(8e) Kinetic Study and Safe Production-Scale Synthesis of Sodium Nitrotetrazolate in Silicon Microreactor Systems

Murphy, E. R., DuPont Central Research and Development
Kralj, J. G., National Institute of Standards and Technology
Zaborenko, N., Massachusetts Institute of Technology
Jensen, K. F., Massachusetts Institute of Technology

Microreactor systems offer significant advantages over their macro-scale counterparts in the field of chemical reaction optimization. With reduced thermal mass and rapid mixing, conditions within a microreactor can be more tightly controlled than those in a traditional apparatus. Furthermore, the use of a microreactor limits the quantity of reactive intermediates to only that which is required for immediate processing, thus simplifying containment in the event of a reactor failure and dramatically decreasing environmental impact by removing the need for intermediate storage and making the processes more sustainable and environmentally friendly. Microfluidic reactor systems have been designed and used to optimize chemical reactions. The advantages afforded by silicon-based microreactors allow for safe and efficient kinetic analysis of hazardous reactions.

One very promising use of microreactor systems is the possibility to perform a quantitative study and on-demand synthesis of hazardous intermediates. The direct two-step synthesis of sodium nitrotetrazolate (NaNT) was selected for kinetic analysis. NaNT is an energetic material used in the construction of fire suppression systems and is too dangerous to test with traditional techniques. In the first step of the direct synthesis of NaNT, 5 aminotetrazole (5-AT) reacts with nitrous acid to produce a highly energetic diazonium intermediate. In the second step, the intermediate undergoes a Sandmeyer type reaction that displaces the diazonium group by the nitrite ion. Both steps, but especially the second one, are highly pH dependent; hence, fast mass transfer was required for accurate kinetic studies without mass transfer limitations.

The high mass transfer rates and safety advantages of microsystems were incorporated into a flexible architecture, presenting an improved ability to safely probe the conditions of the reaction. The continuous flow and low concentrations enabled by microreactors allowed the kinetics of direct synthesis of NaNT to be safely and rapidly characterized. The modular design of this system also enabled the set of modules to be rearranged as parallel reactor chains for small-scale production, and a microsystem for commercial production has been constructed. Because of the reduced scale and reduced associated hazard, the microreactor method also eliminates the need for a number of processing steps, thereby reducing the cost and potentially improving the effective yield of the final product.