Transparent Monolithic Metal Ion Containing Nanophase Aerogels

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.

Facile One-Pot Synthesis of Mechanically Robust Biopolymer–Silica Nanocomposite Aerogel by Cogelation of Silicic Acid with Chitosan in Aqueous Media

“…In previous studies, tetraethoxysilane (TEOS) based monolithic aerogels containing chitosan and silica have been prepared and characterized. [24][25][26][27]35 Such chitosan-silica aerogels with a typical biopolymer weight loading of 10% have a density of about 0.27 g cm −3 , average pore sizes in the 15 to 20 nm range and BET surface areas from 600 to 975 m 2 g −1 with a refractive index around 1.15. 25,27,36 The high density of the composites is indicative of significant shrinkage (>50% by volume) and the generally poor compatibility of the alcohol solvent with the biopolymer coprecursors.…”

“…One of the most common alternatives to fiber-based reinforcement strategies is the chemical modification of the silica gel network structure by means of organic- or biopolymers. Hybrid silica aerogels modified with numerous different biopolymers such as cellulose-silica, − chitosan-silica, − pectic acid/pectin-silica, − and alginic acid-silica , as well as silica-free biopolymer aerogels such as aeropectin and cross-linked chitosan aerogel have recently been reported. The biopolymers themselves typically are polysaccharides and, given the high number of hydroxyl groups, are very well soluble in aqueous solution but poorly in nonaqueous media.…”

“…The presence of polar functional groups (amine and hydroxyl) on the polymer backbone introduce strong dipole moments which favor the interaction with polar molecules (solvent, silica but also other chitosan units) through hydrogen bonding or dipole–dipole interactions. In previous studies, tetraethoxysilane (TEOS) based monolithic aerogels containing chitosan and silica have been prepared and characterized. − , Such chitosan-silica aerogels with a typical biopolymer weight loading of 10% have a density of about 0.27 g cm –3 , average pore sizes in the 15 to 20 nm range and BET surface areas from 600 to 975 m 2 g –1 with a refractive index around 1.15. ,, The high density of the composites is indicative of significant shrinkage (>50% by volume) and the generally poor compatibility of the alcohol solvent with the biopolymer coprecursors. The main innovation of this study is the adaptation to a sustainable aqueous waterglass-based silica gel chemistry: the gelation of ion exchanged sodium silicate (aqueous silicic acid) in the presence of chitosan can be initiated by a change in pH value.…”