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Tracking ageing of bitumen and its SARA fractions using high-field FT-ICR mass spectrometry Florian Handle, Mourad Harir, Josef Füssl, Ay#e N. Koyun, Daniel Grossegger, Norbert Hertkorn, Lukas Eberhardsteiner, Bernhard Hofko, Markus Hospodka, Ronald Blab, Philippe Schmitt-Kopplin, and Hinrich Grothe Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03396 • Publication Date (Web): 11 Apr 2017 Downloaded from http://pubs.acs.org on April 12, 2017

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Tracking ageing of bitumen and its SARA fractions using high-field FT-ICR mass spectrometry Florian Handlea§, Mourad Harirb§, Josef Füssld, Ayşe N. Koyuna, Daniel Grosseggera, Norbert Hertkornb, Lukas Eberhardsteinerc, Bernhard Hofkoc, Markus Hospodkac, Ronald Blabc, Philippe Schmitt-Kopplinb,e*, Hinrich Grothea* a Vienna University of Technology, Institute of Materials Chemistry, Getreidemarkt 9/BC/01 A-1060 Vienna, Austria b Helmholtz Zentrum München, Department of Environmental Sciences, Research Unit Analytical Biogeochemistry, Ingolstädter Landstrasse 1, D-85764 Neuherberg, Germany c Vienna University of Technology, Institute of Transportation, Gußhausstrasse 28-30, A-1040 Vienna, Austria d Vienna University of Technology, Institute for Mechanics of Materials and Structures, Karlsplatz 13, A-1040 Vienna, Austria e Technische Universität München, Chair of analytical Food Chemistry, Alte Akademie 10, 85354 Freising, Germany §

Contributed equally to the work.

*Corresponding authors: Emails: [email protected] & [email protected]

KEYWORDS: Bitumen, SARA fractions, ageing, [ESI(-)] FT-ICR-MS ABSTRACT: Bitumen is a widely used material, but its ageing behaviour is only understood at a macroscopic level as hardening and embrittlement over time. To assess bitumen ageing behaviour on the long run, the pressure ageing vessel (PAV) testing procedure was developed. However, this procedure including high-pressure and high-temperature oxidation of the bitumen has not been understood on a molecular level yet. Here, a bitumen sample and its SARA fractions, i.e., saturates, aromatics, resins and asphaltenes, were investigated in comparison with their aged samples to study changes of their chemical compositions. Negative electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry [ESI(-)] FT-ICR-MS was used to analyze samples. The effect of ageing was characterized using aromaticity equivalent (Xc), double bond equivalent (DBE) and van Krevelen plots. It was found that ageing induces reduction of condensed aromatic compounds to alicyclic and open chain aliphatic compounds, while small aromatic compounds have been found to be relatively stable (or altered only slightly). Abundant alterations were detected in unaged bitumen. These changes can be assigned to resins and asphaltenes as compared to saturates and aromatics. Overall, alterations of highly condensed compounds were found to be ageing in a related way. Furthermore, molecular series of CHO, CHNO and CHOS fragments were more susceptible to oxygenation in bitumen, aromatics, resins and asphaltenes as compared to saturates. In addition, molecular changes in asphaltenes showed significant difference from classical assessment with high content of condensed aromatic compounds. Rather, the most abundant compounds in asphaltenes appear to be more saturated and apolar.

INTRODUCTION Bitumen is a highly viscous, very complex material comprised of high-molecular weight hydrocarbons with oxygen, nitrogen and sulfur containing functional groups with widespread industrial use and petrochemical potential. Its properties tend to vary dependent on source, production cycle, additives, and additional chemical-physical treatments. According to European regulations, bitumen is described simply as a “virtually involatile, adhesive and waterproofing material derived from

crude petroleum or present in natural asphalt, which is completely or nearly completely soluble in toluene, and very viscous or nearly solid at room temperature”.1 Nowadays bitumen is mostly obtained as residue of vacuum distillation in the refinery process of crude oil,2 while natural asphalt shows more limited economic importance in comparison. The question of ageing and other time dependent alterations is one of the most important topics regarding product life-cycle and recycling of asphalt. In both areas, significant improvements would result from a

