Preparation and Characterization of Magnesium Chromite and Magnesium Ferrite Aerogels

A supercritical drying process has been used to prepare MgO-Cr2O3 and MgO-Fe2O3 powders from a mixture of magnesium acetate and the metallic acetylacetonates hydrolyzed in a methanolic solvent. The resultant aerogels have small particle sizes in the range of 10 to 20 nm and high surface areas in the range of 50 to 250 m2/g. After removal from the autoclave, characterization by X-ray diffraction (XRD) showed that the MgO-Cr2O3 was amorphous and that the MgO-Fe2O3 was readily in the form of a spinel structure for magnesioferrite (MgFe2O4). Fourier transform infrared spectroscopy (FTIR) and thermogravimetric and differential thermal analyses (TG-DTG-DTA) showed a high level of retained surface methoxy and carboxylate species for the MgO-Cr2O3 aerogel (over 40% by weight). The MgFe2O4, on the otherhand, had less than 10% organic matter. The differences observed between MgO-Cr2O3 and MgFe2O4 have been attributed to the formation of Cr+6 in the former case. After heating in air at 970K. MgO-Cr2O3 crystallized to the spinel structure MgCr2O4. The IR transmittance of pressed disks of MgFe2O4 aerogel showed behavior typical for a n-type semiconductor. The MgCr2O4, however, was a p-type semiconductor due to mainly Cr+6 present on the surface. Lewis acid centers were prevalent on both materials. The acid strength was found to be higher on the MgFe2O4 compare to the MgCr2O4.