Objective

This project describes innovative and potentially scalable synthetic approaches toward production of energetic materials that eliminate or dramatically reduce the hazardous waste streams from nitration processes used in their manufacture. The objective of this work was to identify and optimize a protocol for performing electrochemical nitration without the use of large excesses of nitric and sulfuric acids. Within this work, nitrating agents have been prepared from electrically conductive salts and nitrogen dioxide (NO2) in an aprotic solvent under electrolysis conditions. These nitrating agents have the potential to provide improved nitration selectivity while reducing or eliminating acidic and/or toxic waste streams. Ideally, a process identified by this effort will be applicable to the nitration of a variety of substrates including aromatic hydrocarbons and polyols, maximizing impact across the portfolio of energetic materials currently in use by the Department of Defense.

Technical Approach

For this effort, syntheses of 2,4,6-trinitrotoluene (TNT), dinitroanisole (DNAN), nitroglycerin (NG), and 1,2,4-butanetriol trinitrate (BTTN) were investigated to prove out this novel electrochemical synthesis method. Historically, these “energetics” have been produced using mixed acid nitration conditions which employ excess acids creating problematic waste streams. Further, the synthesis of TNT would benefit from higher nitration selectivity due to a toxic waste stream, “red water”, generated during a sulfiting process used to achieve the required purity post-synthesis.

Results

This research by Nalas in collaboration with Yale University has resulted in green approaches that form active nitrated nitronium species like nitronium nitrate (N2O5) or nitronium triflate by electrolyzing NO2 in aprotic solvents like acetonitrile and ethylene carbonate with charge carrying salts like lithium nitrate or lithium triflate. Notably, electrochemical nitronium production by the methods outlined herein is robust and straightforward, in contrast to traditional routes to nitronium production. After optimizing batch synthesis of nitronium via electrolysis, a flow process was developed, and a series of proof-of-concept continuous syntheses were conducted with the aim of de-risking further continuous development. Electrochemically generated nitronium was then

used in either batch or continuous nitration of the precursors to TNT, DNAN, NG, and BTTN. Nitration of toluene to the TNT precursor dinitrotoluene was completed with a yield of 86%. Nitration of anisole to 2,4-DNAN was completed with 89% in situ yield and an isolated yield of 70% that resulted in the added benefit of improving the necessary 2,4-DNAN to 2,6-DNAN isomer ratio. Nitration of glycerol to NG and 1,2,4-butanetriol to BTTN were completed with quantitative in situ yields of 100% for both substrates.

Benefits

This process of electrochemical nitronium generation and nitration uses only a small quantity of nitric acid and no sulfuric acid with recoverable organic solvents and reagents. Only electrons, stoichiometric NO2, and stoichiometric substrate are consumed resulting in a green process.