Synthesis of Monolithic Al2O3 with Well-Defined Macropores and Mesostructured Skeletons via the Sol−Gel Process Accompanied by Phase Separation

Abstract

Alumina (Al 2 O 3 ) monoliths with well-defined macropores and mesostructured skeletons have been synthesized via a spontaneous route from the aqueous and ethanolic solution of aluminum salts in the presence of propylene oxide and poly(ethylene oxide) (PEO). The addition of propylene oxide to the starting solution controls the gelation, whereas the addition of PEO induces the phase separation. Appropriate choice of the starting composition, by which the phase separation and gelation concur, allows the production of bicontinuous macroporous Al 2 O 3 monoliths in large dimensions (10 × 10 × 10 mm 3 ). The size of macropores is controlled in the range of 400 nm to 1.8 μm, depending on the PEO content in starting solutions. The dried gel is amorphous, whereas heating at temperatures above 800 °C leads to the formation of crystalline phases without spoiling the macroporous morphology; nanocrystalline γ-Al 2 O 3 is precipitated at 800 °C, α-Al 2 O 3 starts to form at 1000 °C, and complete transformation into α-Αl 2 O 3 is achieved at 1100 °C for 5 h. Nitrogen adsorption−desorption measurements revealed that the skeletons of dried gels possess the mesostructure with a median pore size of about 2.6 nm and a surface area as high as 396 m 2 /g. Heat treatment at 300 °C increases the pore size and surface area to 3.5 nm and 512 m 2 /g, respectively. Even after heat treatment at 800 °C, which results in the formation of nanocrystalline γ-Al 2 O 3, the surface area is 182 m 2 /g, with the pore size being 4.5 nm.