Synthesis and Characterization of Nano-Aerogels

Chapter 4. Synthesis and Characterization of Silica Aerogel Particles

In this chapter, the synthesis of silica nano spherical particles by a direct sol-gel process in scCO₂ is described. Part of this chapter is reproduced from the published article by the author: Synthesis and Formation of Silica Aerogel Particles By a Novel Sol-Gel Route in Supercritical Carbon Dioxide,^171 with permission from the Journal of Physical Chemistry B, 108 (32), 11886-11892, Copyright [2004] American Chemical Society.

A new approach for synthesizing SiO₂ nanoparticles was carried out in CO₂ by polycondensation of silicon alkoxides with carboxylic acids at 40-70 °C and 1300-3000 psig. After formation of SiO₂ colloidal particles in the high-pressure vessel, CO₂ was vented quickly before gelation, resulting in formation of submicron SiO₂ particles in the vessel. By using a nozzle, the supercritical fluid was sprayed into a collection vessel, and SiO₂ nanoparticles with a diameter of about 100 nm were prepared.

4.1. Introduction

Because SiO₂ has high-temperature stability, biocompatibility, and insulating properties, SiO₂ aerogel particles have many potential applications, e.g. catalyst matrixes, chromatography resins, and precursors for ceramic powders for biomaterials.^{112, 172-174} An aerogel is a gel that has a lower density than the fully condensed form of the material forming it. Aerogels are typically produced by replacing the liquid of a gel by air or another gas without allowing complete collapse of the structure.^175 Early work led to aerogels through the use of supercritical fluids to extract liquid, and it led to the hypothesis that the gel structure itself can be preserved in the supercritical drying process.^109 SiO₂ aerogels are conventionally prepared by hydrolyzing and condensing silicon alkoxides in water and/or organic solvents under ambient pressure with an inorganic acid or base as catalyst, gelating for a long time, then drying in scCO₂.^{115, 176} A route of SiO₂ aerogel particle synthesis was previously developed using formic acid to react with tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS) in a supercritical fluid. The first monolithic SiO₂ aerogel was synthesized by employing this methodology in 1997,^177 and M. Moner-Girona et al.^178 in 2003 who obtained micron-size spheres and fibers in supercritical acetone, but found it difficult to have separated particles in scCO₂ because of agglomeration. Advantages of this route include a shorter time for gel aging, obtaining a dry product directly after CO₂ venting, and the ability of the size and morphology of the SiO₂ aerogel to be tailored by modifying the temperature, pressure, relative amounts of reactants, and supercritical fluid venting rate. However, the mechanism and kinetics of the reactions are not well understood.

Recent literature has shown that attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy is technically feasible for in situ measurement of concentration, solubility, and supersaturation in crystallization processes for the chemical and pharmaceutical industries.^{149, 150} The goal of the present work was: (1) to study the particle formation by means of two techniques of (a) destabilizing the sol-gel solution by pressure reduction and (b) the RESS process; (2) to investigate the technical potential and efficacy of ATR-FTIR spectroscopy for the in situ monitoring of solute concentration during the modified aerogel procedure in scCO₂; and (3) to study the kinetics of the sol-gel reactions by using in situ FTIR results.

4.2. Experimental Details

Materials. Reagent-grade tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate (TMOS), acetone (HPLC grade), acetic, benzoic acid, and chloroacetic acid were obtained from the Aldrich Chemical Co. (Ontario, Canada), and used without further purification. Formic acid (Aldrich, reagent grade) was further dehydrated by refluxing with phthalic anhydride (Sigma-Aldrich, 99% reagent grade) for 6h and then distilling. Instrument-grade carbon dioxide (99.99%) was obtained from BOC Canada and further purified by passage through columns containing molecular sieves (Aldrich) and copper (II) oxide supported on alumina (Aldrich) to remove excess water and oxygen, respectively. Purified water was produced by using a Barnstead Easypure LFsystem.

Experimental Setup.

The experimental setup was described in detail in Chapter 3. The polycondensation was carried out in both the view cells (Figure 3.2) and the reactor connected with the in situ FTIR (Figure 3.4).

Polymerization Procedure. To determine the activity of the various acids (acetic, formic, benzoic, and chloroacetic) for catalyzing the condensation reaction, 20-mL of acetone was added to all acids to aid in dissolution (benzoic and chloroacetic acid were not soluble in scCO₂ without this cosolvent). The acid, cosolvent, and orthosilicate precursor were added to the autoclave before heating and addition of carbon dioxide. In the second group of experiments, the 0.176 M acid (acetic or formic) was added to the autoclave before heating and addition of carbon dioxide to the desired pressure. Then 0.044 M TEOS (or TMOS) was pumped in with a separate syringe pump. FTIR spectra were obtained in situ. After the reactions were complete, excess carboxylic acid was washed with fresh scCO₂ at a fixed flow rate (≈2 ml/min). The results provided were the mean of three separate experiments (n=3), and the error bars represent the standard deviation.

Particle Formation and Collection Techniques. Two techniques for particle formation were employed. One was direct formation of particles in the stirred reactor during depressurization with a fixed CO₂ venting rate. For the other one, the RESS process was used, where particles were sprayed through a variable-volume nozzle into a closed stainless steel chamber with heating jacket to collect particles sprayed at a constant depressurizing rate.

Online Characterizations. In situ FTIR monitoring of solution concentration in the stirred autoclave was performed using a high-pressure diamond immersion probe (Sentinel-ASI Applied Systems). The probe is attached to an ATR-FTIR spectrophotometer (ASI Applied Systems ReactIR 4000), connected to a microcomputer, supported by ReactIR software (ASI) (see Figure 3.7).

Product Characterization Techniques. Scanning electron microscopy (SEM) measurements were used to determine the size and morphology of particles. These measurements were made at either Surface Science Western on a Hitachi S-4500 FE instrument using an accelerating voltage of 5kV, or at the UWO Photonics and Nanotechnology Laboratory using a LEO 1530 Field Emission Scanning Electron Microscope. Solid aerogel powders were characterized by means of a Bruker Vector 22 FTIR instrument using a MIRacle Single Reflection HATR (Pike Technologies).