Environmentally friendly hydrogenation of nitrobenzene to p-aminophenol using heterogeneous catalysts.
François Figueras (Institut de Recherches Sur la Catalyse et l'Environnement de Lyon, France)
Location: Aula 118, ETSEQ
Start time: Nov. 12, 2010,
12 p.m.
Abstract
The selective catalytic hydrogenation of nitrobenzene (NB) to p-aminophenol (PAP) is a reaction of industrial importance since PAP is an important pharmaceutical intermediate for the manufacture of paracetamol, a widely used analgesic drug (world market 160.000 t/year). The industrial process reported for the preparation of PAP, involves first nitrobenzene partial hydrogenation to phenylhydroxylamine (PHA) on Pt, followed by the Bamberger rearrangement of PHA to PAP in sulphuric acid. After 3 h of reaction, a 75% conversion of nitrobenzene has been reported with a PAP selectivity of 79% using 27 mL of concentrated sulphuric acid for 57 mL of nitrobenzene in 406 mL of water. The PAP thus formed needs to be isolated by neutralising the acid, leading to the production of salts as effluents (about 1kg of salt per kg of PAP) and more complex separations. Most patents also report the addition of surfactants which increase the area of contact between nitrobenzene and water, usually used as solvent. There is therefore an interest for processes, which would be highly selective towards PAP, would not produce salts as effluents, and could avoid complex separation.
It can be pointed out that this is a difficult task since the coupling of hydrogenation with acid catalysed rearrangement is rather uncommon in organic synthesis. The formal reaction scheme has some common points with the bifunctional isomerization of paraffins, with the major difference that, in paraffin isomerization the concentration of the olefin intermediate is governed by the thermodynamic equilibrium of hydrogenation, while here all steps are controlled by kinetics, therefore a kinetic study is of paramount importance.
We describe a process in which NB is converted to PAP at 353 K, using water as solvent and a bi-functional catalyst composed of a mechanical mixture of supported Pt catalyst with zirconium sulphate calcined at 773-923K. The performance of this system is independent of the support used for Pt, since pure or sulphated zirconia and titania, carbon, Mg-La mixed oxide, etc., give similar results. At low Pt content, the reaction rate is first order relative to NB, and the slow step is the partial hydrogenation of NB to PHA, requiring only minute amounts of Pt. At higher Pt loadings, the rate of hydrogenation of NB to PHA and consequently to aniline takes over that of the acid catalysed Bamberger rearrangement of PHA to PAP. The selectivity of this step depends critically on the solid acid: strong acids such as sulphated zirconia or zeolites give poor selectivities because they tend to decompose the hydroxylamine intermediate. This process does not require sulphuric acid or additives such as DMSO or alkylsulfides, thereby simplifying the down-stream processing.
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