No abstract

Novel transition-metal containing hybrid biopolymer-silica aerogels have been synthesized as transparent monolithic structures. The compositions include Ru(III), Rh(III), Co(II), and Pd(II) species, silica and chitosan, and amine-group-containing biopolymer derived from chitin. Due to its aqueous solubility and hydrogen bonding properties, chitosan was homogeneously incorporated into the silica network. These aerogels have densities in the range of 0.25-0.30 g/cm3, BET surface areas in the range of 600-975 m2/g, and refractive indexes below 1.17 (at 632.8 nm). Infrared spectroscopy shows that chitosan is effectively introduced into the silica aerogels, and the transition metal ions can coordinate with the amine sites on chitosan. This combines the metal-ion interaction of chitosan with that of silica aerogels. Transmission electronic microscopy indicates that the particle sizes of silica are about 2 nm. Small angle neutron scattering (SANS) has been used to study the microstructure of these aerogels. A new Small-Particle Mass-Fractal model scattering function, derived from the Teixeira Mass-Fractal model scattering function, was used to fit the SANS data. It was found that chitosan helps to form an open aerogel structure. It supports a structural model in which there are primary particles that connect with each other closely to form clusters, and these clusters serve as a secondary structural unit to form the chitosan-reinforced aerogel network. It also indicates that chitosan reinforces the interparticle connections. The local environments, structures and chemistries of the transition metal ions have been explored. Of special interest in this regard are the magnetic properties of the Ru(III) containing materials, which are consistent with anti-ferromagnetic coupling, and the reactions of the Rh(III), Ru(III), and Pd(II) species with small gaseous molecules.

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.