Synthesis and Characterization of Nano-Aerogels

2.3. Direct Sol-Gel Process in CO₂

A direct sol-gel process for synthesizing oxide nanomaterials in scCO₂ or subcritical CO₂ is a promising technique, because of its environmental benignness, and low temperature. There are three approaches for the direct sol-gel process in CO₂: (1) using a surfactant to stabilize the dispersion of water in CO₂, in which the water droplets react with the alkoxide to start the sol-gel reactions; (2) alkoxides reacting with water without surfactants; and (3) using carboxylic acid instead of water to initialize the polycondensation of the alkoxides to form macromolecules.

2.3.1. Surfactant-Assisted Hydrolysis

In 1996, Tadros et al. reported the formation of TiO₂ nanoparticles in scCO₂ by hydrolysis and condensation of titanium alkoxide. Before addition of CO₂, the alkoxide and water + anionic fluorinated surfactant, [F(CF₂CF₂)_zCH₂CH₂O]_xP(O)(ONH₄)_y (where x = 1 or 2, y = 2 or 1, z = 1 – 7), were put in two vials that were located in an autoclave. The surfactant was able to stabilize the tiny water droplets in scCO₂ at 45 C and 1400 psig, and spherical particles of titanium dioxide with low anatase crystallinity were formed.^136

In 2003, Stallings et al. performed a more systematic study on the direct sol-gel process of titanium isopropoxide (TIP) in scCO₂ by using the same anionic fluorinated surfactant used by Tadros et al. TIP was injected into water-in-CO₂ dispersions using a HPLC pump. Spherical titania particles with a broad particle size distribution (20-800 nm) were obtained. The surface area of the as-prepared material, 100 ~ 500 m²/g, was attributed to the internal porosity of the spherical titania particles, as evidenced by scanning transmission electron microscopy (STEM). After calcination, the surface area of the as-prepared titania powder at 300 °C in air decreased from ca. 300 to 65 m²/g and increased the mean cylindrical pore diameter from 2.6 to 4.9 nm.^97

In 2003, Hong et al. prepared TiO₂ nanospherical particles by the direct sol-gel processing of titanium isopropoxide (TIP) in water-in-carbon dioxide microemulsion using ammonium carboxylate perfluoropolyether (PFPE-NH₄) as a surfactant. A narrow distribution of particle sizes was observed. The crystallinity and crystallite size of nanoparticles produced increased with an increase of molar ratio of water to surfactant (W₀). Room temperature and a pressure of 4000 psig was used.^137

Keith Johnson’s group also produced TiO₂ spherical particles by the direct sol-gel process of TIP with water in CO₂ at 25 °C and 4000 psig in the presence of surfactants, PFPE-NH₄ and poly (dimethyl amino ethyl methacrylate-block-1H,1H,2H,2H-perfluorooctyl methacrylate). The particle size was also found to be a function of the molar ratio of water to surfactant headgroup (W₀), and the precursor concentration and TIP injection rate as well. The hydrodynamic diameters of the hydrated micelles and particles were characterized using a dynamic light scattering (DLS) measurement, and the larger size of the TiO₂ particles compared to the micelles was attributed to surfactant reorganization. As the surfactant remained attached to the particles, coalescence of the particles was prevented.^53

2.3.2. Hydrolysis without Surfactant

In 2003, Reverchon et al. prepared amorphous titanium hydroxide nanoparticles on the pilot scale in scCO₂ without the assistance of surfactant. The process was based on the reaction:

Ti(OCH₃H₇)₄ + 4H₂O ⇌ Ti(OH)₄ + 4C₃H₇OH (2.9)

The reaction was carried out in an autoclave, which had three inlets and one outlet. The inlets were connected with a CO₂ supply, a CO₂-TIP contactor, a CO₂-water contactor, while the outlet led to a liquid separation tank. Since the solubility of water in scCO₂ is as low as 0.16% (w/w), while that of TIP is 2% (w/w) at 2200 psig and 40 °C, the flow rates of the CO₂-TIP and CO₂-water were 9 and 20 kg/h, respectively, to ensure enough water was available for the hydrolysis. The residence time was 180 seconds, which led to complete conversion. The formed particles were separated with the liquid in a downstream separator. The synthesized Ti(OH)₄ particles were amorphous and had a narrow particle size distribution.^138

In 2004, Yoda et al. synthesized titania-pillared clay, montmorillonite (MNT) in scCO₂. This scheme consists of two steps: (1) ion exchange of interlayer cation by hydrophobic cation, using alkyltrimethylammonium cation in aqueous solution, and (2) the intercalation of a metal alkoxide dissolved in scCO₂ followed by hydrolysis with adsorbed water present in the interlayer space. Nano-sized anatase crystal structure was observed in the calcined samples. The titania-pillared clay sample showed potential as a catalytic adsorbent for toxic volatile organic compounds in air.^139

In 2005, Sun et al. grafted mono and bi layers of TiO₂ on mesoporous SiO₂ molecular sieve SBA-15 via a surface sol-gel process in scCO₂. The process involved grafting of TIP onto the surface of the molecular sieve, extraction of extra TIP, followed by hydrolysis of the impregnated TIP with an excess amount of water and calcination. The XPS results confirmed the formation of Ti-O-Si and Ti-O-Ti bonds. The TiO₂ grafted SiO₂ molecular sieve has potential for photocatalysis and as a catalyst support.^{140, 141}

In 2006, Jensen et al. prepared anatase titania at a temperature as low as 100 °C in scCO₂ by a “seed enhanced crystallization (SSEC) process”. A crystallinity as high as ca. 60% was obtained without downstream calcination. Polypropylene, ceramics, metal fibers and wool fibers were used as seeding materials. To prevent the premixing of the reactants before addition of CO₂, TIP and water were injected in the upper third and lower third, respectively, of the seeding material that was loaded into an autoclave. After addition of CO₂, the reactants were able to mix and the sol-gel reaction started. The authors claimed a mechanism that water dispersed into CO₂ and reacted with TIP sitting on the surface of the seeding material, and the seeding material acted as heterogeneous seeds for the nuclei formation and facilitated the formation of crystalline nuclei. The results were encouraging, however, the proposed mechanism is suspect. As the solubility of TIP is much higher that that of water in scCO₂, it is unlikely that water will be “mechanically dispersed with scCO₂” while TIP was “homogeneously dispersed” by the seeding material rather than CO₂.^142 Recently, Jensen et al. studied the growing of TiO₂ particles with reaction time using in situ small-angle X ray scattering (SAXS) and wide-angle scattering (WAXS),^143 and it was found that the sol-gel process in scCO₂ was faster than in the organic solvent.

Recently, Cabanas et al. synthesized highly porous SiO₂ aerogels in scCO₂ using polystyrene latex particles decorated with methacrylic acid. TEOS or TMOS were used as a precursor reacting with water.^{144, 145}

2.3.3. Carboxylic Acids as Polycondensation Agents

In 1997, Loy et al. prepared a silica aerogel monolith for the first time by polymerization of silicon alkoxide, tetramethyl orthosilicate (TMOS), with formic acid in CO₂ at 40 °C and 6000 psig.^77 The results showed that the sol-gel polymerization could be conducted directly in CO₂ as solvent by using formic acid as the condensation agent/co-solvent. The resulting materials exhibited a highly porous architecture with both meso- and macropores. Following this idea, Moner-Girona et al. prepared silica aerogel micro-particles by using formic acid as a polycondensation agent in CO₂.^78