A Reliable Synthesis of Cubic Mesoporous MCM-48 Molecular Sieve

Abstract

Powder X-ray diffraction (XRD), 29 Si magic-angle-spinning (MAS) NMR spectroscopy, and transmission electron microscopy (TEM) as well as N 2 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 1:0.23:0.55:112SiO 2 /Na 2 O/CTAB/H 2 O. XRD shows that a disordered tubular mesophase (H 1 ) rapidly forms and then transforms to a layered phase (L 1 ) upon heating at 373 K for 5−10 h. After the hydrothermal treatment continues for 72 h, the layered phase (L 1 ) 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 (L 2 ). 29 Si MAS NMR reveals that the L 2 layered phase has a more regular atomic arrangement than the other three mesophases. However, the silica condensation increases monotonicly in the order H 1 → L 1 → V → L 2 with hydrothermal treatment time. The cubic MCM-48 mesophase is not completely stable under hydrothermal synthesis conditions since it converts to the L 2 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 N 2 adsorption data show well-defined three-dimensional channels for the cubic MCM-48.