UV Raman spectroscopic study on the crystallization of microporous boron-aluminum crystalline BAC(10)

Boron aluminum oxo chloride [BAC(10)], a microporous material, and its crystallization were investigated by X-ray diffraction (XRD), adsorption isotherms of probing molecules, infrared (IR), magic-angle spinning nuclear magnetic resonance (MAS NMR), visible Raman and ultraviolet (UV) Raman spectroscopy. XRD and adsorption isotherms indicate that BAC(10) consists of novel microporous crystals, and IR and NMR results confirmed that the primary building units are triangular BO₃, tetrahedral BO₄ and octahedral AlO₆. No Raman signal in the visible Raman spectrum of BAC(10) was detected owing to the weakness of Raman scattering and strong fluorescence interference. However, strong Raman bands of this material were observed in the UV Raman spectra owing to the avoidance of fluorescence. The UV Raman spectra show that, during the crystallization process, the number of BO₃ groups (bands at 300-700 cm^-1) decrease with increasing number BO₄ of groups (bands at 900-1000 cm^-1) in the solid phase. These results suggest that in the crystallization of BAC(10), the polyborate anions are quickly depolymerized to monoborate species including BO₃ and BO₄ groups, which are then gradually crystallized to the framework of microporous BAC(10).