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

Well-crystallized layered double hydroxides (LDH) containing Mg2+ or Ni2+ a s divalent cations and Al3+ as trivalent cations were obtained either by co precipitation (cp) or with the sol-gel (sg) method. After calcination, mixe d oxide structures were achieved, as shown by XRD. Their acid-base properti es were investigated through CO2 and NH3 TPD experiments and FT-IR spectros copy, using CO2 and NH3 as probe molecules. Basic and acidic sites of mediu m-high strength were simultaneously revealed: the basic sites were mainly O 2- 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 m aterials. Several parameters, such as the nature of cations and anions invo lved in the LDH structure, the N2+/M3+ ratio, the synthesis method, and the activation conditions, influenced the acid-base properties of the mixed ox ides. In particular, (i) Mg-containing samples showed higher basicity than Ni-containing samples, (ii) sg materials were more basic than the cp materi als 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 enr ichment in Al3+, and (iv) mixed oxides obtained from carbonate-exchanged LD Hs showed the highest basicity while those containing chlorine showed predo minantly acidic character. Except for the inactive structures containing; h igh amounts of chlorine impurities, the OH--intercalated structures of the meixnerite type, obtained by the rehydration of mixed oxides under appropri ate conditions, showed high activity and selectivity in the acetone condens ation reaction. The Lewis-type basicity of the mixed oxides correlated well with the Bronsted-type basicity of the meixnerite-like LDHs, responsible f or their remarkable selectivity toward the formation of diacetone alcohol.