Revealing the effects of high Al loading incorporation in the SBA-15 silica mesoporous material

High aluminum loading incorporation in the SBA-15 silica structure was investigated. Different Si/Al molar ratios (15, 10, and 2) were evaluated. The SBA-15 and the aluminum-containing materials (Al-SBA-15) were prepared by the "pH adjusting" method with modifications. The mesoporous structure of the materials was demonstrated by the type IV isotherms. The SBA-15 pore changed from a cylindrical to a slit-like structure in the presence of higher aluminum content. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) pointed out that the structural order is compromised in the presence of a higher aluminum load in the Al-SBA-15 materials, although the mesoporous structure was preserved. Higher Al loading increases the total quantity of Lewis acid sites as well as generates Bronsted acid sites. CO adsorption FTIR spectroscopy suggests aluminum incorporation into the SBA-15 and generation of acid sites. The Si-O-Al linkage in the aluminum-containing materials was corroborated by UV-Vis DRS due to the presence of a peak centered at 241 nm related to the Al-O bond, which is ascribed to four-coordinated framework aluminum in the SBA-15 structure. XPS spectra of Al 2p suggested that the Al species are less oxidized than the Al2O3 phase giving some indication of Al incorporation into the SBA-15 framework. Al-27 MAS NMR results revealed that the aluminum species are in a tetrahedral oxygen coordination environment for Al-SBA-15 with Si/Al molar ratios of 15 and 10. Aluminum species in both tetrahedral and octahedral environments were evidenced for Al-SBA-15 with a Si/Al molar ratio of 2.