Alumination and Ion Exchange of Mesoporous SBA-15 Molecular Sieves

Abstract

Mesoporous silica SBA-15 molecular sieve has been synthesized and incorporated with aluminum via three different postsynthesis procedures by reacting SBA-15 with AlCl3 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, N2 adsorption, electron probe microanalysis, powder X-ray diffraction, 27Al 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 AlCl3 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.