F. Prinetto et al., Mg- and Ni-containing layered double hydroxides as soda substitutes in thealdol condensation of acetone, CATAL TODAY, 55(1-2), 2000, pp. 103-116
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.