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better understanding of molecular diversity of bitumen in molecular features. Studies of bitumen chemistry, bitumen oxidation/aging and even bitumen oxidation in PAV were performed at molecular level already for decades.3 However, bulkresolution analytical methods 4 lead only to limited understanding due to the complex chemical nature of bitumen. Here, separation of bitumen into saturate, aromatic, resin, and asphaltene fractions (SARA) offers significant reduction of complexity. SARA analysis is a Standard method of Bitumen Analysis. The separation was conducted according to ASTM D 4124 – 01 standard test methods for separation of asphalt into four fractions 1. A detailed scheme of the separation process is given in Figure 1. In short, it includes two main steps: a) the separation of the solid asphaltenes by n-heptane extraction and b) the column chromatography separating of the soluble fraction according to polarity into colorless saturates, yellow aromates and dark brown resin solutions. The individual steps and the used solvents are described in detail in the section Materials and Methods. All separation experiments were performed in compliance with the standard and its repeatability requirements. All extractions and column separations were double checked for compliance. When the obtained fraction was measured the uncertainty is higher than the acceptability criteria of standard deviation (0,32wt% of the standard ASTM D 4124 – 01 1). Asphaltenes are hereby defined as the n-heptane insoluble and toluene soluble part of bitumen, whereas the soluble fractions are called maltenes. Maltenes can then be separated by column chromatography. 5, 6, 7 To avoid confusion, the SARA nomenclature was preferred here over the Corbett and ASTM nomenclatures. 6, 7Hence, aromatics refer to naphthenic aromatics and resins to polar aromatics in the ASTM-standard. 8 Although concerns regarding repeatability of the chromatographic separation process have been raised, it is still the most validated and practical separation technique for bitumen. 9 Owing to excellent mass accuracy (≤ 200 ppb) and mass resolution (m/Δm50%≥ 400000, where Δm50% is the mass spectral peak full width at half-maximum peak height (FWHM)).

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Fourier transform ion cyclotron resonance mass spectrometry FT-ICR-MS is highly suitable to characterize molecular compositions of the most demanding environmental samples such as natural organic matter of various origin and in the field of petroleomics.3, 10 - 21 In the latter, complementary ionization techniques such as electrospray ionization (ESI), atmospheric pressure photoionization (APPI), electron ionization (EI) and liquid injection field desorption ionization (LIFDL) were used to emphasize detection of certain molecular subfractions with an outstanding range of applications.22 - 29 In this study, negative electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry [ESI(-)] FTICR-MS which offers a substantial coverage of functionalized hydrocarbons was used to assess changes in the chemical space of unaged and aged bitumen and its SARA fractions. Selective depletion of condensed aromatics was observed in aged samples as compared to unaged samples. To date, these data provide most comprehensive assessment of ageing behaviour of bitumen and its SARA fractions on a molecular level.

MATERIALS AND METHODS Materials. A typical bitumen 70/100 was provided by Austrian mineral oil refinery obtained from vacuum distillation. The sample was classified and named by needle penetration value according to EN 1426.30 Bitumen 70/100 is typically used in the production of polymer modified bitumen. All samples were stored in sealed metal cans between working steps to avoid uncontrolled ageing. All solvents used in the experiments were obtained from Carl Roth GmBh + Co. KG and of ROTIPURAN quality (≥99% p.a.). The aluminum oxide used in the chromatographic column had a grain size of 63-200µm and a pH value of 3,5-4,5. Toluene, methanol and formic acid for FT-ICR-MS analysis were obtained from Sigma-Aldrich and were used without additional purification. Separation and Fractionation. The ASTM Standard 4124 separation scheme was employed as the basis to fraction bitumen into four major fractions. The procedure was slightly modified to allow a more differentiated analysis of the material (Figure 1).7

Paragon Plus Environment Figure 1. Separation scheme ACS of SARA fractions.

