Preparation of macrospherical magnesia-rich magnesium aluminate spinel catalysts for methanolysis of soybean oil

In order to fully exploit the green characteristics of solid base catalysts they should be fabricated into macrostructured rather than powder form. Magnesia-rich magnesium aluminate spinel (MgO·MgAl₂O₄) framework catalysts with tunable basicity have been prepared by using γ‐Al₂O₃ macrospheres (0.5-1.0 mm in diameter) as a hard template. The process involves in situ growth of magnesium-aluminum layered double hydroxides (MgAl-LDHs) in the channels of the γ‐Al₂O₃ macrospheres by the urea hydrolysis method, followed by calcination, tuning of the basicity through etching of excess aluminum with aqueous alkali and a final calcination step. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), elemental analysis and low temperature N₂ adsorption-desorption studies demonstrate that the composite MgO·MgAl₂O₄ materials are composed of nanosized rod-like particles aggregated into a spherical framework. Catalytic reactivity was investigated by using methanolysis of soybean oil as probe reaction. The MgO·MgAl₂O₄ composite shows a higher biodiesel yield compared to an MgO/MgAl₂O₄/γ‐Al₂O₃ material with the same loading of magnesium prepared by a conventional impregnation method. The enhanced catalytic activity of the former material can be ascribed to its higher basicity, specific surface area, pore volume and pore size.