Formation Process of MCM-41 Precursor and Porous Texture of MCM-41

Abstract

The formation process of the MCM-41 precursors (silicate/surfactant complex) was investigated on the basis of the pH titration curves of Na4SiO4 in the presence of [C16H33N(CH3)3]Cl. Measurements of the pH titration curves were carried out using the computer-controlled gravimetric titrator constructed in our laboratory. The white precipitate (MCM-41 precursor) was abruptly formed at pH 11.1 (298 K) and at pH 9.9 (343 K). Formation of the MCM-41 precursor can be explained by coagulation of the rod-like micelle colloids whose surface is covered by the condensed silicate anions of (HSiO3)nn−. The porous texture of the MCM-41 samples whose precursors were synthesized under different conditions was analyzed on the basis of the adsorption isotherms of nitrogen at 77 K. It has been shown that the MCM-41 sample whose precursor was prepared at pH 9.9 and 343 K shows one sharp peak (rp = 1.65 nm) in the pore size distribution curve, but the MCM-41 samples whose precursors were prepared at pH 6.5–5.0 and 343 K give two peaks (rp = 1.66 nm and rp = 2.12–2.36 nm). The appearance of the second peak (rp = 2.12–2.36 nm) has been considered to be in connection with the destruction of the MCM-41 precursor into small fragments in acidic medium.