(641g) On the Reaction Pathways and Intermediates of Selective Ringopening of Furanics By Iridium

As the chemical economy gradually shifts from fossil fuels to renewable resources, it becomes important to develop technologies to upgrade renewable platform molecules to final products of industrial relevance. It has been shown the renewable heterocyclic molecules 5-hydroxymethylfurfural and furfural, can be produced in high yields by depolymerizing and dehydrating lignocellulosic biomass. These heterocyclics can undergo reduction of their side groups with a subsequent hydrogenolysis to produce liquid fuel additives and value added products for the chemical industry. Selective hydrogenolysis of these heterocyclic molecules at the C–O bond in the aromatic ring has been shown to be a viable route towards the production of long-chain hydrocarbons, alcohols, and polyols. The selective hydrogenolysis of furfural can result in the production of 1,5-pentanediol (1,5-PeD), a valuable precursor for the production of polyesters and polyurethanes, with current research showing iridium to have a high selectivity for this process.

While the hydrogenolsysis of cyclic compounds to long chain alcohols and polyols has been known for decades, the factors that control the selectivity remain either uknown or debated. It has long been thought the hydrogenolysis of aromatic cyclic compounds proceeds after saturation of the aromatic ring; however recent studies have shown that full ring saturation is not required. Futhermore, any aromatic ring side groups are capable of undergoing reduction and oxidation, and the impact of such reactions on the selectivity are unknown. In the current study, we combine density functional theory and high resolution electron energy loss spectroscopy (HREELS) to probe the hydrogenolysis of saturated and unsaturated heterocylic molecules.  The reduction of the aromatic ring side group in conjunction with the selective hydrogenation of the aromatic ring shifts the selectivity towards either terminal or secondary diols, with the selectivity being determined via the rearrangement of the electronic density of the aromatic π-system.  Furthermore, there is a competition between the kinetics and thermodynamics governing the hydrogenolysis, indicating a strong temperature and time effect on the selectivity.  Comparison of DFT and HREELS spectra allow for several key intermediates to be identified and characterized on the Ir(111).  These insights may allow for the development of more efficient catalysts and processes that can render renewable chemicals commercial.