Toward sustainable chemoselective nitroarene hydrogenation using supported gold as catalyst

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/TiO2 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-H2 (5-12 atm) increased the rate and o-CAN selectivity (to 86%). In contrast, Au/TiO2 promoted exclusive o-CAN production regardless of P-H2 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 x 10(3) -> 86 x 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 x 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.