The Hydrogenation of Adiponitrile

Raney nickel is the most commonly used catalyst for the hydrogenation of adiponitrile and seems to be the only one now employed in industry. It is more active and less expensive than the Raney cobalt previously used. 

Aluminium dissolves with H2 evolution, and this hydrogen remains chemisorbed on nickel, presumably in a dissociated form. Raney nickel catalysts are often doped with other metals in order to improve the catalytic activity the selectivity decreases in the order. Mo Cr Fe Cu Co. These metals are fused with the Ni-Al alloy and remain on the final catalyst, probably as oxides. It is believed that the role of the doping metals is to strengthen the selective adsorption of nitrogenous substrates. 

Raney nickel is an alternative to dispersing nickel on a support to obtain high surface area particles. It is made by treating of a Ni-Al alloy with a concentrated alkaline solution. Aluminium is selectively dissolved, forming soluble aluminates, and leaving porous nickel metal that retains, at least in part, the structure of the starting alloy with channels easily accessible to the reactants. 

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A process for the hydrogenation of adiponitrile and 6-aminocapronitrile to hexamethylenediamine in streams of depolymerized Nylon-6,6 or a blend of Nylon-6 and Nylon-6,6 has been described. Semi-batch and continuous hydrogenation reactions of depolymerized (ammonolysis) products were performed to study the efficacy of Raney Ni 2400 and Raney Co 2724 catalysts. The study showed signs of deactivation of Raney Ni 2400 even in the presence of caustic, whereas little or no deactivation of Raney Co 2724 was observed for the hydrogenation of the ammonolysis product. The hydrogenation products from the continuous run using Raney Co 2724 were subsequently distilled and the recycled hexamethylenediamine (HMD) monomer was polymerized with adipic acid. The properties of the polymer prepared from recycled HMD were found to be identical to that obtained from virgin HMD. 

More recently, dihydrogen complexes have been patented for nitrile hydrogenation. For example, the complex Ru(7/2-H2)2(H)2(PCy3)2 (Fig. 3.6) catalyzes the hydrogenation of adiponitrile to hexamethylenediamine (HMD) in toluene at 90 °C, 70 bar H2 with TON 52, TOF 5 IT1 [68]. At intermediate conversions, the... 

Adiponitrile. Adiponitrile is an important intermediate in polyamide manufacture. 1,6-Hexamethylenediamine formed by the hydrogenation of adiponitrile is used in the production of nylon-6,6, one of the most important polyamides in commercial production. 

C before use. The catalyst was used for the hydrogenation of adiponitrile and stearonitrile to the corresponding primary amines in high yields.94,95... 

Excellent yields (93-97%) of 1,6-hexanediamine were obtained in the hydrogenation of adiponitrile over Raney Co (eq. 7.24), Co-A1203 (Raney type), and Co-kieselguhr as catalysts with addition of ammonia.22 The yields decreased from 97.2 to 93.1% in the absence of ammonia and from 93 to 77% with nickel catalyst in place of cobalt catalyst. 

The byproducts of the hydrogenation of adiponitrile are small amounts of hexahydroazepine, 1,2-diaminocyclohexane and 2-(aminomethyl)cyclopentylamine, besides acyclic secondary products. In an example of a continuous process, a 1 12.5 molar ratio of adiponitrile and ammonia that was mixed with 10-fold amount of a hydrogenated reaction mixture from a prior run to give a 1 44 adiponitrile-ammonia mixture... 

Scheme 7.2 The formation pathways of the carbocyclic byproducts in the hydrogenation of adiponitrile.

Currently the global production of hexamethylenediamine exceeds 1.2 Mt/a and production (e.g. ICI, BASF and Rhone-Poulenc in Europe) is based on the hydrogenation of adiponitrile, largely obtained by catalytic addition of HCN to butadiene. Celanese produced hexamethylenediamine by reaction of ammonia with hexane-1,6-diol, coming from the hydrogenation of adipic acid. However, production by this method was abandoned in 1984. 

