Mesoporous molecular sieve MCM-41 synthesis from fluoride media

Abstract

A study of the synthesis of MCM-41 mesoporous molecular sieves in fluoride media, having no alkaline metal ions, was performed by changing the gel composition and crystallization temperature and time. X-ray diffraction and nitrogen adsorption analyses showed that highly ordered MCM-41 samples were obtained from gels with a NH4OH/SiO2 molar ratio in the 3.25-4.3 range (room temperature synthesis) or in the 4.3-20 range (24 hours at 373 K). During calcination, unit cell shrinkage, caused by high temperature polycondensation of the SiOH groups, was observed for all samples. The samples synthesized at high temperature (373 K) or using low pH gels (7.5) underwent lower unit cell shrinkage than those obtained at room temperature or high pH (9.0), indicating that the former samples had lower SiOH groups content than the latter. These highly-ordered samples showed large surface area (ca. 1100 m²/g) and pore volume (ca. 0.80 cm³/g), also presenting a narrow pore size distribution. Due to higher silicate polycondensation and a thicker pore wall, the samples synthesized at 373 K were more hydrothermally stable than those obtained at room temperature.