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First, 10g (± 1g) of bitumen were placed in a 2L Erlenmeyer flask. N-heptane was added until a ratio of 30mL/g of bitumen was reached. The Erlenmeyer flask was placed on a heating plate and heated under stirring to 90°C until there was no bitumen sticking to the bottom of the flask. Stirring and heating was continued for another 1,5h after this point. Afterwards, the sample was removed from the heating plate. The flask was closed and stored in the dark for 12h. The suspension was filtered by a Büchner funnel (Qualitative Filter Paper 410, VWR International, slow filtration speed, particle retention value: 2-3µm). The liquid phase was stored in another Erlenmeyer flask, and was again sealed to avoid oxidation. The solid phase was washed twice with 100mL n-heptane at room temperature. Then, the solid phase was enclosed in a filter paper and put into a Soxhlet extractor with 750mL n-heptane. High purity of asphaltenes can be gathered by continuing the extraction for 48h and testing the reflow from the extractor by putting one drop of the liquid on a glass plate. Only when no significant residue was visible under a microscope, the extraction was stopped; else the extraction was continued for another 24h. The two solid nheptane insoluble phases (asphaltenes) were merged and enveloped in filter paper and subjected to toluene extraction with 750mL toluene in a Soxhlet extractor. Duration and abortion criteria were the same as for the n-heptane extraction. The toluene was then evaporated and the solid phase was analyzed by gravimetry and by FT-IR spectroscopy to confer the purity of the solvent. The filter-paper was also weighed to examine the toluene-insoluble content, which was well below 0,05wt% for the studied bitumen. Likewise, the liquid phase was filtered and the two n-heptane phases were merged and concentrated to a volume of about 50mL (±5mL). The concentrated maltene phase was transferred onto a chromatographic column filled with 450g of dry aluminium oxide to a column length of 80cm (±1cm), which was stabilized with 400mL of n-heptane. The solvent schedule was chosen according to ASTM 4124. However, due to the stabilizing of the column, forerunning of 350mL were collected. The eluate was collected in 50mL (±2mL) glass beakers, sealed airtight and kept at -15°C in a fridge to slow down degradation and oxidation processes. The solvent was removed by evaporation according to EN 1297-3 28 and the fractions were analyzed as the solid phase. Pressure aging vessel (PAV). PAV procedure is commonly used to simulate long-term aging effects on bitumen in the laboratory. In this study, PAV were carried

out in accordance with the European Standard EN 14769 at 100°C and 2.10 MPa for 20h.32 FT-ICR Mass Spectrometry. Negative electrospray ionization Fourier transform ion cyclotron resonance [ESI(-)] FT-ICR mass spectra were acquired using a 12T Bruker Solarix mass spectrometer (Bruker Daltonics, Bremen, Germany) and an Apollo II electrospray ionization (ESI) source in negative mode. A bitumen sample as well as its SARA fractions were dissolved in toluene to produce a total concentration of about 1000 mg/mL. Subsequently, an appropriate concentration of each sample was prepared in Toluene/Methanol/Formic acid for [ESI(-)] FT-ICR-MS analysis. Infusion of samples was done with a micro-liter pump at a flow rate of 120 µL h−1 with a nebulizer gas pressure of 138 kPa and a drying gas pressure of 103 kPa. A source heater temperature of 200°C was maintained to ensure rapid desolvation in the ionized droplets. The spectra were acquired with a time domain of 4 megawords in [ESI(-)], and 500 scans were accumulated for each mass spectrum. All spectra were internally calibrated using naphtenic acids as mass list. Data processing was done using Compass Data Analysis 4.0 (Bruker, Bremen, Germany) and formula assignment by an in-house made software (NetCalc).33 Molecular Formula Assignments. Molecular formula assignments were generated based on the exact mass differences using NetCalc software. 33 The assigned formulas were based on a restricted list of selected small molecular units with defined mass differences, corresponding to common chemical functional groups (CH2, H2, O, CO2, S, SO3 and NH) and transformations.33 Molecular formula assignments correspond to a multiple Kendrick analogue mass defect analysis and generate all homologous series according to chosen transformations simultaneously. Here, the compositional networks enable assignment of elemental formulae out of mass spectra and allowed alignments according to compositional relationships. The final assigned molecular formulas were categorized into groups containing CHO, CHNO, CHOS and CHNOS molecular compositions, which were used to reconstruct the group-selective mass spectra. Number and percentage of molecular formulas containing CHO, CHNO, CHOS or CHNOS molecular series as well as the computed average of H,C,N,O,S atoms and H/C,O/C,C/N,C/S as well as m/z (mass-to-charge) ratios and double bond equivalent DBE are shown in Table 1.

