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Detailed chemical analysis by using multidimensional gas chromatography – mass spectrometry, and bulk properties of low temperature oxidised jet fuels Renee L. Webster, David John Evans, and Philip John Marriott Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b00264 • Publication Date (Web): 20 Mar 2015 Downloaded from http://pubs.acs.org on March 26, 2015
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Detailed chemical analysis by using multidimensional gas
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chromatography – mass spectrometry, and bulk properties of low
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temperature oxidised jet fuels
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Renee L. Webster a,b, David J. Evans a and Philip J. Marriott b*
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a. Defence Science and Technology Organisation, 506 Lorimer Street, Fishermans Bend, Victoria, Australia 3207
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b. Australian Centre for Research on Separation Science, School of Chemistry, Monash University, Wellington Road, Clayton, Victoria, Australia 3800
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Submitted to
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Manuscript ID: ef-2015-002647 (revised)
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*To whom correspondence is to be addressed:
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PJM: email Philip.marriott@monash,edu
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Tel: +61 3 99059630
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Fax: +61 3 99058500
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KEYWORDS:
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Multidimensional gas chromatography; fuel oxidation products; alternative fuels; fuel thermal stressing
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Abstract
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Sequential multidimensional gas chromatographic separations were used to identify
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secondary and tertiary oxidation products in a number of conventional fossil and
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alternatively-derived aviation fuels. The samples were produced by low temperature thermal
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oxidation of the fuels, and the higher dimensional separations were required due to the
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difficulty in quantification of the products at both bulk and molecular levels. The presence of
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certain oxidation products is a concern, due to functional similarities to restricted
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contaminants, contribution to solid deposit formation, and impact on physical properties.
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Accurate identification assisted in characterisation of the range of products. Primary alcohols
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dominated the secondary oxidation products in most cases, comprising 24-78% of the total
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oxygenated species identified in each fuel. Fuels were also subjected to several standardised
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physical property tests, showing that thermal oxidative stress affects certain fuel properties in
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a quantifiable manner which in some cases is comparable to the effect of adding polar
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dopants.
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1
Introduction
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Thermal stability of fuels is a property which is becoming increasingly important. Modern jet
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engines operating at increasingly higher temperatures employ fuel in a dual-use role; the
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second role is in a heat-sink capacity as a cooling fluid for hydraulics, avionics and engine
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lubrication systems. In these scenarios fuels may be heated to well over 100 °C, initiating
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auto-oxidation reactions which are known to contribute to the formation of gums, sediment
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and other insoluble species which block fuel nozzles and filters. Thermal oxidation may also
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affect other fuel characteristics including sooting tendency, surfactant formation and water
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separability.1.
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The increasing uptake of synthetic and biologically derived fuels adds further complications.
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Their suitability in high heat-demand applications is potentially diminished due to a lack of
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naturally-occurring antioxidants2, which mitigate the effects of peroxides and radical species,
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and potentially leads to oxidative instability. The initial stages of fuel oxidation are well
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understood3, with hydroperoxides being the primary products, and alcohols, carbonyls and
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carboxylic acids the secondary products. The subsequent formation of tertiary oxidation
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products such as heterocycles and anhydrides has not been thoroughly explored. Methods of
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fuel production and processing will also affect the chemical composition, and therefore may
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affect the formation of oxidation products, thermal stability characteristics and properties4.
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Of particular interest are those secondary and tertiary oxidised species which are likely to
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have an effect on the water shedding and separation characteristics, since they possess a
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variety of oxygen-containing polar functional groups. Previous work5 has identified
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furanones as a class of compounds that may be of concern, due to the similar functionality
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shared with fatty acid methyl esters (FAME). FAME are considered contaminants which are
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not tolerated in jet fuels, due to deleterious effects on the fuel’s stability and water
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separability6. Interactions of fuel and water are undesirable due to effects on the fuel system,
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particularly the coalescer. The presence of water in fuels may also accelerate corrosion and
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microbial growth. The presence of other oxygen containing compounds generated through
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low temperature thermal oxidation on fuel properties is an area that has not yet been
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thoroughly investigated7-10.
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There are a multitude of methods in use for isolating and quantifying oxidation products in
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the complex fuel matrix. Fuel oxidation is commonly assessed via a number of bulk chemical
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and physical property tests (including but not limited to, the jet fuel thermal oxidation test
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(JFTOT), automatic accelerated oxidation or quartz crystal microbalance), with varying
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degrees of suitability towards the parameter being measured. Complications arise in the
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analysis of trace compounds where specificity, selectivity and carryover from the matrix
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become problematic. Additionally, there is no consensus on which, if any, of the available
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bulk fuel property tests are suitable indicators as to the degree of oxidation of a fuel. Trace
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oxidised species have generally been analysed through the use of lengthy separation
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processes such as column chromatography, liquid−liquid extraction, solid phase extraction
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and preparative high performance liquid chromatography. While group-type analyses, which
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focus on analysis of particular functional groups, can be useful, they do not allow for the
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precise analysis of individual oxidised compounds.
