Mechanochemical versus co-precipitated synthesized lanthanum-doped layered materials for olefin oxidation

Abstract Layered catalytic materials with a double hydroxide structure and Mg 2+ /(Al 3+  + La 3+ )  = 3 (Al 3+ /La 3+ molar ratio of 1) were prepared via co-precipitation and mechanochemical routes. In the second one, stoichiometric amounts of nitrates in mixture with Na 2 CO 3 and NaOH were directly milled in a ceramic mortar under ambient conditions. XRD characterization confirmed the formation of hydrotalcite structure for the samples containing Mg and Al, and of mixtures of La 2 (CO 3 ) 2 (OH) 2 and poor crystallized Mg,Al-layered double hydroxides for the samples modified by La. Catalytic tests carried out in the oxidation of cyclohexene by hydrogen peroxide in acetonitrile shown that the presence of lanthanum enhances the activity of the MgAlLa catalyst compared to unmodified MgAl. For MgAl the reconstruction of the LDH structure is a fast process occuring even under the reaction conditions that is favorized by the replacement of CO 3 2− by OH − . This in situ anion replacement allows an increase of the catalytic activity. The addition of La to MgAl eliminates the advantages induced by the reconstruction but preserves the system epoxidation activity. On the other hand, the presence of La generates catalysts with improved stability and recyclability. This behavior has been assigned to the cooperative participation of the different crystallographic phases.