Multistep Continuous Heterogeneous Synthesis of C.I. Reactive Red 195 and Safety Evaluation of the Continuous Diazotization Process

Integrating the diazotization and coupling reactions in solid–liquid heterogeneous systems to achieve large-scale, multistep continuous flow synthesis of water-soluble azo dyes remains a significant challenge. During the diazotization process of water-soluble azo dyes, considerable diazonium salt may precipitate, posing potential safety risks. In this study, we established a continuous dynamic tubular reaction system to achieve the multistep continuous heterogeneous synthesis of C.I. Reactive Red 195, a representative water-soluble azo dye. The optimal conditions for continuous diazotization and coupling reactions were determined, achieving a high throughput of 120 L/h and a yield of up to 736 kg/day. The purity of the synthesized dye increased by 20% compared to the commercial C.I. Reactive Red 195, with the K/S value rising from 19.07 to 22.16, indicating enhanced dyeing performance. Density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations show that 2-naphthylamine-1,5-disulfonic acid diazonium salt (NADA-DS) spontaneously aggregates into stable clusters due to dispersion forces, which leads to precipitation. Furthermore, the thermal stability, impact sensitivity, explosive propagation, and decomposition activation energy of NADA-DS were investigated. The severity and possibility of thermal runaway during the continuous diazotization process are classified as level 1. The risk matrix indicates that the continuous diazotization process risk is acceptable, with the Stoessel criticality diagram categorizing the hazard level as grade 1, signifying a low level of risk. This study promotes safer, more efficient, and sustainable production of water-soluble azo dyes.