Table 1. Computed of mass peaks of singly charged negative ESI ions for bitumen 70/100 and its SARA fractions in both age stages. Members of Molecular series CHO CHOS CHNO CHNOS

Bitumen original aged 1552 1375 998 864 1051 1035 236 196

Saturates unaged aged 1706 1366 514 215 663 51 24 0

Aromatics unaged aged 1978 1873 2244 2055 832 1066 71 135


Resins unaged aged 1821 1844 1112 1149 2249 2247 579 416

Asphaltenes unaged aged 839 955 95 163 345 578 48 73

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number of assigned mass peaks average H [%] average C [%] average O [%] average N [%] average S [%] computed average H/C ratio computed average O/C ratio computed average C/N ratio computed average C/S ratio mass weighted average DBE weighted average DBC/C weighted average

3837 56.9 39.6 2.9 0.3 0.3 1.43 0.07 118 121 404.0 8.7 0.30

3470 58.6 37.1 3.7 0.4 0.3 1.58 0.1 96 121 376.3 6.2 0.23

2907 60.5 33.7 5.3 0.3 0.2 1.79 0.15 131 138 348.4 3.2 0.15

Plots of the assigned molecular formulas retrieved from the FT-ICR-MS datasets were processed using van Krevelen plots, DBE and Xc. These parameters are reported in details elsewhere. 13-15, 34

RESULTS AND DISCUSSION FT-ICR-MS data of Unaged Bitumen and its SARA Fractions. Negative electrospray ionization high-

1632 60.7 33.7 5.3 0.0 0.3 1.8 0.15 1731 119 349.4 3.1 0.15

Page 4 of 26 5125 60.1 35.3 3.9 0.1 0.5 1.7 0.11 311 66 373.5 4.5 0.19

5129 60.0 35.0 4.2 0.2 0.5 1.71 0.12 183 68 377.1 4.4 0.18

5761 55.6 40.5 3.0 0.7 0.2 1.376 0.07 57 160 400.0 9.6 0.34

5656 57.4 37.1 4.6 0.7 0.3 1.54 0.12 56 122 390.5 6.7 0.25

1327 60.0 34.2 5.4 0.2 0.2 1.75 0.15 173 190 322.2 3.5 0.17

field Fourier transform ion cyclotron mass spectra [ESI(-)] FT-ICR-MS of bitumen and its SARA fractions provided several thousand of mass peaks of which ~ 50% could be attributed to molecular series CHO, CHNO, CHOS and CHNOS using conservative assignment criteria.10-15, 34 The patterns of these molecular series in bitumen and its SARA fractions are shown in Figure 2 (likewise Figures S5 and S6).

Figure 2. [ESI(-)] FT-ICR mass spectra of the unaged bitumen and its SARA fractions. Top: H/C ratios versus mass-edited m/z of the mass spectra from 150-850 Da. Bottom: H/C versus O/C van Krevelen diagram elemental ratios. Insert histograms represent the molecular composition based on CHO (blue), CHOS (green), CHNO (orange), and CHNOS (red) atom combinations and numbers represent the counts of assigned molecular formulas (more details are shown in Figures S4, S5, and S6). The aromaticity index AI provided denotes single aromatic compounds for AI > 0 (blue triangle). 35 Bubble areas indicate relative mass peak intensity.


