Synthesis and Characterization of Periodic Mesoporous Organosilica Materials
Date
2006-11-15T07:32:33Z
Authors
Tshavhungwe, Alufelwi Maxwell
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Abstract
Periodic mesoporous organosilica (PMO) materials (consisting of ethane groups
in the framework) and bifunctional periodic mesoporous organosilica materials
(consisting of ethane groups in the framework and either glycidoxypropyl groups
or aminopropyl groups in the channels) were synthesized by the sol-gel method
under basic conditions. Ethanesilica materials were synthesized by condensation
of 1,2-bistrimethoxysilylethane (BTME) and by co-condensation of BTME with
tetraethylorthosilicate (TEOS). Bifunctional periodic mesoporous organosilica
materials were synthesized by the co-condensation of BTME with either 3-
glycidoxypropyltriethoxysilane (GPTS) or 3-aminopropyltriethoxysilane (APTS).
Cetyltrimethylammonium bromide was used as the structure-directing template.
Cobalt ion incorporated ethanesilica and modified ethanesilica materials were
synthesized in situ by adding cobalt nitrate to the reaction mixture. Cobalt was
also supported on ethanesilica materials and APTS-modified materials by using
the incipient wetness impregnation method.
Raman spectroscopy and diffuse reflectance infrared spectroscopy (DRIFTS)
results confirmed the formation of organosilica materials and showed that the
surfactant was removed by solvent extraction. Thermogravimetric analysis
(TGA) and differential thermal analysis (DTA) showed that the ethane portion of
the materials (originating from the bridging ethane group in BTME) only
decomposed at temperatures > 400 oC. These techniques also showed that the
surfactant is removed by solvent extraction. Cobalt ion incorporation was
confirmed by Raman spectroscopy and UV-vis diffuse reflectance spectroscopy.
Powder powder X-ray diffraction (XRD) and nitrogen adsorption data indicated
that the mesophase and textural properties of the materials are dependent on the
reaction conditions (i.e. ageing duration, ageing temperature, amount of silica
precursor(s), amount of water and amount of base (NH4OH)). The periodicity of
the materials was indicated by the presence of low angle diffraction peaks in powder X-ray diffraction patterns. Cubic and hexagonal mesophases were
identified using powder X-ray diffraction. When solvent extraction is prolonged,
the BET surface area and the pore volume increase, while the average pore
diameter decreases. Materials with more dominant XRD structural features and
larger d values, higher surface areas, lower pore volumes and average pore
diameters are obtained when low ageing temperatures are used.
For samples prepared from a mixture of BTME and TEOS at a given temperature,
the surface area was found to increase with increasing amount of TEOS added.
This trend was observed for materials with and without cobalt. Type IV
isotherms, typical of mesoporous materials, were obtained for ethanesilica and
modified ethanesilica materials prepared without cobalt.
For cobalt incorporated periodic mesoporous ethanesilica materials, the XRD
lattice parameter (d100) increased whereas surface area and pore volume decreased
with increasing cobalt loading.
Nitrogen gas adsorption on samples with varying ratios of BTME:GPTS or
BTME:APTS revealed that increasing the amount of GPTS or APTS affects pore
size, surface area and pore volume as well as shapes of the isotherms and
hysteresis loops. The hysteresis loops of the Type IV isotherms obtained for
GPTS-modified ethane silica materials (without cobalt) change from Type H3 to
Type H4. There is a tendency for pore sizes to change from mesopore to
micropore when the amount of GPTS is increased. Isotherms of cobalt
incorporated GPTS-modified ethane silica materials changed from Type IV to
Type I. The surface area, pore volume and pore diameter decreased with
increasing loading of GPTS or APTS as well as after cobalt incorporation.
Description
Student Number : 0107507J -
PhD thesis -
School of Chemistry -
Faculty of Science
Keywords
periodic mesoporous organosilica, ethanesilica, cobalt incorporation