Drying Nitrate-Containing Protic Ionic Liquids Resulted in an Explosion

Key Learnings

A batch of the protic ionic liquid pyrrolidinium nitrate exploded while drying using a rotary evaporator and an oil bath. Be mindful of the strong oxidizing properties of concentrated nitric acid under anhydrous conditions. Mixing nitric acid with secondary amines has been reported to cause violent reactions.   

Effects of Incident

A glass round-bottom flask was scattered and hot silicon oil was blasted by the shock to the laboratory walls and ceiling. Additionally a cloud of reddish-brown nitrogen dioxide gas was visible. There were no injuries. 

Description 

"A batch of the protic ionic liquid pyrrolidinium nitrate exploded while drying it under reduced pressure at 110° C, using a rotary evaporator with an oil bath." So says a paper in Green Chemistry authored by researchers from KU Leuven and Umicore (Green Chem. 2013). 

While ionic liquids typically have low vapor pressures and high flash points, that doesn't mean they're hazard-free, as the documented experience indicates. KU Leuven, Koen Binnemans and colleagues were investigating a series of pyrrolidium ionic liquids with different coordinating groups, one of which was nitrate. The researchers report that after stirring at room temperature for four hours, the remaining pyrrolidine, water and nitric acid were removed on a rotary evaporator under reduced pressure (16 mbar) at 70° C. Heating of the flask on the rotary evaporator was done by means of a hot silicon oil bath. Not all of the water could be distilled off, and the temperature was increased stepwise to 110 °C. Suddenly an explosion occurred. The glass round-bottom flask was scattered and part of the hot silicon oil in the heating bath was blasted by the shock to the walls and ceiling in the neighborhood of the rotary evaporator. At the same time, a cloud of reddish-brown nitrogen dioxide gas was visible. Luckily, nobody was injured and there was only material damage. 

Causation

The explosion can be attributed to the strong oxidizing properties of concentrated nitric acid under the anhydrous conditions, resulting in a violent oxidative decomposition of the organic compounds. A search of the literature revealed that mixing of nitric acid with secondary amines like pyrrolidine has been reported to cause violent reactions.

Corrective Actions to Prevent Reoccurence 

Dry nitrate-containing protic ionic liquids via freeze-drying rather than heating. 

Be mindful of the hazards associated with nitric acid. Nitric acid is a strong acid, very corrosive, and decomposes to produce nitrogen oxides. The fumes are very irritating, and inhalation may cause pulmonary edema. Nitric acid is also a powerful oxidant and reacts violently, sometimes explosively [with] reducing agents (e.g., organic compounds) with liberation of toxic nitrogen oxides. Contact with organic matter must be avoided. Extreme caution must be taken when cleaning glassware contaminated with organic solvents or material with nitric acid. Toxic fumes of NOx are generated and explosion may occur. Separate nitric acid from organics in both storage and waste.