1769 59.9 33.4 5.8 0.6 0.3 1.79 0.17 58 106 347 3.3 0.16

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As shown in Figure 2 (likewise Figures S5 and S6), H/C versus O/C van Krevelen diagrams and mass edited ratios showed extended, smooth distributions of all four molecular series across considerable ranges of unsaturation (H/C ratio) and mass, down to rather low degrees of oxygenation (O/C ratio < 0.05). Furthermore, CHO, CHNO and CHOS compounds appeared more unsaturated on average in bitumen and resins than in aromatics, saturates and asphaltenes, suggesting elevated abundance of polyaromatic compounds. The asphaltenes were largely devoid of high mass molecules (m/z > 550) and showed high proportions of oxygen atoms mainly involved in CHO and CHNO molecular series at a considerable range of H/C ratios. Overall, abundance of CHO-molecular series was significant in bitumen, asphaltenes and saturates, whereas aromatics and resins were rich in CHOS and CHNO molecular series, respectively (Figures 2 and S4; Table 1). Average values of H, C, N, O and S (atom %), H/C, O/C, C/N and C/S elemental ratios as well as average mass-to-charge (m/z) ratios and DBE per molecule and DBE/C values were computed based upon intensity weighted averages of assigned mass peaks (Table 1). The average values of oxygenation was elevated in aromatics, saturates and asphaltenes (in increasing order), whereas observed proportions of heteroatoms were elevated in aromatics (CHOS) and resins (CHNO), respectively (Table 1). Furthermore, the relative oxygenation appeared to follow the average H/C ratios, suggesting functionalization of aliphatic rather than aromatic groups. Weighted mass average of unaged bitumen and its SARA fractions followed the order asphaltenes < saturates < aromatics < resins < bitumen, respectively (Table 1). Accordingly, FTICR-MS-based hierarchical cluster analysis (HCA) of unaged bitumen and its SARA fractions clearly differentiate between the three groups of bitumen and resins, aromatics and asphaltenes and saturates, respectively (Figure S1A).

Bitumen and resins showed sizable proportions of low oxygen (O/C < 0.1) condensed aromatics and appreciable proportions of hydrogen-deficient (H/C < 1.2) CHOS compounds. Saturates and asphaltenes showed high proportions of CHO and CHNO aliphatic compounds in comparison at overall fewer but more intense mass peaks (Figure S1D). In contrast, aromatics showed high proportions of hydrogen-rich (H/C > 1) CHOS compounds across a substantial mass range (m/z ~ 170 – 750 Da), entirely different from those observed in the bitumen and resins (Figure S1C). CHO compounds in bitumen and resins and aromatics showed higher relative convenience of nominal compositions at still distinctive overall differences. DBE and Aromaticity Equivalent of Unaged Bitumen and its SARA Fractions. The count of double bond equivalents (DBE) despites the level of unsaturation (i.e., sum of double bonds and rings) in a molecule or, alternatively normalized to the number of carbon (DBE/C) has been extensively used in petroleomics.35-37 Iso-abundance plots of DBE versus number of carbon for the unaged bitumen and its SARA fractions revealed large counts of DBE for bitumen and resins, lower counts for aromatics and saturates, with asphaltenes in between (Figure 3).

Figure 3. Plots of DBE versus number of carbon of the unaged bitumen and its SARA fractions. Insert diagrams depict the computed percentage of condensed aromatic units (olive color), aromatics with single benzene ring (sky magenta color) and alicyclic and open chain aliphatic units devoid of aromatics (sky blue color).34 Bubble areas indicate relative mass peak intensity.


