Nitrobenzene Hydrogenation to N-phenylhydroxylamine: a New Approach to the Selectivity

S.D Kushch

doi:10.18321/ectj513

Abstract

A new approach to resolve the problem of selectivity with respect to N-phenylhydroxylamine in nitrobenzene hydrogenation is proposed. N-phenylhydroxylamine only is the final product of nitrobenzene electroreduction in aprotic media. In this case nitrobenzene reduction carries out by alternation of electrochemical (electron transfer) and chemical (species formed protonation) stages i.e. by so-called EC mechanism. Such mechanism realization in nitrobenzene hydrogenation is possible if i) a catalyst activates hydrogen as “hydrogen electrode” i.e. serves electrons source; ii) a reaction media contains limiting proton concentration. These limitations are discharged in the media of aprotic dipolar solvent, which solvated both positive and negative species. Really, in aprotic dipolar solvents over reduced platinum complexes or lowpercentage (≤1 wt.%) platinum, iridium or osmium catalyst nitrobenzene is hydrogenated with process discontinuance after nitrobenzene total consumption. Nitrobenzene hydrogenation yields N-phenylhydroxylamine as the main (the yield is 98%) product. As these low-percentage catalysts, complex catalyst in situ is heterogeneous i.e. it represents a platinum colloid (particle size ~ 40 nm) stabilized by aprotic dipolar solvent. So, process of nitrobenzene hydrogenation, which is similar to nitrobenzene electoreduction, can is created. A kinetic scheme proposed is analyzed and kinetic equation for initial reaction rate, which is conformed to kinetic data, is obtained.

Highlights

An abundant literature is devoted to catalytic hydrogenation of nitrobenzene into aniline
As first example of this approach use, NB hydrogenation to PHA as a final product with the high (98%) yield catalyzed by complex Pt(DMSO)2Cl2 reduced by NaBH4 in DMSO was reported [7]
Aprotic dipolar solvent having high donor properties competes with PHA for catalyst active centers and can inhibit PHA disproportionation

Summary

Introduction

An abundant literature is devoted to catalytic hydrogenation of nitrobenzene into aniline. PHA is the final product of NB electroreduction in aprotic dipolar media. As first example of this approach use, NB hydrogenation to PHA as a final product with the high (98%) yield catalyzed by complex Pt(DMSO)2Cl2 reduced by NaBH4 in DMSO was reported [7]. Nitrobenzene anion-radical NBT adsorbed on catalyst surface is an intermediate product in this process.

Results

Conclusion