Investigation of the Surfactant Role in the Synthesis of Mesoporous Alumina

Abstract

Mesoporous alumina with a narrow pore size distribution can be synthesized by hydrolysis of aluminum alkyl ethers in an organic solvent in the presence of an ionic or nonionic surfactant. However, the pores are not ordered and the role of the surfactant as a possible template is not clear as it is in the synthesis of the mesoporous silica of the M41S family and of the SBA-15, 16 type materials. In this work we use continuous wave electron paramagnetic resonance (CW-EPR) spectroscopy, in combination with electron spin−echo envelope modulation (ESEEM) measurements, to provide experimental evidence for the interaction between the surfactant and the alumina species during synthesis. This is achieved by using spin-labeled analogs of the surfactants and following their interaction with 27Al at different stages of the alumina formation. The results show that in sec-butanol solutions the surfactants do not form micellar aggregates. Instead, the anionic surfactant, lauric acid, has a strong tendency to bind to alumina precursors, hydrolysis products of the aluminum alkyl ethers in the starting solution, and remains bound until the final product. Here the surfactant decorates the surface of the alumina particles with an average extended configuration with the carboxylate being the closest to the alumina surface. The interaction with aluminum increases during precipitation as the density of the alumina increases. By contrast, the nonionic polyethyleneoxide type surfactants: Tergitol 15-S-12 and Pluronics P123 and L64 seem to remain unbound even in the as-synthesized precipitates. In this last case, EPR spectra of a small, hydrophobic spin probe have shown that solvent evaporation by room temperature drying brings about the formation of liquid-like “organic zones” confined in the alumina structure, which presumably are at the origin of the pores.