A high-yield reproducible synthesis of MCM-48 starting from fumed silica

A new synthesis pathway for MCM-48 is developed using fumed silica as the silicate source and a Gemini surfactant (GEM 16-12-16) as the surface directing agent. Different synthesis parameters have been optimized in order to obtain a highly ordered mesostructure. The determining factor in the stabilization of the final structure is proven to be a double post-synthesis hydrothermal treatment. This treatment, performed successively in a minimum of pure water at 403 K, reduces the pH of the get, allowing a better condensation of the silicate structure. Other parameters of importance are the OH-/Si ratio, the gelation time, and the synthesis temperature, influencing the solubility of the fumed silica, the condensation and the structuring of the silicate framework, and (lie pore diameter, respectively. The synthesis temperature has also a positive effect on the particle dimension, producing uniform grains of 4-5 mum. The optimized parameters (3-day gelation time followed by two consecutive post-synthesis hydrothermal treatments) give a highly reproducible synthesis, with a high silica yield (70-75%). The synthesized samples have been characterized using XRD, N-2 sorption isotherms, SEM, and infrared (DRIFT) measurements.