Tailoring of mesostructured alumina properties by the one pot incorporation of fluorine

In this study, to modulate the properties of mesostructured alumina synthetized by combining the sol-gel process and the surfactant templating mechanism, the introduction of fluorine has been carried out through a one step process using ammonium fluoride as fluorinating agent. Small angle x-ray scattering, X-ray diffraction, 19 F, 27 Al NMR and manometry nitrogen adsorption-desorption analyses show that even after heating at 500°C, amorphous mesostructured aluminum (hydr)oxyfluoride with an intrinsic mesoporosity are obtained for NH 4 F concentration in water lower than 112.5 g.L -1 . With the increase in the fluorine content, the mesostructuration is lost. For materials recovered after removal of the porogen agent (Pluronic P123) by Soxhlet extraction, aluminum (hydr)oxyfluoride hydrate Al 2 (OH) 2.76 F 3.24 (H 2 O) and ammonium fluoroaluminates, are detected by XRD. When the extracted materials are calcined at 500°C, ammonium fluoroaluminates are decomposed into AlF 3 and AlF 1.96 (OH) 1.04 is also detected. Because of the presence of the crystalline phases, the intrinsic porosity vanishes and the mesoporosity is mainly inter-aggregates. Adsorption of pyridine followed by infrared spectroscopy reveals that whatever the fluorine content, no Brønsted acid site is present. The concentration of the Lewis acid sites first increases to reach a maximum value around 0.8 μmol.m -2 for a NH 4 F concentration in water of 112.5 g.L -1 , then it reaches a plateau. • Mesostructure preserved until 112.5 g.L -1 of NH 4 F in water • Amorphous aluminum (hydr)oxyfluoride are formed at low NH 4 F amount • Crystallization leads to interparticular porosity when fluorine content increases • Addition of NH 4 F increases Lewis acidity of mesostructured alumina • Regardless of the fluoride content no Brønsted acid site is detected