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While coarse correlations were observed between count of DBE and number of carbon atoms for all fractions, a distinctive clustering within bitumen and all SARA fractions suggested structure selectivity, like common abundance of near saturated molecules in saturates, aromatics and asphaltenes and presence of polyaromatic compounds (> 12 DBE at C20-35) in the resins. Saturates had 0-17 DBE and 10-40 carbon atoms, while aromatics and asphaltenes had (0-17 DBE and 8-48 carbon atoms) and (0-25 DBE and 10-40 carbon atoms) respectively. Improved insight was derived from aromaticity equivalent Xc, based on the carbon backbone of bitumen and its SARA fractions, 34 and computed as follows Equation (1):

Equation (1) In Xc and DBE=C+1/2(N+P-H)+1, the atoms (C,H,N,O,S and P) are atoms retrieved from the assigned molecular formulas, m and n are the fractions of oxygen and sulfur involved in double bond structure of the compounds, respectively. If DBE ≤ mO+nS, then Xc=0. Thus, given threshold values (i.e., Xc ≥2.5000 and Xc ≥2.7143) serve as unequivocal parameters for the presence of aromatics and

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condensed aromatic cores, respectively. Assuming m=1 and n=1 bitumen and resins showed > 50% of condensed aromatic compounds, whereas more than 70% of aliphatic and alicyclic compounds without benzene rings were found in saturates and asphaltenes, as demonstrated in Figure 3 (inserts). In addition, > 50% of aliphatic and alicyclic versus 18% and 27% for condensed aromatics and aromatic compounds were found in aromatics. The obvious differences in relative unsaturation for unfractioned bitumen and its SARA fractions indicate a variable propensity/susceptibility for ageing processes. Effect of Ageing Process on Bitumen and its SARA Fractions. An unambiguous definition of ageing status in complex materials is often demanding. Information-rich high-field FT-ICR mass spectra of bitumen and its SARA fractions showed alterations within all CHO, CHNO, CHOS and CHONS molecular series when changing from unaged to aged bitumen and its SARA fractions (Figures S2, S7 and S8). No uniform trends were observed across all studied materials. The computed counts and percentages of common and respective unique molecular compositions between related pairs of samples varied considerably and were found largest in saturates, lowest in aromatics, and intermediate in bitumen, resin and asphaltene fractions, respectively (Figure 4).

Figure 4. Number of common and unique assigned molecular formulas of unaged and aged bitumen 70/100 and its SARA fractions. Insert numbers illustrated the computed percentages of common and unique assigned molecular formulas when matching pairs of samples.


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Here, PAV(EN)-aged bitumen was compared to an unaged sample. For accelerated laboratory ageing, a PAV apparatus was employed. Comparison of two states of ageing (i.e., short-term and long-term aging) revealed

that ageing seemed to target aromatic CHO compounds which were very prominent in the unaged and rather attenuated in the PAV-aged stage (Figure 5).

Figure 5. Comparison of unaged (top) and aged (bottom) bitumen 70/100 and its SARA fractions. Van Krevelen diagrams of the unique CHO, CHNO, CHOS and CHNOS molecular series when comparing pairs of samples separately, obtained from [ESI(-)] FT-ICR mass spectra. Color code represents: blue CHO-, orange CHNO-, green CHOS- and red CHNOS and numbers present the count of the unique assigned molecular formulas in each pairs of samples. The aromaticity index AI provided denotes single aromatic compounds for AI > 0 (blue triangle).34 Bubble areas indicate relative mass peak intensity. The PAV-aged stage exhibits increased abundance of more oxidized compounds. Furthermore, the unique molecules show that specifically condensed aromatic compounds are influenced (Figure 6).

