Optimization of experimental conditions for the synthesis of Si-MCM-41 molecular sieves using different methods and silica sources

Abstract

The present work evaluates the effects of the silica/surfactant and solvent/co-solvent molar ratios, for synthesis under ambient conditions (using commercial tetraethyl orthosilicate (TEOS)), and the temperature and aging time, for hydrothermal synthesis (using silica from rice husk ash (RHS), an agro-industrial waste), in order to identify the best conditions for synthesis of Si-MCM-41. The porous materials obtained were characterized using N2 physisorption, XRD, SEM/EDS, FTIR, and TEM. The specific surface areas ranged from 213 to 1357 m2 g−1, the pore volumes were from 0.06 to 0.81 cm3 g−1, and the mean pore diameters were from 15.65 to 17.79 Å. The best results were obtained using a co-solvent/solvent molar ratio of 0.28, in the synthesis under ambient conditions, and 160 °C and aging for 3 days, using the hydrothermal methodology. Under these conditions, the materials presented better formation of mesopores, characteristic of Si-MCM-41, with specific surface areas of 575 and 551 m2 g−1, pore volumes of 0.76 and 0.73 cm3 g−1, and pore diameters of 17.79 and 17.85 Å, for the syntheses performed under ambient and hydrothermal conditions, respectively. The results indicated that despite using different synthesis methods and silica sources, the two materials presented very similar characteristics, evidencing the feasibility of using an inexpensive waste material, such as RHS, as silica source in the synthesis of these molecular sieves, once the synthesis conditions have been satisfactorily adjusted.