Reduction of Nitrobenzene in a Parallel‐Plate Reactor: I . Differential Conversion Using Periodic Cell‐Voltage Control

Abstract

The effect of steady and periodic, step cell‐voltage control on the selectivity of the reduction of nitrobenzene in a parallel‐plate reactor was theoretically investigated. Under step cell‐voltage control, the potential is continuously oscillated from a constant high to a constant low value. Nitrobenzene (NB) is electroreduced to the intermediate phenylhydroxylamine (PHA) which may undergo further electrochemical reduction to aniline (AN) or may rearrange chemically to form the desired product, p‐aminophenol (PAP). A reactor model was developed which assumes a well‐mixed core, mass‐transfer boundary layers for NB and PHA at the cathode, concurrent hydrogen evolution, oxygen evolution at the anode, and negligible migration transport. An analytic, stationary‐state solution to the transient‐diffusion equation within the boundary layers was coupled through the reaction kinetics to an analytic solution of the one‐dimensional Laplace equation for electrolyte potential. Differential‐conversion simulations were performed in which the homogeneous reaction need not be considered because its characteristic reaction time is large compared to both those of the other reactions and to the cycle time of the control. The best periodic cell‐voltage control strategy employed in the simulations was found to produce a PHA selectivity 250% of that under steady control at the same cycle‐averaged PHA production rate. Under periodic control, PHA selectivity was shown to increase asymptotically with decreasing cycle time; high‐frequency operation lowered the depletion of NB and overabundance of PHA in the boundary layer which otherwise would occur during the high‐polarization portion of the cycle. Decreasing the duty cycle (fraction of cycle time spent at high polarization) increased the PHA selectivity since smaller duty cycles resulted in a higher NB concentration and a lower PHA concentration at the cathode surface, but simultaneously the PHA production rate decreased. Increasing the voltage of the high‐polarization portion of the cycle kinetically favors the production of PHA over AN. An increase in the mass‐transfer rate causes a like effect on the PHA selectivity and to a much smaller extent on the PHA production rate for any set of steady or periodic‐control parameters. The differential‐conversion results provide a basis for the control strategies used in integral‐conversion simulations and experiments presented in Part II of this work.