Activated layered double hydroxides (LDHs) with high crystallinity, obtained bycalcination/rehydration of LDH precursors synthesized by urea decomposition, havehigher catalytic activity in acetone self-condensation and Knoevenagel reactions thanless crystalline materials obtained from LDH precursors synthesized by titration coprecipitation.The activated LDHs possess both basic and acidic sites. High resolutiontransmission electron microscopy (HRTEM) confirms that the highly crystalline activatedLDHs retain the lattice structure of the LDH precursors with lattice parametersa b=0.31 6±0.01nm and a 60.28 An acid-base catalytic mechanism hasbeen proposed to interpret the catalytic behavior based on the fact that acid-basehydroxyl group pairs on the activated LDH surface have a separation of 0.31nm. It isproposed that the active sites are mainly located on the ordered array of hydroxyl siteson the basal surfaces rather than on the edges, as has been previously suggested.

In order to fully exploit the green characteristics of solid base catalysts they should be fabricated intomacrostructured rather than powder form. Magnesia-rich magnesium aluminate spinel (MgO • MgAl2O4)framework catalysts with tunable basicity have been prepared by using _-Al2O3 macrospheres (0.5–1.0mmin diameter) as a hard template. The process involves in situ growth of magnesium–aluminum layereddouble hydroxides (MgAl-LDHs) in the channels of the _-Al2O3 macrospheres by the urea hydrolysismethod, followed by calcination, tuning of the basicity through etching of excess aluminum with aqueousalkali and a final calcination step. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmissionelectron microscopy (TEM), high-resolution TEM (HRTEM), elemental analysis and low temperatureN2 adsorption–desorption studies demonstrate that the composite MgO•MgAl2O4 materials are composedof nanosized rod-like particles aggregated into a spherical framework. Catalytic reactivity was investigatedby using methanolysis of soybean oil as probe reaction. The MgO • MgAl2O4 composite shows a higherbiodiesel yield compared to an MgO/MgAl2O4/_-Al2O3 material with the same loading of magnesiumprepared by a conventional impregnation method. The enhanced catalytic activity of the former materialcan be ascribed to its higher basicity, specific surface area, pore volume and pore size.