Direct synthesis and the morphological control of highly ordered mesoporous AlSBA-15 using urea-tetrachloroaluminate as a novel aluminum source

Abstract

The synthesis of AlSBA-15 under mild acidic conditions was performed through adjusting the molar H₂O/HCl ratio, which indicates the formation of Si–O–Al linkages that lead to isomorphous substitution of Si⁴⁺ by some Al³⁺ ions. In this paper, the direct incorporation of Al³⁺ onto a SBA-15 framework in acid mediated synthesis with a nSi/nAl molar ratio of 7 was optimized using urea tetrachloroaluminate ionic liquid as a new aluminum source. The hydrothermal temperature was varied from 80 to 140 °C and the samples were denoted as AlSBA-15(U_x). The conventional AlSBA-15 also was prepared using direct synthesis [AlSBA-15(D)] and post synthesis [AlSBA-15(P)] aiming to study the influence of aluminum sources and preparation conditions on their structural, textural, and physicochemical properties. The synthesized materials were characterized by N₂ physisorption, XRD, FT-IR, Py-FT-IR, NH₃-TPD, XRF, HRTEM and SEM. All preparation methods led to the formation of aluminum containing SBA-15 samples with different Si/Al contents. Nevertheless, depending on the preparation methods, the AlSBA-15 samples exhibited different structural, morphology, and surface characteristics, especially in terms of Brønsted and Lewis acid site content. AlSBA-15(U₁₀₀) had high surface area (813 m² g⁻¹) and high acidity. TEM images of synthesized AlSBA-15(U₁₀₀) showed well-ordered hexagonal arrays of uniform cylindrical channels. The effectiveness of AlSBA-15, as an acid catalyst, was studied for the esterification of acetic acid with butanol and cumene cracking. The high activity of AlSBA-15(U₁₀₀) is attributed to its good ordered structure and high acidity.