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.