de Jw Jan Haan

The effect of sulfidation on the Ni distribution in Ni/USY zeolites

Ni/USY zeolite catalysts have been prepared by either an ion-exchange or an impregnation procedure. The modification of support and the effect of sulfidation on the Ni distribution were examined by various techniques (water adsorption, FT i.r. spectroscopy of chemisorbed NO, XPS, 29 Xe n.m.r. and Xe adsorption). In all catalysts the sulfidation of Ni is incomplete and the method of Ni introduction influenced the Ni distribution in Ni/USY zeolites. In unsulfided samples ion-exchange technique led to Ni located essentially in hexagonal prisms, whereas in the other samples prepared by impregnation procedure the Ni concentrates were located at or near the outer zeolite surface. Some nickel redistribution has been observed during catalyst sulfidation.

Adsorption of linear alkanes in the alpha-Cages and gamma-Cages of H-ZK-5 and K-ZK-5

The adsorption properties of linear alkanes in H-ZK-5 and K-ZK-5 were studied at room temperature by means of sorption and 13C NMR measurements and by molecular simulations. Alkane molecules up to n-heptane are able to adsorb in the α- and γ-cages of ZK-5, but adsorption of longer alkane molecules is limited to the large α-cages as a result of size exclusion from the small γ-cages. These γ-cages are the preferred adsorption sites for propane and n-butane because of the more favorable heats of adsorption. In contrast, n-hexane and n-heptane adsorb preferentially in the large α-cages because of the larger entropy in these cages. For n-pentane, the γ-cages are the preferred adsorption sites in H-ZK-5, whereas in K-ZK-5 the α-cages are the preferred adsorption sites. This difference is caused by the presence of the potassium cations in the γ-cages of K-ZK-5 which constrains the adsorption in these cages. The difference in siting of molecules with different chain lengths is the result of a steep decrease of the entropy of the adsorbed molecules in the γ-cages with increasing chain length. The 13C NMR measurements and molecular simulations show that the molecules in the γ-cages are in general more coiled than those in the α-cages. Although the conformational equilibria of the molecules in the α-cages of H-ZK-5 and K-ZK-5 are comparable, the molecules in the γ-cages of K-ZK-5 are considerably more coiled than those in the γ-cages of H-ZK-5. The simulation parameters describing the zeolite-alkane interaction were tested by a comparison of the simulation results with the experimental findings.

n-Pentane hopping in zeolite ZK-5 studied with 13C NMR

Using 1D‐ and 2D‐exchange 13C NMR techniques it is shown that n‐pentane translation in zeolite ZK‐5 occurs by hopping between neighboring α and γ cages. Owing to the structure of the ZK‐5 pore network, these techniques give direct information about the translational part of molecular intercage motion, which represents the elementary step of sorbate self‐diffusion. Between 247 and 317 K the hopping rates obtained for n‐pentane in H‐ and K‐ZK‐5 at respective loadings of 8.1 and 9.9 molecules per unit cell are of the order of 1–103 s−1. This corresponds to intra‐zeolite self‐diffusion constants of 10−18–10−15 m2 s−1. Non‐exponential time‐correlation functions indicate heterogeneity, possibly reflecting the different loading states of the individual cages at the microscopic level. The temperature dependence of hopping shows that it is an activated process with a barrier of 28 ± 5, kJ mol−1 for both H‐ZK‐5 and K‐ZK‐5. The apparent activation entropy determined from a two‐site exchange analysis is 130 ± 20 J K−1 mol−1 for both H‐ and K‐ZK‐5. Copyright

Silver ion assisted ring expansions of some geminal dibromobicyclo[n.1.0]alkanes. Evidence for free cationic intermediates

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13C Solid state NMR study of ethylene-vinyl alcohol copolymer

In the present study ethylene-vinyl alcohol copolymers (E-VOH) with an increasing content of ethylene were synthesized and investigated using high-resolution solid state 13c NMR. A splitting of the methine carbon resonance into three peaks could be observed. This splitting could be explained taking into account tacticity and sequence effects.