EVOLUTION AND CATALYTIC ACTIVITY OF PD GAMMA-AL2O3 CATALYSTS PREPAREDFROM CHLORINE-FREE COMPOUNDS/

Characterization of chlorine-free alumina-supported Pd catalysts at di fferent stages of their lifecycle showed variations in the state of th e metal precursor and its dispersion. ion exchange of Pd(NH3)(4)(NO3)( 2) with gamma-Al2O3, followed by precursor decomposition al 500 degree s C in He, yields extremely small reduced metal particles. However, a subsequent reduction in Hp and He purge at 500 degrees C results in a considerable sintering of palladium. In general, conditions for the re moval of NH3 ligands do not have a large effect on the final palladium dispersion. Another Pd source, palladium acetylacetonate, binds more strongly to Al2O3 than to SiO2; the organic part of the Pd(acac)(2)/Al 2O3 precursor is subjected to considerable splitting before being stri pped off from the support during calcination. In addition, an oxidativ e pretreatment at greater than or equal to 350 degrees C is essential for removing all the carbon from the catalyst surface. In a 2,2-dimeth ylpropane reaction, chlorine-free, mildly reduced Pd/Al2O3 catalysts s how catalytic properties that are not dependent on metal loading; more over, they behave similarly to Pd/SiO2, which was studied previously(1 ). This finding suggests that 2,2-dimethylpropane conversion occurs on the metal, without any special participation of the support. However, upon activation by high-temperature reduction at 600 degrees C gamma- alumina, in association with Pd, starts to play an important catalytic role. It appears that on highly reduced Pd/Al2O3, 2,2-dimethylpropane isomerizes on Pd-Al centers or on the alumina that is in close proxim ity to Pd, whereas more distant alumina species are very active in hyd rocracking. The role of hydrogen spilled over from metal to gamma-Al2O 3 seems to be important, and the gradient in hydrogen concentration al ong the support may be crucial in regulating the overall catalytic beh avior.