Hydrogenation of aniline

Hydrogenation of anilines normally gives cyclohexylamines and varying amounts of the coupled products, dicyclohexylamines. The ratio of these products is influenced by temperature, catalytic metal, support, additives, and solvent. It is possible to exert substantial control over the product composition. Coupled products increase with increasing temperature (23). 

The amount of coupled product was found to depend importantly on the catalytic metal a sequence for increased coupling to dicyclohexylamine was found to be Ru < Rh Pd Pt (59), a sequence that reflects one reason for the industrial preference for rhodium and ruthenium in hydrogenation of anilines. 

The extent of coupling is also influenced by the solvent. In the hydrogenation of aniline over ruthenium oxide, coupling decreased with solvent in the order methanol > ethanol > isopropanol > t-butanol. The rate was also lower in the lower alcohols, probably owing to the inhibiting effect of greater concentrations of ammonia (44). Carboxylic acid solvents increase the amount of coupling (42). 

Figure 2 Hydrogenation of aniline, toluene, p-toluidine, 4-tert-butylaniline, over M1273. Temperature 338 K and 2 barg hydrogen pressure.

The literature on the hydrogenation of aniline and substituted anilines suggests that the amine functionality can act as a poison [3, 4, 12 and 13] especially over platinum catalysts. The more basic the nitrogen the more... 

Various modifications of the reaction of an alcohol with ammonia provide the most common commercial routes to alkylamines. Some others routes that are used to make certain individual amines include aldehyde/amine additions, nitrile reduction, the Ritter reaction, amination of isobutylene, and hydrogenation of anilines. Capacities of many plants depend on the product mix of mono/di/tri products as well as the variety of amines (ethyl, propyl and butyl). One must know the product mix that the capacity is based upon and the actual scheduled output of produces) to determine what amounts can actually be manufactured. Capacities are often in excess of anticipated demand to satisfy seasonal demands for pesticide uses116. 

A manufacturing process that is specific for cyclohexylamines is the catalytic hydrogenation of anilines or phenols in the presence of ammonia. The catalytic hydrogenation of aniline is the classical method for the manufacture of cyclohexylamine. Hydrogenation of phenol in the presence of ammonia produces predominantly cyclohexyl or dicyclohexylamine depending upon catalysts, reaction conditions and reactant ratios116. 

Hydrogenation of anilines usually affords a saturated amine as the major product, but several side reactions, including hydrogenolysis, reductive hydrolysis, and reductive coupling, may accompany and even dominate the reduction. Hydrogenolysis is important only in certain activated molecules (10). 

Reductive Coupling. Formation of dicyclohexylamine (25) in hydrogenation of aniline probably involves addition of cyclohexylamine to an imine and subsequent hydrogenolysis of a carbon—nitrogen bond (11). 

The per cent of dicyclohexylamine formed in hydrogenation of aniline increases with catalyst in the order ruthenium < rhodium platinum, an order anticipated from the relative tendency of these metals to promote double bond migration and hydrogenolysis (30). Small amounts of alkali in unsupported rhodium and ruthenium catalysts completely eliminate coupling reactions, presumably through inhibition of hydrogenolysis and/or isomerization. Alkali was without effect on ruthenium or rhodium catalysts supported on carbon, possibly because the alkali is adsorbed on carbon rather than metal (22). 

The temperature required for the reduction of cobalt oxides to the metal appears to be somewhat higher than for the reduction of nickel oxide. The catalyst with a higher catalytic activity is obtained by reduction of cobalt hydroxide (or basic carbonate) than by reduction of the cobalt oxide obtained by calcination of cobalt nitrate, as compared in the decomposition of formic acid.91 Winans obtained good results by using a technical cobalt oxide activated by freshly calcined powdered calcium oxide in the hydrogenation of aniline at 280°C and an initial hydrogen pressure of 10 MPa (Section... 

This paper examines the hydrogenation of aniline, /Moluidinc. and 4-fcrt-butylanilinc over a series of 2.5 % Rh/Si02 catalysts, comparing reaction rates and product selectivities. Further studies concentrated on examining support particle size and average metal crystallite size effects on /Moluidinc hydrogenation and the support pore size effects on 4-tert-butylaniline hydrogenation. 

In general, the above equations are best solved in a computer. Goto et al.9 obtained a solution to the case where species C is in excess so that the reaction is pseudo-first-order. They used the solution to analyze the efficiency of reaction systems such as oxidation of ethanol, hydrogenation of a-methyl styrene, and hydrogenation of aniline. They defined... 

---