Synthesis and characterization of a boron-aluminum oxochloride

A novel microporous crystalline boron-aluminum oxochloride with a cationic framework, designated BAC(3), has been synthesized hydrothermally in the system B₂O₃-Al₂O₃-Na₂O-H ₂O and characterized using ICP, ion chromatograph, X-ray diffraction (XRD), magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR), infrared spectroscopy (IR), thermal analysis, and adsorption measurement. Some factors, such as temperature, pH value, and anions have important effects on the crystallization. Chemical analysis shows that the as-synthesized solid has the molar composition 0.2B₂O₃ · 1.0 Al₂O₃ · 0.6HCl · 5.7H₂O. XRD analysis shows that the crystal has a unique framework structure. The crystal is indexable on an orthorhombic unit cell with a = 22.55, b = 14.42, and c = 8.75 Å. The three-dimensional framework is built up from triangular BO₃ and tetrahedral BO₄ units with BO₃/BO₄ of 2.6, as well as octahedrally coordinated Al. XRD, IR, and DTA-TG studies demonstrated that the microporous crystal is unstable to thermal treatment above 300°C, with the decomposition of structural H₂O and HCl confirmed by mass spectroscopy analysis. On calcination above 800°C, this microporous crystal is converted via an amorphous phase to a known crystalline Al₁₈B₄O₃₃. The measurement of adsorption of water indicates that it possesses characteristic micropore adsorption properties. Ion-exchange analysis shows that the Cl^- anions in BAC(3) can be partially exchanged by Br^- anions, suggesting that this novel microporous crystal has a cationic framework.