Effect of nonionic structure-directing agents on adsorption and structural properties of mesoporous alumina

Abstract

Mesoporous aluminas (MAs) with tailored adsorption, framework, and surface basic–acidic properties were prepared using different structure-directing agents (SDAs) such as Brij®78, Pluronic F108, poly(ethylene oxide)–poly(butylene oxide)–poly(ethylene oxide) B50-6600 and Vorasurf™ 504 block copolymers, which so far were not explored for the synthesis of this important material viasolvent evaporation-induced self-assembly (EISA) strategy. Thermogravimetry, Fourier transform infrared spectroscopy, small-angle X-ray diffraction, nitrogen adsorption, transmission electron microscopy, and CO2 and NH3 temperature programmed desorption techniques were used to characterize the samples studied. Depending on the SDAs and aluminium precursors used in the EISA process, the BET surface area, pore volume and the average pore size of the MA samples were tailored in the range of 300–433 m2 g−1, 0.33–1.07 cm3 g−1, and 2.7–11.5 nm, respectively. Importantly, the use of F108 as a SDA and aluminium isopropoxide as an aluminium precursor gave a highly ordered MA with a large pore volume of 1.07 cm3 g−1 and enlarged mesopores centering at 11.5 nm, which are larger than those reported for the P123 and F127 Pluronic block copolymer-templated MA samples. However, the use of F108 as the SDA and AlCl3 as the aluminium precursor resulted in disordered MA with much smaller mesopores centering at 2.7 nm, enhancing the adsorption affinity towards CO2 (1261 μmol g−1) and NH3 (350 μmol g−1). This study shows that the EISA strategy does not require a stringent selection of SDA, and that the F108 block copolymer is an excellent SDA for the preparation of MA with a large pore volume and high adsorption capacity towards CO2, which exceeds substantially the value obtained for conventional alumina samples and for the previously reported ordered MA prepared in the presence of P123 and F127, which are commonly used as SDAs.