Synthesis And Characterization Of Ordered Mesoporous Silica, Aluminosilicate And Titanosilicate Materials

Ordered mesoporous materials possess regular pore structure and tunable pore size of 1.5-50nm, break the pore size constraint (<1.5 nm) of microporous zeolite. The M41S materials brought a new approach in the synthesis of molecular sieves, which used self-assembled molecular aggregates as the structure-directing agents where as the synthesis of molecular was usually used single molecular as template. Beside this, mesoporous materials are currently under intense investigation with respect to their utilities as industrial catalysts, adsorbents, biological catalysts, photocatalysis, environmental protection, drug carriers, optics, electrode materials and host-guest chemistry.Using a trimeric surfactant C_14-2-14-2-14 as template, highly ordered mesoporous silica and mesoporous aluminosilicate and mesoporous titanosilicate were synthesized. The effect of different experimental conditions on the mesostructure, surface properties and morphologies of samples were investigated by small angle X-ray diffraction (SXRD), N₂ adsorption-desorption technique , scanning electron microscopy (SEM) and transmission electron microscopy (TEM) , ²⁷Al NMR, FTIR and Temperature Programmed Analysis (TGA).Highly ordered two-dimensional hexagonal structure mesoporous silica was synthesized using trimeric surfactant C_14-2-14-2-14 as template. HRTEM revealed that the presence of highly ordered mesopores. The ordering of mesostructure was improved with the increase of surfactant concentration. The amount of silicate (x value) in synthesis gels had great influence on the mesostructure and surface properties of mesoporous silica. The ordering of mesostructure, the BET surface area and the total pore volume was improved with the decrease of the amount of silicate. When x value was no larger than 25, the samples possessed highly ordering of mesostructure. SEM showed that the sample was mainly consisted by aggregated particles and the diameter of the particle decreased with the increase of the molar ratio of silicate to surfactant. The amount of ethyl acetate that added to control the hydrolysis and aggregation of silicate also had great influence on the. synthesis of mesoporous silica. When the amount of ethyl acetate was equal to or larger a little than the stoichiometric amount, the highly ordering of mesostructure was maintained. However, the large amount of ethyl acetate added in the synthesis gel made the ordering of mesostructure decrease. Mesoporous silica materials were synthesized under different hydrothermal temperatures. When hydrothermal temperature was low (30℃ and 50℃), the highly ordered mesoporous silica was obtained. Further increasing hydrothermal temperature resulted in the lost of mesostructure ordering. The BET surface area, total pore volume and pore size of these samples showed the same tendency, also decreased with the increase of hydrothermal temperature. These indicated that the low hydrothermal temperature favored the formation of highly ordered mesoporous silica. Addition of small organic molecular trimethylbenzene (TMB) into synthesis gels resulted in the expansion of pore size, and this didn't change the ordering of mesostructure, which indicated that C_14-2-14-2-14 had great solubilization ability for TMB, the adding of TMB didn't decrease the assemble ability of surfactant to form ordered mesostructure. Three template agents were used to synthesize mesoporous silica in order to investigate the effect of the molecular structure of templating agent on mesostructure. The samples templated by TTAB and C_14-2-14-2-14 showed 2D hexagonal structure, however, as for the sample templated by C_14-2-14 showed cubic structure with Pm_3|-n symmetry. The assemble ability of TTAB and C_14-2-14-2-14 to assemble inorganic species to form mesostructure was further investigated. In the similar synthesis conditions, C_14-2-14-2-14 can assemble more silicates to form ordered mesostructure, which showed higher assemble ability than TTAB. The different of assemble ability is arise from the different molecular structure.Using trimeric surfactant C_14-2-14-2-14 as templating agent, the low cost and facile inorganic species sodium aluminate and sodium silicate as inorganic precursors, highly ordered 2D hexagonal aluminosilicate MCM-41 with high Al content were successfully synthesized. The mesostructure, surface properties, morphology and acid properties were studied in detail. The ordering of mesostructure was increased with the decrease of the molar ratio of Si/Al. All samples showed type IV adsorption isotherms except for the sample with Si/Al=l. When Si/Al ratio increased, the BET surface area and total pore volume increased. The results of XRD and N₂ adsorption-desorption technique indicated that when the Si/Al ratio equal to 2.0, the sample showed relative high mesostructure ordering. SEM showed the sample contained small particles with irregular shape and with a size of Ca. 0.5μm. HRTEM showed the existing of highly ordered pore structure in the sample, which coincided with the results of XRD and N₂ adsorption technique. The ²⁷Al MAS NMR showed that the relative content of the four-coordinated aluminum was decreased with the increase of Si/Al ratio, meanwhile the relative content of six-coordinated aluminum increased. By the calculation of the Si/Al^? molar ratio in the bulk phase, the total amount of four-coordinated aluminum was increased with the decrease of Si/Al ratio. The FTIR spectra of the pyridine adsorbed samples indicated the presence of Bronsted acid and Lewis acid. The Bronsted acid content of the samples were further measured quantitatively by thermally programmed desorption of cyclohexylamine. The results showed the samples had high Bronsted acid content, and the Bronsted acid content increased with the increase of the amount of four-coordinated aluminum. These results above indicated that highly ordered mesoporous aluminosilicate with high Al content could be obtained using the trimeric surfactant with high assemble ability as template. The samples with high Bronsted acid content are helped to improve the catalytic properties of this kind of materials.Ordered mesoporous titanosilicate was synthesized using trimeric surfactant C_14.2-14-2-14 as templating agent. The results of XRD, N₂ adsorption and HRTEM proved the presence of the lamellar and 2D hexagonal structure in the sample. The sample was composed of small particles about several hundred nanometers in size and the pores in these particles were arranged like concentric rings. The adding of acetyl acetone can slow down the hydrolysis rate of titanium species, which is favored for titanium species to be assembled by template to form mesostructure. The samples with different Si/Ti molar ratios were synthesized successfully. The XRD results showed that the mesostructure of resulting sample of mesoporous silica without Ti species belonged to 2D hexagonal symmetry, whereas for the mesoporous titanosilicate synthesized under same condition had lamellar structure in sample, so that the addition of Ti induced the phase transition of the samples. With the increase of the molar ratio of Si/Ti, the ordering of mesostructure, the BET surface area and total pore volume increased. The samples showed type IV adsorption isotherms except for the sample with Si/Ti=1. As for the sample with Si/Ti=5, the XRD pattern showed one clear diffraction peak and the N₂ adsorption isotherm showed a clear sorption step in the low relative pressure and the BET surface area was higher than 400 m²/g, indicating that when Si/Ti was as low as 5, the material already had ordered mesostructure. These results above indicated that ordered mesoporous titanosilicate with different Si/Ti molar ratio could be obtained using trimeric surfactant C_14-2-14-2-14 as templating agent and the Si/Ti ratio could be as low as 5 without the losing of mesostructure ordering. The synthesis of ordered mesoporous high Ti content titanosilicate is very important to the catalytic application of this kind of materials.