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Multidimensional gas chromatographic (MDGC) separations provide a suitable technique for
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the detection of individual compounds of interest within the complex fuel matrix. An increase
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in the available separation space permits improved resolution of trace compounds in complex
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fuels, allowing for the identification of oxidised species without the extent of interferences,
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analyte losses and researcher time accompanying other analytical techniques. It has been
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demonstrated previously11,
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analysis of trace compounds in complex matrices, particularly fuels11, 13. Previous work by
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Mitrevski et al.14 combined MDGC and GC×GC separations, affording the advantages of
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each in the isolation and identification of trace oxygenated species and broad sample
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profiling, respectively. This study expands upon the previous work, utilising multiple heart-
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cut separations to identify a range of oxidation products, including tertiary oxidation products
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which have not been reported before using a comparable technique. Physical properties of a
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number of thermally stressed fuels are also reported for the first time, with insights into
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possible consequences of the presence of oxidation products.
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that sequential heart-cut techniques are well suited to the
Experimental
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2.1
Samples and Standards
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2.1.1 Standards
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A selection of oxygenated compounds covering the main classes of anticipated secondary
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oxidation products were employed as qualitative reference standards for development of the
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gas chromatography-mass spectrometry (GC-MS) analytical method. The components of this
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standard mixture are given in the Supplementary Information, Table S1. All oxygenated
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compounds except FAMEs were obtained from Sigma-Aldrich (Castle Hill, Australia) and
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used without further purification. The FAME mixture used in demulsification and
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separability analyses was obtained from a commercial biodiesel facility and consisted
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primarily of C16.0, C18.0 and C18.2 FAMEs. The furanone mixture used in these tests
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contained approximately equal volumes of each furanone from the methyl through to octyl
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substituted analogues, and was diluted to the given concentration in the same fuel to which it
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was added.
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2.1.2 Fuel Samples
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A total of seven aviation fuels were used in this study, details of which are given in Table 1.
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Samples were selected to include a range of processing technologies, and new generation
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alternatively derived fuels (Fuels A-D) to compare with more established conventional fuels
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(Fuels E-G). Fuels E-G are certified for use in jet engine aircraft, whilst fuels A-C meet the
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requirements of Annex A1 and A2 in ASTM D756615 for synthetic aviation fuels. Fuel D
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utilises a new pathway for production and has not been certified at the time of publication.
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The fuels were used ‘as-is’, with no blending with other feedstocks, and contained such
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additives that may have been included at the refinery. No sample cleanup or preparation was
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carried out in order to represent ‘real-world’ usage. All fuels and samples were stored at -18
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°C.
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2.2
Thermal stressing
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Fuels were subjected to dynamic thermal stressing by circulating through a custom-built
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thermal stability rig (TSR) described previously5. Fuel samples were sparged with air for 60
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min at ambient temperature before the cyclic thermal stressing commenced. The fuels flowed
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through a heated stainless steel tube 5 m long, with the sand bath temperature adjusted to
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provide a bulk fuel temperature of 180 °C. Fuel temperature is measured from inside the
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stainless steel tube directly after the portion submerged in the heated sand bath. The fuel was
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supplied at 18,600 kPa at a flow rate of 40 mL/min and collected in a separate flask, resulting
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in a residence time of 120 s for each pass. The fuel was returned to ambient temperature
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between each pass through the heated tubing by a small length of tubing being immersed in a
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recirculating chilled water bath.
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A sample of fuel was removed from the TSR at eight intervals throughout the thermal
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stressing period for analysis of thermal oxidative degradation products and fuel properties.
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The sampling interval became progressively longer throughout the duration of the
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experiment, in order to more effectively capture changes that occur in the earlier stages of
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thermal oxidation. Table 2 shows the protocol describing the sampling intervals and
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residence times. Fuel is returned to the collection reservoir every 20 min at the beginning of
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the experiment, gradually shortening to every 5 min by the end.
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2.3
Instrumentation
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MDGC analysis was carried out on an Agilent 7890A gas chromatograph with a model 7000
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triple quadrupole mass spectrometer (Agilent Technologies, Santa Clara, CA, USA). The
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spectrometer was operated in single quadrupole mode only by using total transfer of ions at
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the first quadrupole. Operating conditions and instrumental set up are given in Table 3. A
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traditional non-polar/polar column set was selected in order to aid identification of
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compounds by order of elution on the 2D column based on their increasing polarity. A 30 m
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5% phenyl column was used in the first dimension, with a 30 m wax column in the second
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dimension. Heart-cutting was performed using a Deans switch assembly, with a liquid CO2-
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cooled cryogenic tee trap at the beginning of the second column for cryofocussing. Data
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collection, processing and analysis were performed with MassHunter version B.06.00.
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Our previous work on sequential heart-cutting of an algae-derived jet fuel14 determined that
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under these conditions, optimum resolution of the significant complexity in the fuel matrix
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was achieved using narrow heart-cuts of 12 s taken from the first dimension, and the column
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dimensions allows the selected heart-cuts to elute within a 2 min time period on this column.
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As this study incorporates conventional fossil-derived fuels, which are even more complex
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than the neat alternatively derived fuels, a similarly narrow heart-cut region of 12.5 s was
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chosen for this analysis. However, successive heart-cuts were made 2.5 min apart in each run
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such that 12 – rather than 10 – successive injections would sample the entire chromatogram.
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This slightly longer elution region was required for the more complex fossil-derived fuels, the
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heart-cuts of which did not always elute from 2D in
