Mesostructured γ-Al2O3with a Lathlike Framework Morphology

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Abstract
A novel three-step assembly pathway is reported for the formation of a mesostructured alumina with framework pore walls made of crystalline, lathlike γ-Al2O3 nanoparticles. In the initial supramolecular assembly step of the pathway a mesostructured alumina with a wormhole framework morphology and amorphous pore walls is assembled through the hydrolysis of Al13 oligocations and hydrated aluminum cations in the presence of a nonionic diblock or triblock poly(ethylene oxide) surfactant as the structure-directing porogen. The walls of the initial mesostructure are then transformed in a second hydrolysis step at a higher temperature to a surfactant−boehmite mesophase, denoted MSU-S/B, with a lathlike framework made of boehmite nanoparticles. A final thermal reaction step topochemically converts the intermediate boehmitic mesophase to a mesostructure with crystalline γ-Al2O3 pore walls, denoted MSU-γ, with retention of the lathlike framework morphology. The boehmitic MSU-S/B intermediates formed from the chloride salts of aluminum incorporate chloride anions into the mesostructure. Chloride ion incorporation tends to disorder the nanoparticle assembly process, leading to a broadening of the slit-shaped framework pores in the final MSU-γ phases and to the introduction of intra- and interparticle textural mesopores. However, the well-ordered MSU-γ phases made from aluminum nitrate as the preferred aluminum reagent exhibit narrow framework pore size distributions and average pore sizes that are independent of the surfactant size and packing parameter, in accord with a lathlike framework assembled from nanoparticles of regular size and connectivity. The high surface areas (∼300−350 m2/g) and pore volumes (∼0.45−0.75 cm3/g) provided by these mesostructured forms of γ-Al2O3 should be useful in materials and catalytic applications where the availability of surface Lewis acid sites and the dispersion of supported metal centers govern reactivity.