Synthesis of highly porous aluminas mediated by cationic surfactant: Structural and textural properties
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khimi Institute of Physics, The National University of Mexico, A.P. 20-364, 01000 México D.F., Mexico
C. Lopez, C. Angeles, F. Hernandez, and J.J. Fripiat Instituto Méxicano del Petróleo, Prog. de Ingeniería Molecular, Eje Central Lázaro Cárdenas # 152, A.P. 07730 México, D.F. México (Received 12 April 2005; accepted 25 May 2005)
The cationic surfactant cetyltrimetylammonium bromide was used to synthesize mesostructured ␥–Al2O3. The effects of the surfactant concentration and of the sol aging (at 95 °C for 24 h) were studied by x-ray powder diffraction, nuclear magnetic resonance, transmission electron microscopy, and analysis of the low-temperature nitrogen adsorption-desorption isotherms. Mesostructured alumina with wormhole morphology and amorphous walls was obtained through the precipitation by ammonium hydroxide of a 0.1 M aluminum nitrate aqueous solution in presence of 0.1 M surfactant. The pore size was smaller than 5 nm. After digesting the milky suspension under atmospheric pressure at 95 °C, a crystallized boehmite-surfactant phase, with fiber morphology, is formed which at 550 and 700 °C is transformed into a highly porous ␥−Al2O3. A similar evolution was observed using 0.01 M CTAB solution and aging. Pore volume up to 1.1 cm3/g and pore size up to 16 nm were obtained. Without surfactant, the same aging treatment led to aggregated fibers: the pore size is less than 8 nm and the pore volume is smaller than 0.6 cm3/g. The ␥-alumina surface area is determined mainly by the organization generated by the surfactant and to a lesser extent by the boehmite precursor particle size. From the point of view of catalyst preparation, the surfactant at the concentration of 0.01 M in 0.1 M aluminum nitrate and the aging treatment in solution play a beneficial role. I. INTRODUCTION
Transition aluminas with high specific surface areas are widely used as adsorbent, catalysts, and catalyst supports in many chemical processes related to oil refining, petrochemical, and fine chemical industries.1–4 Alumina has also applications in the complex catalyst used for cracking and hydro cracking,5,6 hydrodesulfurization of gasoline and diesel,7 naphtha reforming,8,9 isomerization of alkanes,10,11 steam reforming of hydrocarbon feedstock,12 and the control of automotive emissions.13 Fluid cracking catalysts containing transition alumina exhibit matrix activity higher than those containing untreated clays or silica.14 Large pores coupled to relatively high acidity are required in the alumina matrix of the face-centered cubic (fcc) catalysts for an efficient cracking of heavy feeds.15 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0363 J. Mater. Res., Vol. 20, No. 11, Nov 2005
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Conventional ␥-alumina is obtained by dehydroxylating boehmite around 500 °C.16,17 The transformation of boehmite into ␥–Al2O3 is pseudomorphic; the morphology and texture of boehmite precursor rules the textural properties and morphology of the resulting ␥−alum
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