Continuous diazotization of aromatic amines with high acid and sodium nitrite concentrations in microreactors

We design a new continuous diazotization microreaction process in which concentrated acid and sodium nitrite solutions are used. We form oil in water (O/W) microdroplets by mixing amine with sodium nitrite aqueous solution, which react with mineral acid to make the diazotization. In this way, the formed particles are dispersed in the aqueous phase, which can flow with the main stream, and consequently are consumed. Then the fluid can smoothly flow without clogging in the microchannels and we can achieve continuous operation. We use aniline, o-methylaniline, m-methylaniline, and o-ethylaniline as the raw materials, 6 M HCl and 6.5 M H₂SO₄ as the acid as well as sodium nitrite to demonstrate the feasibility. The resulting diazonium salts are converted to iodo-substitute aromatic compounds to measure the yield. The results show that, SIMM-V2 micromixer is efficient to form the emulsion. Mixing the microdroplets with acid in a co-axial flow way can prevent clogging. In hydrochloric acid medium, high reaction yield can be obtained when the reaction temperature is 4 °C and the residence time is 13–15 s. In sulfuric acid medium, high conversion of aromatic amine of up to 99% and yield of approximately 90% can be achieved at the reaction temperature of 9 °C and the residence time of 14–16 s. This process provides efficient continuous diazotization reactions using concentrated acid and sodium nitrite solutions, minimizing the production of waste water.