A vesicle-aggregation-assembly approach to highly ordered mesoporous γ-alumina microspheres with shifted double-diamond networks.

Alumina materials have widely been used in industrial fields, such as catalysis and adsorption. However, due to the fast sol-gel process and complicated crystalline-phase transformation, the synthesis of alumina materials with both highly ordered mesostructures and crystallized frameworks remains a great challenge. Herein, we report a novel vesicle-aggregation-assembly strategy to prepare highly ordered mesoporous γ-alumina microspheres with unique shifted double-diamond networks for the first time, by using diblock copolymer poly(ethylene oxide)--poly(methyl methacrylate) (PEO--PMMA) as a template and aluminum isopropoxide as a precursor in a tetrahydrofuran (THF)/hydrochloric acid binary solvent. During the gradual evaporation of THF and HO, the as-made Al-based gel/PEO--PMMA composites can be obtained through a co-assembly process based on the hydrogen bonding interaction between hydroxyl groups of alumina oligomers and PEO segments of the diblock copolymers. The formed composites exhibit a spherical morphology with a wide size distribution (diameter size 1-12 μm). Furthermore, these composite microspheres possess an inverse bicontinuous cubic mesostructure (double diamond, 3[combining macron]) with Al-based gel buried in the PEO--PMMA matrix in the form of two intertwined but disconnected networks. After a simple calcination at 900 °C in air, the structure of the resultant mesoporous alumina changes to a relatively low symmetry (shifted double diamond, 3[combining macron]), ascribed to the shifting of the two alumina networks due to loss of the templates. Meanwhile, the unit cell size of the alumina mesostructure decreases from ∼131 to ∼95 nm. The obtained ordered mesoporous alumina products retain the spherical morphology and possess ultra-large mesopores (∼72.8 nm), columnar frameworks composed of γ-alumina nanocrystalline particles (crystal size of ∼15 nm) and high thermal stability (up to 900 °C). As a support of Au nanoparticles, the formed Au/mesoporous γ-alumina composite catalysts have been used in the catalytic reduction of 4-nitrophenol with a high kinetic constant of 0.0888 min, implying promising potential as a catalyst support.