Catalysis of Organic..

Ostgard et al. 231metal atoms. These newly formed ensembles would then prefer reactions like thehydrogenation of the nitrile to the primary amine that can take place on smaller sites,while disallowing the formation and adsorption of larger molecules such as the alphaamino secondary amine and the Schiff’s base that are depicted in Figure 1.Figure 3 displays the effectiveness of other modifiers for Grewe diamineformation and interestingly CO itself is not as effective as formaldehyde. Apparentlythe presence of hydrogen in the modifying molecule improves its ability to generatemore selective sites, meaning that the formaldehyde generated carbonaceous layer onthe catalyst is more than just strongly adsorbed CO. However the modification withCO is still better than that with acetone and acetaldehyde. While the acetone andacetaldehyde modified catalysts are similarly slightly more selective than theuntreated catalyst, the acetone modified one produces far more secondary aminesand less of the other side products than the acetaldehyde modified catalyst. Clearlyacetaldehyde forms a different type of residue on the metal than acetone and that isto be expected by the structures of these molecules.Figure 4 shows the benzonitrile hydrogenation results over fresh andformaldehyde treated catalysts both with and without ammonia. In the absence ofammonia, the formaldehyde treated catalyst is distinctly better than the fresh one andits primary amine selectivity is comparable to that of the fresh catalyst withammonia. Logically, using the treated catalyst with ammonia gave the best results.Since less ammonia was used for the hydrogenation of benzonitrile than pynitrile, itsprimary amine selectivity was also lower, however the selectivity trends for both ofthese nitriles were the same for the formaldehyde treated catalyst. Treating thecatalyst with carbon dioxide also increased its benzylamine selectivity to plainlyshow that carbon dioxide is not as inert in the presence of an activated Ni catalyst asit is sometimes thought to be. The data suggest that carbon dioxide can decomposeon the catalyst to form an effective carbonaceous layer that enhances benzylamineformation. Benzaldehyde treatment also improved benzylamine selectivity in asimilar fashion. It is known that benzaldehyde decarbonylates on Pd and Pt catalysts(14) and it is not surprising that it could also do so on Ni to generate in-situ CO forthe effective modification of the catalyst.Figure 5 displays the structures of the other nitriles studied here and Figure 6shows their hydrogenation data. Due to the different amounts of ammonia (Table 1),it is difficult to directly compare the absolute primary amine selectivities of thesetests, however the formaldehyde treatment clearly improves the primary amineselectivity more for aromatic nitriles than for aliphatic ones. Unlike the aliphaticnitriles, the aromatic ones adsorb stronger and longer on clean catalytic surfaces viaboth the aromatic ring and the nitrile moiety leading to higher levels of secondaryamines. Hence weakening the adsorption of aromatic nitriles with smaller ensemblesvia the deposition of carbonaceous residues inhibits the formation and adsorption oflarger molecules leading to lower secondary amine levels. Nonetheless, thevaleronitrile data distinctly show that this treatment is also useful for improving thealready high primary amine selectivity of aliphatic nitrile hydrogenation.