Mesoporous silica SBA-15 molecular sieve has been synthesized and incorporated with aluminum via three different postsynthesis procedures by reacting SBA-15 with AlCl 3 in dry ethanol (route 1), with aluminum isopropoxide in dry hexane (route 2) and with an aqueous solution of sodium aluminate (route 3) followed by calcination. Characterization by transmission electron microscopy, N 2 adsorption, electron probe microanalysis, powder X-ray diffraction, 27 Al magic-angle-spinning NMR, and electron spin resonance spectroscopies has been carried out to evaluate the efficiency of these alumination methods and their effect on the pore structure and ion exchange capacity of SBA-15. The Si/Al ratios in the resulting materials are in close agreement with the composition in the postsynthesis mixtures over a range of 40-10, indicating that the aluminum is mostly incorporated into silica SBA-15. The silanol groups on the internal wall surfaces of SBA-15 are suggested to be the sites for aluminum incorporation. The incorporation procedure has a strong effect on the aluminum coordination, pore structure and ion exchange capacity of the resulting materials. At comparable Si/Al ratios around 20, the percentage of aluminum with tetrahedral symmetry in the synthesized materials is about 100% (route 3) > 76% (route 1) > 71% (route 2), indicating that alumination by aqueous sodium aluminate is most effective. This is probably due to the presence of Na + ions which balance the negative charges associated with tetrahedral aluminums. The BET specific surface areas are 85% (route 1) > 60% (route 2) > 42%(route 3), showing that alumination by AlCl 3 in ethanol best maintains the mesoporous structure of SBA-15. Electron-spin resonance reveals a linear dependence of the Cu(II) ion exchange capacity on the tetrahedral aluminum content of the aluminum-containing SBA-15 materials. This suggests that the incorporated aluminums in the tetrahedral framework do serve as ion exchange sites.
A range of mesoporous aluminosilicate molecular sieves with the MCM-41 structure has been synthesized using different sources of aluminum and characterized in detail by powder X-ray diffraction (XRD), 29Si and 27Al magic-angle-spinning (MAS) NMR, and transmission electron microscopy (TEM). NMR clearly shows that, when Catapal alumina or sodium aluminate is used, virtually all Al in the solid is 6-coordinate. On the other hand, by using aluminum sulfate, MCM-41 can be easily prepared with all aluminum in 4-coordination and a framework Si/Al ratio as low as 10. Resolution of the XRD pattern of aluminosilicate MCM-41 rapidly deteriorates as the aluminum content of the solid increases, and TEM shows that the mesopore system is no longer strictly hexagonal. On the other hand, the resolution of the XRD patterns improves upon calcination of the samples at 550 "C for 24 h, indicating that heat treatment enhances the formation of the mesopores. The walls of the mesopores are essentially amorphous, and the local atomic arrangement is similar to that in amorphous aluminosilicates.
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