Synthesis and Characterization of Nano-Aerogels

4.3. Results and Discussion

4.3.1. In situ FTIR Analysis of the Sol-gel Reactions.

Figure 4.1 provides a series of traces for a typical experiment in CO₂ taken at fixed time intervals, from the in situ ATR-FTIR analysis of the condensation of TEOS using acetic acid. Analysis of the spectra shows that the absorptions at 1066, 923, and 796 cm^-1 correspond to the stretching frequency of the Si-O-Si bond from the product,^{170, 179} whose absorbance increases as the reaction proceeds. The produced SiO₂ aerogel powder was further analyzed by a separate offline FTIR instrument, which is designed for measuring solid materials. Figure 4.2 shows the results of this analysis, which confirm the peaks at 1066, 923, and 796 cm^-1 in Figure 4.1 belong to the aerogel product, although a slight peak shift can be observed. The in situ FTIR analysis showed that the product peaks were found to increase during the course of the polymerization and then would remain constant if the reaction approached equilibrium, or the saturated concentration of polycondensate was reached. This reaction was repeated in an agitated view-cell reactor, and the solution remained clear during the course of the condensation reaction. As the condensation product is most likely polydisperse with a broad range of molecular weights, it is not possible to directly determine concentration of the molecules by FTIR, only of the actual bond concentration. Analysis of the FTIR spectrum also shows a product peak at 3344 cm^-1, which increases during the course of the reaction from the production of the condensate molecule. The condensate molecule is a mixture of H₂O and CH₃CH₂OH. Both the alcohol and water will merge into an indistinguishable broad peak in this range. More detailed in situ IR studies of the sol-gel process will be discussed in Chapter 7.



Figure 4.18. ATR-FTIR plot of the progress of an aerogel reaction in CO₂, where the lines are experimental data: 4.4 mmol TEOS + 22 mmol HOAc + 4.4 mmol H₂O at 3000 psig and 60 C from 1 to 7 h at 1-h intervals.


Figure 4.19. FTIR spectrum of the SiO₂ aerogel powder, there are three obvious absorptions (1065, 928 and 797cm^-1) in the range between 4000 and 600 cm^-1 (The small absorptions at 3294 and 1709 cm^-1 are due to water and acetic acid residue respectively).

4.3.2. Activity of Various Acids

To compare the relative activity of the various acids studied in the condensation of the orthosilicates, the main absorbance from the oxo bond at ~1065 cm^-1 was plotted versus reaction time (see Figure 4.3). The results showed that among the four acids, benzoic and chloroacetic acid were the most active in promoting conversion to product. However, several problems make these acids poor candidates for the polymerization agent. First, as we will see later, slower condensation of orthosilicates is preferred to form discrete particles. Second, as both of these acids are solid at room temperature, and as they have poor solubility in CO₂, this makes separation from the final product difficult. Both anhydrous acetic and formic acids were less active than the other two acids.

Figure 4.20. Effect of acid type on TEOS condensation activity in CO₂. The points are experimental data from the ATR-FTIR absorbance at 1066 cm^-1 (n = 3). The experimental conditions are 60 °C and 3000 psig, 0.044 M TEOS, 0.176 M acid, and 2.75 M acetone.

Figure 4.4 shows that production of the condensation product could rise remarkably by addition of (a) a small amount of water (TEOS: H₂O = 1 ~ 2) or (b) more acid. NMR studies in aqueous sol-gel polymerizations show similar accelerations with increased concentrations of water.^180 However, too much water was found to make the reaction rate too high, causing precipitation and agglomeration of aerogel particles. Also, too much acid is not economically feasible and causes downstream separation problems.

Figure 4.21. Effect of water and acid concentration on TEOS condensation activity in CO₂. The points are experimental data from the ATR-FTIR absorbance at 1066 cm^-1 (n = 3). Series 1 (♦) : 0.088 M TEOS + 0.352 M HOAc; Series 2 (■): 0.088 M TEOS + 0.352 M HOAc + 0.088 M H₂O; Series 3 (▲): 0.088 M TEOS + 0.704 M HOAc. The experimental conditions are 50 °C and 3000 psig.

