Synthesis of Aluminum Oxide (Al2O3) for Catalytic Uses in Chemical Reactions

The aim of this project is to produce pure and relatively high quality powdered aluminum/aluminium oxide, for use (mainly) as a catalyst in chemistry.

The main method that people use to make aluminum oxide is by dissolving metallic aluminum in hydrochloric acid to make aluminum trichloride. The aluminum is then displaced by the addition of a base such as NaOH, KOH or Na2CO3. The aluminum is then oxidised and the chlorine is reduced to form a solution of sodium/potassium chloride and a suspension of aluminum hydroxide (Al(OH)3) particles, which are then heated to decompose it into aluminum oxide (Al2O3).

Here, we will be following a very similar method, but we will dissolve the aluminum in sodium or potassium hydroxide, which, in some places, are much easier to buy than hydrochloric acid.

Caution:

• Some reactions shown here use caustic/corrosive chemicals. Please refer to the packaging of the chemicals to know about all hazards caused by them, and take appropriate precautions.

• Some reactions and chemicals also pose a strong fire hazard.

• Proper protection equipment is mandatory.

• Do not ingest or swallow ANY of the chemicals used or seen here.

The author of this paper will not accept any responsibility for any accidents or mishaps caused.

This paper has been written while taking safety as a high priority. Only if you already know how to perform the reaction and know ALL the safety precautions, you can skip the things written in bold and italics. But I still encourage EVERYONE looking to perform this reaction, to read the full paper properly.

Also, sorry if this Instructable is too long.

With all that said, enjoy reading!

Supplies

I have tried to, as far as possible, use easy-to-obtain materials and chemicals, which are as listed below:

• Chemicals:

• Sodium or potassium hydroxide (NaOH/KOH), which can be purchased online or as a drain cleaner. (Caution: Caustic, highly corrosive to skin, metals and can even sometimes dissolve/soften/damage glass)

• Aluminum metal. (Powder or foil is best). (Be careful while handling aluminum powder. Do not inhale the particles.)

• Citric acid, which can be purchased locally in stores or online as a food additive/preservative.

•Sodium carbonate or sodium bicarbonate (Baking soda)

• A lot of distilled or de-ionized water. (Not mandatory (can be substituted with drinking water), but highly recommended for best results.)

• Ice, just to cool the reaction vessel.

• Materials/tools:

• A few beakers/flasks/containers made of glass or any material that can withstand caustic soda.

• Another vessel to contain all washing water contaminated with NaOH, citric acid, baking soda.

• A container larger than the reaction vessel to prepare an ice bath for cooling.

• Filtration setup (Funnel/strainer, stand, filter paper, coffee filter etc.)

• Oven

• Small weighing scale

• Airtight storage container for final product.

• A small spatula and stir rod.

• Tissues for cleaning (Optional).

• For safety- Gloves (Both latex and nitrile should work well.), tray for spill protection, goggles/face shield, proper body protection from splashes and spills.

Step 1: Preparation and Safety

Before we start the actual reactions, we need to first make sure that we have taken appropriate precautions and safety measures for the reactions. Individual safety measures have been mentioned with each step, but these are some general things to follow:

• All procedures are to be conducted in an extremely well ventilated area with sufficient lighting.

• All reactions, tests and filtrations are to be conducted above a plastic or chemical-resistant surface and are preferably supposed to be done in a tray of similar nature, while the person performing them must be wearing proper protection equipment.

• Check that all containers are compatible with the chemicals used. (Chemical-resistant plastic or glass is recommended.)

• Make sure that all containers, beakers, flasks etc. are clean before use.

• Do not leave any reaction mixture (especially caustic soda) in a vessel (glass or plastic) longer than 12 hours. Wash the reaction vessels as soon as possible after use. Wash or dispose gloves or anything contaminated as soon as possible.

• Dispose of contaminated objects in a proper way. Do not pour non-neutralised reaction mixtures into the drain.

• Weigh all solids on a piece of paper or a beaker and measure liquids with an accurate volume measurement vessel like a graduated cylinder. Remember- the more accurate your measurements are, the purer your product will be and the fewer number of washings you will have to do.

Wear gloves and other protection equipment before starting the reaction.

Step 2: Reaction of Aluminum With NaOH Solution to Precipitate a Salt

When metallic aluminum is introduced to a solution of sodium hydroxide (or potassium hydroxide), the following reaction takes place:

2Al + 2NaOH + 6H2O → 2Na[Al(OH)4] + 3H2

(If potassium hydroxide is used, K will be used as the symbol of potassium instead of Na for sodium.)

This equation shows that two molecules of aluminum will react with two molecules of sodium hydroxide and six molecules of water to form two molecules of a salt called sodium tetrahydroxoaluminate and three molecules of hydrogen gas.

Knowing that one mole of any substance contains 6.022 * 10^23 molecules/atoms of that substance, we can calculate the amount of each substance required for the reaction.

Taking the molar mass of each substance, we get:

54 grams of aluminum + 80 grams of sodium hydroxide + 108 grams or milliliters of water → 236 grams of sodium tetrahydroxoaluminate + 6 grams of hydrogen gas.

When I did this reaction, I wanted to do it on a smaller scale, so I divided all the values by 6. When 9 grams of aluminum foil pieces were added to 13 grams of sodium hydroxide and 18 ml of water, the products were 39 grams of sodium tetrahydroxoaluminate and 1 gram (0.5 moles- 11.2 liters) of hydrogen gas.

Note- It might be good to use an excess of water for this reaction. You can also use around 0.2 grams excess NaOH to ensure that all the aluminum reacts.

