Synthesis and characterization of sol-gel Mg/Al and Ni/Al layered double hydroxides and comparison with co-precipitated samples

Mg/Al and Ni/Al layered double hydroxides (LDI-Is) were prepared by a sol-gel(sg) route performing hydrolysis of alkoxide or acetylacetonate precursors with HCl or HNO3. For comparison, other samples were prepared from nitrate salts by the classical co-precipitation (cp) method at constant pH. Pure sg Mg/Al LDHs with a Mg2+/Al3+ ratio in the range 2.47-4.29 were obtained, as shown by XRD. Almost pure sg Ni/Al LDHs resulted when using HCl for the precursor hydrolysis, while unsuccessful trials were done with HNO3. The mixed oxides, obtained by calcination of sg materials, exhibited alcogel like texture and specific surface areas, at least 10% higher than those of the cp ones, as evidenced by N-2 volumetric measurements. Furthermore, TEM analysis revealed different morphologic features of the sg and co-precipitated materials, both as-prepared and calcined. TG-DSC analysis and IR spectroscopy evidenced the presence of organic species remaining as compensating anions in the interlayer space, inducing a rather different decomposition process for the sg than for the co-precipitated samples. IR spectroscopy also indicated the higher basicity of the Mg-compared to the Ni-containing LDHs. Furthermore, it evidenced that Al3+ was more easily segregated from Ni(Al)O than from Mg(AI)O mixed oxides. (C) 2000 Elsevier Science B.V. All rights reserved.