Hydrogenolysis of C–C and C–Cl bonds by Pd–Re/Al2O3 catalysts Original Research Article

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 (ζ) appears a useful diagnostic parameter for probing Pd–Re interactions. Extensively multiple cracking of 2,2-dimethylpropane on Re-rich Pd–Re/Al2O3 samples (ζ ∼ 5 for the range 25–100 at% Re) is steadily tempered over the range of 0–25 at% Re, reaching the level of ∼ 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.