To start this reaction, take a beaker or a reaction vessel (Make sure that the vessel is big enough to fit all the reaction components and still be less than one-third full. This should ensure that the mixture doesn't bubble over.) and place it in an ice bath for cooling. (This is not absolutely necessary but it will help a lot to keep the reaction under control.) Next, pour the required amount of cold water into the beaker. Then, add the sodium hydroxide.

(Be careful: The dissociation of sodium hydroxide in water is quite exothermic. Add the base to the water, and not the other way around, to prevent splashing.)

After all the sodium hydroxide has dissolved, make sure that the solution is cool and then slowly add small pieces of aluminum while stirring.

Note- It might be better to freshly tear up the pieces just before adding them, as this will expose some fresh aluminum under the oxide layer that naturally forms around aluminum. This will make it easier for the NaOH to attack it.

This reaction takes place slowly. Even if you do not see hydrogen bubbles and but can still feel pieces of aluminum, do not add more. Just let the reaction take place. Keep stirring. The mixture can suddenly bubble and rise a lot. Transfer it to a bigger vessel if the current one is too small.

(This reaction produces a lot of highly flammable hydrogen gas. Do it in a fume hood or an extremely well-ventilated area, away from sparks and extremely hot objects.)

Optional- After the reaction is done, place it on a hot plate or heating surface and stir vigorously to push the reaction to completion.

After the reaction is done, let all the insoluble Na[Al(OH)4] settle at the bottom and then decant off the water. When I did it, the particles didn't settle down properly, and started floating again when I tried to pour it, even though I did it very carefully. If you face the same problem, I suggest filtering it through some filter paper, a coffee filter or some cotton. After filtering it, mix the solid remaining in the filter with the main stuff. After doing this, wash the solid two times with more distilled water. (Pour some more water in, let the solid settle and then decant or filter off the water.) This is optional, but I think it's better to do it for a purer product.

Step 3: Oxidation of Na[Al(OH)4] With Citric Acid

After washing, you should have a pretty good slurry of sodium tetrahydroxoaluminate. The next thing to do is to displace the tetrahydroxoaluminate ion with a citrate ion.

We know that each citrate ion can react with three sodium ions to form trisodium citrate. Therefore, for every three sodium tetrahydroxoaluminate molecules, we need one citrate ion. This can also be said as- the molar ratio between the Citric acid (C6H8O7) and the sodium tetrahydroxoaluminate (Na[Al(OH)4]) is 1:3. Taking the molar masses of each of the two reactants into account, we get the mass ratio- 192:354 grams, which means that for every gram of sodium tetrahydroxoaluminate, we need appropriately 0.542 grams of citric acid. In my case, I had around 39.33 grams of Na[Al(OH)4], for which I needed around 21.15 grams of C6H8O7. You can do the calculations yourself to get the exact amount you need, or if you want to do it with a different acid such as acetic acid. Just take the number of ions each substance's molecule has, the number of ions it takes to neutralize the other substance by looking at the number of ions in the formula of the final salt, the molar mass of each substance and use this to get the mass ratio.

Just remember- if you are using vinegar instead of citric acid, first check it's exact concentration by weight. Also, use around 0.1 grams extra acid to ensure that all the Na[Al(OH)4] reacts.

In the picture, the citric acid is pre-dissolved and is being added with a pipette into the beaker, which is being cooled with an ice bath, but I don't think cooling it or adding the acid in the form of a solution is super necessary, as long as you add the acid slowly.

(Be careful: This reaction is quite exothermic.)

Step 4: Neutralizing Excess Citric Acid and Washing the Product

After adding all the acid to the reaction mixture and ensuring that it is cool, try adding small amounts of baking soda (sodium bicarbonate) or soda ash (sodium carbonate). If it releases bubbles of carbon dioxide, slowly add more in very small amounts until it stops bubbling. This will react with any remaining acid. The reason for this is that aluminum hydroxide is a base, which can react with acids to form more salts like aluminum citrate. The basic nature of the solution after adding the sodium bicarbonate/carbonate will ensure that all the aluminum ions are in the form of Al(OH)3.

Note- After the bubbling stops, you can heat the solution slightly or stir it vigorously to remove all the carbon dioxide from the solution.

After adding the base, let all the Al(OH)3 sediment at the bottom and decant or filter off all the liquid. After this, wash this sediment at least five times by adding water, stirring vigorously, letting the solid settle and decanting/filtering off all the water.

Now, you should have a reasonably pure slurry of aluminum hydroxide. Next, transfer all of it to a large tray or a printer paper kept on paper towels/tissue paper. Dry the powder by letting all the water evaporate. You can do this by just letting it sit, or blowing air at it with a small fan, or placing it in an oven at around 100-150°C for a while.

Step 5: Decomposing Al(OH)3 to Al2O3

Now, you should have a nice, dry powder of reasonably pure aluminum hydroxide.

The last step is just to heat the aluminum hydroxide to about 250-350°C, preferably in an oven, for at least a few hours. I think this can also be done on a stove. In this step, the heat converts the aluminum hydroxide in to the final product- aluminum oxide, by the following reaction:

2Al(OH)3@300°C → Al2O3 + 3H2O

By taking the molar mass of each, we can tell that 156 grams of aluminum hydroxide will get converted to 101 grams of aluminum oxide. Therefore, the mass ratio between the final and initial products is 101:156, which is equal to approximately 0.6474. This tells us that the final product should weigh about 64.74% of its initial mass.

After taking it out of the oven, let the aluminum oxide dry and then transfer it to a clean container for storage.

Keep in mind, even if the decomposition is incomplete, some reactions, like the dehydration of ethanol to make diethyl ether or ethene, can work with aluminum hydroxide.

And there you have it! Aluminum oxide, a useful catalyst in chemical reactions.