Homogeneous and heterogeneous rhenium catalysts for sustainable transformations of polyols, alcohols, and amines
Biomass-derived molecules, such as sugar polyols and lignin, are promising feedstock for making small and useful organics (SUO) and high value-added organics (HVO). However, the high oxygen to carbon ratio (C:O ≈ 1) prohibits the direct use of biomass-derived molecules as energy carriers and chemicals. Deoxygenation of polyols has become an important challenge in the utilization of biomass resources. Oxorhenium complexes show excellent oxophilicity to transfer oxygen and deoxygenate alcohols. Oxorhenium(VII) complexes can efficiently catalyze the transformation of glycerol to allyl alcohol, acrolein, and propanal. The volatile SUO products were easily separated from the nonvolatile residues via simple distillation. Based on kinetic studies and isotope labeling experiments, an oxo-Re(V) diolate intermediate was proposed as both a proton acceptor and a hydride acceptor to facilitate hydrogen transfer and deoxygenation reactions. In contrast to high valent oxorhenium complexes, a nanoparticulate Re 0 metallic catalyst shows great ability to activate C-H bonds in alcohols and amines. Dehydrogenation of alcohols into carbonyl groups is an important reaction. Acceptorless dehydrogenation is a novel, green, and atom-economical reaction, which has attracted attention recently. Metallic Re0 nanoparticles catalyze acceptorless dehydrogenation in the neat alcohol to give the corresponding ketone with concurrent release of hydrogen gas. X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM/TEM) demonstrated that the Re0 catalyst is a well-structured 2 nm nanocrystalline particles (Re NPs) covered by ReIVO 2 oxide. The Re NPs can also be utilized to amine acceptorless dehydrogenation reactions. Unlike alcohols, amines dehydrogenation was followed by homo-coupling to give imine as the final product. Another difference between amine and alcohol acceptorless dehydrogenation is γ-C-H activation for alcohols versus β-C-H activation for amines, which might be caused by the different electronegativities of oxygen and nitrogen. Similar to other metal hydrogenation catalysts, Re NPs catalyze hydrogenation of N-heterocycles. When combined with its dehydrogenation activity, Re NPs supported on carbon (Re/C) acts as a catalyst for reversible hydrogenation/dehydrogenation that has applications in hydrogen storage. This reaction was demonstrated with various N-heterocyclic substrates, which provide stable and recyclable liquids. Five cycles of hydrogenation/dehydrogenation was achieved with Re/C catalyst without significant loss of activity accounting for 1962 turnovers.
Abu-Omar, Purdue University.
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