Design of nanostructured multifunctional Pd-based catalysts from layered double hydroxides precursors
Three types of multifunctional supported Pd catalysts (0.2 wt.% Pd) have been obtained from layered double hydroxides (LDH) precursors by: (i) impregnation with Pd(acac)2 of a Mg(Al)O support, (ii) coprecipitation of a multicationic Pd/Mg/Al LDH, and (iii) synthesis of a Pd colloid Mg/Al LDH nanocomposite. The precursors have then been calcined at 773 K and reduced at 523 K to obtain the catalysts, which exhibit very different basic properties and Pd particle size distributions. Their potential was evaluated in the one-pot synthesis of 2-methyl-3-phenyl-propanal (MPPAL) from benzaldehyde and propanal. The full one-pot synthesis of MPPAL was performed sequentially operating in two stages, i.e. for 20 h in N2, then for 7 h in H2 atmospheres. The activities of the catalysts in the first stage of the reaction (condensation and dehydration steps) are related to their reconstruction ability in the propanol/water solvent to give Brønsted basic sites. The hydrogenation ability of the catalysts decreases when the average Pd0 particle size decreases. The specific behavior of the nanosized particles accounts for a decrease of the adsorption strength of CC bond due to the enhancement of the metal–support interaction. The catalyst prepared from a Pd nanocomposite precursor ensures the best balance between the different functions. It leads to a MPPAL selectivity of 77% at 64% benzaldehyde conversion. Benzyl alcohol formed by hydrogenation of benzaldehyde on Pd and hydrogen transfer from propanol is the main secondary product. However, the process is negatively affected by the high consumption of propanal from self-condensation.
Graphical abstractThree types of multifunctional supported Pd catalysts have been obtained from LDH precursors by: impregnation, coprecipitation and synthesis of a Pd colloid Mg/Al LDH nanocomposite, respectively. That prepared from the nanocomposite precursor ensures the best balance equilibrium between the different functions and leads to the higher performances in the one-pot synthesis of 2-methyl-3-phenyl-propanal (MPPAL) from benzaldehyde and propanal. Figure optionsDownload full-size imageDownload as PowerPoint slide
Journal: Applied Catalysis A: General - Volume 318, 20 February 2007, Pages 170–177