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

Mesoporous alumina represents a very interesting molecular sieve exhibiting a narrow pore size distribution and relatively large pore size compared with conventional alumina. As a solid acid catalyst, the surface acidity performance of alumina is not good, which would be improved by doping niobium into mesoporous alumina. In this study, mesoporous niobia-alumina were prepared via a combined Sol-Gel process with complexation of the inorganic species and surfctant-assisted templating method using amphiphilic triblock copolymer P123 as a template, anhydrous AlCl₃ and niobium oxalate/oxalic acid aqueous solution as precursors in ethanol. The samples were characterized by thermogravimetric analysis(TGA), X-ray powder diffracion(XRD), Fourier transform infrared(FT-IR), nitrogen adsorption-desorption method. Also, the surface acidity of samples were determined by Hammett indicator method. The effects of calcined conditions and Nb/Al(mol ratio) on structure and surface acidity of composite materials were examined, so the optimum conditions and suitable Nb/Al were obtained.The results show that the optimal calcined condition is 450℃ for 6h under flowing air and the temperature raising rate is 1℃/min. The sample obtained on the above condition has strong small angle XRD diffraction and better long-range, which virtually contains nanocrystalline domains within relatively thick amorphous wall. The pore size and surface area of mesoporous composite molecular sieves will be increased than mesoporous alumina. The amount of Nb incorported should be controlled strictly. When Nb/Al=0.01, the composite is of the characteristic mesoporous materials.Acid feature characterization results show that the acidity of the mesoporous alumina is improved comparing with commerical γ-Al₂O₃, though the acid strength is not changed. With the increasement of calcined temperature and time, the total amount of acid sites on mesoporous alumina will be decreased. Doping Nb into alumina will introduce new acid centers which improve the acid feature of mesoporous alumina. The effect of Nb content on acid amount and strength of composite molecular sieves is not very much.