Temperature programmed desorption of pyridine and 2,6-dimethylpyridine from differently pretreated Pd/Al2O3 catalysts

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 600degreesC. 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 600degreesC. 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.