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

Skotak M., Karpiński Z.

Polish Journal of Chemistry

2004

Vol. 78 / nr 10

1953-1959

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.

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

Skotak M., Kijeński J., Karpiński Z.

Polish Journal of Chemistry

2003

Vol. 77 / nr 5

757-765

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.