Investigation On The Selective Reduction Of Nitroarenes In Fe-CO₂-H₂O System

In this work, CO2-H2O system employing iron, where the carbonic acid, in situ formed, takes the place of the traditional acid, is developed for the selective reduction ofp-(and o-) chloronitrobenzene (CNB),1-nitroanthraquinone (1-NO2AQ) etc.. The carbonic acid above promotes the reduction of nitroarenes during the reaction employing iron under relatively mild reaction conditions. After the reduction, releasing CO2 makes the system self-neutralize, as a result, there is no need for the base to neutralize the acid and no formation of the salt in the system above. The water in the system can be reused rather than discharged resulting in that the environmental pollution can be avoided. The selective reduction of nitroarenes using iron in CO2-H2O system is an environmentally benign process worth to develop not only in practice but also in theory study.Different reaction parameters, such as temperature, pressure the molar ratio of iron to substrate etc., influencing the reduction ofp-(and o-) CNB and 1-NO2AQ are systematically examined in order to optimize the reaction conditions. Some optimized reaction results are as follows:n (Fe)/n (CNB)=3/1, n (H2O)/n (CNB)=194/1,120℃,7.0 MPa,360 r/min. After 8 h, p-CNB is reduced quantitively to p-chloroaniline (p-CAN) and the selectivity to p-CAN is near to 100%; After 6 h,600 r/min, for o-CNB, both the conversion of o-CNB and the selectivity to o-CAN are 100%; 1-NO2AQ is reduced to give 98%conversion of 1-NO2AQ and near 100%selectivity to 1-aminoanthraquinone (1-NH2AQ) when the reaction is run under the reaction conditions below:n (Fe)/n (1-NO2AQ)=3/1, n (H2O)/n (1-NO2AQ)-777/1, 140℃,7.0 MPa,600 r/min for 8 h.Other nitroarenes can be reduced to relevant aromatic amines. For example, p-aminoacetophenone can be obtained from p-nitroacetophenone with near 100%selectivity to p-aminoacetophenone employing iron in CO2-H2O system and 4,4'-diaminostilbene-2,2'-di-sulfonic acid has the same result with that ofp-aminoacetophenone.In this research, to make the reduction of nitroarenes in CO2-H2O system more environmentally benign, based on the solubility of aromatic amines in CO2, removing the aromatic amines from CO2-H2O system by scCO2 rather than the organic solvent after the reduction is investigated. The results show that under the conditions of 35℃,8.1-9.0 MPa, after 11 h, the extraction recovery of 0.63 gp-CAN is 79%using 6.01/h CO2 and under the conditions of 35℃,7.4-8.0 MPa, after 9 h, the extraction recovery of 0.63 g o-CAN is 85% using 6.0l/h CO2. This indicates that scCO2 can remove aromatic amines from CO2-H2O system efficiently.