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n-Pentane and 3-Methylpentane Conversions on Differently Pretreated Supported Palladium Catalysts

Bonarowska M., Karpiński Z.

Polish Journal of Chemistry

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2006

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Vol. 80, nr 11

1877-1887

EN

Metal-support interactions in silica- and alumina-supported palladium catalysts have been investigated by using n-pentane and 3-methylpentane conversions as test reactions. High temperature reduction (HTR) at 600 graduate C results in a considerable increase of the selectivity towards isomeric and cyclic products, i.e. causes a similar effect as in the case of previously studied reactions of 2,2-dimethylpropane and various C6-alkanes. However, the silica-supported palladium catalysts behave differently from the Pd/Al2O3. For the former catalysts subjected to HTR, the selectivity for cyclic products is enhanced, whereas a similarly pretreated Pd/Al2O3 gives mainly isomers. This observation, and the changes in both the hydrogen reaction order and in the activation energy caused by different catalyst pretreatment, were both analyzed in detail. All mentioned quantities appeared to be new convenient diagnostic parameters towards determining whether or not palladium interacts with a support.

2

n-Butane Conversion on Differently Pretreated Supported Palladium Catalysts

Bonarowska M., Karpiński Z.

Polish Journal of Chemistry

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2005

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Vol. 79 / nr 11

1821-1828

EN

Metal-support interactions in silica- and alumina-supported palladium cata lysts have been re-investigated by using n-bu tane converion as a test reaction. High temperature reduction at 600°C results in a consider able in crease in the isomerization selectivity, i.e. a similar effect as in the case of previously studied rea tions of 2,2-dimethylpropane and C6-alkanes. There fore, changes in the selectivity for isomerization and also in the activation energy can be re garded as use ful diagnostic parameters to wards determining whether or not palladium interacts with a support.

3

Temperature Programmed Desorption of Pyridine and 2,6-Dimethylpyridine from Differently Pretreated Pd/Al2O3 Catalysts

Skotak M., Karpiński Z.

Polish Journal of Chemistry

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2004

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Vol. 78 / nr 10

1953-1959

EN

The temperature programmed desorption (TPD) of pyridine and 2,6-dimethylpyridine from alumina and two alumina-supported palladium catalysts of different metal loadings (0.3 and 2.77 wt.% Pd) confirmed the presence of strong acid sites in the samples subjected to high temperature reduction at 600°C. Roughly similar amounts of both organic bases were desorbed from the catalysts which underwent similar pretreatments. However, 2,6-dimethylpyridine appears less strongly bonded than pyridine to Lewis acid sites in alumina, apparently because of some steric hindrance produced by the presence of methyl substituents in 2- and 6-position to the nitrogen lone pair. Thus, pyridine is better suited for probing evolution of Lewis acidity in alumina, brought about by high temperature reduction at 600°C.With increasing temperature during thermodesorption, both organic bases adsorbed on palladium-containing samples undergo transformation, leading to desorption of several products, among which hydrogen and nitrogen predominate. Introduction of increasing amounts of palladium to alumina makes the acidity probing difficult, because a considerable part of adsorbed organic base is decomposed on metal sites. Decomposition of pyridine and 2,6-dimethylpyridine may serve as a convenient probe of availability of palladium surface. After high temperature reduction of Pd/Al2O3 a considerable part of Pd surface is blocked by support species.

4

Temperature Programmed Desorption of Triethylamine from Differently Pretreated Pd/Al2O3 Catalysts

Skotak M., Kijeński J., Karpiński Z.

Polish Journal of Chemistry

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2003

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Vol. 77 / nr 5

757-765

EN

The temperature programmed desorption (TPD) of triethylamine from alumina and two alumina-supported palladium catalysts of different metal loadings (0.3 and 2.77 wt.% Pd) confirmed the presence of strong Lewis acid sites in the samples subjected to high temperature reduction at 600_C. With increasing temperature during thermodesorption, triethylamine adsorbed on Lewis acid sites of alumina undergoes transformation, leading to desorption of several products, among which hydrogen, ethylene and acetonitrile predominate. However, introduction of increasing amounts of palladium to alumina makes the acidity probing difficult, because a considerable part of adsorbed triethylamine is decomposed on metal sites. Temperature programmed oxidation (TPO) shows that the organic coke left after TPD of triethylamine is associated with acid sites of alumina, not with palladium sites. Another observation that pure alumina and 0.3 wt.% Pd/Al2O3 retained larger amounts of coke than the 2.77 wt.% Pd/Al2O3 catalyst reveals a beneficial role of palladium in desorbing organic material in the course of TPD runs.

5

The effect of carburization of palladium catalysts on the hydrogenation of acetylene-ethylene mixtures

Borodziński A.

Polish Journal of Chemistry

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1998

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Vol. 72, nr 11

2455-2462

EN

During the hydrogenation of acetylene-ethylene mixtures on palladium catalysts, the rate of acetylene disappearance and the concentration of carbon dissolved in the bulk palladium increase similarly for samples metal crystallite size<25nm. Such a positive correlation supports the hypothesis that carburization of palladium is responsible for the increase of the rate of acetylene disappearance during againg of the catalyst. The maximum concentration of carbon in palladium at the steady-state depends on the temperature of the reaction and on the crystallite size. This concentration increases with carburization temperature and decreases with the increase of Pd crystallite size. The presence of supersaturated solution of carbon in Pd at 50 degree C is possible, due to a diffusion-hindered frozen state in the bulk. The catalytic properties of such a sample match the behavior of partially carburized Pd catalyst at steady-state conditions. This result strongly suggests that, in both cases, concentrations of surface carbon are similar.