The hydrogenation of adiponitrile has additional complications in some kinetic studies 6-aminohexanonitrile was detected in solution and is considered a reaction intermediate (Figure 9). Alternatively, it may be a by-product if the hydrogenation is incomplete. Furthermore, the addition of the amines to the imine intermediates leads to an increased number of possible by-products, including cyclic compounds, due to the bifunctional character of the substrate. 

Figure 9 The hydrogenation of adiponitrile, an a,(n-dinitrile, to hexamethylenediamine via 6-aminohexanonitrile.

Activated hollow spheres have been found to be advantageous for the hydrogenation of carbonyl compounds, nitriles, aromatics, and unsaturated C-C bounds. In the case of carbonyl conqrounds, promoters (e.g.. Mo and Cr) that exist as surface cations were found to be the most effective. In the case of nitriles, the use of promoters to stabilize the residual A1 content of the catalyst so that it can be used with base modifiers was found to be the most useful combination. An example of this was the improved performance of the LiOH treated Cr / Ni promoted Co hollow spheres for the hydrogenation of adiponitrile to hexamethylenediamine. Some reactions were found to be more sensitive to the type of promoter they require. In the case of l,4-dihydroxy-2-butyne, it was found that Mo worked satisfactory as a promoter while the Cr / Fe combination led to worse results. Nonetheless, for all of the reactions studied here it was found that the activate hollow spheres were more active than the activated tablets on both a volume and weight basis, thereby allowing increased flexibility in the use of promoters and other selectivity enhancing additives. 

Catalytic sites of formation of DCH seemed to be specific and rather reactive. They were easily deactivated or poisoned. The addition of sodium carboxylates (5% of nickel surface coverage) lowered the 1,2-DCH formation by 40% during the hydrogenation of adiponitrile without any loss of activity (semibatch test). 

In the case of the hydrogenation of adiponitrile we have shown that cyclic and linear secondary imines are in equilibrium through a transimina-tion reaction (8). 

The relative rate of formation of the impurity comparatively to the rate of formation of the corresponding diamine (vficArfdiamine) at iso conversion. For each impurity, we fixed arbitrarily this ratio equal to 1 for the hydrogenation of adiponitrile. When this ratio is 0 this by-product has not been detected by gas chromatography that means its content is less than 100 ppm. 

The tertiary amine is formed in a similar maimer from the imine and a secondary amine. This side reaction can he minimized hy carrying out the hydrogenation in the presence of ammonia, which tends to shift the equihhrium hack towards the imine. When a compound with two or mote nitrile groups is hydrogenated, the formation of hoth cychc and acychc secondary and tertiary amines is possible, depending on whether the side reaction is intramolecular or intermolecular. For example, for the hydrogenation of adiponitrile ... 

The hydrogenation reaction takes place in the presence of a Raney nickel catalyst at 120-150°C (250-300°F) and 5000 psig. Nylon properties are sensitive to impurities in the feedstock so the reaction and purification steps for both adiponitrile and hexamethylene diamine are carried out to minimize by-products and impurities in the HMDA product. The hydrogenation of adiponitrile is carried out in a large quantity of ammonia which prevents the formation of hexamethyleneimine by-product. Ammonia also acts as a heat transfer fluid and helps maintain the reaction temperature. The conversion of adiponitrile is nearly 100% and the selectivity to HMDA is 97% giving an overall yield of HMDA from adiponitrile of 96.7%. 

Another process to obtain hexamethylenediamine by hydrogenation has been developped by Toyo Rayon. Caprolactam and ammonia are contacted in the gas phase at 350°C to form aminocapronitrile in the presence of a phosphate catalyst. Aminocapronitrile is hydrogenated to give hexamethylenediamine in a process very simi-mar to that of the hydrogenation of adiponitrile. 

The hydrogenation of functionalities in the carboxylic acid oxidation state can also be useful for small- or large-scale syntheses. For example, the hydrogenation of adiponitrile generates hexamethylenediamine that is one of the two monomers in the production of nylon. This reaction is conducted with a heterogeneous catalyst, but homogeneous catalysts for the reduction of nitriles to amines would be convenient for the conversion of nitriles to amines on a laboratory scale. The hydrogenation of esters to aldehydes would... 

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