Process optimization, thermal hazard evaluation and reaction mechanism of m-xylene nitration using HNO3-Ac2O as nitrating reagent

Nitro-m-xylenes (NMX) was synthesized in semi-batch mode via m-xylene nitration using nitric acid-acetic anhydride (HNO3-Ac2O) as nitrating reagent in this work. The nitration process was optimized by response surface methodology. The yield of NMX could reach 92.40% under the optimal conditions of jacket temperature of 20 °C, 0.15 mol of HNO3 and 0.19 mol of Ac2O. Corresponding large reaction enthalpy (ΔH=240.9 kJ/mol) and adiabatic temperature rise (ΔTad,r=398.1 K) indicated the serious hazards of nitration in a thermal runaway scenario. Pyrolysis of nitration products occurred at about 320–400 °C by differential scanning calorimetry tests. The reaction order of the pyrolysis was determined to be 1.2 by adiabatic kinetic analysis. Thermal risk of the nitration process was assessed to be acceptable and level 1 via the risk matrix and Stoessel criticality diagram analyses, respectively. Furthermore, detailed nitration mechanisms for NO2+ generation and aromatic substitution were presented. The activation free enthalpy and free energy parameters for each step were calculated by density functional theory (DFT). These findings can help understand the exothermic sources of m-xylene nitration and guide the intrinsically safer design and scale-up of the process.