Characterization of MCM-48 silicas with tailored pore sizes synthesized via a highly efficient procedure

Abstract

MCM-48 silicas have been prepared via a high-yield synthesis procedure using mixtures of cationic alkyltrimethylammonium surfactants (alkyl chain length from 12 to 20 carbon atoms) and neutral cosurfactants. This is the first reported successful synthesis of MCM-48 using eicosyltrimethylammonium surfactant. The samples, especially those synthesized using cationic surfactants with cetyl, octyl, and eicosyl chains, were found to exhibit a high degree of structural ordering, which manifested itself in very well-resolved X-ray diffraction (XRD) patterns, and remarkably narrow capillary condensation steps after the template removal. The latter could be achieved not only via calcination, but also via ethanol/HCl washing. The unit-cell size and pore size of MCM-48 were found to be determined by the alkyl chain length of the cationic surfactant used. The calcined MCM-48 samples had BET specific surface areas of about 1100 m(2) g(-1), large primary mesopore volumes (up to 1.15 cm(3) g(-1)), XRD d(211) interplanar spacings from 3.6 to 4.5 nm, and average primary mesopore sizes covering the range from 3.5 up to as large as 4.5 nm. The percentage of template in the as-synthesized MCM-48 samples assessed using thermogravimetry (TGA) was found to be correlated with the primary mesopore volume of the calcined materials. The as-synthesized MCM-48 had inaccessible primary mesoporosity and its external surface exhibited relatively weak interactions with nitrogen adsorbate, similar to alkylsilyl-modified silicas and contrary to bare silicas. This indicated that the external surface was covered with surfactant ions attached to the silicate framework, as previously demonstrated for MCM-41. The differences between nitrogen interactions with surfactant-silicate and silica surfaces provided a possibility for detecting the residual surfactant left after the ethanol/HCl washing using nitrogen adsorption.