Multiple functionalization of mesoporous silica in one-pot: Direct synthesis of aluminum-containing plugged SBA-15 from aqueous nitrate solutions

Aluminum-containing plugged mesoporous silica has been successfully prepared in an aqueous solution that contains triblock copolymer templates, nitrates, and silica sources but without using mineral acid. The acidity of the solution can be finely tuned from pH 1.4 to 2.8 according to the amount of the introduced aluminum species which ranged from an Al/Si molar ratio of 0.25/1 to 4.0/1. The aluminum nitrate additive in the starting mixture, along with the weak acidity produced by the nitrates, contributes to the formation of plugged hexagonal structures and the introduction of different amounts of aluminum species into the mesostructure. Characterization by X-ray diffraction, transmission electron microscopy, and N₂ sorption measurements show that the Al-containing plugged silicas possess well-ordered hexagonal mesostructures with high surface areas (700-860 m² g^-1), large pore volume (0.77-1.05 cm³ g^-1) and, more importantly, combined micropores and/or small mesopores in the cylindrical channels. Inductively coupled plasma-atomic emission spectrometry results show that 0.7-3.0 wt % aluminum can be introduced into the final samples. ²⁷Al MAS NMR results display that about 43-60% aluminum species are incorporated into the skeleton of the Al-containing silicas and the amount of the framework aluminum increases as the initial added nitrates rises. Scanning electron microscopy images reveal that the directly synthesized Al-containing plugged silica has a similar morphology to that of traditional SBA-15. Furthermore, the Al-containing plugged samples have excellent performances in the adsorption and the catalytic decomposition of isopropyl alcohol and nitrosamine. Finally, the direct synthesis method is used to produce plugged mesoporous silicas that contain other metals such as chromium and copper, and the resultant samples also show good catalytic activities.