Characterization and catalytic activity of differently pretreated Pd/Al2O3 catalysts:: the role of acid sites and of palladium-alumina interactions

The effect of catalyst pretreatment on the performance of chlorine-free gamma-alumina-supported palladium catalysts with metal loadings in the range of 0.3 to 2.77 wt% has been investigated in the hydroconversion of n-hexane (nH) and 2,2-dimethylbutane (22DMB) at 290degreesC and atmospheric pressure. The catalyst properties were modified by varying the catalyst pretreatment: low-temperature reduction (LTR) at 300degreesC; high-temperature reduction (HTR) at 600degreesC; and oxidation of highly reduced catalysts followed by LTR (regeneration). The isomerization activity for both nH and 22DMB conversion is enhanced by HTR of the Pd/Al2O3 as compared to LTR and regeneration. It is concluded that HTR increases the Lewis acidity of the alumina and the participation of Lewis acid sites improves catalytic performance. The influence of Lewis acid catalyst sites on the macroscopic kinetics overshadows the modest catalytic contribution from palladium. Ammonia effectively blocks the acid centers and limits the overall catalytic performance to metal-only catalysis. The role of support acid sites in the conversion of C-6-hydrocarbons was further demonstrated by isomerization activity associated with 22DMB hydroconversion catalyzed by an alumina-supported Ni catalyst. Based on the results of FTIR studies of adsorbed CO and results of temperature-programmed palladium hydride phase decomposition (TPHD), it was concluded that a part of the alumina-supported Pd transforms into a Pd-Al alloy during the HTR pretreatment. The catalytic performance of the regenerated catalysts in the C-6-alkane conversion is consistent with the suggestion that the Pd-Al alloy formed during HTR decomposes during the regeneration treatment to form Al2O3 moieties on the Pd surface. These moieties, most likely. interact strongly with nearby palladium species, creating sites which are active for alkane isomerization and, simultaneously, resistant to coking. (C) 2004 Elsevier Inc. All rights reserved.