mesopores do not significantly affect intracrystalline diffusion in USY. This result indicates that the intracrystalline mesopores of USY zeolite do not form a connected network, which would allow diffusion through crystals only via mesopores.
Journal of the American Chemical Society 10/2005; 127(37):13055-9. · 9.91 Impact Factor
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P. Kortunov,
S. Vasenkov,
J. Kärger,
M. Fé Elía,
M. Perez, M. Stöcker,
G. K. Papadopoulos,
D. Theodorou,
B. Drescher,
G. McElhiney,
B. Bernauer,
V. Krystl,
M. Kočiřík,
A. Zikánová,
H. Jirglová,
C. Berger,
R. Gläser,
J. Weitkamp,
E. W. Hansen
[show abstract]
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ABSTRACT: Diffusivities of n-octane in particles of industrial fluid catalytic cracking (FCC) catalysts and in zeolite USY, which is the main zeolitic component of the particles, are reported. Diffusion measurements have been performed by using pulsed field gradient (PFG) NMR for a broad range of molecular displacements and temperatures. The recorded diffusivities are used to evaluate the relevance of various transport modes in the particles of FCC catalysts, such as diffusion in the micropores of the zeolite crystals located in the particles, diffusion through the surface layer of these crystals, and diffusion in the meso- and macropores of the particles, for the rate of molecular exchange between catalyst particles and the surrounding atmosphere. This rate is shown to be primarily related to the diffusion in the meso- and macropores of the particles under the condition of fast molecular exchange between these pores and the zeolite crystals located in the particles. The diffusivity associated with this type of diffusion (i.e., the intraparticle diffusivity) is found to correlate well with the catalytic performance of FCC catalysts having the same fractions of the same zeolite USY but different systems of meso- and macropores.
03/2005;
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P Kortunov,
S Vasenkov,
J Kärger,
M Fé Elía,
M Perez, M Stöcker,
G K Papadopoulos,
D Theodorou,
B Drescher,
G McElhiney,
B Bernauer,
V Krystl,
M Kocirik,
A Zikanova,
H Jirglova,
C Berger,
R Gläser,
J Weitkamp,
E W Hansen
[show abstract]
[hide abstract]
ABSTRACT: Pulsed-field gradient nuclear magnetic resonance (PFG NMR) has been applied to study molecular diffusion in industrial fluid catalytic cracking (FCC) catalysts and in USY zeolite for a broad range of molecular displacements and temperatures. The results of this study have been used to elucidate the relevance of molecular transport on various displacements for the rate of molecular exchange between catalyst particles and their surroundings. It turned out that this rate, which may determine the overall rate and selectivity of FCC process, is primarily related to the diffusion mode associated with displacements larger than the size of zeolite crystals located in the particles but smaller than the size of the particles. This conclusion has been confirmed by comparative studies of the catalytic performance of different FCC catalysts.
Magnetic Resonance Imaging 03/2005; 23(2):233-7. · 1.99 Impact Factor
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ABSTRACT: The effect of pore surface hydrophilic/hydrophobic character on n-hexane diffusion within MCM-41 particles was investigated by PFG NMR at different pore fillings. Starting with totally filled pores, the n-hexane diffusivity within MCM-41 particles is significantly enhanced as the pore filling is decreased until it reaches a maximum value. The diffusivity is shown to decline again at a very low pore filling. This is rationalized in terms of a a three-component parallel diffusion mechanism, in which a surface diffusion, an interface diffusion and a liquid phase diffusion are involved. The influence of surface property variations on the molecular diffusivity in MCM-41 is enhanced in partially filled samples.
Microporous and Mesoporous Materials.
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ABSTRACT: Mesoporous MCM-41 materials are ideal model systems for fundamental adsorption studies owing to their regular and well-defined pore geometry. The effect of pore dimension and pore surface hydrophobicity on the diffusion of n-hexane confined in MCM-41 was investigated by modifying the pore surface chemistry and the pore dimension of a mesoporous MCM-41 reference material with a pore diameter of 37.5 Å and a crystal dimension of approximately 6 μm. The modification of the MCM-41 reference material was carried out in a controlled manner by silanation as discussed in the text.PFG-NMR measurements at room temperature revealed a 40% reduction in the diffusion coefficient when changing only the hydrophobicity (by reaction of the pore surface with SiCH3Cl3) of the pore surface. Moreover, an increase in the mobility of n-hexane of more than 80% was observed when decreasing the pore dimension from 37.5 Å to approximately 33.5 Å (by reaction of the pore surface with SiCl4). The surface-to-volume (S/V)-ratio of the materials (as derived by NMR diffusion measurements) was found to be three orders of magnitude less than the S/V-ratio determined by N2-adsorption and will be discussed in the text.
Microporous and Mesoporous Materials.
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ABSTRACT: The pore dimension of a reference MCM-41 material was reduced in a controlled manner by successive silylation with tetrachlorosilane. Also, the surface of these materials was modified by temperature treatment, resulting in a reduction of the concentration of surface silanol groups. Pulsed field gradient (PFG) NMR diffusion measurements of n-hexane confined in these modified MCM-41 samples have been performed and the results discussed within the Gaussian phase distribution and short gradient pulse approximations. The observed increase of the mean residence time with increasing diffusion time of the confined n-hexane suggests a distribution of channel lengths to exist.
Microporous and Mesoporous Materials.
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ABSTRACT: The hydrothermal treatment of zeolite NH4Y was investigated as a function of steaming time in the range between 0.5 h and 20 h at 773 K, 873 K and 973 K, respectively. The evolution of the lattice parameter (ao) and the frequency of the double ring vibration mode in the infrared absorption spectra (wDR) show saturation effects for the Si/Al ratios of about 4 and 5 at 773 K and 873 K, respectively. The average crystal size remains in this case temperature and time independent between about 90 and 110 nm. Short time steaming, i.e. up to 5 h at 973 K, also indicates a saturation of the Si/Al ratio at about 7, which is accompanied by a strong decrease in the average crystal size. The Si/Al ratios calculated using the results of 29Si MAS NMR show similar dependecies on the steaming temperature and time as the data obtained by the IR spectroscopy. However, the absolute values of the Si/Al ratios obtained by the former method are significantly larger than those obtained by the latter. This difference is attributed to the sensitivity of the MAS NMR data to the presence of non-framework species, which leads to the overestimation of the Si/Al ratios. The steaming-time dependences of the micropore volume and micropore surface area are closely related to the steaming-time dependences of the Si/Al ratios. At 773 K and 873 K there is an increasing formation of mesopores with increasing steaming time. However, the increase in the steaming temperature up to 973 K does not lead to a faster development of the mesopore surface area, which implies that a suitable fine tuning of mesopores in steam is only possible at the lower temperatures.
Studies in surface science and catalysis 154:1411-1417.
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