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 C{double bond, long}C 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.