High-temperature stable transition aluminas nanoparticles recovered from sol–gel processed chitosan-AlO_x organic–inorganic hybrid films

Five and ten weight percent-alumina-containing chitosan-AlO_x films were prepared via sol–gel processing. The films were AlO_x-agglomerate-free. These organic–inorganic films were degraded by heating at 500 °C. The solid powder residues were found by means of thermogravimetry, X-ray diffractometry, infrared spectroscopy, and electron microscopy to consist of alumina (Al₂O₃) nanoparticles entraping volatile components, whose thermal removal encouraged ambient oxygen uptake. The surface microstructure and morphology of the recovered alumina nanoparticle were inspected by high-resolution transmission and scanning electron microscopy. Also, the surface chemistry and texture were evaluated by X-ray photoelectron spectroscopy and N₂ sorptiometry. Coalescences of globular nanoparticles of γ-/η-Al₂O₃ were the dominant composition of the 800 °C calcination product of the recovered alumina, irrespective of the alumina-content of the film. Upon increasing the calcination temperature up to 1100 °C, an enhanced polymorphic transition into agglomerated nanoparticles of the seldom encountered Iota-(ι-)Al₂O₃ took place. The high thermal stability of the otherwise unstable transition aluminas (at ≥950 °C) may suggestively owe to its polymorphic interdependence and/or persistent nanoscopic nature (average particle size = ca. 3–4 nm; specific surface area = ca. 80–60 m²/g). The surface chemical composition for the recovered transition aluminas nanopowders promises versatile acid–base properties for catalysis applications. Accordingly, the highly abundant bio-waste, chitosan, was successfully utilized as a novel synthesis medium for catalytic-grade alumina nanoparticles.