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 N2 adsorption−desorption. Decomposition products of sol−gel HTs were identified by MS and included, among others, C x H y fragments from alkoxy groups, which were partially withdrawn between 250 and 450 °C. Alkoxy groups, from the alcohols employed as solvents, are probably intercalated within the HT layers, as indicated by XRD and DRIFT analyses. C x H y groups in these alkoxy groups were detected by DRIFT at ∼2945 and ∼2831 cm-1 in dried HTs, even after annealing at 500 °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 μm), sol−gel HTs showed very small particle aggregates (ca. 300∼350 nm). Calcined HTs presented specific surface area and pore volume values between 254∼332 m2/g and 0.81∼1.39 cm3/g, respectively, depending on the alcohol used in the sol−gel procedure.