| Layered niobate has a special laminate structure and the cation located at interlayer can be exchanged. The physical and chemical properties of the material could be adjusted through proton-exchanging, exfoliating, pilling etc. modification methods. The layered niobate nanosheets can be obtained by the method of exfoliation. Oxide-pillared hexa niobate has been successfully prepared by an exfoliation-restacking route, and then the structure and functional performance tunable composite material would be obtained.Here in, the host potassium hexaniobate KNb3O8and K4Nb6O17were prepared by a polymerizable complex method and solid-state method, respectively, and through proton-exchanging, exfoliating, pilling etc methods to modify the structure and properties of the niobate. A monodispersed TiO2and Fe (OH)3sol were prepared by hydrolysis route, and then the TiO2/Nb6O17and Fe2O3/Nb6O17samples were successfully prepared by restacking with Nb6O174-nanosheets sol. The structure, morphology and stability were characterized by X-ray powder diffraction (XRD), UV-Visible diffuse reflectance spectroscopy (UV-vis DRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermal analysis (TG-DTA), and other physical and chemical testing means. The regioselective nitration of toluene was used as a probe to evaluate the acid catalytic activity of the H3ONb3O8samples, and the experimental result was detected by gas chromatographic analysis.The results show that:the layered niobate KNb3O8can be prepared by a polymerizable complex method and the best conditions for the preparation in the present experimental is calcined at700℃for4h. The particle size of the as-prepared sample KNb3O8is relatively small compared to the sample obtained with the high-temperature solid-phase method. The thermal analysis showed that KNb3O8sample has a higher thermal stability and the weight loss rate of samples proton-exchanged and exfoliated significantly increased, which is caused by the interlayer protons hydrated. The BET surface area of the samples almost unchanged before and after the proton-exchanging (KNb3O8is9.23m2·g, H3O Nb3O8is12.78m2·g). The BET surface area of the sample that exfoliated significantly increased, and the value is167.13m2·g. XRD and HRTEM results also show that the interlayer spacing is significant increased after exfoliation. The regioselective nitration of toluene was used as a probe to evaluate the acid catalytic activity of the H3ONb3O8sample. The experimental results show that the amount of catalyst is the minimal impact on orthogonal experiment, and the pretreatment temperature of the catalyst is the main impact factor. The higher of the pretreatment temperature is, the stronger of the solid acid strength is. Therefore, the acid strength of the solid acid is the main factor that influenced the acid catalytic activity.The XRD and SEM results indicated that the layered niobate K4Nb6O17prepared by high temperature solid phase method own perfect layered structure, but the debris could be observed on the surface of the samples after proton-exchange. This phenomenon caused by the acidification sustained and vigorous stirring. The two dimensional sheet sol could be obtained by exfoliating the H4Nb6O17samples, and the nanotubes would be formed by adjusting the pH of the sol. HRTEM and SEM results show that the diameter of the nanotubes is uniform and tube length is longer. TiO2/Nb6O17and Fe2O3/Nb6O17composite samples own the properties of nanosheets and nanocrystals (TiO2or Fe2O3) structure characteristics.The spectral response characteristics of the catalyst samples was characterized by UV-Visible diffuse reflectance spectroscopy (UV-vis DRS). The results showed that the absorption threshold have almost no change before and after proton-exchange or exfoliation. When the oxide particles TiO2and Fe2O3were introduced into the interlayer, the absorption threshold existed significantly red-shift. The most obvious red-shift is the Fe2O3/Nb6O17sampes. |
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