Preparation and characterization of sol-gel MgAl hydrotalcites with nanocapsular morphology

A series of MgAl hydrotalcites (HTs) were synthesized by a sol-gel procedure with ethanol, 2-propanol, and 1-butanol as solvents. Particular attention was paid to controlling hydrolysis reactions by means of using a substoichiometric H2O/Al molar ratio equal to 1. The sol-gel HTs were characterized by means of thermogravimetry, mass spectrometry (MS) of the decomposition products, X-ray diffraction, diffuse reflectance infrared spectroscopy (DRIFT), electron microscopy, and N-2 adsorption-desorption. Decomposition products of sol-gel HTs were identified by MS and included, among others, CxHy fragments from alkoxy groups, which were partially withdrawn between 250 and 450 degrees C. Alkoxy groups, from the alcohols employed as solvents, are probably intercalated within the HT layers, as indicated by XRD and DRIFT analyses. CH, groups in these alkoxy groups were detected by DRIFT at similar to 2945 and similar to 2831 cm(-1) in dried HTs, even after annealing at 500 degrees C. The HTs obtained by the sol-gel procedure described in this work showed nanocapsular morphology. Transmission electron microscopy and Rietveld refinement techniques evidenced the curvature of the HT layers. In comparison with coprecipitated MgAl-CO3 HT particles (> 10 mu m), sol-gel HTs showed very small particle aggregates (ca. 300 similar to 350 nm). Calcined HTs presented specific surface area and pore volume values between 254 similar to 332 m(2)/g and 0.81 similar to 1.39 cm(3)/g, respectively, depending on the alcohol used in the sol-gel procedure.