Unraveling the Role of H₂ and NH₃ in the Amination of Isohexides over a Ru/C Catalyst

High Resolution Image Download MS PowerPoint Slide The direct amination of biomass-derived isohexides with NH 3 over a Ru/C catalyst was systematically investigated to understand the role of H 2 and NH 3 in the production of isohexide diamines vs aminoalcohols, i.e., the transformation of one or both OH-groups in isohexides into NH 2 groups. Only aminoalcohols with an exo -OH group were generated starting from isosorbide, which contains both an exo -OH and an endo -OH group, while a moderate yield of diamines was obtained from isomannide with two endo -OH groups due to the higher reactivity of the latter. The main byproducts were identified, including a variety of N- and O-containing cyclic compounds, such as 2,5-dimethylpyrrolidine, that arise from a decomposition path driven by hydrolysis/hydrodeoxygenation of a tricyclic amine intermediate. By combining density functional theory calculations with microkinetics, NH 3 was found to adsorb strongly on the catalyst surface and generate adsorbed NH 2 and NH species with variable coverage depending on the temperature and the nominal H 2 /NH 3 ratio. Isomerization of isohexides was greatly suppressed by adsorbed NH 3 . Meanwhile, adsorbed NH 3 discouraged the formation of byproducts driven by competing side reactions promoted by H 2 . The H 2 /NH 3 ratio, which conditions the distribution of NH 2 and NH species on the Ru surface, influences drastically the catalytic performance.