Solvothermal synthesis of gamma aluminas and their structural evolution

Mesoporous boehmite with different morphologies have been prepared by a surfactant-free solvothermal technique with an alcohol-toluene mixed system, and the corresponding γ-Al2O3 was obtained after sequential calcination. The effects of the type of alcohol on the crystalline structure, textural properties, morphology, and acidity of the products were investigated through a series of characterizations, including XRD, SEM, TEM, nitrogen physisorption, 27Al MAS NMR, and Py-IR. The results indicate that alcohol molecules with different number of carbon atoms make a strong impact on the growth of boehmite crystallites and their sequent assemblies. Longer alkyl chain in the alcohol molecule is favorable for the formation of boehmite with hierarchical structures. The changes in crystalline structure further affect the pore structure and surface acidity of the alumina samples. Besides, different inorganic salts including Ca(NO3)2·4H2O, Cu(NO3)2·3H2O, Co(NO3)2·6H2O, MgCl2·6H2O, Zn(NO3)2·6H2O, and Fe(NO3)3·9H2O were used instead of Al(NO3)3·9H2O to synthesize the corresponding hydroxides, and the results show that other hydroxide hierarchical structures with similar morphologies can be also obtained through this simple solvothermal route.

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► γ-Al2O3 hierarchical structure can be obtained under solvothermal condition.
► The physicochemical properties of γ-Al2O3 are dependent on the solvent.
► Pore structures are related to the crystallite size and morphology of γ-Al2O3.
► The behavior of solvent in the boehmite synthesis can be used to prepare other inorganic hierarchical structure.