Self-assembly of hierarchically mesoporous - Macroporous phosphated nanocrystalline aluminum (oxyhydr)oxide materials

A hierarchical structure of mesoporous - macroporous phosphated aluminum (oxyhydr)oxide (PAl) materials was prepared via a simple self-assembly process with the use of precursor aluminum secbutoxide in a mixed solution of H ₃PO₄ and Na₂HPO₄. Direct phosphation resulted in the incorporation of phosphorus into the inorganic framework of aluminum (oxyhydr)oxides by the Al-O-P bonds. The X-ray diffraction (XRD) patterns revealed that, despite slight phosphorus incorporation or phosphation, the frameworks of the as-synthesized autoclaved PAl samples remained in a crystalline phase of boehmite AlOOH-type, and their calcined products had a phase of γ-Al₂O₃-type. The macroporous structures are uniform, with sizes of 500-1800 nm, and the macropore walls are composed of accessible mesopores of a scaffold-like nanoparticle assembly. It is shown that the direct incorporation of phosphorus from a phosphate solution of the synthesis system can stabilize the framework of mesoporous-macroporous aluminum oxides with high surface areas and acidic properties. The present study also demonstrated that the surfactant (Brij 56) did not seem to have a direct role in the formation of macroporous structures, but significantly influenced the textural properties of the resultant PAl materials. The surface areas of surfactant-synthesized PAl exceeded 700 m²/g, which is almost two times of that of surfactantless-synthesized samples. The synthesized hierarchical PAl exhibited high thermal stability (at least 800°C), possessing surface hydroxyl groups and acid sites, which may attract much interest for practical applications, including catalysis.