Synthesis of Mesostructured Or Mesoporous Aluminas in the Presence of Surfactants. Comprehension of the Mechanisms of Formation

L Sicard,B Lebeau,J Patarin,F Kolenda

doi:10.2516/ogst:2003039

Abstract

The aim of the present study was double: the synthesis of mesoporous aluminas in aqueous medium and the comprehension of their mechanisms of formation. Mesoporous aluminas were obtained from an acidic solution of the aluminum Keggin polycation and a mixed micelle of sodium palmitate and cetyltrimethylammonium bromide. The pore diameters could be increased by increasing the final pH. Aluminas were also obtained from a basic solution of sodium aluminate. Very important pore volumes and specific surface areas were obtained when washing with ethanol or acetone instead of water. A fundamental study to understand the mechanism of formation of a mesostructured alumina, whose symmetry was hexagonal, was also carried out in order to improve the procedures. The study of the interactions between the surfactant and the alumina framework allowed us to explain why the structure collapses upon calcination. A mechanism of formation, based on fluorescence observations, is also presented. It was established that the aluminum species interact with the micelles and polymerize within the palisade layer. However, the micelles are quasi spherical throughout the experiment and do not organize into a hexagonal array before a precipitate is observed.

Highlights

Mesoporous aluminas were obtained from an acidic solution of the aluminum Keggin polycation and a mixed micelle of sodium palmitate and cetyltrimethylammonium bromide
Claus catalysis for the conversion of hydrogen sulfide into sulfur, in the drying of gases or in the selective adsorption of ions in liquid purification. – Aluminas used as inert catalytic support: the hydrotreatments of hydrocarbon residues require aluminas with high pore volume and specific distribution of pore sizes. – Aluminas used as active carrier: adding promoters to the formulation can boost some specific properties
We studied the nature of the interactions between the surfactant and the inorganic framework within a mesostructured alumina and the mechanism of formation of such a material [26,27,28]

Summary

Introduction

Aluminas constitute a vast family of powders whose specific properties, such as the crystalline structure, the porous texture or the surface activity can be tailored through a good understanding of the effect of the preparation variables on the solid properties.Adsorption and catalysis remain big consumers of transition aluminas (γ or η alumina) derived from the thermal treatment of aluminum trihydroxide such as gibbsite or from aluminium oxy-hydroxides such as boehmite gels.Industrial applications of thermally activated aluminas can be classified into three categories [1, 2]: – Aluminas used solely: illustrations can be found in theClaus catalysis for the conversion of hydrogen sulfide into sulfur, in the drying of gases or in the selective adsorption of ions in liquid purification. – Aluminas used as inert catalytic support: the hydrotreatments of hydrocarbon residues (hydrodemetallation, hydrodesulfurization and hydrodenitrification) require aluminas with high pore volume and specific distribution of pore sizes. – Aluminas used as active carrier: adding promoters to the formulation can boost some specific properties. – Aluminas used as active carrier: adding promoters to the formulation can boost some specific properties. The alumina surface acidity is adjusted by adding small amounts of chloride (formulation of the reforming catalyst) or the thermal stability of the alumina surface is increased by incorporating small amounts of silica or rare earth metals (preparation of catalyst for the automotive exhaust gas treatment). Γ-alumina supports, whose surface area is typically in the range of 180 to 250 m2/g, are preferred. These carriers display pore sizes between 60 to 150 Å with cumulative pore volume between 0.5 to 0.7 cm3/g. Larger pores may range from a few hundred to a few thousand angstroms in size

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Conclusion