Fundamentals of Mass Transfer and Kinetics for the Hydrogenation of Nitrobenzene to Aniline - Air Products

Introduction
From a historical perspective, aniline is perhaps one of the more important synthetic organic chemicals ever manufactured. In 1856, Sir William Henry Perkin, a student at the Royal College of Chemistry in London, discovered and isolated a purple dye during the oxidation of impure aniline. The discovery of this dye, known as mauve, created quite a stir and Perkin, seeing the value of his discovery, proceeded to scale
up the synthetic process for the production of mauve, which included the synthesis of aniline. This process was to become one of the first commercial processes to generate a synthetic organic chemical.

During the last three decades, polyurethane plastics have emerged as a growth industry and aniline once again plays a key role as an industrial intermediate used in the manufacture of MDI, 4,4’-diphenylmethane diisocyanate, a key commercial monomer in the manufacture of polyurethane plastics.

Aniline is produced by the reduction of nitrobenzene, which is produced from the nitration of benzene in a mixture of sulfuric and nitric acid. Originally, nitrobenzene was reacted with dispersed iron in the presence of HCl to generate aniline and an iron oxide sludge.

This process generated large quantities of waste and was eventually replaced by the catalytic hydrogenation of nitrobenzene in a three-phase slurry reactor.

Aniline can also be produced in the gas phase by the reduction of nitrobenzene with hydrogen over fixed catalysts. This paper focuses on the characterization of a slurry process for the reduction of nitrobenzene to aniline.  The catalytic hydrogenation of nitrobenzene to aniline in a continuously mixed slurry reactor is a complex chemical process.  A number of competing mass transfer and kinetic rate processes contribute to the overall observed reaction rate. Scale-up and optimization of the process require that the contributing rate processes are understood individually and that their impact on the total process rate be quantified. Laboratory reactors must be operated under conditions that will allow meaningful process characterization for scale-up.

The key rate processes are illustrated in Figure 1, which shows schematically typical concentration profiles during the hydrogenation of the nitrobenzene: first, the rate of hydrogen mass transfer from the gas phase to the liquid phase; the rate of hydrogen and nitrobenzene mass transfer from the bulk liquid phase to the outer surface of the catalyst; the rate of hydrogen and nitrobenzene mass transfer into the porous catalyst; and finally, the absorption and kinetic rates of the hydrogen and nitrobenzene on the inner catalytic surface of the catalyst particle.

Two classical, mechanistic pathways to aniline from nitrobenzene are possible depending upon process conditions and the effects of gas/liquid and liquid/catalyst mass transfer. Intermediates formed during these competing chemical routes can act as reversible catalyst poisons that can radically change reactor performance. It is not within the scope of this paper to exhaustively review the literature or to optimize the process catalysts, temperature or conditions for the reduction of nitrobenzene to aniline; rather, this paper will describe the principles, laboratory methods and analysis techniques for characterizing these competing processes.

Literature

A review of the literature can yield a confusing description of the overall process kinetics of nitrobenzene hydrogenation with authors reporting reaction orders for nitrobenzene and hydrogen between zero and one depending upon reaction conditions.  A general review of the kinetic studies is given in the doctoral thesis by Füsun Yücelen. Most kinetic studies generate global kinetic rate models based on hydrogen uptake and overall conversion. However, some detailed studies using analytical methods to identify the individual intermediate species have also been used to characterize reaction kinetic processes8.

Additionally, most of the kinetics studies in the literature are conducted at relatively low temperatures with solvents. Optimized industrial processes would rather eliminate the use of solvents and their associated separation processes, which can negatively impact overall process economics. Environmental pressures also tend to favor processes that use fewer solvents.

It would generally be preferred to use the reaction products of the nitrobenzene reduction, i.e., aniline and water, as the reaction solvent. Unfortunately, aniline and water are not miscible at low temperatures; therefore, higher temperatures must be employed to maintain a single liquid phase under process conditions.  No literature studies are reported under these conditions. It is hoped that his study will help shed some light on this process while illustrating the general principles required for characterizing a hydrogenation process from the laboratory scale.

Experimental

For this set of studies the global reaction kinetics was determined by continuously monitoring the reaction exotherm using the METTLER TOLEDO RC1/HP60 highpressure reactor fitted with a single baffle and gas induction agitator supplied by Mettler-Toledo Inc.9.  The METTLER TOLEDO PIC10 pressure controller was operated to allow hydrogen gas into the reactor to maintain the desired pressure setpoint while gas was not allowed to exit the reactor.

The reduction of nitrobenzene to aniline is exothermic and generates –536.6 ± 5.9 kJ/mole as measured in this study, making the calorimetric method ideal for this process. Reagents were all purchased through Aldrich. Individual species were monitored using internal standard gas chromatographic methods. Sponge nickel or Raney^® type catalysts were used in this study at concentrations between 0.05 % and 0.10 %.  All reactions were carried out with the agitator operating at 1000 rpm, which gave a mixing intensity measured at 1.0 watt/liter.

Chemistry

The chemistry for the reduction of nitrobenzene to aniline was originally elucidated by Haber as reported by Strätz10.

Two paths exist to aniline according to this scheme.  The first proceeds from the sequential reduction of nitrobenzene: first to nitrosobenzene, next to phenylhydroxylamine, and finally to aniline. This path, we will see, is favored under conditions in which the concentrations of nitrobenzene are low, < 0.15 %. If the concentrations of nitrosobenzene and phenylhydroxylamine are allowed to increase, the formation of azoxybenzene proceeds rapidly followed by reduction to azobenzene, phenyl hydrazine, and finally, aniline.

The second path, while ultimately yielding aniline, is much slower kinetically on nickel catalysts and is favored under conditions in which the nitrobenzene concentrations are high, > 0.15 %. During high temperature studies, the only stable intermediates identified and monitored were nitrobenzene, azoxybenzene and azobenzene, as we will see later.

Aniline/Water Solubility

The solubility of aniline and water has been well documented.  The stoichiometric composition of the complete reduction product of nitrobenzene is 27.9 % water/72.1 % aniline. From Figure 3, temperatures in excess of 160 °C would be required to keep this mixture homogeneous.  High temperatures can impact byproduct chemistry, reducing aniline yields. However, if aniline is fed to the reactor system along with the nitrobenzene or if water is continuously removed via distillation, the concentration of aniline in a continuous process can be maintained at aniline concentrations higher than that dictated by the stoichiometry.

Therefore, lower temperatures are required to keep the system homogeneous. In this study the continuous feed to the reactor consisted of 50 % aniline and 50 % nitrobenzene, which maintained a nominal steady-state composition in the reactor of 14.3 % water and 85.7 % aniline. Under this composition, temperatures in excess of 135 °C are required to maintain homogeneity of the liquid phase.