Showing papers by "Tewodros Asefa published in 2002"

Periodic mesoporous organosilica with large cagelike pores

Abstract: Ordered mesoporous organosilica with large (10 nm) cagelike pores was synthesized using a triblock copolymer template and characterized using 29Si and 13C NMR spectroscopy, nitrogen adsorption, thermogravimetry, TEM, and SAXS

Synthesis and properties of 1,3,5-benzene periodic mesoporous organosilica (PMO): novel aromatic PMO with three point attachments and unique thermal transformations.

Abstract: A new aromatic periodic mesoporous organosilica material containing benzene functional groups that are symmetrically integrated with three silicon atoms in an organosilica mesoporous framework is reported. The material has a high surface area, well-ordered mesoporous structure and thermally stable framework aromatic groups. The functional aromatic moieties were observed to undergo sequential thermal transformation from a three to two and then to a one point attachment within the framework upon continuous thermolysis under air before eventually being converted to periodic mesoporous silica devoid of aromatic groups at high temperatures and longer pyrolysis times. The mesoporosity of the material was characterized by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), and nitrogen porosimetry, whereas the presence and transformation of the aromatic groups in the walls of the materials were characterized by solid-state NMR spectroscopy, mass spectrometry, and thermogravimetric analysis. ...

Metamorphosis of ordered mesopores to micropores: periodic silica with unprecedented loading of pendant reactive organic groups transforms to periodic microporous silica with tailorable pore size.

Abstract: Ordered porous silicas with unprecedented loadings of pendant vinyl groups have been synthesized via co-condensation of tetraethyl orthosilicate (TEOS) and triethoxyvinylsilane (TEVS) under basic conditions in the presence of cetyltrimethylammonium surfactant. The resulting organosilicate-surfactant composites exhibited at least one low-angle X-ray diffraction (XRD) peak up to the TEVS:TEOS molar ratio of 7:3 (70% TEVS loading) in the synthesis gel. The surfactant was removed from these composites without any structural collapse. Nitrogen adsorption provided strong evidence of the presence of uniformly sized pores and the lack of phase separation up to TEVS:TEOS ratios as high as 13:7 (65% TEVS loading), whereas 29Si MAS NMR and high-resolution thermogravimetry showed essentially quantitative incorporation of the organosilane. Thus, a hitherto unachieved loading level for pendant groups, considered by many to be impossible to achieve for stable organosilicas because of the expected framework connectivity ...

Synthesis and characterization of ordered mesoporous silicas with high loadings of methyl groups

Abstract: Ordered silicas with high loadings of pendant methyl groups were synthesized via cocondensation of tetraethyl orthosilicate and triethoxymethylsilane under basic conditions in the presence of cetyltrimethylammonium surfactant. The resulting materials were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), 29Si and 13C NMR spectroscopy, nitrogen adsorption and thermogravimetry. For lower loadings of the organosilane in the synthesis mixture, XRD patterns were characteristic of a 2-dimensional hexagonal structure. As seen from XRD, the degree of structural ordering diminished as the loading increased, but an XRD pattern with a single pronounced peak and TEM images characteristic of a 2-dimensional hexagonal structure were recorded even for a methyl-functionalized silica with about 50% of silicon atoms carrying the organic groups. The extent of organosilane incorporation was found to be nearly quantitative. The methyl groups were stable during surfactant removal, both via solvent extraction and pyrolysis at 573 K under nitrogen. The (100) interplanar spacing and the pore diameter were found to decrease from 3.45 to 3.12 nm, and from 2.9 to 2.3 nm, respectively, as the loading of the organosilane increased up to 43%, but no further decrease was observed when the loading increased to 50%. The BET specific surface area was relatively constant (720–800 m2 g−1) for loadings of 20–50%, whereas the primary pore volume decreased as the loading of organics increased. Nitrogen adsorption provided some evidence of phase separation into organosilicate-rich and silicate-rich phases for methyl group loadings of 33% or higher and particularly for the 50% loading. Thermogravimetric data suggested that the methyl groups are highly thermally stable in both nitrogen and air atmospheres.