SCHEME 1 Reactions for the nitration of CB with mixed acid
Sulfuric acid provides a necessary environment for the generation of . In addition, sulfuric acid as a solvent can absorb reaction heat, which can prevent local overheating and reduce the occurrence of side reactions. Therefore, in industrial production processes, the nitration of CB is usually carried out at high sulfuric acid concentrations, which undoubtedly increases the viscosity of the system and the mass transfer resistance for CB traveling from the organic phase to the acid phase;5 in addition, CB nitration is a fast and highly exothermic reaction. It remains a challenge for batch reactors to provide rapid feedback and sufficient mass and heat transfer rates. The uneven distribution in chemical concentrations, residence times and temperatures leads to side reactions and rapidly increasing temperature, making it difficult to control the selectivity of the reaction. Moreover, the volume of traditional nitrification reactors is large. If an accident occurs, the high chemical oxygen demand (COD) value of the waste acid and the emission of nitrogen oxides can cause serious damage to the environment. To ensure a safe production environment, the reaction is often operated at low temperatures and drip rates. Therefore, the problems associated with these conditions, such as a low space-time yield and high energy consumption, are prominent.
The smaller characteristic size and larger specific surface area of microreactors significantly improve heat and mass transfer rates,6-8 easily meeting the requirements of isothermal conditions and effectively controlling the selectivity of the reaction. A lesser degree of liquid holdup is conducive to the realization of intrinsic safety, rapid mixing provides the system with a uniform chemical concentration distribution and precise residence time, and the enclosed space of the microreactor system effectively inhibits the decomposition of concentrated nitric acid.9,10 In addition, microchemical systems have the advantages of easy automatic control and integrated amplification;11 as such, they act as a safe, efficient and green platform for the nitration of CB. In recent years, many examples of continuous-flow nitration have emerged in which the advantages of microreactors and the various needs of aromatic nitration are combined.12,13 Burns et al.2 carried out the nitration of benzene and toluene in a stainless steel and polytetrafluoroethylene (PTFE) capillary microchannel reactor, respectively. Rapid interphase mixing and mass transfer were realized, and the industrial potential of microreactors in chemical production was preliminarily explored. Kulkarni et al.14 carried out the nitration of benzaldehyde safely and continuously in a microreactor system. Given the high heat transfer efficiency of the microreactor, the reaction time was shortened to 2 min by increasing the temperature. Using a microreactor system, Chen et al.15 continuously synthesized dinitro herbicides in a single step. In contrast to the traditional two-step batch process, there was no need to separate the reaction intermediates, which greatly reduced the amount of solvent required. Yu et al.16used p -difluorobenzene as a raw material and fumed nitric acid to continuously synthesize 2,5-difluoronitrobenzene. Compared with the yield achieved with a batch reactor (80%), the yield achieved with the microreactor was increased (98%), and the reaction time was shortened from 1 h to 2.3 min. Luo et al.17 carried out the nitration of acetyl guaiacol in a microreactor using nitric acid/acetic acid as the nitrating agent. The established kinetic model could accurately predict the experimental results observed at high temperature. The reaction conditions were further optimized, and the product yield reached 90.7%. Chen et al. studied the nitration of 2-ethylhexanol18 and trifluoromethoxybenzene19 in a microreactor. The kinetic model could accurately predict the reaction conversion. Somma et al.20 studied the nitration of benzaldehyde in a microreactor embedded in a static mixer. The results showed that the microreactor exhibited better performance than batch reactors. The effect of reduced mass transfer between the two phases on the reaction rate was greatly reduced.
Accurate kinetic data are essential for understanding the characteristics of nitration in depth and guiding the design of reactors. Early studies confirmed that the nitration of CB is a second-order reaction, and the reaction rate for both CB and nitric acid is described by first-order kinetics. Cox et al.3measured the rate of CB nitration in sulfuric acid with a mass fraction of 70.2%. The rate constants at different temperatures and the activation energy were obtained. Tselinskii et al.21used nitric acid as a nitrating agent to determine the rate of homogeneous CB nitration in 70-90% nitric acid and obtained the rate constants and activation energy. The above studies were all carried out in a batch reactor. Because nitration is a fast reaction, the reaction parameters can change significantly within only a few seconds of sampling and quenching, so the residence time cannot be precisely controlled. Therefore, earlier studies were often carried out at low sulfuric acid concentrations to avoid the large residence time error caused by the rapid reaction. As such, kinetic data for CB nitration at high sulfuric acid concentrations are relatively lacking. In addition, the nitration of CB is a liquid-liquid heterogeneous reaction. It is difficult to eliminate the effect of reduced mass transfer between the two phases on the reaction in a batch reactor, which leads to deviations in the obtained kinetic parameters. Unfortunately, to date, accurate kinetic data for the CB nitration with concentrated mixed acids have not been systematically and comprehensively reported.
The purpose of this study was to accurately measure the kinetics of CB nitration at high mixed acid concentrations. A continuous-flow microreactor system and a homogeneous reaction condition were applied. The effects of temperature, residence time and sulfuric acid concentration on the conversion and selectivity of the reaction were investigated systematically. According to the conversion, the observed reaction rate constants were obtained at different temperatures and sulfuric acid concentrations. The concentration of is not easy to measure directly, and is indispensable to explain the relationship between the reaction rate and sulfuric acid concentration. In this paper, by integrating the experimental data reported in the literature, a mathematical model that can accurately estimate nitric acid equilibria in aqueous sulfuric acid as a function of sulfuric acid concentration and temperature is proposed. Using this model, the rate constants based on are obtained, and the activation energy of CB nitration is calculated.