Studies of alkali metal compounds in diesel fuel and small molecule activation chemistry

  • Richard Mackay Gauld

Student thesis: Doctoral Thesis

Abstract

The study of internal diesel engine deposits is a field that has recently begun to be explored in more depth from a chemistry perspective. The many issues caused within diesel engines related to the formation of insoluble, carboxylate-based deposits demanded insight be gained into the chemical make-up and composition of such deposits in the crystalline state. Such insight is challenging as the long chain nature of these deposits complicates the ability to render them crystalline, hence why no detailed studies had previously been carried out onto such species. Of related interest is the expanding field of small molecule activation chemistry, with CO2 being a prime example of an environmentally damaging small molecule produced by diesel engines. Despite the worldwide activity in both small molecule activation and alkali metal-based chemistry, no previous studies had sought to combine these two strands of research as undertaken here. The first part of this project develops the field of diesel deposits in terms of accessing suitable crystals in order to determine the solid-state structures of long chain alkali metal carboxylates, of the type commonly found within diesel engines. The products obtained were characterised by X-ray crystallography and multinuclear NMR spectroscopy. This revealed a remarkable set of structures, ranging from dimeric arrangements in polar solvents to polymeric arrangements in non-coordinating solvents. A highlight is the structure of the long chain sodium 2-ethylhexanoate, 3, which to the best of our knowledge represents the longest chain solvent-free and donor-free alkali metal carboxylate characterised using single crystal diffraction, showing a novel arrangement composed of a central hydrophobic core, with the long alkyl chains arranged around the periphery of this central core, rather than a simple layering arrangement. In the second part of this project the activation of the globally important small molecule CO2 was probed using common alkali metal based reagents. Three novel structures were crystallographically characterised, collectively exhibiting a higher degree of complexity post CO2 addition than would otherwise be anticipated. For example, addition of CO2 to LDA was antici[pated to occur between the reactive Li-N bond to form a molecular lithium carbamate, however in actual fact addition of CO2 furnished a more complex dodecameric structure, composed of two open cubanes linked by planar four-membered (LiO)2 rings Extension to small isocyanate molecules and NacNac alkali metal reagents revealed a common series of transformations, leading to products containing new dual imine and amide functionality at the backbone γ-carbon position. The mechanism leading to this dual functionality was explored via multinuclear NMR spectroscopic studies. Finally, extension to carbodiimides and phosphine oxide molecules revealed attack at the front of the alkali metal NacNac complex, contrasting with the backbone γ-carbon attack seen with isocyanates and CO2.
Date of Award23 Jun 2020
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsEPSRC (Engineering and Physical Sciences Research Council)
SupervisorRobert Mulvey (Supervisor) & Eva Hevia (Supervisor)

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