Figure 6. Comparison of unaged (top) and aged (bottom) bitumen 70/100 and its SARA fractions. Aromaticity equivalent Xc versus number of carbons for condensed aromatic units (2.7143 ≤ Xc ≤ 3) and aromatics with single benzene ring (2.5 ≤ Xc < 2.7143) of the unique molecular formulas when comparing pairs of samples separately, obtained from [ESI(-)] FT-ICR mass spectra. The insert core structures are examples of many possible structures of these molecular formulae.34 The values in brackets indicate the number of compounds with the same Xc value. Bubble areas indicate relative mass peak intensity.


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About 35% and 8% of the unique precursors in both stages of bitumen (i.e., unaged and aged) were defined as condensed aromatic compounds respectively, while aromatics showed only a slight reduction (Figure 6). As shown in Figure 6, number of formulas distributed in each straight segment (constant Xc values) differ in both age stages of the bitumen. For instance, there are 36 formula with benzene units (Xc=2.5000) assigned in unaged bitumen, and a slight increase was observed in aged bitumen with 41 of assigned formulas. In the opposite, numbers of formulas assigned for naphthalene like compounds in unaged bitumen (63) were two-times higher than in aged bitumen (30). Furthermore, a markedly reduction in the condensed aromatic units was observed (Figure 6). Another important question is the effect of PAV ageing on the separated SARA fractions. Often, common molecular compositions occur in both non-aged and aged bitumen in all fractions (Figure 4) which does not necessarily imply identical molecules because of the numerous options to form isomeric compounds. None the less, only the unique chemical compositions of SARA fractions are discussed here. Figure 5 shows the van Krevelen plots of the unique CHO, CHNO, CHOS and CHNOS molecular series which demonstrate distinct evolution of individual SARA fractions in each pairs of samples. Overall, saturates lose many of their CHO, CHNO and CHOS compounds, while a gain in these chemical compositions was detected in asphaltenes. In relation, both resins and aromatics lose many of their hetero-aromatics and S-rich compounds. Except for saturates, aromatic, resin and asphaltene fractions showed increased content of highly-oxidized CHOS and CHNO aliphatic molecules. The resin fraction showed the largest molecular change during ageing, like a three-fold decline of condensed aromatic compounds as compared to aromatics and asphaltenes, while a slight change has been noticed in saturates (Figure 6). Furthermore, alteration of condensed aromatic compounds decrease in resins with about threefolds and only two-folds in saturates, aromatics and asphaltenes, respectively. In addition, alteration of aromatic compounds remained almost constant in aromatics and resins, while reduction and enhancement by factor four and two were observed in saturates and asphaltenes respectively (Figure 6). As shown in Figure 6 (insert), alteration of the unique compounds with benzene (Xc = 2.5000), naphthalene (Xc = 2.7143), anthracene (Xc = 2.8000) and coronene (Xc = 2.8947) core structures were various in bitumen and its SARA fractions. Overall, resins and bitumen showed sizable alterations in comparison with asphaltenes, while aromatics and saturates were more stable (Figures 3 and S3).

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(n=1-9) in CHO, CHNO and CHOS molecular series in Figure 7. Counts of oxygen atoms involved in the unique CHO, CHNO and CHOS molecular series of bitumen and its SARA fractions in both unaged (blue) and aged (red). both age stages of bitumen and its SARA fractions.

Oxygenation Profile of Ageing Process of Bitumen and its SARA Fractions. Sorting assigned molecular formulas according to their count of oxygen atoms allowed to follow the level of their oxygenation associated with the ageing process. Figure 7 shows the numbers and distribution of molecular formulas according to counts of oxygen


