Chemically Selective Reactions in Confined Spaces in Hybrid Aerogels
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0899-N06-06.1
Chemically Selective Reactions in Confined Spaces in Hybrid Aerogels Xipeng Liu, Chunhua Yao and William M. Risen, Jr. Department of Chemistry, Brown University Providence, RI 02912, U.S.A Abstract By employing novel hybrid silica/functional polymer aerogels, control of the course of chemical reactions between reactants confined inside of the aerogels with reactants whose access to the confinement domain is controlled by diffusion has been explored. Thus, monolithic silica/biopolymer hybrid aerogels have been synthesized with coordinated metal ions that can react with amino acids, such as L-cysteine, that are provided externally in a surrounding solution. Metal ions, such as Au(III), that can react in solution with the amino acid to produce one set of products under a given set of stoichiometric or concentration conditions, and a different set of products under a second set of conditions, were selected for incorporation into the aerogel. It was discovered that the course of the reaction could be changed by spatial confinement of the reaction domain in the aerogel. For example, in the case of Au(III) and L-cysteine, the Au(III) ions are confined in nanoscale domains, and when they are reacted with the amino acid, the nature of the reaction products is controlled by diffusion of the L-cysteine into the domains. Exploration of these and related phenomena will be presented. Introduction Silica aerogels have unique properties [1-2]. We have reported the synthesis and characterization of a class of clear monolithic aerogels that contain silica and a bioderived, chemically functional polymer [3-5]. Those made with chitosan; derived from chitin, have high surface areas (500-1100m2/g) and small average pore sizes (3-5nm), although some pores are as large as 20nm in diameter. The presence of chitosan, a polysaccharide with one amine group per saccharide unit, provides a ligand to coordinate metal ions strongly and homogeneously throughout the aerogel. For example, an aerogel X-SiO2-Au(III) with coordinated Au(III) ions can be formed by introduction of an AuCl4- ion during the synthesis and before the supercritical fluid extraction (SFE). Here, X represents chitosan. If an X-SiO2-Au(III) aerogel is exposed to a reactive compound that is present in a surrounding phase, the compound can be “taken up” by the aerogel and allowed to react with aerogel components or other reactants added previously. In this work, the reactive compounds react with the Au(III). The “taking up” process first involves diffusion through the initial aerogel structure. When the reactive compound first arrives at the metal ion site, the system can be described as a nano reactor in which the effective concentrations of the reactants, i.e. the metal ions and reactive compounds in this case, are determined by the pore structure, the number of coordinated metal ions at the pore site, and the number of reactant molecules. As the reaction proceeds, the relative effective concentrations should change as the number of metal ions in the aerogel change
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