Theoretical Insights into the Catalytic Hydrogenolysis of Biomass Derived Cyclic Ethers and Polyols over Metal and Alloy Promoted Particles in Aqueous Systems

The increasing demand for scarce petroleum resources have generated tremendous interest in the development of sustainable strategies that can convert biomass as well as other renewable feed sources into fuels and chemicals. Plant-based sugars and other biomass sources can be deconstructed into polyols and cyclic ethers that contain excess oxygen which must be removed in order to synthesize useful chemical intermediates. This requires rejection of oxygen as either CO2, CO or water and the efficient use of hydrogen. In this work, we have used first-principles quantum chemical calculations along with detailed kinetic analyses to examine the fundamental mechanisms that control the selective hydrogenolysis of biomass-derived cyclic ethers and polyols in aqueous media.

Recent experimental efforts have shown that metal alloys comprised of reducible and oxophillic metals such as Re-promoted Rh, Pt, Ir or Pd catalysts selectively activate biomass-derived cyclic ethers and polyols such as tetrahydrofurfural alcohol (THFA) at the more substituted carbon to form α,ω-diols with high selectivity and activity. The reaction pathways observed over the non-promoted Rh, Pt, Ir and Pd catalysts are markedly different as they demonstrate high catalytic selectivities to activate the C-O bonds of the least-substituted carbon centers which tend to form α,β-diols. First principle density functional theory (DFT) calculations clearly show that the non-promoted Rh catalyst preferentially activates cyclic ethers and polyols such as THFA and 1,2-propanediol, respectively at the less-substituted carbon center in order to reduce steric repulsion that occurs in activating at the more-substituted carbon center. The direct activation via the metal as was found for Rh cannot be used to explain the very different catalytic behavior of the Re-promoted Rh system. Our theoretical results together with detailed kinetic experiments strongly suggest that the presence of the very oxophillic Re sites on the surface of the Rh-Re catalyst can form strong acid sites in the presence of water. These sites catalyze an acid mechanism that controls the hydrogenolysis of THFA and other cyclic ethers and polyols on the Rh-Re catalysts.

The nature of the active site for hydrogenolysis is still actively debated in the literature. Some studies suggest that Re is partially oxidized and that the active sites may be Re-OH groups while others indicate that the Re is fully reduced. We show that hydroxyl groups as well as water are strongly bound to the Re sites and result in Brønsted acid sites that can catalyze the ring opening of THFA. The unoccupied metallic Re sites on the Rh-Re surface act as Lewis acid sites and can also catalyze the ring opening of THFA. The activation barriers and overall reaction energies for the adsorption, desorption and the dissociation of water are used together with microkinetic models in order to try to predict the relative amounts of the different acid sites on the Rh-Re surface under the reaction condition to elucidate the most plausible active acid sites on the Rh-Re catalyst.

In aqueous solution, solid acids can dissociate to form the hydronium ions, which can influence the activity for reactions that need to occur on the surface of the catalyst. The heterolytic dissociation of the adsorbed water and hydroxyl at the Re sites on Rh-Re surface in water are examined to understand the form of solid Brønsted acid sites on the Rh-Re surface in water by exploring both the dissociated hydronium ions as well as the non-dissociated surface solid acids. Constrained ab initio molecular dynamics simulations are used to quantify the free energy changes for the dissociation reactions, as well as the ring opening reaction of THFA at the acid sites on the Rh-Re surface to further understand the effects of entropy.