Synthesis of hard mesoporous macro-spheres with silicate and aluminosilicate compositions

Abstract

This study reports the synthesis mechanism and the influence of different variables in the preparation of mesoporous macro-spheres having silicate and aluminosilicate compositions. The spheres possess sizes in the range 200–1000 μm with a narrow particle size distribution and a significant mechanical resistance (hard macro-spheres). In addition, the presence of a highly regular mesoporosity (around 3 nm) and of a high surface area (normally about 1000 m²/g) make these materials as very interesting self-supported adsorbents or catalysts that could be directly applied without no binder, which is a great advantage compared to materials in powder form. The synthesis of the hard mesoporous spheres takes place in a biphasic system, generated by the use of tetrabutoxysilane as silica source, which is reacted with water and NaOH leading to the formation of small primary particles which initially do not present any mesoscopic ordering. However, after about 5 h of synthesis, the presence of cetyltrimethylammonium chloride, as surfactant, gives rise to the detection of a mesostructured silica material. Subsequently, after 9 h of synthesis the small mesostructured particles, with sizes in the micrometer range, acquire a spherical shape. Finally, the reorganization and fusion of these particles cause the formation of the silica macrospheres, with a particle size of several hundreds of micrometers. When the synthesis is carried in the presence of an aluminium source, the aluminosilicate materials so obtained exhibit smaller surface area and pore volume and greater particle size than the pure silica ones.