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.