Understanding Sodium Carboxylate Structures in Diesel Fuel and Deposits

Lead Research Organisation: University of Strathclyde
Department Name: Pure and Applied Chemistry

Abstract

There have been reports of internal injector deposits causing problems in diesel engines since 2008. Such problems cause rough idling, power loss, high emissions, high-pressure fuel pump wear, injector sticking, internal component corrosion and engine failure. These reports coincided with introduction of common rail diesel injection systems and of ultra-low sulphur fuels introduced because of emission regulations. This is clearly a green issue which requires resolution by the industry.

Injection systems have design features that are more conducive to deposit formation. The changes to fuels have also affected their ability to solubilise deposits. Deposits formed are complicated, frequently mixtures of inorganic and organic compounds.

One sub-group of this complexity of major interest is "sodium soaps" (carboxylates). Work with different sodium precursors, sodium hydroxide and sodium 2-ethylhexanoate, a fuel soluble salt, shows that interaction with monoacid lubricity additives produces filter blocking or injector sticking. With the use of various soluble sodium salts to undertake research on internal diesel injector deposits (IDID), and possible standard engine tests being introduced, it is imperative to gain more understanding of this chemistry. Current interest in IDID from sodium carboxylate sources is escalating both in the USA and Europe. Recent literature has seen a proliferation of engine testing studies, some using "fuel soluble" sodium salts as candidate reactive precursors for possible interaction with different carboxylate species.

The field based reaction the community is trying to mimic is:

NaOH (aq) + RCOOH ----------------> RCOONa + H2O

Obtaining repeatability of this simple "soap" forming process when contacting aqueous base with fuel is challenging and the current approach of using "fuel soluble" sodium salts as substitutes is extremely complex. Reaction chemistry is being attempted throughout the diesel fuel system, from fuel tank to injector tip. There are added complications of a two-phase system, ligand coordination stoichiometry, competing metals, unknown solubility factors, acids from fuel impurities, and impurities from water including metals such as calcium.

Recent unpublished engine test work by Innospec shows that the NaOH to ligand molar ratio dictates injector sticking as a result of different structures forming on the injector needle. Though surprising from a traditional ionic carboxylate model from aqueous systems, it is unsurprising viewed in the light of modern metal-organic and coordination chemistry from organic diluents. Molecular organic frameworks of ladders, clusters and inverse micelle structures are known.

Work Programme: To explain the engine test results it is proposed to synthesise compounds and to grow single crystals and determine the structures of these carboxylates with changes in stoichiometry, first in simple organic solvents, then with for example trace water present. Resultant complexes will be studied with regard to their structure and ability to be solubilised by diesel. Attempts will be made to synthesise these same complexes in diesel and compare their properties, and to correlate structurally to injector needle deposits.

Overarching Aim: To prepare and characterise these different structures to enable understanding of their different properties in fuel. Results will have major implications for fuel technology worldwide.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509760/1 01/10/2016 30/09/2021
1810875 Studentship EP/N509760/1 01/10/2016 31/03/2020 Richard Mackay Gauld
 
Description Have expanded the use of lithium amides in CO2 fixation chemistry, obtaining a range of crystalline architectures. Have recently shown unusual backbone activity at commonly used 'NacNac' molecules towards a range of small molecules, which should allow for a range of new structures to be obtained and potentially probed for catalytic activity. Recently submitted a paper showing the sodium carboxylates do not adopt simple structures within diesel engines, but instead form complex species that would contribute to the deposit issue under investigation. Each of these objectives lends itself to further investigation, with future work focusing on expansion of the small molecules used to a comprehensive understanding of the structures adopted by longer chain examples of alkali metal carboxylates.
Exploitation Route The outcomes looking at the structures adopted by sodium carboxylates in the solid state will be of use to those in the fuel and general chemical industries, showing how simple reactions that occur within processes can lead to much more complexity than initially envisioned. The results looking at small molecule activation will have particular impact in academic settings, as it will hopefully push for more interest in studying the structures that can be obtained from insertion of reactive small molecules. The NacNac work in particular could provide a cheap and easy route to functionalised NacNac ligands that may act as catalysts in academic settings.
Sectors Chemicals

 
Description Poster Presentation at the Dalton Younger Members Event at the University of Cardiff 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact Poster showing recent work delivered at the DYME event in Cardiff. This conference is specifically designed for early carrier researchers and allowed the presentation of recently published work to this group of people. The poster attracted a significant amount of attention and follow-up questions.
Year(s) Of Engagement Activity 2019
 
Description Poster Presentation at the Universities of Scotland Inorganic Chemistry Conference- University of Glasgow 2019 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact Poster presentation showcasing recent work carried out in the field of small molecule fixation using a common NacNac scaffold. This sparked interest from a number of fellow conference participants
Year(s) Of Engagement Activity 2019
 
Description Poster presentation at ICOMC 2018 in Florence, italy 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Poster presentation based on workcarried out involving CO2 fixation reactions, which lead to discussion and debate.
Year(s) Of Engagement Activity 2018
 
Description Talk given at USIC 2018 conference held at the University of Edinburgh 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact Talk given at a national conference based on recent work carried out looking at CO2 fixation reactions
Year(s) Of Engagement Activity 2018