Sciencemadness Discussion Board

Homemade aerogel with hexafluoroisopropanol?

clearly_not_atara - 6-9-2016 at 14:32

Aerogels are produced by supercritical drying. This is because they are prepared in a solvent by precipitation and the ordinary evaporation of a solvent destroys the nanostructure via surface tension (and freeze-drying would destroy it by crystallization).

However, supercritically drying aerogel with water as the solvent leads to the dissolution of the silica as silicic acid, which is surprising and inconvenient. So they were dried with alcohols as the solvent. However:
Quote:
The first pilot plant for the production of silica aerogel monoliths using the TMOS method was established by members of the Lund group in Sjobo, Sweden. The plant included a 3000 liter autoclave designed to handle the high temperatures and pressures encountered for supercritical methanol (240 degrees C and 80 atmospheres). However, in 1984 the autoclave developed a leak during a production run. The room containing the vessel quickly filled with methanol vapors and subsequently exploded. Fortunately, there were no fatalities in this incident, but the facility was completely destroyed.


Due to the extreme danger of supercritically drying alcohols, a multi-stage process is used:

* the aerogel is prepared in water

* the water is replaced by alcohol

* this is pressurized

* the alcohol is replaced by CO2

* this is heated to criticality and dried

Unfortunately the fourth step involves replacing a solvent at high pressure, which requires complicated equipment and is difficult to perform at large scales. Worse, the solvent is flammable. Alcohols or acetone are required as an intermediate solvent because water reacts with carbon dioxide to form carbonic acid, which makes replacement impossible.

I was thinking for a while about using 2,2,2-trifluoroethanol as a single supercritical drying solvent, replacing both alcohol and CO2. It turns out that 2,2,2-trifluoroethanol actually has a lower critical pressure than CO2 (48 atmospheres vs 71), but it also happens to be flammable (albeit with a low flame energy). I switched to hexafluoroisopropanol, which is harder to synthesize but essentially nonflammable. There are two advantages: no intermediate solvent is required, and the high-pressure apparatus is greatly simplified, as it only needs to be suitable for boiling and not solvent replacement.

I couldn't find a table that listed the critical properties of hexafluoroisopropanol, but while looking, I found that some Russians had had my idea two years ago.
Quote:
Sols (3–5 mL) were poured into cylindrical polypropylene containers, sealed and left to gelate and age at room temperature for 24h. The resultant gels were soaked in ethanol or HFIP for 24h to exchange the pore liquid for the solvent chosen. This procedure was repeated five times. Then the gels formed were placed into an autoclave for supercritical drying.

2.3. Supercritical drying
Supercritical drying was performed as follows: a gel sample (3–5 mL) in a glass tube containing ∼14–16 mL of an appropriate solvent was placed into a stainless steel autoclave (V = 38 mL). The autoclave was sealed and heated to a temperature exceeding the critical temperature of the solvent. The heating rate was approximately 100◦ C/h. For ethanol and HFIP, the drying temperatures were 260–270 ◦C and 210–215◦ C, respectively. The measured pressure in the autoclave at the beginning of the drying procedure reached 7.5–9.0 MPa and 4.5–5.0 MPa, respectively. After reaching the desired temperature, the pressure was evenly decreased to atmospheric level during 2h. Then the hot autoclave was evacuated in vacuum for 30 min, cooled to room temperature and opened.


It appears that hexafluoroisopropanol shares the low critical pressure (5.0 MPa ≈ 48 atm) and temperature of trifluoroethanol. The paper reports hexafluoroisopropanol as nonflammable, in accordance with other data. In addition, aerogels dried by HFIP process have hydrophobic surface chemistry owing to etherification with hexafluoroisopropyl groups.

In fact, it seems like the procedure could be carried out in a home laboratory, as long as sufficient precautions were taken to control emissions of hexafluoroisopropanol (which is irritating and corrosive). In this study an ordinary autoclave was use to make aerogel.

Anything to watch out for?

[Edited on 6-9-2016 by clearly_not_atara]