Synthesis of fatty amines. Selectivity control in presence of multifunctional catalysts

Fatty and N-methyl (or ethyl) substituted amines used, for example, for the preparation of tensioactive agents are generally obtained from the reaction of fatty acids (or esters), ammonia, light alcohols and hydrogen or nitriles, light alcohols and hydrogen over copper or nickel catalysts. In these processes, the rate-limiting step is either the light alcohol (R'OH) dehydrogenation for the formation of RNR'(2) or the secondary amine formation (R2NH). In both cases the light alcohol and the by-product water have inhibiting effects under the rather severe experimental conditions. Over supported copper-chromite catalysts there exists a correlation between the selectivity to RNR'(2) and the presence of a well dispersed copper chromite phase stabilized with alkaline-earth elements. However, over such catalysts the R2NR' amines cannot be obtained with high selectivity. In a second part of our study stimulated by the fact that environmental legislation will soon mandate replacements of copper-chromium catalysts with non-chromium catalysts, we investigated promoted nickel catalysts. In order to suppress the inhibiting effect of light alcohols, hexamethylene tetramine (HMTA) was used as methylating agent. Under these conditions, the nitrile is transformed either into dimethylalkylamine or into methyldialkylamine with a selectivity (or a yield) higher than 90%. Moreover, it was shown that HMTA was catalytically converted into monomethylamine (MMA), dimethylamine (DMA) and trimethylamine (TMA) during the reaction. These reagents specially MMA, DMA act as methylating agents of primary alkylamines (RNH2) while the corresponding imines (CH2=NH or CH2=N-CH3) would be the methylating species of the secondary alkylamine (R2NH).