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. This 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 relativey stable arylhydroxylamines. Especially when carried out in batch reactor, this may become a severe problem.

Arylhydroxylamines can undergo spontaneous decomposition, are known carcinogenes, and are responsible for unwanted by-product formation through condenations.

Solvias AG (EXTERNAL LINK) has managed to overcome the intermediary formation of arylhydroxylamines by use of Vanadium promoted precious metal powder catalysts (US 6096924).

Vanadium affects the mechanism of the nitro group hydrogenation by catalysing the disproportionation of the intermediate arylhydroxylamine. This avoids the intermediary accumulation of the arylhydroxylamines.

Evonik owns an exclusive licence for this technology and offers V promoted catalysts in its portfolio. You can find them under Reactions and Products (e.g. CF 1082 R/W 3% Pt + 2% V, or CF 1082 B/W 1 % Pt
+ 3 % V).