A Reliable Synthesis of Cubic Mesoporous MCM-48 Molecular Sieve

Abstract

Powder X-ray diffraction (XRD), 29Si magic-angle-spinning (MAS) NMR spectroscopy, and transmission electron microscopy (TEM) as well as N2 adsorption have been employed to study the formation of various mesophases that lead to the synthesis of cubic mesoporous MCM-48 molecular sieve. A typical synthesis is performed at 373 K, pH = 11.8 using tetraethyl orthosilicate as the silicon source and cetyltrimethylammonium bromide (CTAB) as the structure-directing agent with a molar gel composition of 1:0.23:0.55:112SiO2/Na2O/CTAB/H2O. XRD shows that a disordered tubular mesophase (H1) rapidly forms and then transforms to a layered phase (L1) upon heating at 373 K for 5−10 h. After the hydrothermal treatment continues for 72 h, the layered phase (L1) gradually transforms to a cubic MCM-48 mesophase (V), which is accompanied by a slight pH increase of about 0.2 units. Prolonged hydrothermal treatment for over 120 h results in further structural transformation from the cubic mesophase V to a second layered phase (L2). 29Si MAS NMR reveals that the L2 layered phase has a more regular atomic arrangement than the other three mesophases. However, the silica condensation increases monotonicly in the order H1 → L1 → V → L2 with hydrothermal treatment time. The cubic MCM-48 mesophase is not completely stable under hydrothermal synthesis conditions since it converts to the L2 phase. This may account for the poor repeatability of prior syntheses of MCM-48 material. We show that cubic MCM-48 can be stabilized either by addition of acetic acid to maintain a constant gel pH or by selection of the reaction time to prevent further mesophase transformation. TEM and N2 adsorption data show well-defined three-dimensional channels for the cubic MCM-48.