Synthesis Of Mesoporous Silica Using CO₂/N₂ Switchable Surfactants As Soft Templates

Mesoporous silica is widely applied in fields of chemical, biomedical and environment protection. Template method is used widely to synthesize mesoporous silica, while mesoporous structure can be obtained only when the template is removed from mesopore.Calcination, solvent extraction and oxidation is commonly used to remove templates.However there are some drawbacks about these template removing method, for example calcination brings collapsed mesoporous structure, solvent extraction needs plenty of solvent and oxidation needs strict condition etc. In this thesis, CO2/N2 switchable surfactants which can transform between surface active and surface inactive via bubbling CO2 and N2 were used as templates to synthesize mesoporous silica. Compared to conventional template method, N2 was bubbled into heated mixture to switch off surfactants when reaction was completed,resulting collapse of micelles in mesopores. At last, water and acetone were used to wash out inactive surfactants to obtain mesoporous structure.Firstly, CO2/N2 switchable surfactant N’-dodecyl-N,N-dimethylacetamidinium bicarbonate(DDAB) was used as template to synthesize surfactant-silica complex. 24% of the template was removed via bubbling N2 into the heated complex and ultrawashing process.51.7% of template was removed after that ethylenediamine tetraacetic acid disodium salt(Na2EDTA) was added into reaction. At last, mesoporous silica with uniform pore size(2nm), long-range ordered mesopore, large surface area(826.2 m2/g) and high pore volume(0.49m3/g) was obtained.Secondly, hollow-structured mesoporous silica with narrow pore size distribution,ordered mesopore, large surface area(992.2m2/g) and high pore volume(0.86cm3/g) was synthesized using mixed surfactants of switchable surfactant DDAB and anionic surfactant sodium laurate(SL) as template. The mixed surfactants were also switchable and 54.4% of the template was removed via switching off mixed surfactant by bubbling N2 at 70 ℃ and heated ultrasonic washing.Both switchable surfactant DDAB and switchable mixed surfactants(DDAB and SL)were used to obtain mesoporous structure, however there is still almost half of templates remain in mesopores. Therefore switchable surfactant dedecyl dimethyl tertiary amine(DDTA)was used to synthesize mesoporous silica with ordered mesopore, large surface area(565.8m2/g) and pore volume(0.4cm3/g) under CO2 atmosphere because DDTA is much easier to be switched off. There was 2 wt% of templates in mesoporous silica via bubbling N2 at 70 ℃ and ultrasonic washing at room temperature when reaction was completed.Compared to conventional template removing method, removing template via switching off switchable DDTA is not harmful to mesoporous structrue and silicon hydroxyl, which is also much easier and more environmental friendly.