Toward Sustainable Chemoselective Nitroarene Hydrogenation Using Supported Gold as Catalyst

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We have taken a systematic approach to the clean production of functionalized aromatic amines, adopting the hydrogenation of o -chloronitrobenzene ( o -CNB) to o -chloroaniline ( o -CAN) as a case study. We tested a laboratory-synthesized Au/TiO 2 catalyst against a commercial benchmark (Pd/C). Both catalysts exhibited nanoscale (1–9 nm)-supported metal particles ( d TEM = 4.0 nm (Au) and 5.4 nm (Pd)). In batch liquid phase operation, Pd/C was nonselective, generating aniline and nitrobenzene as undesired byproducts, where elevated P H 2 (5–12 atm) increased the rate and o -CAN selectivity (to 86%). In contrast, Au/TiO 2 promoted exclusive o -CAN production regardless of P H 2 but at a lower rate. Reaction exclusivity extended to (ambient pressure) gas phase continuous processing with 100% o -CAN yield and catalyst stability up to 140 h on-stream. A switch from batch to continuous operation was accompanied by an increase in projected o -CAN production capacity (5 × 10 3 → 86 × 10 3 kg o -CAN kg Au –1 year –1 ). Water (as o -CNB carrier) served as an additional source of reactive hydrogen to deliver an order of magnitude increase in the selective hydrogenation rate (vs ethanol) and a production capacity of 14 × 10 5 kg o -CAN kg Au –1 year –1 . Our ultimate catalytic process explicitly addresses nine of the 12 green chemistry principles with an E-factor = 0.28.