Synthesis of mesoporous silica material with sodium hexafluorosilicate as silicon source under ultra-low surfactant concentration

Abstract

The discovery of novel inorganic porous materials of MCM-41 family, which are composed of regular hexagonal array of silica tubes with diameters in the range 20–100 A, has opened up new possibilities in the field of catalysis, adsorptions, advanced materials design, etc. [1, 2]. Since its first report in 1992, numbers of research works have been focused on the techniques of synthesis and application of MCM materials [3–7]. In the synthesis of MCM-41 materials, long carbon chain surfactants, e.g. trimethylammonium chloride (CnTAC), were usually employed as template to form the ordered inorganic-organic mesophases. The concentration of surfactants used in the synthesis of materials is one of the most considering factors because of its high expense and harmfulness to environment. Therefore, it’s certainly preferable to synthesize MCM-41 materials by using small amount of surfactants. Some researchers have strived to decrese the concentration of surfactant while keeping the well order of mesoporous silica [8–10]. However, to our knowledge, the concentration of surfactant in the aqueous solution was still higher than 0.5 wt% up to now [10]. In the classical method of synthesis of MCM-41, sodium silicate and silicon alkoxides were generally used as silicon source, and there are rare works employing other compounds of silicon [11, 12]. In the present work, we introduce an easy pathway to synthesize highly ordered mesoporous silica materials by using inexpensive sodium hexafluorosilicate (Na2SiF6) as silicon source under ultra-low surfactant concentration (about 0.2 wt%). In this work, a series of samples with different recipes or procedures were synthesized. In a typical process, an aqueous solution of cetyltrimethylammonium chloride (CTAC) and industrial purity Na2SiF6 were mixed for 30 min under 50 ◦C until solid Na2SiF6 dissolved completely. Then 28 wt% ammonia solution was added to the mixed solution and left the gel to stir for 10 min. After the reaction gels loaded static for another 2 h, the products were filtered, washed with distilled water and heated in a drying oven at 75 ◦C. The as-synthesized samples were calcined in air at 540 ◦C for 18 h to obtain mesoporous silica materials. The X-ray powder diffraction measurements of samples were carried out on a Rigaku RINT 2000 X-ray diffractometer with Cu Kα radiation. Thermogravimetric analysis (TGA) was carried out using a Seiko RTG-