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

Abstract

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 CO 2 , 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 oxophilic metals such as Re-promoted Rh, Pt, Ir or Pd catalysts selectively activate biomassderived 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 CO 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 oxophilic Re sites on the surface of