Study Of Preparation, Characterization And Complexation Properties To Phosphate Of Mesoporous Alumina

Due largely to high surface area, adjustable pore channel, excellent adsorption properties, mesoporous alumina had shown a series of important performances in the optics, electronics, thermodynamics, molecular catalysis and adsorption, and had been usually used as an adsorbent, a catalyst, a catalyst support and so on. Many research groups had been focusing on synthesis and characterization of mesoporous materials. However, it's still an advanced subject for preparing mesoporous alumina with high surface area, adjustable pore size and thermal stability and investigating the complexation properties. The aim of our research was to synthesize mesoporous alumina with high surface area and adjustable pore size and then discussed the effect of alkali ions, alkyl amine as well as calcination on the synthesis of mesoporous alumina. The surface complexation properties of phosphate at the surfaces of alumina were also investigated.In this paper, mesoporous alumina with high surface area and narrow pore size was synthesized by using low-cost aluminum metal powder as an alumina source, PEG as a structure directing agent in the presence of alkalis ions. TEM,TG/DTA,XRD,N2 adsorption/desorption techniques were used to characterize the properties of synthesized mesoporous alumina. The results showed that alkalis ions played an important role in preparing process of mesoporous alumina. The decreases of surface area of mesoporous alumina synthesized in the presence of alkali metal ions is in the order MA-K600(1100) > MA-Na600(1100) > MA-Li600(1100), which is in accord with the adsorption energy of these ions at the surfaces i.e. the more negative the adsorption energy, the lower the surface area of MA synthesized. Among the tested alkali metal ions, K+ has outstanding properties in promoting the formation of higher surface area mesoporous alumina with excellent anti-sintering characteristics. The pore wall structure of synthesized mesoporous alumina precursor using aluminum powder as an aluminum source was well crystalline bayerite, which could be transformed into amorphous alumina,γ-Al2O3 orθ+α-Al2O3 with increasing calcination temperature.Mesoporous alumina with high surface area, adjustable pore size was synthesized by using low-cost inorganic aluminum salt or aluminum metal powder as aluminum sources respectively, PEG as a structure directing agent, and alkyl amine as the pore expander. The effect of calcination temperature to wall structure and pore size of mesoporous alumina was discussed. TEM, TG/DTA, XRD, N2 adsorption/desorption techniques were used to characterize the properties of synthesized mesoporous alumina. The results showed that mesoporous alumina with high surface area and adjustable pore size could be obtained by the two ways. Alkyl amine is a kind of efficient pore expander and can adjust pore size efficiently. Comparing the two methods, inorganic aluminum salt displayed better performance for synthesizing mesoporous alumina with larger pore size, which can be adjusted from 8.35 to 13.85 nm, while aluminum metal powder did better performance for synthesizing mesoporous alumina with smaller pore size, which can be adjusted between 3.62 and 6.96 nm and narrow distribution. PEG could prevent the nano particles from aggregation in the process of calcinations. The wall structure, surface area and pore size of mesoporous alumina correlated closely with calcination temperature. When the calcination temperature was at 550°C, the wall crystalline structure of MA was in an amorphous form. The wall crystalline structure dominated inγform was obtained when the calcination temperature reached 800°C. At 1100°C, however, the wall crystalline structure was a mixture ofα- andθ-alumina. When the calcination temperature was over 1100°C, the BET surface area of the sample decreased drastically and the pore structure collapsed, which correspondes to the pureα-alumina phase. The testing results indicated that the pore size increased with increasing calcination temperature, while the BET surface area and pore volume decreased when the calcination temperatures went up between 550 and 1100°C.The complexation properties of phosphate to synthesized mesoporous alumina were investigated by in situ Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy. The results indicated that, at pH 4.15-9.01, the adsorption capacity was quite varing. With the decreasing of pH, adsorption capacity increased. The adsorption isotherms at pH 4.15 and 9.01 were performed. At pH 9.01, it fitted with Langmuir formula when phosphate concentration in adsorption systerm was lower and Freundlich formula at higher concentration. At pH 4.15, adsorption isotherms followed with Freundlich formula. Adsorption kinetics curve showed that the equilibrium time was connected with initial phosphate concentration, i.e. the higher phosphate concentration, the shorter to reach balance. The process was individed into two parts, the fast course at the beginning and the lower course later. Curve fitting results showed that surface complexes were quite different at varing pH.