Chemoselective Reduction of Nitroarenes using Hydrogen Sulphide under Phase Transfer Catalysis

Abstract

Hydrogen Sulphide gas (H2S) is the major source of sulphur as an impurity in gasification process of fossil fuels, biogas plant, syngas production plant, petrochemical and various industrial gases. H2S gas is highly corrosive, toxic and odorous in nature. It is very necessary to remove H2S from gas streams as it can damage mechanical and electrical components of any control system, corrode energy generation and heat recovery units. In the present work, our main aim is utilise this toxic unwanted H2S and synthesise value added fine chemicals such as aromatic amines. In order to achieve our aims two industrially used alkanolamines such as mono ethanolamine (MEA) and n-methyldiethanolamine (MDEA) have been used to absorb H2S and this H2S-laden aqueous alkanolamine solution is used as a reducing agent. Mono nitro, dinitro, polynitro, heterocyclic nitro compound have been reduced selectively to their corresponding aromatic amines in the liquid-liquid or liquid-liquid-solid phase transfer catalysis mode of reaction in the presence of phase transfer catalysis. In this current work insoluble PT catalyst have been used such as Amberlite IR400 (Cl) and a number of soluble PT catalyst have been used such as Tetrabutylammonium bromide (TBAB), Tetrabutylphosphonium bromide (TBPB), Tetramethylammonium bromide (TMAB), Tetrabutylammonium iodide (TBAI) and Ethyltriphenylphosphonium bromide (ETPPB), Tetrapropylammonium bromide (TPAB). The main objectives of this work are to maximise conversion of the organic substrate, maximise selectivity of the desired product and to deveolop a suitable mechanism to explain the whole reduction process. Six different system have been studied and in those five system chloronitrobenzene (CNB) reduction have been studied in L-L and L-L-S PTC mode of reaction and 1-nitronapthalene (1-NN), nitroacetophenone (NAP), dinitrotoluenes (DNT) have been studied in the biphasic mode of reaction. In last system total sixteen nitroaromatic compounds have been reduced under an identical set of parameters. For all the system parametric study, mechanistic investigation was performed and kinetic and statistical model have been established. The studied parameters are stirring speed, catalyst concentration, temperature, reactant concentration, sulphide concentration, MDEA loading, elemental sulphur loading. The developed model have been validated with the experimental data and the model predicts the conversion well.