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

The amination of isohexides to diaminesover Ru/C is affectedby ammonia poisoning and formation of byproducts by hydrogenolysis. The direct amination of biomass-derived isohexides withNH(3) over a Ru/C catalyst was systematically investigatedto understandthe role of H-2 and NH3 in the production ofisohexide diamines vs aminoalcohols, i.e., the transformation of oneor both OH-groups in isohexides into NH2 groups. Only aminoalcoholswith an exo-OH group were generated starting fromisosorbide, which contains both an exo-OH and an endo-OH group, while a moderate yield of diamines was obtainedfrom isomannide with two endo-OH groups due to thehigher reactivity of the latter. The main byproducts were identified,including a variety of N- and O-containing cyclic compounds, suchas 2,5-dimethylpyrrolidine, that arise from a decomposition path drivenby hydrolysis/hydrodeoxygenation of a tricyclic amine intermediate.By combining density functional theory calculations with microkinetics,NH3 was found to adsorb strongly on the catalyst surfaceand generate adsorbed NH2 and NH species with variablecoverage depending on the temperature and the nominal H-2/NH3 ratio. Isomerization of isohexides was greatly suppressedby adsorbed NH3. Meanwhile, adsorbed NH3 discouragedthe formation of byproducts driven by competing side reactions promotedby H-2. The H-2/NH3 ratio, which conditionsthe distribution of NH2 and NH species on the Ru surface, influences drastically the catalytic performance.

Automated summary

The direct amination of biomass-derived isohexides over a Ru/C catalyst showed that the presence of hydrogen gas (H-2) and ammonia gas (NH3) played important roles in the selectivity of isohexide conversion to diamines and aminoalcohols. Isohexides with an exo-OH group formed aminoalcohols, while isohexides with two endo-OH groups produced a moderate yield of diamines due to higher reactivity. The adsorption of NH3 on the catalyst strongly suppressed isomerization of isohexides and discouraged the formation of byproducts promoted by H-2.