Transparent Monolithic Metal Ion Containing Nanophase Aerogels
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ABSTRACT The formation of monolithic and transparent transition metal containing aerogels has been achieved through cooperative interactions of high molecular weight functionalized carbohydrates and silica precursors, which strongly influence the kinetics of gelation. After initial gelation, subsequent modification of the ligating character of the system, coordination of the group VIII metal ions, and supercritical extraction afford the aerogels. The structures at the nanophase level have been probed by photon and electron transmission and neutron scattering techniques to help elucidate the basis for structural integrity together with the small entity sizes that permit transparency in the visible range. They also help with understanding the chemical reactivities of the metal-containing sites in these very high surface area materials. These results are discussed in connection with new reaction studies. INTRODUCTION There is considerable interest in the formation and properties of gels and aerogels based on silica. Some of this arises from the possibility of incorporating metal-containing species in the materials so that they can be used as precursors of catalysts. The catalyts then would be formed as composites containing metal particles [1 ] or metal oxide [2 ] or other metal complex compounds. That has led to a range of studies of transition metal species in silica areogels[3,4]. In this work, a general synthetic approach has been taken that is designed to provide for both the incorporation of metal ions and the preservation of other advantageous features of aerogels. These advantages can include more than just low density and high porosity. Indeed, some silica-based aerogels are monolithic structures with useful optical, thermal and electronic applications. Formation of transparent, monolithic metal ion containing aerogels potentially provides a way to attain a class of materials with these attributes as well as those imparted by metal ions. The primary reported methods for incorporation reported to date include adsorption of metals from solution by functionalized aerogels. This is a method for the clean up of metal-ion contaminated sites [5]. It also includes the incorporation of transition metal complexes, such as copper amine complexes [3]. And, it includes the formation of complexes in aerogels through the reactive attachment of a complexing ligand into the sol-gel structure [4]. The approach reported here is to form a silica-and polymer-based aerogel in which the polymer can provide metal ion coordination sites for a range of metal ions. In order to be transparent, the gel must be formed at low enough pH to obtain nanometer scale fundamental particle sizes. The polymer must have the ability to strengthen the gel sufficiently to allow for the formation of an aerogel monolith. The polymer also must have the ability to provide useful metal ion coordinating sites in pH ranges at which the silica structure and particles size distribution are stable. It is difficult to find materials and approaches to achieve this
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