Enhancing the Nitration Efficiency of MPO for FOX-7 Production via Ultrasound-Assisted Technology

Abstract 1,1-Diamino-2,2-dinitroethene (FOX-7) is a promising insensitive high-energy material with excellent thermal stability and low sensitivity. However, its synthesis via the nitration of 6-hydroxy-2-methylpyrimidin-4-one (MPO) suffers from low nitration efficiency. In this study, ultrasound-assisted intensification technology was systematically investigated to enhance the nitration reaction of MPO. The effects of ultrasonic irradiation on reaction rate, product yield, and reaction mechanism were explored using both an ultrasonic bath and an ultrasonic probe. The results showed that ultrasound significantly accelerates the nitration process. Under ultrasonic assistance, the isolated yield of FOX-7 reached 77.8% within 30 min, compared to only 35.7% under silent conditions. Increasing acoustic input further accelerated the early nitration process, whereas prolonged probe irradiation decreased the FOX-7 yield, indicating that excessive ultrasonic exposure may promote degradation of nitration products or intermediates. Control experiments showed that improved macromixing and thermal effects contributed to, but did not fully account for, the ultrasonic enhancement. Quantum-chemical calculations were therefore performed to examine an •OH-mediated hydrogen atom transfer (HAT) pathway. The modeled HAT steps exhibited moderate local free-energy barriers, while the localization of unpaired electrons at the resulting carbon centers supported the proposed site-specific hydrogen-abstraction sequence. Subsequent •NO2 coupling and HSO4–-assisted proton transfer provide plausible routes for nitro-group incorporation and intermediate conversion. Moreover, differential scanning calorimetry (DSC) results, together with the observed product yield, suggest that the temperature near the cavitation bubbles (≤100.7°C) was insufficient to cause substantial thermal decomposition of the product under the applied ultrasonic conditions. This work demonstrates that ultrasound is an effective and mild process intensification tool for the nitration synthesis of FOX-7, offering both mechanistic insight and practical potential.