Hydrogenation of nitroalkanes to hydroxylamines

#This invention relates to the preparation of N-alkyl hydroxylamines by the catalytic hydrogenation of a nitroalkane.

#Hydroxylamine and its N-alkyl derivatives are valuable oxygen scavengers in boiler feed waters (see, for example, U.S. Pat. Nos. 4,067,690 and 4,350,606) with the N-alkyl hydroxylamines being particularly useful. They are conventionally added to the boiler waters as an aqueous solution of the free base, or, optionally (but less preferably) as the salt. The free base form is regarded as greatly preferred for this use as salt residues are undesirable in the boiler. The salts must be converted to the free base before use, thus adding another process step either at the processing site or at the application site.

#Various methods for synthesizing alkyl hydroxylamines are known. Catalytic hydrogenation of a nitroalkane to the corresponding N-alkylhydroxylamine has received considerable attention. This process, as presently conducted, has several serious disadvantages, including: (1) the yield is relatively low; (2) substantial amounts of alkylamine are formed; (3) the free base, when finally recovered, is unstable (that is, not stable under ordinary storage conditions which such material must undergo); and (4) the catalyst rapidly loses its activity and selectivity.

#Utility of N-alkylhydroxylamine as an article of commerce depends to a considerable extent on its stability. For example, in boiler feed water applications, it is normally made up as a dilute aqueous solution, then shipped, and finally stored on-site for extended periods of time (weeks or even months) before use. It must not decompose or deteriorate during this time.

#As an indication of the magnitude of the problem, a 55-gallon steel drum of 15% aqueous solution of free N-isopropylhydroxylamine (NIPHA) formed according to current technology begins deterioration immediately on sealing, and within 30 days will analyze zero product. NIPHA free base formed from its commercially salt (NIPHA.HCl 97% purity) also exhibited instability so that it can not be considered a storage stable material. The conversion of the salt to the free hydroxyl amine was carried out entirely in a metal-free environment and the resultant NIPHA crystals were made into an aqueous solution and stored at 25.degree. C. under nitrogen. After only four days, an assay of the solution showed that only 30% of the original amount of NIPHA remained. One can conclude from this that, when an acid is used in the process to prepare NIPHA sat, as in the prior art, the NIPHA free amine prepared from this salt would not be expected to be storage stable.

#In the course of describing the present invention herein, certain terms used have the following meanings: