Selective Liquid‐Phase Hydrogenation of a Nitro Group in Substituted Nitrobenzenes over Au/Al₂O₃ Catalyst in a Packed‐Bed Flow Reactor

Abstract

A series of substituted nitrobenzenes with the general formula XC6H4NO2 (X=Cl, CH=CH2, or C(O)CH3) dissolved in toluene were reduced with hydrogen over the 1.9 % Au/Al2O3 catalyst at 60–110 °C and 10–20 bar in a three‐phase packed‐bed reactor operating in up‐flow mode. Under these conditions, hydrogenation of isomeric ClC6H4NO2 gives exclusively chloroanilines. Hydrogenation of 3‐CH2CHC6H4NO2 and 4‐CH3C(O)C6H4NO2 leads to the formation of 3‐CH2CHC6H4NH2 and 4‐CH3C(O)C6H4NH2 with selectivities of up to 93 and 97 % at substrate conversions of 98 and 100 %, respectively. Smooth catalyst deactivation was observed regardless of which substituted nitrobenzene was taken for hydrogenation. According to the results obtained by temperature‐programmed oxidation of the spent catalyst, a carbonaceous deposit formed that might block the catalyst surface. Almost complete regeneration of the supported gold catalyst with retention of its high selectivity to hydrogenation of a nitro group was achieved in a flow of air at temperatures up to 400 °C to eliminate carbonaceous deposits.

Unconventional Pd@Sulfonated Silica Monoliths Catalysts for Selective Partial Hydrogenation Reactions under Continuous Flow

“…A reason for this may reside in the fact that, among the chloro‐nitrobenzene isomers, the meta one is the most recalcitrant to (selective) hydrogenation . The continuous hydrogenation of 1 to 1 a , in 99 % conversion and 98 % selectivity, was described using 1.9 % Au@γ‐Al 2 O 3 packed catalysts in toluene at 80 °C, 10 bar H 2 pressure and very high H 2 / 1 ratio (107) . However, the catalyst deactivated rapidly, because approximately 20 % of its starting activity was lost after 2 h continuous stream.…”

“…Nonetheless, the reaction conditions could be optimized ( τ 17 s, H 2 : 6 ratio 9) so as to achieve a 73 % 6 a selectivity at 90 % 6 conversion, with a remarkable 6 a STY of 0.29 kg L −1 h −1 (Table , entry 4). As above described for 1 , the liquid‐phase continuous‐flow hydrogenation of 6 was previously reported in 99 % conversion and 99 % selectivity using 1.9 % Au@γ‐Al 2 O 3 packed catalysts, however under harsher conditions than that with Pd@MonoSil‐ArSO 3 catalyst (80 °C, 10 bar H 2 , H 2 : 6 ratio 107, catalyst regeneration at 400 °C required) . The batch hydrogenation of 6 by Pd catalysts was previously reported using Pd@SiliaCat and 5 % Pd@C in scCO 2 , however with complete and significant (16 %) dechlorination, respectively.…”

Recent Progress in Continuous‐Flow Hydrogenation

“…Another example of the hydrogenation of substituted nitrobenzenes was reported by Bukhtiyarova and co‐workers . Using Au/Al 2 O 3 as catalyst, prepared by the deposition‐precipitation method, 1‐nitro‐3‐vinylbenzene ( 48 ) and 1‐(4‐nitrophenyl)ethan‐1‐one ( 49 ) as reaction models were hydrogenated (60–110 °C, 10–20 bar H 2 pressure, 1.9 % catalyst loading) in a three‐phase packed‐bed reactor to afford 50 and 51 , respectively.…”

Flow synthesis of secondary amines over Ag/Al2O3 catalyst by one-pot reductive amination of aldehydes with nitroarenes

“…As a result of special tests, we found earlier that there is no inuence of external or internal mass transfer limitations under the conditions used. 9 The performance of the catalyst was evaluated by analysis of the samples taken 30-34 min aer the beginning of the experiment. The reaction products were analyzed by GC (Agilent 6890N instrument with a HP 5-MS capillary column 60 m  320 mm  0.25 mm) using n-decane as the internal standard.…”