Alumina Aerogels (Alkoxide Method)

Materials

  • Aluminum tri-sec-butoxide, Al(OC4H9)3
  • Absolute (200-proof) ethanol, C2H5OH
  • Deionized water
  • Methanol, CH3OH
  • Glacial acetic acid, CH3COOH
  • Acetone

Gel Preparation

In this gel preparation you will be using aluminum tri-sec-butoxide. Aluminum tri-sec-butoxide is an annoying, viscous liquid that flows like cold honey and crusts over (hydrolyzes) on contact with water from the air. A good way to handle it is to pull the plunger out of a syringe, scoop some of the alkoxide into the barrel, replace the plunger, and then squirt it out as needed. For best accuracy, you’ll want to weigh it instead of trying to measure by volume. We don’t really care if it hydrolyzes a little bit in handling (that is, if it turns white or crusts) since the first step of making the gel is to hydrolyze it.

Download an Excel calculator to scale this recipe according to your needs.

  1. Mix 7.37 g ethanol (9.34 mL), 0.108 g (0.108 mL) water, and 2.46 g (2.54 mL) aluminum tri-sec-butoxide in a beaker/flask and cover/plug.
  2. Heat mixture to 60°C and stir for 45 min. The solution will go cloudy and then clear up on its own as it hydrolyzes.
  3. Cool the mixture (sol) to room temperature.
  4. To your 9.94 g of sol, add 1.99 g (2.51 mL) methanol, another .030 g water, and 0.508 g (0.485 mL) glacial acetic acid.
  5. Stir mixture for 30 min.
  6. Pour mixture into molds. Gel time is ~120 min.

Gel Processing Conditions

  1. Once the gel has set, place it under methanol and allow the gel to age for at least 24 h.
  2. Exchange into acetone 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 approximately:

  • Density 0.040 g cm-3
  • Surface area 376 m2 g-1

although the actual values you get will probably vary since these are based on alumina aerogels that were dried by high-temperature supercritical drying from methanol.

Crystallization

Alumina aerogels are cool because they don’t sinter until about 950°C, where silica aerogels sinter around 600°C. You can actually increase their crystallinity by heat treating them in an oven at a temperature around 270°C for about 4 hrs and even at 1050°C they will only shrink about 2%.

Useful Information

Aluminum tri-sec-butoxide:

  • Molecular weight 246.32 g mol-1
  • Density 0.967 g mL-1
  • Form is a viscous, honey-like substance that crusts over on contact with water from the air
  • Sigma-Aldrich part number 201073

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

Methanol:

  • Molecular weight 32.04 g mol-1
  • Density 0.791 g mL-1
  • Sigma-Aldrich part number 322415

Acetic acid:

  • Molecular weight 60.05 g mol-1
  • Density 1.049 g mL-1
  • Sigma-Aldrich part number 320099

References

  1. John F. Poco, Joe H. Satcher, Jr., and Lawrence W. Hrubesh, “Synthesis of high porosity, monolithic alumina erogels”, Journal of Non-Crystalline Solids, 285, 57-63 (2001).  DOI: 10.1016/S0022-3093(01)00432-X

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