C. LeBlond, Jian Wang, R. Larsen
1997
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Journal
Studies in Surface Science and Catalysis
Abstract
The nitro group in 1-(4-nitrobenzyl)-1,2,4-triazole (4NBT) was selectively hydrogenated to the amine using a Pd/C catalyst. Kinetics and reaction pathways were studied using in-situ kinetic tools, i.e. , measurements of heat flow by reaction calorimetry and rate of hydrogen uptake, in addition to analysis of samples taken from the reactor. The hydrogenation reaction follows predominantly a two-step monomeric pathway from 4NBT to the hydroxylamine intermediate and then to the aniline. The first step, hydrogenation of 4NBT to the hydroxylamine, follows nearly zero-order kinetics. Activation energies and heats of hydrogenation associated with each step in the consecutive hydrogenation were determined. The heat of hydrogenation of the second step, hydrogenation of the hydroxylamine (−58 kcal/mol), is slightly less than that of the first step (−65 kcal/mol). However, the activation energy associated with the second step is higher than that associated with the first step. Consequcently, the rate of hydrogenation of the hydroxylamine increases with increasing temperature at a faster pace than that of hydrogenation of the 4NBT does. Hydrogenation via the dimeric pathway, as evidenced by presence of a trace amount of the azoxy intermediate (formed from a coupling reaction of the nitroso and the hydroxylamine intermediates) contributes only ∼0.1% to the overall hydrogenation reaction. The low probability of the reaction following the dimeric pathway may be attributed to the high rate of hydrogenation of the nitroso on the catalyst.