Laboratory of Crystallography and Coordination Chemistry

National Centre for Scientific Research "Demokritos", Institute of Materials Science

Athens, Greece

 

 

Preparation of a Manganese oxide column for inert gas purification from oxygen

     
  Various inert-atmosphere preparations and manipulations (e.g. on a Schlenk line) require the strict absence of oxygen, down to the ppm scale. However, inert gases usually contain traces of oxygen at much higher levels. For their purification prior to use, it is common to use a "sacrificial" air-sensitive material which will react with oxygen and capture it before it enters the reaction vessel.

Here, we will describe the preparation of a manganese oxide column for use with a Schlenk line. The principle of its operation is that oxidizable manganese(II) oxide (MnO, green) will be oxidized to manganese(III) oxide (Mn2O3, black) thereby removing oxygen  traces. It is useful because its color change is a straightforward indicator of when it has been depleted.

     
 

Apparatus:

1) Heating mantle for columns, or custom-built oven with heating tape (used here)

2) Thermocouple for temperature monitoring

3) H2 bottle (pure or mixture)

4) Vacuum pump

5) Cold trap

 

Materials

1) Manganese oxalate dihydrte (Mn(C2O4)∙2H2O), or

Manganese(II) acetate terahydrate (Mn(O2CMe)2∙4H2O), oxalic acid and distilled water

2) Vermiculite

3) Glass wool

4) Liquid nitrogen

 

Glassware

Column with two stopcocks (~80 cm total length)

Two 2 L beakers

One 3 L beaker

 

     
1. Preparation of manganese(II) oxalate dihydrate Mn(C2O4)∙2H2O  
  MnO has to be prepared by thermal decomposition of a MnII source in order to be susceptible to oxidation-reduction cycles. Manganese oxalate is an ideal source for this. Manganese oxalate is commercially available, but for reasons of reducing costs, it can be easily prepared.

A solution of Mn(O2CMe)2∙4H2O (160.0 g, 652.8 mmol) in water (0.85 L) is slowly added to a solution of oxalic acid (123.5 g, 979.2 mmol) in water (1.5 L). The solution gradually starts to form a white suspension, which after ~5 min turns to light pink.

The solution is heated to ~95 C and allowed to remain at that temperature for ~15 min. The color of the suspension must now be white. The solid is filtered, washed with distilled water, and dried at 50 C overnight. The yield is almost quantitative.

 
     
2. Packing of the column  
  200 g of the previously prepared (or purchased) manganese oxalate are thoroughly mixed with the appropriate quantity of vermiculite, so that the column is packed. The vermiculite will create the empty spaces through which the gas may easily flow while possessing the thermal stability to withstand the subsequent thermal treatments.

The mixture is introduced into the column, with a quantity of glass wool on its two sides keeping the powder from clogging the stopcocks.

     
3. Thermal decomposition  
  The column is then placed inside a heating mantle, or wrapped with a heating tape and placed inside an insulating material (the custom-built oven on the right is insulated with stone wool).

One end of the column is connected to a vacuum pump, through a cold-trap. After the column has been brought under vacuum, the cold-trap is immersed in liquid N2, in order to capture the H2O, CO and CO2 that will be formed during dehydration and thermal decomposition.

Our heating element is powered through a Variac (variable transformer).

Attention! Use of a thermostat is not recommended because of its discontinuous on-off operation (maximum power - zero power) that may lead to large jumps in temperature. Instead, a variac can allow a steady and adjustable thermal output.

The temperature is initially set to 150 C at which the salt starts to dehydrate. It is possible that the process will have to be stopped for the cold-trap to be emptied from the ice. The intake of the cold-trap should be wide enough so that it doesn't clog with ice (e.g. 2.5 cm in our case).

Then the temperature is increased to ~330 C for the decomposition to begin. This is the case when the column has obtained a green-brown color throughout its length.

The column is left to cool and then filled with the working inert gas.

Attention: From this point on, the column should be handled with extreme care. Opening of the oxide to ambient air will cause a highly exothermic oxidation that may lead to combustion.

     
     
  At that stage, the column does not contain pure MnO, but a mix of MnO and Mn2O3. However, it can be used normally and it will successfully purify the inert gas. After some time it will exhibit two colors. When it has all turned black it is time for its regeneration.

     
     
4. Regeneration  
  The column is connected to a H2 bottle on its intake and an oil bubbler on its outlet. A slow H2 flow is supplied and then we start heating. A temperature of ~310-330 C should be maintained throughout the process. After 4-5 hours the regeneration should be complete.

This can also be done with a H2 mix. However, a larger flow is required and the process will take more time.

     
  After the regeneration is complete, the column is allowed to cool and then filled with the inert gas. Its color now should be a bright green.
   
   
 
 

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