Pd/C Catalyzed selective hydrogenation of nitrobenzene to cyclohexanone oxime in the presence of NH2OH·HCl: Influence of the operative variables and insights on the reaction mechanism

Consecutive, parallel reactions and equilibria affecting the selectivity in cyclohexanone oxime. [Display omitted] •Nitrobenzene hydrogenation catalyzed by Pd/C in the presence of NH2OH HCl forms Aniline and cyclohexanone oxime.•The best solvent are ethers followed by dichloromethane and dimethylcarbonate.•NH2OH HCl hydrogenates to NH4Cl in the presence of Pd/C as a catalyst.•Aniline in the presence of NH2OH HCl gives cyclohexanone oxime and NH4Cl favors the selectivity to cyclohexanone oxime.•An imine intermediate on catalyst surface gives CON, COX and PCNA all in equilibrium. We studied the influence of temperature, solvent, pressure, catalysts type on the selectivity of nitrobenzene hydrogenation to cyclohexanone oxime (COX) in the presence of NH2OH. The best reaction conditions are: pressure 0.8 MPa, temperature 333 K, solvent ethers, and catalyst Pd/C5%. Other hydrogenation metal catalysts did not give comparable results. The amount of Pd/C influences the yield in COX, which rises above to 90 % at the highest load. The reaction profile shows that aniline is the reaction intermediate. Indeed, aniline as a substrate gives COX, though in lower yield than that achieved employing nitrobenzene. The NH2OH parallel hydrogenation to NH4Cl, influences positively the selectivity to COX. It has been observed that COX, cyclohexanone and N-cyclohexylideneaniline are in equilibrium in the reaction solution and all likely derive from nucleophilic substitutions to a common imine intermediate formed on the Pd surface, whose high activity does not need any further metal catalyst.