Microstructures of Mesophases, MCM-41 and Gibbsite Formed in CTAB/Water Systemwith Negatively Charged Silicate and Aluminate Species

• Main Authors: Yauh-Yarng Fahn, 范樂陽
• Other Authors: Pouyan Shen
• Format: Others
• Language: en_US
• Online Access: http://ndltd.ncl.edu.tw/handle/74918252226015055397

博士 === 國立中山大學 === 材料科學研究所 === 88 === Abstract Cationic surfactant cetyltrimethyl ammonium brombide (CTAB) was used as template to synthesize aluminosilicate MCM-41 (plane group P6m, hexagonal array of uniform mesopores derived from crystalline colloidal array (CCA)) molecular sieve and lamellar phases in colloidal solution with negatively charged silicate and aluminate species at pH=10. In the first part, sodium aluminate (up to 0.25 molar ratio) and sodium silicate were the precursor of Al and Si, respectively; in the second part, kaolinite (Al4[Si4O10](OH)8) was used instead. The hydrothermally reacted (100oC in a Teflon sealed container) materials subject to room temperature drying, calcination (540oC) or ethanol rinsing were studied by X-ray diffraction (XRD), optical microscopy under plane polarized light or transmission electron microscopy (TEM) with emphasis on the microstructures and formation mechanism of mesophases, MCM-41, and gibbsite (Al(OH)3) at a relatively low CTAB/water ratio and the effect of Al/Si ratio on micelle interspacing in terms of micelle size and aluminosilicate wall thickness. In the first part, both calcination and ethanol rinsing were shown to remove the template successfully. The resultant MCM-41 particulates were more or less coalesced and the elongated ones tended to be folded. The hexagonal MCM-41 has a tube interspacing 4.5-5.4 nm and tube wall thickness 1.9-3.7 nm, both generally increasing with the increase of sodium aluminate/sodium silicate ratio up to 0.1 molar. ratio. The tube diameter also increased slightly presumably because of competitive electrostatic coordination of the hydrophilic head of CTAB with the negatively charged aluminate (AlO2-) vs. silicate (SiO4-4) species stable at pH=10. The MCM-41 particulates have well-developed {100} faces, the close-packed plane of 2-D hexagonal structure, and rigid amorphous tube walls, suggesting interface-controlled assembly of rod-like CTAB micelles with their polar head already incorporated with aluminosilicate. Tubules-within-a-tubule were corrugated and folded when extended beyond a certain persistence length, typically 1 mm. Spherical particles with disordered mesopores (typically ca. 4 nm in mesh size) due to entanglement of micelles under semi-dilute condition were also formed. In the second part, the CTAB-saturated solution at pH=10 was separated from mud-like kaolinite to form translucent hydrous gel. Upon drying on a glass slide at room temperature, the gel became whitish because of the following crystallization events. First, whitish gibbsite nucleated preferentially at gel/air and gel/glass interface to form spherulites. The spiral and lateral growth of plate-like gibbsite crystal with {100} and {110} growth front was rapid enough to entrap solution droplets. Subsequently, dendritic lamellar (basal spacing ca. 2.6 nm according to XRD and TEM observations) mesophase exhibiting length fast and clino-extinction with extinction angle 42o was formed via 2-D growth near the edge of the drying gel. This lamellar phase was incorporated with aluminate according to TEM-EDX analysis. Finally, explosive nucleation and dendritic growth of isotropic phase concluded the crystallization. This final event involved surface nucleation as best exhibited at the droplets trapped in gibbsite host. Upon calcination to remove the surfactant, the aluminosilicate MCM-41 retained while aluminate-incorporated lamellar mesophase disappeared as indicated by XRD. In an additional experiment to understand the crystallization behavior of CTAB in drying water, we found that the plate-like and then dendritic monoclinic lamellar phase (space group P21/c) with optical extinction angle of 37o was formed as the growth dimensionality decreased toward the edge of the gel. This nucleation and growth process is analogous to the CTAB/water system with negatively charged silicate and aluminate species derived from kaolinite at pH=10.