Organic compounds -- part of the class 532-570 series – Organic compounds – Amino nitrogen containing
Patent
1997-11-13
2000-08-01
O'Sullivan, Peter
Organic compounds -- part of the class 532-570 series
Organic compounds
Amino nitrogen containing
564168, 564417, 564418, 564421, 564422, 564423, 562 47, 562 60, C07C30340, C07C23112, C07C20936
Patent
active
060969245
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for the hydrogenation of unsubstituted or substituted aromatic nitro compounds with hydrogen in the presence of known hydrogenation catalysts, typically Rh, Ru, Pt, Pd, Ir, Ni or Co, at which hydrogenation catalytic amounts of at least one vanadium compound must be present. The invention also relates to the use of vanadium compounds in the catalytic hydrogenation of aromatic nitro compounds with hydrogen in the presence of known hydrogenation catalysts.
The catalytic hydrogenation of aromatic nitro compounds is a reaction which is industrially important, for example for the preparation of intermediates for agrochemicals, dyes and fluorescent whitening agents. For the preparation of stilbene fluorescent whitening agents, for example, 4,4'-dinitrostilbene-2,2'-disulfonic acid has to be reduced to 4,4'-diaminostilbene-2,2'-disulfonic acid, which may be achieved by classical reduction processes or by catalytic hydrogenation. The preparation of azo dyes requires large amounts of diazonium salts which in turn are prepared from the corresponding amines.
Catalytic hydrogenations of aromatic nitro compounds to the corresponding aromatic amines proceed via several intermediary stages. Important among these are the corresponding nitroso compounds and, in particular, the hydroxylamine intermediate, as is described, inter alia, by M. Freifelder in Handbook of Practical Catalytic Hydrogenation, Verlag Wiley-Interscience, New York, 1971.
This hydroxylamine intermediate poses a special problem in practice, because under specific conditions it can accumulate in large amounts in reaction solutions. This applies in particular to aromatic nitro compounds, the hydrogenation of which results in relatively stable arylhydroxylamines. This is particularly critical when the hydrogenation is carried out in a slurry batch reactor. In the extreme case, several tons of arylhydroxylamine can thus be formed.
Arylhydroxylamines are in many respects problematical. For one thing it is known that such compounds are often thermally instable and can disproportionate during heating with or without H.sub.2 with strong emission of heat. The liberated heat can trigger further decomposition reactions which can then result in incidents with heavy explosions. W. R. Tong et al, AICHE Loss Prev. 1977, (11), 71-75 describe such an incident during the reduction of 3,4-dichloronitrobenzene to 3,4-dichloroaniline.
This instability makes a thorough and elaborate thermal examination of hydrogenation mixtures imperative. In particular, the thermal behaviour of the possible hydroxylamine intermediates must be thoroughly examined. F. Stoessel, J. Loss Prev. Process Ind., 1993, Vol 6, No 2, 79-85 describes this procedure, using the hydrogenation of nitrobenzene to aniline as an example.
Arylhydroxylamines are also known as strong carcinogens and therefore constitute a high hazard potential in the case of interrupted or incomplete hydrogenation (J. A. Miller, Cancer Res. 3 (1970),559).
The preparation of a pure amine constitutes a third complex of problems. If, during the hydrogenation or at the end of the reaction, significant amounts of arylhydroxylamine are present, then this may lead to condensations with formation of unwanted and dyed azo or azoxy products. Since the amount of arylhydroxylamine can change from batch to batch, the resulting product quality differs in purity and aspect.
The problems indicated above are further aggravated by the fact that the resulting concentrations or even the maximum possible concentrations of this hydroxylamine intermediate cannot be predicted even in processes which are known and well-studied. The presence of impurities in the trace range can trigger the spontaneous accumulation of hydroxylamine intermediates in unpredictable manner. In, for example, Catalysis of Organic Reactions, Vol 18, (1988), 135, J. R. Kosak relates that the simple addition of 1% of NaNO.sub.3 increases the accumulation during the hydrogenation of 3,4-dichloronitrobenzene from the initial <5% to about 30%
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Baumeister Peter
Studer Martin
Novartis AG
O'Sullivan Peter
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