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

A series of layered double hydroxides (LDHs) containing Mg2+ or Ni2+ as div alent and Al3+ or Ga3+ as trivalent cations were used as catalysts in order to substitute NaOH in the selective condensation of acetone into diacetone alcohol. With this aim the LDHs were totally exchanged with OH- anions and acquired the meixnerite-like structure, showing a high concentration of act ive basic sites. These meixnerite-like structures were prepared by reconstr uction under appropriate conditions of the mixed oxides obtained by calcina tion. The complete reconstruction was easily achieved in water at ambient t emperature and pressure for Mg-containing samples, while more severe condit ions 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 marke dly improved by hydrothermal treatments under ammonia. The nature of the ca tions influenced the acid-base properties of the mixed oxides, as evidenced by NH3 and CO2 TPD experiments, and consequently their catalytic activitie s. 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 deact ivation. (C)2000 Elsevier Science B.V. All rights reserved.