Synthesis Of MCM-41 Materials Research Articles

Effect of cationic template on the adsorption of aromatic compounds in MCM-41

The effect of cationic template on the adsorption of aromatic compounds in MCM-41 was investigated in the present work. Various MCM-41 samples were prepared through controlling template removal during the synthesis of MCM-41 materials in which cationic surfactant was used as a template. The properties of synthesized samples were characterized with X-ray diffraction (XRD), nitrogen adsorption, FTIR and thermogravimetric analysis etc. The adsorption equilibriums of toluene, cumene and water on MCM-41 samples were measured using a digital microbalance. Compared with parent MCM-41 with template removed completely (denoted as MCM-41), the samples with template partially removed (denoted as C-MCM-41) exhibited moderate adsorption capacity for aromatic compounds. However, the adsorption equilibrium of water showed that the hydrophobicity on the surface of C-MCM-41 was significantly enhanced. The combination of moderate adsorption capacity for aromatics and hydrophobicity for water is desirable for the applications including removal of VOCs. It was found that the adsorption behaviors of aromatic compounds and water in various MCM-41 samples were governed by both porosity and cationic spots generated by cationic templates; and the former played a more important role than the latter when aromatics were adsorbed. The porosity and cation density of the MCM-41 material can be tailored by controlling the removal of the cationic template from the pore structure. The influence of template extraction methods on the structure and adsorption properties of MCM-41 materials has also been addressed.

Removal of the Template Molecules from MCM-41 with Supercritical Fluid in a Flow Apparatus

Supercritical CO2 modified with a methanol/dichloromethane mixture was used to quantitatively remove the template molecules (cetyltrimethylammonium bromide) from the pores of as-synthesized pure siliceous MCM-41 in a flow apparatus. The resulting mesoporous MCM-41 materials, characterized by powder X-ray diffraction, nitrogen adsorption, and high-resolution transmission electron microscopy, exhibited higher surface areas, larger pores, and narrower pore size distributions than those obtained by conventional calcination at high temperature. The removed template retained its structures and properties and could be reused in the synthesis of MCM-41 materials.

Characterization of Pore Structure and Coordination of Titanium in TiO2 and SiO2–TiO2 Sol-Pillared Clays

Titania sol-pillared clay (TiO2 PILC) and silica–titania sol-pillared clay (SiO2–TiO2 PILC) were synthesized by the sol–gel method. Supercritical drying (SCD) and treatment with quaternary ammonium surfactants were used to tailor the pore structure of the resulting clay. It was found that SCD approach increased the external surface area of the PILCs dramatically and that treatment with surfactants could be used to tailor pore size because the mesopore formation in the galleries between the clay layers follows the templating mechanism as observed in the synthesis of MCM-41 materials. Highly mesoporous solids were thus obtained. In calcined TiO2 PILC, ultrafine crystallites in anatase phase, which are active for photocatalytic oxidation of organics, were observed. In SiO2–TiO2 PILCs and their derivatives, titanium was highly dispersed in the matrix of silica and no crystal phase was observed. The highly dispersed titanium sites are good catalytic centers for selective oxidation of organic compounds.

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

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-

Control of mesostructure and morphology of surfactant-templated silica in a mixed surfactant system

Mixed cationic surfactants were used in the synthesis of MCM-41 materials. Synthetic conditions for controlling the nanometer-scale channels size and micrometer-scale morphology in mixed alkyltrimethylammonium halides (CnTMAX) systems (n=8–18; X=Cl or Br) were studied. The d100 spacing of MCM-41 synthesized using the C18TMABr–C14TMABr system decreases linearly with increase in the C14TMABr molar fraction. Such linearity was also reproduced in other systems with some deviation at high molar fractions of short chain length surfactants (n<14). The morphology in mixed CnTMABr system can be changed from particle form to a tubules-within-tubule (TWT) structure by adjusting the average surfactant chain length. It was found that the optimum average chain length for preparing a TWT structure is 15–16 for the mixed CnTMABr system and 16–17 for the mixed CnTMACl system. A high yield TWT morphology also can be prepared by adding the cosolvent acetone to a suitable value of the hydrophobic–lipophilic balance (HLB). The change in morphology of the products with respect to average chain length, counter ions and cosolvents are explained by the principle of hydrophilic–hydrophobic (lipophilic) balance (HLB).

Synthesis of MCM-41 materials: an in situ FTIR study

A new approach has been developed to monitor the synthesis of MCM-41 mesoporous materials based on in situ ATR FTIR measurements. The transformation between Q 4 and Q 3 species, which is an essential step in the synthesis and structural development of MCM-41 materials, can be followed using this technique.

MCM-41 type materials with low Si/Al ratios

The synthesis of MCM-41 type materials with high amounts of tetrahedral aluminum is reported. Transmission electron microscopy with EDX elemental analysis shows that when using a tetraalkylammonium base in the synthesis of MCM-41 materials Si/Al ratios as low as 12 are possible. In all cases the MCM-41 materials contain an aluminum-rich dense phase in which the aluminum is also tetrahedrally coordinated. This dense phase consumes most of the aluminum of the synthesis mixture.