Textural and Acidic Properties of Mixed Alumina-Silica Oxides Prepared with Commercially Available Sols
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ABSTRACT In this study we have used commercially available preformed sols as building blocks to systematically explore the effects of composition, particle size, and packing on the textural and acidic properties of alumina-silica. We have prepared single oxides and alumina-silica mixed oxides with varying AI:Si atomic ratios using commercial sols from Vista Chemical Co. (alumina) and Eka Chemicals, Inc. (silica). Simple particle packing models based on the structure and experimentally determined particle size distributions of the sols explain the textural and acidic properties of both the single and mixed oxides. Comparisons with aerogels prepared from alkoxides show that materials with different atomic-scale homogeneity can be obtained. This continuum of precursor sizes from monomer through colloid allows a measure of control over textural and acidic properties in the mixed oxides, even at a fixed composition. These results show that systematic studies using preformed sols add insight into the effect of preparation upon catalytic materials. INTRODUCTION The distribution, or extent of mixing, of the individual components in a mixed oxide can be manipulated using sol-gel synthesis. Such a synthesis allows materials with different homogeneity to be obtained at a single composition. Control of the "goodness of mixing" of a sample allows some control over textural and acidic properties of catalytic interest such as thermal and hydrothermal stability or pore structure. One approach to controlling homogeneity is to use precursors of different sizes, and in this study we have used commercially available preformed sols of varying sizes as our building blocks. Preformed sols are less well studied and more difficult to manipulate than the commonly used alkoxide sol-gel precursors, but offer the advantages of lower cost and ease of handling. Preformed sols also allow the effect of sol particle size to be studied more easily than is possible in an alkoxide-based synthesis. We have chosen to work with the alumina-silica system, historically the most studied mixed oxide due to its many industrial applications. Mullite (3A12 0 3 .2SIO 2 ) is the subject of many studies because it is the most stable compound within the alumina-silica binary system. Mullite is considered to be a promising candidate for use as a high-temperature structural material due to its chemical and thermal stability, high-temperature strength and creep resistance, low thermal expansion, low density, and low thermal conductivity. 1,2 Other potential applications for mullite include its use as a substrate in multi-layer packaging for 3 microelectronics and as infrared-transparent windows for high-temperature optical applications. Aluminosilicates such as zeolites and clays also have useful catalytic properties such as thermal and hydrothermal stability, high activity, and suitable pore structures which make them useful for cracking catalysts and catalyst matrices. 4 However, other porous aluminosilicates with potential such as high temperature catalysis or sepa
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