Synthesis Of Highly Ordered Mesoporous Silicas Using Lamellar Liquid Crystals As Templates

Mesoporous materials have been used in the fields of adsorption-separation,catalysis,biology,chromatographic analysis and drug transportation due in large part to their uniform,adjustable pore structure and large specific surface area since Mobil company synthesized mesoporous M41S materials.The superior characteristics of mesoporous materials are closely related to its various microscopic pore structures and macroscopic morphology.This paper describes a new and effective method for the synthesis of mesoporous silica.The method uses a lamellar liquid crystal(LC)formed by self-assembly of mixed anionic and cationic surfactants:cetyltrimethylammonium bromide(CTAB)and sodium dodecyl sulfate(SDS)as a template.The formation region of LC was determined by investigating the effect of temperature on LC structure and the phase behavior of the system at different surfactant concentrations and ratios.In addition,a clear "cross flower”structure of lamellar LCs was obtained by observation with a polarizing electron microscope.The influences of synthesis temperatures,surfactant concentrations,pH,relative ratios of CTAB and water to silica ratios(molar ratios of solvent to silicon source,R)on the synthesis of mesoporous materials were also investigated and characterized by electron microscopy and N2 adsorption/desorption tests on the basis of lamellar LC.The results show that:the temperature affects the speed of the thermal motion of surfactant molecules,too high or too low temperatures are not conducive to the formation of LC structure.In the temperature range of LC,the higher the temperature of the synthesized sample,the larger the specific surface area of the sample.The temperature of 50℃ is the optimum temperature for the formation of LC and layered silica structures.The LC becomes denser and the degree of silica is increased with the increase of surfactant concentration.Hydrochloric acid was used as catalyst in the study which affected the hydrolysis and condensation of silicon sources.When the pH value of hydrochloric acid decreases,the acidity,the specific surface area of the sample and the order gradually increase.However,if the acidity is too strong,the structure of the template will be compressed,resulting in decrease of the pore size.Therefore,the pH value of 2 is the best catalytic condition.Changes in the amount of CTAB affect the formation of microstructure in a mixed system.The specific surface area of mesoporous silica increases initially and then decreases with the increase of CTAB in the excess CTAB region,reaching a maximum of about 800 m2/g when the CTAB ratio is 72%.This is the result of changes in the microstructure of the system from micelles to LCs to micelles.The ratios of water and silicon are an important factor affecting the properties of silica.The right amount of silicon source(R is 50 to70)is not only beneficial to its own condensation,but also can make the template structure more stable,thus forming a highly ordered silica.A small or excessive amount of silicon source not only reduces the amount of silicon dioxide,but also destroys the LC structure.In addition,SEM and TEM images show that the synthesized sample has a obvious layered structure.