A vesicle-aggregation-assembly approach to highly ordered mesoporous gamma-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 gamma-alumina microspheres with unique shifted double-diamond networks for the first time, by using diblock copolymer poly(ethylene oxide)-b-poly(methyl methacrylate) (PEO-b-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 H2O, the as-made Al3+-based gel/PEO-b-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 mu m). Furthermore, these composite microspheres possess an inverse bicontinuous cubic mesostructure (double diamond, Pn (3) over barm) with Al3+-based gel buried in the PEO-b-PMMA matrix in the form of two intertwined but disconnected networks. After a simple calcination at 900 degrees C in air, the structure of the resultant mesoporous alumina changes to a relatively low symmetry (shifted double diamond, Fd (3) over barm), 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 similar to 131 to similar to 95 nm. The obtained ordered mesoporous alumina products retain the spherical morphology and possess ultra-large mesopores (similar to 72.8 nm), columnar frameworks composed of gamma-alumina nanocrystalline particles (crystal size of similar to 15 nm) and high thermal stability (up to 900 degrees C). As a support of Au nanoparticles, the formed Au/mesoporous gamma-alumina composite catalysts have been used in the catalytic reduction of 4-nitrophenol with a high kinetic constant k of 0.0888 min(-1), implying promising potential as a catalyst support.