Grafting and anchoring of molecular complexes as metal precursors for alumina-supported Pd catalysts

The surface coordination chemistry of Pd complexes on alumina has been studied in the framework of synthesizing Pd/γ-Al2O3 catalytic materials. Two methodologies were explored: the direct grafting of Pd complexes on hydroxyl functions present at the alumina surface and the anchoring of the precursors via amine-bearing silanes previously grafted on the support. Suitable conditions to graft and anchor Pd complexes on alumina surface were found and experimental proofs of grafting and anchoring processes are provided. The results show that covalent grafting indeed took place for samples prepared in acetonitrile with [Pd(CF3CO2)2(bipy)] and [PdCl2(PhCN)2] complexes or with [Pd(OAc)2] and [Pd(CF3CO2)2] in acetone. The anchoring was successful for catalysts prepared in acetone with 1 wt.% of [Pd(CF3CO2)2] loading. Grafting and anchoring were found to stabilize palladium in its Pd(II) oxidation state. This has an adverse effect on the activation step that should lead to reduction of the complex to give the metallic catalytic supported active phase.