Synthesis and characterization of europium-doped ordered mesoporous silicas

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Abstract
Silica mesoporous molecular sieves doped with europium were synthesized using Cab-O-Sil M5 silica and europium chloride in the presence of cetyltrimethylammonium bromide surfactant. The materials obtained with lower Eu content (Si∶Eu molar ratio of 100) in the synthesis gel exhibited two-dimensional hexagonally ordered structure of the MCM-41 type, whereas larger Eu contents resulted in lower size of hexagonally ordered porous domains (for Si∶Eu = 33) or in disordered porous structures (for Si∶Eu = 20). The pore size increased from 3.4 to 4.0 nm as the amount of Eu in the synthesis mixture increased. Pore size distributions (PSDs) for Eu-doped silicas were narrower than the PSDs of the pure-silica MCM-41 material synthesized under the same conditions. A decrease in the specific surface area and primary mesopore volume was observed for higher Eu loadings. The volume of secondary (interparticle) pores increased as the content of Eu in the synthesis gel increased, which is consistent with the transmission electron microscopy images that provided an indication of the decrease in the particle size, although the latter was not uniform throughout the materials under study. Energy dispersive X-ray fluorescence (EDX) data indicated that europium is indeed present in the structure of the materials studied, although its content tended to be somewhat lower than that in the synthesis gel and its distribution within the materials appeared to be non-uniform. The synthesis procedure reported herein was used to synthesize silicas doped with other lanthanides, and for given lanthanide loadings, the resultant materials exhibited porous structures similar to those of the Eu-doped samples.

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