Hydrogenolysis of C-C and C-Cl bonds by Pd-Re/Al2O3 catalysts

A series of chlorine-free Pd-Re/Al2O3 catalysts were studied in catalytic conversions of 2,2-dimethylpropane and dichloromethane in excess hydrogen. Rhenium appeared very active in the former and inactive in the latter reaction, and adding this metal to Pd/Al2O3 produced profound changes in catalytic behaviour. All Pd-Re bimetallic samples strongly deactivated with time-on-stream. The steady-state activity of Pd-Re/Al2O3 catalysts in the reaction of 2,2-dimethylpropane shows a pronounced maximum at 50 at% Re, resembling the catalytic behaviour of Pt-Re/Al2O3 catalysts. Hydrogenolysis is a predominant reaction, the presence of only 10 at% Re in the Pd-Re bimetal eliminates the modest isomerization capability of palladium. The fragmentation factor (zeta) appears a useful diagnostic parameter for probing Pd-Re interactions. Extensively multiple cracking of 2,2-dimethylpropane on Re-rich Pd-Re/Al2O3 samples (zeta similar to 5 for the range 25-100 at% Re) is steadily tempered over the range of 0-25 at% Re, reaching the level of similar to 2 typical for pure Pd catalysts. The presence of Re suppresses hydrogen coverage and gives rise to a more significant alkane splitting and decrease of apparent activation energy. For CH2Cl2 hydrodechlorination, the steady-state activity and apparent activation energy of the bimetallic samples decreased with increase of Re content. Temperature-programmed hydrogenation of used Pd and Pd-Re catalysts uncovered that the reaction of CH2Cl2 results in the deposition of large amounts of carbon in the bulk (as a Pd-C solution) and surface layer of the catalysts, On the other hand, chlorine left by the reactant in the catalysts is located on the alumina. Since for both reactions the addition of very small amounts of Re introduced drastic changes in the catalytic behaviour, it is speculated that a majority of introduced rhenium occupies the bimetal surface. Such a distribution follows from catalyst preparation, Similarities and differences found for hydrogenolyses of C-C and C-Cl bonds are discussed. (C) 1999 Elsevier Science B.V. All rights reserved.