Biopolymer-silica Hybrid Aerogels Containing Transition Metal Species; Structure, Properties, and Reactions

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Biopolymer-silica Hybrid Aerogels Containing Transition Metal Species; Structure, Properties, and Reactions Xiangjun Hu, Shuang Ji, and William M. Risen, Jr. Department of Chemistry, Brown University, Providence, RI 02912, USA. 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. INTRODUCTION As more naturally occurring and naturally derived polymers are employed in composites, it is important to investigate both the nature of their molecular level interactions with matrix materials, such as silica, and the special properties they offer. In the case of chitosan, this includes the ability to take up metals and place them in particularly valuable molecular environments. Monolithic, transparent hybrid aerogels have been prepared with the bioderived polymer chitosan and silica as the principal components, and their structures have been studied by small angle neutron diffraction (SANS), transmission electron microscopy (TEM) and spectroscopic techniques ranging from infrared to several nuclear magnetic resonance (NMR) methods [1-4]. They are characterized by having densities of about 0.27 g cm-3, an ultimate particle size of less than or equal to 2 nm diameter, average pore size of 3 to 5 nm, and high surface