Alumina Aerogels (Epoxide-Assisted)

Materials

• Aluminum chloride hexahydrate, AlCl₃•6H₂O
• Propylene oxide, C₃H₆O
• Absolute (200-proof) ethanol, C₂H₅OH
• Deionized water

Note: If your propylene oxide comes in a SureSeal® bottle from Aldrich or something like it (has a bottle cap with a diaphragm you have to puncture) you may also need

• Syringe with a Luer lock connection
• Needle with coring (sharp) tip, 18-22 gauge
• Nitrogen gas line with a small needle at the end to backfill bottle

Optional

• Epichlorohydrin
• Acetone

Warning!

Propylene oxide is a known carcinogen (exposure can cause cancer), and epichlorohydrin is probably too. If you plan on doing this procedure, take the proper precautions to prevent your exposure to the vapors of these substances by using a fume hood in a lab, if possible, or at the very least a fitted respirator (gas mask) with the right organics cartridges and a well-ventilated space, on top of the usual splash goggles, gloves, long pants, and closed-toe shoes.

Look under Explore > Information About Chemicals to see where you can find health and safety information about these and other chemicals.

If you can’t use these substances safely, don’t use them until you can!

Gel Preparation

1. Mix 10.0 mL (7.89 g) ethanol and 10.0 mL (10.0 g) water in a beaker.
2. Weigh 2.96 g AlCl₃•6H₂O and dissolve in the ethanol/water solution.
3. Weigh 7.86 g (9.47 mL) propylene oxide and add to the beaker slowly (~1-2 mL per minute) with stirring. Weighing instead of measuring by volume is important. The gel time is sensitive to the amount of propylene oxide.
4. A transparent, colorless gel will result. Gel time is about 45 min.

What Doesn’t Work

• Dumping all the propylene oxide in at once. It can cause the solution to get hot very quickly and start to boil.

Problems You Might Have

• Your gels might shrink after forming. This is normal.
• You might observe the gel time to be longer than 45 min. This can occur if your propylene oxide is old and gets hydrolyzed by water from the air and loses its kick. Fix this by getting new propylene oxide.
• You might see the solution get hazy upon mixing all the stuff together. This means your glassware was dirty. Might be caused by residual Alconox or other detergents. Use clean glassware.

Variables You Can Play With

• Try using epichlorohydrin instead of propylene oxide. Epichlorohydrin seems to have a longer shelf-life, usually works better, and might be easier to work with.
• You can adjust the density of the gels by adjusting the amount of solvent (50% ethanol/50% water mix).
• Try making the gels in ethanol only. You will find they shrink up to 50% after setting.
• Try making the gels in water only. You will find the gel time is ~14 h and shrinkage is reduced to only 5-10%

Gel Processing Conditions

1. Once the gel has set, place it under 50% ethanol/50% water solution and allow the gel to age for at least 24 h.
2. Exchange into 200-proof ethanol at least four times over the course of several days to a week.
3. Supercritically dry. A suggested procedure would be to heat the CO₂ through its critical point (31.1°C and 72.9 bars) to ~45°C while maintaining a pressure of ~100 bars. Depressurize at a rate of ~7 bar h^-1.

What You Should Get

Transparent, monolithic alumina aerogel with a blue cast from Rayleigh scattering that resembles silica aerogel with:

• Density 0.060 g cm^-3
• Surface area 701 m² g^-1
• Pore volume 4.5 cm³ g^-1
• Average pore diameter 24 nm

Using ethanol only as the solvent will increase the bulk density and decrease the surface area. Using water only will decrease the bulk density but will also decrease the surface area.

Useful Information

Aluminum chloride hexahydrate:

• Molecular weight 241.43 g mol^-1
• Sigma-Aldrich part number 237078

Propylene oxide:

• Molecular weight 58.08 g mol^-1
• Density 0.83 g mL^-1
• Sigma-Aldrich part number 110205 or 82320

Ethanol:

• Molecular weight 46.07 g mol^-1
• Density 0.789 g mL^-1
• Sigma-Aldrich part number 459836 or 459844, or get Everclear from a liquor store

Epichlorhydrin:

• Molecular weight 92.52 g mol^-1
• Density 1.183 g mL^-1
• Sigma-Aldrich part number 240699

References

1. Theodore F. Baumann, Alexander E. Gash, Sarah C. Chinn, April M. Sawvel, Robert S. Maxwell, and Joe H. Satcher, “Synthesis of High-Surface-Area Alumina Aerogels without the Use of Alkoxide Precursors”, Chemistry of Materials, 2005, 17 (2), 395-401. DOI: 10.1021/cm048800m

2. Alexander E. Gash, Thomas M. Tillotson, Joe H. Satcher Jr., Lawrence W. Hrubesh, Randall L. Simpson, “New sol-gel synthetic route to transition and main-group metal oxide aerogels using inorganic salt precursors”, Journal of Non-Crystalline Solids, 2001, 285 (1-3), 22-28. DOI: 10.1016/S0022-3093(01)00427-6