Potassium Hydride-mediated Hydrodesulfurization Catalyzed by Cobalt

• Author / Creator

  Vorapattanapong, Asama

• New triethylphosphoranimide-supported thiolate-capped clusters of cobalt and nickel were synthesized, isolated, and characterized as part of an investigation into homogeneous catalysts for the hydrodesulfurization of refractory organosulfur substrates. In particular, p-toluenethiolate and i-propylthiolate ligands were installed onto heterocubane phosphoranimide-supported cobalt and nickel halides via simple room temperature metathesis. The compounds were isolated, purified, and characterized principally by elemental analysis and X-ray crystallography.

  The thiolate clusters, along with analogous phosphoranimide clusters capped by Cl, NPEt3, and Me anions, were investigated as precatalysts for hydrodesulfurization (HDS) under mild conditions. All of these clusters, functioning as homogeneous catalysts, promote HDS under very mild reaction conditions (1 atm H2 and 110 °C) in the presence of stoichiometric amount of KH to afford high conversion within 2 hours. Among these clusters, the chloride-capped cobalt cluster yields the highest conversion, giving a high yield of the fully desulfurized product. The high reactivity of the chloride-capped cluster led us to explore simple late transition metal halides (Fe, Co, Ni, and Cu) as precatalyst for our KH-mediated desulfurization. Surprisingly, these metal halides, likely function as heterogeneous catalysts, are also competent precatalysts, giving high conversions, albeit not as efficiently as the phosphoranimide-supported clusters.

  As part of our investigation into the mechanism of the hydrodesulfurization reactions catalyzed by these heterocubane clusters, we have synthesized and partially characterized various phosphoranimide-supported cobalt hydride and sulfide clusters. The cobalt hydride and sulfide clusters were prepared from either the homoleptic cobalt bis(phosphoranimide) cluster or the simple chloride cluster using a range of hydride and sulfide sources. While these clusters were never isolated as pure materials, cobalt hydride and sulfide complexes were partially characterized via IR spectroscopy, as well as the characterization of the reaction by-products.

  In addition to the synthesis of putative intermediates, the mechanistic roles of the transition metal and KH, along with the nature of the active catalysts, were investigated. Control experiments indicate that potassium hydride itself, as the sole reductant, remains competent for the “hydrogenolysis” of C¬–S bonds. Desulfurization by KH in the absence of hydrogen and catalyst can occur via two pathways: ortho deprotonation and nucleophilic aromatic substitution at the ipso-carbon of DBT. In addition to the nucleophilic hydride, the potassium cation plays an important role in activating the substrate by π-coordination. Augmenting the K+ ion, the transition metal catalyst further promotes the desulfurization of DBT. In contrast, the presence of H2 has only minimal effect on any of the KH-assisted desulfurization reactions.

  Finally, the hydrogenolysis of C–S, C–O, C–N, and C–Br bonds of a range of heteroaromatic substrates were explored using catalytic CoCl2 and KH under very mild reaction conditions (110 °C). Under these conditions, the system desulfurizes a range of diarylsulfides and promotes the hydrogenolysis of aromatic C–O, C–N, and C–Br bonds.

• Subjects / Keywords

  • cobalt
  • hydrodesulfurization
  • homogeneous

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R33N20W4P

• License

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