Study On Synthesis And Surface Acidity Of Nb-containning Zirconia Mesoporous Molecular Sieves

In this study, mesoporous ZrO₂ was synthesized via a combined Sol-Gel process with surfactant-assisted templating method. cetyltrimethylammonium bromide(CTAB) as template, zirconyl chloride octahydrate(ZrOCl₂·8H₂O) as predecessor, acetylacetone (acac) as a stabilizer and ethanol as solvent. The effects of the processing parameters have been investigated, including calcined temperature, calcined time and amounts of surfactant, etc. Then, niobium was added to the mesoporous system to synthesize mesoporous Nb₂O₅-ZrO₂, with niobium oxalate as predecessor.The resulting oxide was characterised with several tools: TG-DTA, XRD, FT-IR, nitrogen adsorption-desorption isotherms measurements. The results showed that the optimal calcined condition is 450℃for 6h under flowing air with temperature raising rate is 1℃/min,with Zr/CTAB=2.The sample obtained on the above condition has better small angle XRD diffraction and the phase structure of ZrO₂ is single tetragonal, with BET specific surface area of 72.98m²/g, and average pore diameter of about 2.2 nm. The pore size and surface area of mesoporous composite molecular sieves will be increased than mesoporous ZrO₂. The amount of Nb incorported should be controlled strictly. When Nb/Zr=0.05, the composite is of the characteristic mesoporous materials, with BET specific surface area of 122.42m²/g, and average pore diameter of about 3.68nmThe surface acidity of samples were determined by Hammett indicator method. Acid feature characterization results show that the calcination has litter effect on surface acidity, but the total amount of acid sites on mesoporous ZrO₂ will be decreased, with the increasement of calcined temperature and time. So as the decresement of CTAB. Although the acid feature of mesoporous ZrO₂ will be improved while doping Nb into Mesoporous ZrO₂, it cann't introduce new acid centers, When Nb/Zr=0.05 ,the total amount of acid sites is biggest.