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Again, non-uniform behaviour was observed: the number of oxygenated compounds in aged stage increased in resins and asphaltenes across all CHO, CHNO, and CHOS molecular series. The largest relative abundance profile of oxygen atoms was observed in aged asphaltenes, which contained the highest proportions of multi-oxygenated compounds, especially for CHO and CHNO molecular series. Abundant of oxygen atoms were found in both CHNO and CHOS molecular series of bitumen, whereas only the aromatic CHNO molecular series showed a near Gaussian profile covering oxygen atom range from 1 to 9 Implications and Perspectives. The lack of suitable fieldaged samples is a problem for most ageing studies of bitumen: Rarely a sufficiently preserved sample of the unaged bitumen has been set aside, when a road was built. Hence, the opportunity to study the symptoms of field ageing is scarce. Hence, we have conducted a fieldageing trial field with the particular bitumen investigated in this study which will be available for comparison also in the future. In general, the PAV (pressure ageing vessel) procedure was found to oxidize small aromatic molecules mainly by addition reactions, which lead to increased polarity and lesser solubility in the bitumen causing the abundance of asphaltenes to increase substantially. Gravimetric analysis has shown an increase in asphaltenes fractions by 50% of initial value, while the aromatic fractions and to a lesser extent the resin fractions were decreased in abundance (results not shown). However, the results stated in this study can vary for different bitumens (asphalts).

atoms (Figure 7). Aged saturates were in fact depleted of oxygen atoms relative to unaged materials. The combined data demonstrate a lesser abundance of condensed aromatic compositions and higher degree of oxygenation in aged samples as compared with unaged samples (Figures 6 and 7). These trends in oxygen atoms profiles gives evidence of the intrinsic compositional (and structural) variety of compounds within and across CHO, CHNO and CHOS molecular series in bitumen and its SARA fractions in both age stages, which is an acknowledged aspect of progressive oxidation.

CONCLUSIONS

ACKNOWLEDGMENTS

We have shown that the pressure ageing vessel (PAV) procedure applied to bitumen and its SARA fractions could be characterized with high-field FTICR mass spectrometry providing new insight into changes of their molecular composition changes. In general, unaged samples showed that the resin fraction was very closely related to the unaged bitumen but polar condensed aromatics were considerably more abundant. Asphaltenes and saturates were mostly linear and acyclic aliphatics with a high content of oxygen functionalities, whereas the aromatic fraction was unique with a high proportion of aromatic compounds. Ageing behaviour of bitumen and its SARA fractions showed that while the degree of unsaturation of compounds was abundant in unaged samples they were converted into highly oxygenated and more saturated compounds in aged samples. Especially condensed aromatics of low polarity seem to be degraded in mass by oxygenation processes. This corresponds reasonably well to the micro-structural observations found by CLSM and 39, 40 The implications of the fluorescence analysis earlier. updated micelle model for bitumen presented therein are directly supported by these findings. Embrittlement and corrosion of bitumen can be understood as the vanishing of the micelle mantle, which consists mainly of polycyclic aro-

We express our gratitude towards our governmental funding partners, the Austrian Research Promotion Agency (FFG) in the project “OEKOPHALT Project Number 834203 – Chemical and physical fundamentals of bitumen ageing for ecological asphalt recycling”. Furthermore, we thank our cooperation and funding partners from asphalt industry, namely: Pittel+ Brausewetter GmbH, Swietelsky Baugesellschaft m.b.H., Nievelt Labor GmbH, and the OMV AG for their continued support.

matic compounds of low to average polarity. This mantle is considered to bridge the polarity gap between the high polar volume elements and the surrounding matrix. Hence, the elimination of this class of molecules directly causes embrit36 tlement and material failure. The question whether asphaltenes consist mainly of condensed aromatics in archipelago or a more continental geometry with highly polar side chains remains to be elevated. If we take into account that the most polar molecules are mostly saturated linear or acyclic functionalized hydrocarbons with an average mass of ~ 350 Da, then the condensed aromatic rings have to be virtually apolar in nature. Pi-Pi stacking to higher stack-sizes than the assumed 5-7 molecules could be achieved, if no steric hindrance from side-chains has to be considered. This novel hypothesis concerning asphaltenes actually being two main groups of molecules easily differentiated from one another reconciles former studies on average molecular weight with more mass spectrometric investigations and the use of complementary ionization techniques for less polar compounds (atmospheric pressure photoionization).

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Tracking Aging of Bitumen and Its Saturate ... - ACS Publications

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