Increasing the basicity and catalytic activity of hydrotalcites by different synthesis procedures

Hydrotalcites have been synthesized by three different procedures: conventional precipitation-aging, aging under microwave irradiation, and sonication during the coprecipitation step. The synthesis procedure has an effect on the crystal size and textural properties of the hydrotalcite (HT) and the Al2O3–MgO mixed oxides formed upon calcination. HT samples prepared under sonication at 298 K are formed by dispersed and homogenous particles of 80-nm average particle size. They also produce upon calcinations the mixed oxides with the largest surface area (∼300 m2 g−1). This method of preparation increases not only the surface area but also the number of defects in the solid, leading to sites of higher basicity. This was determined by means of catalytic reactions such as Knoevenagel and aldol condensations which demand basic sites of different strengths. Hydrotalcites were regenerated from mixed oxides by hydration while giving Brönsted basic sites. Samples originally prepared by sonication present smaller crystallite size and have a larger number of accessible active sites. With these samples acetone/citral condensations with 96 and 99% conversion and selectivity, respectively, are achieved in a 15-min reaction time.