Preparation And Mechanism Of The Mesoporous Silica In Situ Thermochemical Study

Ordered mesoporous materials are pores solids that pore channels with diameter in the range of 2～50nm are arrayed orderly in three dimensions,which templated by self-assembly system of surfactant.Mesoporous materials are widely used in many fields such as catalysis and cataloger carrier,adsorption and separation,environment science,host-guest chemistry, biomimetic and else functional materials,because of their bigger and uniform pores,high surface areas and comparatively favorable stability.The process of mesoporous materials is very complex but the study about the mechanism of the synthesis of mesoporous silicon dioxide is very less.And its interpretation isn't comprehensive enough.In situ studies of mechanism of synthesis mesoporous materials have not been reported at home and abroad.In this paper we study the mechanism ofmesoporous silica by microcalorimetry.According to the power-time(p-t)curve,we synthesis the intermediate products and characterize them by XRD,TEM,TG and N₂ adsorption-desorptionand techniques then Using these results to study the mechanism of the silica.At the same time,we tried new synthetic routes -using supramolecular self-assembled template(CTAB/PAM,β-CD/DTAB &β-CD/TTAB)to synthesis ordered mesoporous silica(MCM-41).The obtained results were as follows:1.Using the self-assembly ofβ-Cyclodextrin(β-CD)and polyacrylamide(PAM)as structure-directing agent,high-quality ordered silicas were prepared under the condition of 50℃,water-bath 4 h.The addition of a small amount of PAM is conductive to improve the order of the sample.But when the concentration of PAM increased to a certain concentration, a reunion phenomenon occurred.2.Using respectively the self-assembly of Myristyltrimethylmethylammonium bromide(TTAB),β-CD and Dodecyltrimethylammonium bromide(DTAB),β-CD as structure-directing agents, mesoporous silicas were prepared under the condition of 65℃,water-bath 4 h.Study the packages of different length of the alkyl chain andβ-CD.Come to the following conclusions: DTAB withβ-CD form 1:1 complex but the TTAB andβ-CD can form 1:1 and 1:2 two kinds of complex.And the addition ofβ-CD is conductive to form spherical particles.When the concentration ofβ-CD increased to a certain concentration,the morphology of Samples become amorphous.3.Synthesis of mesoporous materials is done through successive steps,and different process release different heat.with the increasing of time,the morphology of sample changes a little, but the specific surface area,wall thickness,diameter have great change.According to p-t curves and analysis the results of SXRD,N2 adsorption - desorption,SEM,TEM of intermediate product.We believe that the synthesis of the mesoporous silica contains at least two processes. First,inorganic/surfactant complexes are formed which spontaneously pack into an ordered arrangement.In this process,the main interaction between cationic surfactant and silicate anions was electrostatic interaction.Second,the silicate further hydrolysis and condensation.We think that the "phase separation model" may be used in basic solution.The growth of inorganic oligomers may results in a new liquid phase rich in oligomer and surfactant and giving rise to micelle.So the d_BJHmay changed.However,this change is not inevitable.It was controlled by the charge density of the inorganic layer and the change match of inorganic and organic surfantant.The main innovation of this thesis:using micro-calorimeter measurement methods to obtain the p-t curves and according to the curve to prepared intermediate product.The intermediate products were characterized by small angle X-ray diffraction(SXRD),N2 adsorption-desorption and scanning electron microscope(SEM),thermo gravimetric(TG)techniques.The mechanism of the synthesis of mesoporous silica was study according to these results.The characteristic of this method is that we can obtain the change of energy in the process without affecting the reaction system.