Nanoparticle Assembly of Mesoporous AlOOH (Boehmite)

The first examples of a new mesoporous form of crystalline AlOOH (boehmite) were prepared using aluminum sec-butoxide as the alumina precursor and an amine surfactant as a hierarchical structure director. The hydrolysis of the alkoxide in the presence of various amine surfactants at ambient temperature afforded boehmite−surfactant nanocomposites, denoted MSU−S/B. TEM images of the as-made nanocomposites indicated the presence of a scaffold-like structure formed through the aggregation and intergrowth of boehmite nanofibers. Thermal treatment of the nanocomposite at 325 °C topochemically removed the surfactant component and afforded a surfactant-free boehmite phase, denoted MSU−B, with retention of the scaffold structure. The surface areas of the MSU−B products prepared at AlOOH:surfactant mass ratios of 1:1 were 1.27−1.43 times as large as the surface area obtained for boehmite prepared under equivalent hydrolysis conditions in the absence of an amine structure modifier. No correlation was found between the mesostructure pore size and the size of the structure-directing surfactant, which is consistent with a hierarchical scaffold structure formed through the assembly of nanoparticles of more or less uniform size. Both the surfactant−boehmite MSU−S/B nanocomposite and the surfactant-free MSU−B boehmite phase could be converted to mesostructured γ-Al2O3 through calcination at 500 °C. Substantially higher surface areas and pore volumes were obtained for the γ-Al2O3 phases derived from the nanocomposite, indicating that the surfactant mediates nanoparticle assembly even during the topochemical transformation of mesostructured boehmite to γ-Al2O3.