Investigation of acid-base properties of catalysts obtained from layered double hydroxides

Well-crystallized layered double hydroxides (LDH) containing Mg2+ or Ni2+ as divalent cations and Al3+ as trivalent cations were obtained either by coprecipitation (cp) or with the sol-gel (sg) method. After calcination, mixed oxide structures were achieved, as shown by XRD. Their acid-base properties were investigated through CO2 and NH3 TPD experiments and FT-IR spectroscopy, using CO2 and NH3 as probe molecules. Basic and acidic sites of medium-high strength were simultaneously revealed: the basic sites were mainly O2- atoms and Mn+-O2- pairs, while the Lewis acid sites were provided mainly by Al3+ in Mg-containing materials and by Al3+ and Ni2+ in Ni-containing materials. Several parameters, such as the nature of cations and anions involved in the LDH structure, the N2+/M3+ ratio, the synthesis method, and the activation conditions, influenced the acid-base properties of the mixed oxides. In particular, (i) Mg-containing samples showed higher basicity than Ni-containing samples, (ii) sg materials were more basic than the cp materials due to different textural and morphological features, (iii) the surface basicity increased with the calcination temperature up to 723-773 K while it markedly decreased after calcination at 873-923 K due to the surface enrichment in Al3+, and (iv) mixed oxides obtained from carbonate-exchanged LDHs showed the highest basicity while those containing chlorine showed predominantly acidic character. Except for the inactive structures containing; high amounts of chlorine impurities, the OH--intercalated structures of the meixnerite type, obtained by the rehydration of mixed oxides under appropriate conditions, showed high activity and selectivity in the acetone condensation reaction. The Lewis-type basicity of the mixed oxides correlated well with the Bronsted-type basicity of the meixnerite-like LDHs, responsible for their remarkable selectivity toward the formation of diacetone alcohol.