Facile strategy for synthesis of mesoporous crystalline γ-alumina by partially hydrolyzing aluminum nitrate solution

Abstract

The facile synthesis of mesoporous γ-alumina is developed through the partial hydrolysis of Al(NO₃)₃ aqueous solution with (NH₄)₂CO₃ without organic surfactants. In this synthesis, the stable NH₄NO₃/Al species (AN/Al) hybrid containing Keggin-Al₁₃ polycations is first prepared, which is the key for the successful formation of mesoporous γ-alumina. XRD, ²⁷Al MAS NMR, TEM, and N₂ adsorption and desorption results demonstrate that the as-prepared AN/Al hybrid can be transformed to γ-alumina by treatment at 200 °C and exhibit a wormhole-like mesoporous structure with large surface area up to ∼450 m² g⁻¹, pore volume of ∼0.3 cm³ g⁻¹ and narrow pore size distribution peaked at ∼3.9 nm after completely removing NH₄NO₃ at 300 °C. The obtained mesoporous γ-aluminas have high thermal stabilities up to 900 °C and excellent hydrothermal stability. The investigation shows that the synergetic effect of NH₄NO₃ and Al₁₃ species promotes crystallization of Al species to γ-alumina, which may have a unique mechanism distinct from the mesoporous aluminas reported previously. CO₂ adsorption measurements indicate that these mesoporous γ-aluminas have a much higher CO₂ adsorption capacity than ordered mesoporous alumina synthesized by the surfactant-templating method and conventional γ-alumina derived from aluminum oxyhydroxides. We believe that this research should be useful for providing new insights into the transformation of transition alumina phases and for synthesizing mesoporous γ-alumina with promising properties for various chemical applications.