Reforming Dawsonite by Memory Effect of AACH-Derived Aluminas

Ammonium aluminum carbonate hydroxide (AACH), a material isostructural with the mineral dawsonite (NaAl(OH)₂CO₃), leads to finely dispersed and high-surface area Al₂O₃ upon thermal decomposition. This family of compounds exhibit memory effect, i.e., the dawsonite structure can be recovered upon treatment of the derived aluminas in (NH₄)₂CO₃ solutions at 323 K. Treatment of the oxides in water or NH₄OH, NH₄Cl, and Na₂CO₃ solutions led to aluminum (oxi)hydroxides. The structural, morphological, and porous properties of the materials were investigated by ICP-OES, elemental analysis, XRD, FTIR and Raman spectroscopies, He pycnometry, TEM, gas (N₂ and Ar) adsorption, and thermal analysis (TGA and DSC). The reconstruction of dawsonite in ammonium carbonate was complete for the solids calcined at 523 and 723 K, consisting of a highly amorphous and low-skeleton-density alumina phase with well-developed porosity. The recrystallization was incomplete upon formation of larger gamma-Al₂O₃ crystals by calcination at 1073 K, and no reconstruction occurred when alpha-Al₂O₃ was obtained by calcination at 1473 K. Characterization of the reconstructed samples indicates the attainment of NH₄-dawsonite with higher purity than in the parent material. This is due to the presence of amorphous Al-containing phase(s) in the as-synthesized sample coexisting with AACH. All the phases were selectively converted into dawsonite by thermal decomposition and (NH₄)₂CO₃ treatment. The reformed samples consist of relatively large crystals with newly developed microporosity as compared to the parent material. These results extend the unique memory property of oxides derived from particular layered materials such as hydrotalcites to other families of mineral-like compounds.