Heck reaction catalyzed by colloids of delaminated Pd-containing layered double hydroxide

A novel ligand-free catalyst, Pd(II)-doped colloidal layered double hydroxide (LDH) has been prepared from delamination of glycinate-intercalated ternary-component MgPdAl-LDH (Mg_0.95Pd_0.05Al_1/3(OH)₂]Gly_1/3·mH₂O]) in formamide. The catalytic performance of these mono-dispersed ultrathin nanosheets of LDH for Heck reaction was evaluated. Owing to the largely enhanced accessibility for reactant molecules resulting from the nature of high inner surface area of LDHs, these palladium-bearing nanosheets showed excellent efficiency in Heck reactions in a wide range of substrate molecules. The presence of Mg, Al and OH⁻ ions in the LDH-lamellas favors control the size growth of the in situ formed Pd(0) species implanted in the LDH layer matrixes, helping to prevent the formed Pd(0) from extraction from the lamellas followed by Pd-black formation. Moreover, the Pd sites and basic sites on the brucite-like nanosheets are combined at a molecular level and interact with each other closely because of the co-precipitation method employed; so the basic sites on the LDH monolayers might function as basic ligands. This new catalyst, delaminated MgPdAl-LDH, was characterized by XRD, TEM, HRTEM and EDX analysis.