Reaction Pathways and Intermediates in Selective Ring Opening of Biomass-Derived Heterocyclic Compounds by Iridium

While the catalytic hydrogenolysis
of biomass-derived aromatic
cyclic compounds to functionalized long chain alcohols and polyols
has been known for decades, the factors that control the selectivity
remain either unknown or controversial. Previous reports have hypothesized
full ring saturation of the aromatic ring is necessary prior to hydrogenolysis.
Contradictorily, recent studies have shown hydrogenolysis occurs prior
to the saturation of the conjugated bonds. Furthermore, it has been
assumed the functional groups present are fully reduced prior to hydrogenolysis;
however, this has not been shown a priori. In order to resolve these
controversies, we combine density functional theory and high-resolution
electron energy loss spectroscopy (HREELS) to probe the catalytic
hydrogenolysis of saturated and unsaturated heterocyclic molecules
(furan, furfural, furfuryl alcohol, and tetrahydrofurfuryl alcohol)
on iridium. Our results reveal that full saturation of the aromatic
ring is not only unnecessary but leads to slower kinetics and differing
selectivities. In contrast to previous studies, we show selective
partial ring saturation can enhance the kinetics of the hydrogenolysis
process. Reduction/oxidation of the functional group leads to a change
in the electronegativity, resulting in a change in selectivity. These
results provide important mechanistic insights allowing for further
improvement of catalysts for the effective transformations of biomass-derived
oxygenates to value-added products.