A New Approach to Molecular Hydrodesulfurization Catalysts: Allyl-capped Trialkylphosphoranimide-bridged Late First-row Transition Metal Clusters

  • Author / Creator
    Nkala, Fiona M
  • A series of new trialkylphosphoranimide allyl-capped clusters [M(ƞ3-allyl)(NPR)3]2, (M = Fe, Co or Ni, R = Et, tBu or Ph) were synthesized. The nickel dimers [Ni(ƞ3-allyl)(NPR3)]2, R = Et, Ph are diamagnetic and have been fully characterized using NMR spectroscopy, X-ray crystallography, elemental analysis and IR spectroscopy. The paramagnetic clusters [Ni(ƞ3-allyl)(NPtBu3)]2, [Co(ƞ3-allyl)(NPtBu3)]2, and [Fe(ƞ3-allyl)(NPtBu)3]2 have only been characterized by elemental analysis and IR spectroscopy. Addition of alane•dimethylethylamine to [Ni(ƞ3-allyl)(NPEt3)]2 affords the paramagnetic heterotrimetallic alane adduct [Ni(ƞ3-allyl)(NPEt3)]2•AlH3. The catalytic hydrogenation activity of [Ni(ƞ3-allyl)(NPR3)]2, R = Et, Ph and [M(ƞ3-allyl)(NPtBu)3]2, M = Fe, Co was investigated under mild conditions (1 atm H2, RT-80 °C, 16h). Hydrogenation studies suggest that these clusters react with hydrogen and decompose to heterogeneous material(s) at elevated temperatures. The nickel precatalyst [Ni(ƞ3-allyl)(NPEt3)]2 was the most active in diphenylacetylene partial hydrogenation, although the reactions proceed primarily to either cis or trans-stilbene, depending on conditions. [Ni(ƞ3-allyl)(NPPh3)]2 exhibited much lower reaction rates, attributed to slower precatalyst activation. The iron and cobalt clusters, in contrast, activate under hydrogen at room temperature, although hydrogenation proceeds much more slowly than that of the nickel dimers. The activity of [Ni(ƞ3-allyl)(NPEt3)]2 in DBT hydrodesulfurization was investigated in the presence of various basic reagents (MTMP, MOtBu. M= Na or K) in stoichiometric amounts and under variable reaction conditions (1-34 atm H2, 110-150 °C, 16 h). Hydrodesulfurization of DBT in the presence of the nickel precatalyst is highly dependent on the pKa of the base and the alkali metal. The highest conversion was obtained in the presence of highly basic potassium 2,2,6,6-tetramethylpiperidide whereas reactions using KOtBu as promoter are slower. Lower conversions were obtained with the corresponding sodium analogues. The slow reactions rates are attributed to varying rates of catalyst activation and decomposition. Supporting the allyl dimer on γ-Al2O3 dramatically improves HDS activity in the presence of KOtBu, suggesting improved catalyst stability and dispersion.

  • Subjects / Keywords
  • Graduation date
    Fall 2017
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.