pubs.acs.org/Langmuir © 2010 American Chemical Society
Mesoporous γ-Alumina Formed Through the Surfactant-Mediated Scaffolding of Peptized Pseudoboehmite Nanoparticles Zhaorong Zhang and Thomas J. Pinnavaia* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322 Received January 14, 2010 Mesoporous γ-alumina with precisely controlled mesoporosity is synthesized through the scaffolding of pseudoboehmite nanoparticles in the presence of a nonionic surfactant as the porogen. In the initial step of the synthesis, a colloidal suspension of pseudoboehmite is prepared by peptizing pseudoboehmite in dilute acidic solution. The nanoparticles in the peptizate are then assembled into a scaffold structure using nonionic Tergitol 15-S-7 (C15H33(OC2H4)7OH) as the surfactant porogen. Calcination of the resulting surfactant-containing composites at 500 C removes the surfactant and concomitantly converts the pseudoboehmite crystallites to γ-alumina through topochemical transformation with the retention of the scaffold structure. Depending on the surfactant to alumina ratio used to form the scaffold structures, the average pore size can be precisely controlled over the range of 3.5-15 nm. Also, the BET surface areas of the scaffold structures are substantially larger in comparison to the γ-alumina formed from pseudoboehmite at the same calcination temperature in the absence of surfactant (296-321 vs 238 m2 g-1). The substantial improvement in surface area provided by the scaffold structures, together with the ability to provide narrow pore size distributions over a wide range of average pore sizes by simply adjusting the surfactant content, should substantially improve the effectiveness of this oxide as an adsorbent and as a catalyst or catalyst support.
Introduction High-surface-area transition aluminas, also known as activated aluminas,1 are a group of fundamental catalyst components and adsorbents for various commercial chemical processes, including the cracking and hydrocracking of petroleum,1,2 the purification of gas oil fractions,3,4 the steam reforming of hydrocarbon feedstocks to produce hydrogen,5-7 and the control of automotive emissions,8,9 to name a few. The usefulness of a specific transition alumina in each of these processes requires a favorable combination of textural properties, including surface area, pore volume, and pore size distribution, as well as surface acid/base characteristics. Among the seven polymorphs of transition alumina identified so far, namely, phases γ, η, δ, θ, χ, κ, and F, the γ form is one of the most extensively used in industrial catalysis owing to its comparatively large surface area, unique surface characteristics, and exceptional structural stability.1 However, conventional γ-alumina formed through the thermal dehydration of a crystalline aluminum oxyhydroxide (boehmite) at a temperature above 450 C typically exhibits interparticle pores associated with limited surface areas (
