Comprehensive characterization of highly ordered MCM-41 silicas using nitrogen adsorption, thermogravimetry, X-ray diffraction and transmission electron microscopy

Highly ordered MCM-41 silicas were synthesized using a new procedure and th eir structures were thoroughly characterized. Using the new synthesis metho d and the previously reported similar one, the pore diameter of MCM-41 can be tailored from 3.1 to 4.9 mn with about 0.35 nm increments and reproducib ility usually better than +/-0.1 nm using surfactants of different chain le ngth. The surfactant content and structural parameters of MCM-41 synthesize d using the same single surfactant or surfactant mixture were highly simila r for different silica:surfactant molar ratios in the synthesis mixture. Mo reover, the silica:surfactant molar ratio in as-synthesized MCM-41 was quit e independent of the chain length of the surfactant used and approximately equal to 8. This may be related to the well-defined synthesis conditions im posed by the pH adjustment procedure employed. The results indicate that th e silica: surfactant ratios in starting mixtures suitable for the synthesis of high-quality MCM-41 under the present synthesis conditions should not e xceed about 8 to avoid contamination with amorphous silica. (C) 2001 Elsevi er Science B.V. All rights reserved.