4.3.3. Effect of Temperature and Pressure

Figure 4.5 shows that higher temperatures resulted in significantly higher polycondensation rates. The rate of condensation clearly increases with temperature, indicating the dominating reaction of the sol-gel process is either an irreversible reaction or an endothermal reversible reaction.^181 Figure 4.6 shows an interesting effect from experiments varying the reactor pressure. Increasing the reactor CO₂ pressure caused lower initial reaction rates, but led to higher final concentrations of the product. The behavior of the curve at 1300 psig was probably due to the lower solubility of the colloidal particles when the pressure was low, which resulted in a lower absorbance of the oxo bond in the CO₂ phase.

It should be noticed that the absorbance of the oxo bond peak around 1066 cm^-1 is close to the TEOS peaks at 1085 and 1108 peaks, and they may overlapping with one another. This will be further discussed in Chapter 7.

Figure 4.22. Effect of temperature on TEOS condensation activity in CO₂. The points are experimental data from the ATR-FTIR absorbance at 1066 cm^-1 (n = 3). The experimental conditions are 3000 psig, 0.088 M TEOS, and 0.352 M acetic acid.

Figure 4.23. Effect of pressure on TEOS condensation activity in CO₂. The points are experimental data from the ATR-FTIR absorbance at 1066 cm^-1 (n = 3). The experimental conditions are 50 °C, 0.088 M TEOS, and 0.352 M acetic acid.

4.3.4. Experimental Phase Behavior

Using the view-cell system for the experimental conditions studied, it was found that TMOS, TEOS, acetone, acetic acid, and formic acid were miscible with CO₂. Benzoic and chloroacetic acid had poor solubility, but were soluble in CO₂ with added acetone cosolvent. When the FTIR experimental conditions were further examined by means of the view cell reactor, it was found that precipitates appeared spontaneously after the silicon alkoxide and 96% formic acid (≈ 4% water) were pressurized with CO₂ and heated. Figure 4.7a shows an example of the typical agglomerated micron size spheres that were produced. The situation could be improved by adding excess formic acid as cosolvent in CO₂, and by decreasing the concentration of reactants (Figure 4.7b), but precipitation of the products was still inevitable. However, when using anhydrous acetic or formic acid and small amounts of water (water/TEOS = less than 2/1), the fluid remained clear, indicating that the products remained in solution.


Figure 4.24. SEM of SiO₂ aerogel powder. The experimental conditions are: (a) 1.1 mmol TEOS + 7.7 mmol 96% HCOOH in the 25-mL view cell, at 40 °C and 2000 psig; (b) 0.176 mmol TEOS + 2.64 mmol 96% HCOOH in the 25-mL view cell, at 40 °C and 2000 psig.

4.3.5. Particle Formation

Particles were formed either by depressurizing the reactor vessel or expanding the particles using a modified RESS process, into a heated collection chamber. When depressurizing the reactor vessel, particles would form at pressures of approximately 900 ~1000 psig. Results from the view cell observation and SEM showed that separated micro-spheres were formed when the pressure dropped to about 900 ~1000 psig. When using the modified RESS process, the particle size was found to decrease with increasing rate of depressurization and the particles were significantly smaller than those formed from depressurizing the reactor vessel. Generally, a higher venting rate made smaller particles with narrower distributions and lower levels of agglomeration. The particles gathered in the autoclave, spheres in the range between submicron and several microns in diameter (Figure 4.8a), were larger than those formed in the device outside the autoclave by the RESS process (with approximate diameters of 100 nm) (Figure 4.8b).



Figure 4.25. SEM of SiO₂ aerogel powder: (a) collected from the high pressure mixer upon decompression. The experimental conditions are 0.044 M TEOS + 0.176 M HOAc, at 60 °C and 3000 psig; (b) collected using the Rapid Expansion of Supercritical Solutions (RESS) process. The experimental conditions are 0.044 M TEOS + 0.176 M HOAc, at 60 °C and 6000 psig.