Synthesis and Characterization of Nano-Aerogels

8.4. Conclusion

This research shows that the M-acetate group on the colloidal particles is CO₂-philic, which is similar to C-F and carbonyl groups.^{289, 301, 302, 323} According to the FTIR study on the CO₂ impregnated into the aerogels, it can be concluded that a LA-LB interaction exists between CO₂ and the M-acetate groups. The electronic structure of the M-acetate shows the possible association modes, even though the exact modes cannot be discerned through the experimental results. The interactions have also been studied semi-quantitatively using a solubility parameter approach. The results show that the acetate group can decrease the solubility parameters of the macromolecules, which improves the interaction with CO₂. This further confirms that the acetate group is the CO₂-philic domain on the colloidal particles. However, the acetate group cannot make the solubility parameter of the macromolecules as low as that of CO₂ under the operational synthesis conditions. The existence of cosolvent can promote the solubility parameter of the solvent mixture, which also contributes to the stabilization of the colloidal particles.

Chapter 9. Summary and Conclusions

9.1. Outline

The successful synthesis of well-defined SiO₂, TiO₂ and ZrO₂ nanomaterials shows that the direct sol-gel process in CO₂ is a promising technique. The advantages of this method include: (1) a high conversion, e.g. about 98 % conversions for TiO₂ and ZrO₂ aerogels; (2) a high yield, e.g. 20 g of TiO₂ monolith was synthesized in a 100 ml autoclave; (3) a high surface area, e.g. the surface areas of TiO₂ anatase were as high as 105-181 m²/g; and (2) environmental benignness. The disadvantages involve the relatively high-pressure apparatus, although this is not generally considered high pressure by industrial standards.

Although the chemistry of the direct sol-gel process of SiO₂, TiO₂ and ZrO₂ in CO₂ is relatively complex and different from one another, the reactions can be summarized as substitution, esterification, hydrolysis and condensation.

CO₂ was also found to be a superior solvent for the sol-gel process. Because of the CO₂-philicity of the metal-acetate group, the colloidal particles were stabilized by CO₂. In addition, scCO₂ exhibits a low interfacial tension, and is able to penetrate into the nanoparticles.^324 This allows heterogeneous reactions taking place and formation of the well-defined gel network. Furthermore, scCO₂ extraction and drying maintain a high surface area of the resulting materials.

During the sol-gel process in CO₂, the temperature and pressure were in the range of 40-70 C and 2000-7000 psig. This is because, at this temperature and pressure range, the system was homogenous at the initial stage, which is critical for formation of well-defined nanostructures. The optimum temperature and pressure were theoretically justified by the solubility parameters of CO₂ and colloidal particles.

9.2. Synthesis, Characterization and Mechanism Studies

In order to synthesize the oxide nanomaterials, the direct sol-gel process in CO₂ was carried out in either a view cell or an autoclave. The starting materials were mixed before addition of CO₂ using a syringe pump. The system was maintained at a constant temperature and pressure until the nanomaterials were formed. In order to obtain TiO₂ particles, CO₂ was removed through either a quick depressurization or a RESS process before agglomeration took place. In the case of TiO₂ and ZrO₂, fresh CO₂ was slowly passed through the reactor in order to extract the organic species and to maintain the nanomaterial microstructure.

In order to crystallize the amorphous resulting nanomaterials, calcination was carried out in the furnace at a temperature 380-600 °C. The nanomaterials were characterized by using electron microscopy, N₂ physisorption, FTIR, XRD and thermal analysis.

The mechanism of nanomaterial formation, the kinetics of sol-gel reactions, and the interactions between CO₂ and the Ti-acetate were studied by using ATR-FTIR spectrometry. The formation of TiO₂ nanofibers was attributed to presence of the Ti-acetate hexamers that lead to one-dimensional condensation. In the case of SiO₂ synthesis, the conversion of the precursor TEOS was favored by a higher temperature and lower pressure. In the cases of TiO₂ and ZrO₂ synthesis, the precursors quickly reacted with acetic acid to form metal acetate complexes, and the complexes consequently condensed into sol and gel either gradually or quickly after a critical point. The LA-LB interactions between CO₂ and the metal-acetate were studied using ATR-FTIR spectrometry. The solubility parameter calculation results showed that the metal-acetate group is CO₂-philic.

9.3. Chemistry of the Direct Sol-gel Process in CO₂

In the sol-gel process, acetic acid has been used as a polycondensation agent for various alkoxides in CO₂. The first step involves a substitution reaction, which resulted in alcohol. In the second step, the alcohol reacts with acetic acid and water is produced through esterification. Then, the generated water leads to hydrolysis and then subsequent condensation. The reactions can be written as:

Substitution :

≡M-OR + HOAc ⇌ ≡MOAc + ROH (9.48)

Esterification :

HOAc + ROH ⇌ ROAc + H₂O (9.49)

Hydrolysis :

H₂O + ≡M-OR ⇌ ≡M-OH + ROH (9.50)

Condensation :

≡M-OH + ≡M-OR ⇌ ≡M-O-M≡ + ROH (9.51)

≡M-OH + ≡M-OH ⇌ ≡M-O-M≡ + H₂O (9.52)

Where, M = Si, Ti or Zr ; R = Me, Et, Prⁱ, Pr or Bu.

Although the reactions are very complicated after formation of oligomers and metal complexes, reactions 9.1-5 can be used as the general reactions for the sol-gel process in CO₂.