Influence Law of pH Value on Physicochemical Properties of Boehmite and γ-Al2O3 During Hydrothermal Aging
The influence of the initial pH value of the aluminum hydroxide slurry obtained from the hydrolysis of aluminum alkoxides and the addition of HNO3 on the physicochemical properties of the resultant boehmite and γ-Al2O3,such as specific surface area,pore structure,micro-morphology,and thermal stability,was investigated using XRD,BET,TG-DTA,and TEM during the hydrothermal aging process of aluminum hydroxide slurry.The results indicate that the initial pH of the aluminum hydroxide slurry during the hydrothermal aging process can significantly influence the growth of boehmite crystallites.Under acidic aging conditions,the crystallites of boehmite exhibit a deformed hexagonal shape,with the proportion of(010)and(101)faces to the total surface area decreasing and the(100)face increasing.Under alkaline conditions,the boehmite crystallites are in the shape of standard rhomboidal plates,primarily exposing the(010)and(101)faces.Further studies reveal that the addition of HNO3 can result in a significant effect on the pore structure of the product.Specifically,under acidic hydrothermal aging conditions,as the mole ratio H+/Al3+increases,the degree of peptization of aluminum hydroxide increases,the crystallite aggregates are dispersed,the crystallite size decreases,the physical adsorption water content of the obtained boehmite increases,the decomposition temperature of NO3-decreases,the pore diameter distribution of the corresponding γ-Al2O3 becomes more concentrated,and the specific surface area first increases and then decreases.These findings provide a theoretical basis for the flexible manipulation of the physicochemical properties of γ-Al2O3.
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