Probing the Role of Promoters in Zinc Reduction of Nitrobenzene:  Continuous Production of Hydroxylaminobenzene

Abstract

Production of hydroxylaminobenzene (HAB) via zinc reduction of nitrobenzene (NB) is an established batch process based on aqueous electrolytic reactions involving an electrolyte as the promoter. This experimental study explored continuous production of HAB that is needed as an intermediate in a novel zinc/biocatalyst processing train, recently demonstrated by Luckarift et al. to obtain aminophenols. Experiments were conducted using a Zn packed bed to probe the role of promoters under continuous-flow conditions at a temperature of 60 °C and promoter/NB molar ratios of 0.75, 1.15, and 1.5. In addition to the conventional NH4Cl, ammonium formate and acetate were used as promoters. The best HAB yield was 95%, as compared to the batch yield of 68%. The formation of oxidized zinc species caused the packed bed to build back pressure, which appeared to enhance the selectivity of HAB/aniline. The formation of NB and ammonium/zinc complexes, as supported by voltammetric evidence, appears to be one of the key roles of promoters. These findings demonstrated efficacy and advantages of the continuous HAB production process.

Hydroxyl-group-modified polymeric carbon nitride with the highly selective hydrogenation of nitrobenzene to N-phenylhydroxylamine under visible light

“…[4][5][6][7] PHA was usually acquired via chemical reduction of NB by stoichiometric zinc powder, which resulted in the discharging of metal wastes and environment disruption. 8,9 In recent years, reports on the heterogeneous catalytic selective Scheme 1 The proposed reaction pathways of the consecutive hydrogenation of nitrobenzene.…”

An Efficient Continuous Flow Synthesis for the Preparation of N-Arylhydroxylamines: Via a DMAP-Mediated Hydrogenation Process

“…N -Arylhydroxylamines ( N -AHA) are versatile organic molecules used as intermediates to synthesize industrially valuable fine chemicals, bioactive drugs, and polymerization inhibitors [ 1 , 2 , 3 , 4 , 5 , 6 ]. Among the reported protocols for obtaining N -AHA, the selective reduction of nitroarenes is the most convenient, which involves a biocatalytic reduction by baker’s yeast or plant cell [ 7 , 8 ], the use of Fe or Zn in stoichiometric quantity [ 9 , 10 , 11 ], and homogeneous or heterogeneous metal catalysts (with Pt, Ni, Pd, Rh, Ir, Ag, Au) with hydrazine or H 2 [ 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. At present, lots of attention is focused on the development of a heterogeneous metal catalytic system because of its great stability, reusability, and easy isolation.…”

Selective hydrogenation of nitroaromatics to N-arylhydroxylamines in a micropacked bed reactor with passivated catalyst

“…Fortunately, a similar strategy has been successfully proved by pumping the substrate through a packed-bed of zinc powder. 10 Encouraged by this result, we commenced our exploration of selective hydrogenation under continuous-ow conditions using RANEY®-nickel as a catalyst. Our initial strategy speeded up feeding-rate imaging, indicating that the desired N-arylhydroxylamine could be ushed out before the over-reduction.…”