Nitro-products of organic compounds are useful in many applications; they are widely employed as precursors or components of energetic materials. Industrial nitration generates substantial waste and uses aggressive chemicals, making the process unsafe and environmentally objectionable. This work advances the solvent-free process of mechanochemical nitration of aromatic compounds. Feasibility of such nitration with high yield and rate was shown. Nitration is achieved during mechanical milling of the organic precursor, solid powder catalyst, and nitrate, serving as a source of nitronium. However, it remained unclear how important the choices of the catalyst, organic compound to be nitrated, and nitronium source are. Quantifying and understanding such effects will advance the mechanochemical nitration technique and build the foundation for the follow-up work identifying the reaction mechanisms and scaling the process up to pilot-plant level.
A laboratory planetary mill is used for experiments. The type of catalyst, the organic compounds to be nitrated, and the nitrates, serving to provide nitronium were varied systematically. The products were analyzed using gas chromatography–mass spectrometry. Aside from milling parameters, the amounts of liquid organic precursor and nitrate were varied. The results were analyzed correlating properties of catalysts, precursors, and nitronium sources with the rate of reaction, yield of nitro-products, selectivity, and production of byproducts.
Mechanochemical nitration was successful for multiple useful aromatic compounds. The reaction rate and yield are increased when solid catalysts have high acidity and contain both, Bronsted and Lewis sites. Among the tested materials, MoO3 was the preferred catalyst. Homogenizing the catalyst with nitrate by a preliminary milling step accelerates ensuing mechanochemical nitration significantly. The selectivity was enhanced and the yield of the nitroproduct was increased when the volume of the aromatic precursor was reduced while the mass of metal oxide catalyst was fixed. The formation of nitroproducts depends on the aromatic activation by the functional group, gas basicity and enthalpy of vaporization of the aromatic precursor. Reaction enthalpies and kinematic viscosity were found to be important as well. Different nitration rates were observed for different nitrates used as nitronium sources with the highest nitration rate observed for Cu(NO3)2. The reaction rate correlated with the enthalpy of the global nitration reaction for which the decomposition of the nitrate was assumed to form the corresponding metal hydroxide. More exothermic global reactions led to higher reaction yields and rates. A second nitration of the aromatic ring was observed for all the precursors tested. Nearly complete mechanochemical nitration could be achieved in many experiments. In this work, achieving the complete nitration was not targeted; instead, conditions, materials, and process parameters affecting the reaction were established.
A path is established to further scale-up and optimize solvent-free mechanochemical nitration of organic compounds. Catalysts are benign, reusable or recyclable. Reaction rate is controllable and yield is high. Found relationships between the process parameters, materials, and yield and reaction rate for the nitro-products will guide future mechanistic and scale-up efforts. Design of pilot-plant production of single-nitrated products can be a logical next step.