Mg- and Ni-containing layered double hydroxides as soda substitutes in the aldol condensation of acetone

A series of layered double hydroxides (LDHs) containing Mg2+ or Ni2+ as divalent and Al3+ or Ga3+ as trivalent cations were used as catalysts in order to substitute NaOH in the selective condensation of acetone into diacetonealcohol. With this aim the LDHs were totally exchanged with OH- anions and acquired the meixnerite-like structure, showing a high concentration of active basic sites. These meixnerite-like structures were prepared by reconstruction under appropriate conditions of the mixed oxides obtained by calcination. The complete reconstruction was easily achieved in water at ambient temperature and pressure for Mg-containing samples, while more severe conditions were needed for the Ni-containing one, as shown by XRD, TEM and UV-VIS DR spectroscopy. The reconstruction degree of Ni/Al mixed oxides was markedly improved by hydrothermal treatments under ammonia. The nature of the cations influenced the acid-base properties of the mixed oxides, as evidenced by NH3 and CO2 TPD experiments, and consequently their catalytic activities. The catalytic activities of the meixnerite-like forms were related both to the basicity of the corresponding mixed oxide and to the reconstruction degree in the lamellar form, thus to the density of the OH- sites. Indeed, the loss of the lamellar structure was the main cause of the catalyst deactivation. (C)2000 Elsevier Science B.V. All rights reserved.