Reactivities of Aromatic Protons in Crude Oil Fractions toward Br2

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Reactivities of Aromatic Protons in Crude Oil Fractions Toward Br2 Tagging for Structural Characterization by NMR and EPR Spectroscopy and Mass Spectrometry Michael T. Spiegel, Ian G. M. Anthony, Matthew R. Brantley, C. Alton Hassell, Patrick J Farmer, and Touradj Solouki Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02342 • Publication Date (Web): 13 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018

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Reactivities of Aromatic Protons in Crude Oil Fractions Toward Br2 Tagging for

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Structural Characterization by NMR and EPR Spectroscopy and Mass

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Spectrometry

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Michael T. Spiegel, Ian G. M. Anthony, Matthew R. Brantley, Alton Hassell, Patrick J. Farmer,

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and Touradj Solouki*

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Department of Chemistry and Biochemistry, Baylor University, Waco, TX 76798, USA

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ABSTRACT (Word Count: 179): Comprehensive characterization and structural elucidation

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of specific chemical sub-classes within a complex mixture often require analyte separation, a

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variety of spectroscopic techniques, and inquiry into reactivity patterns. Crude oil is one of the

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most challenging and highly complex mixtures and, for classification purposes, it is typically

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separated into low-weight n-heptane-soluble and high-weight toluene-soluble (or asphaltene)

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fractions. Both fractions of crude oil have been studied using several spectroscopic methods, but

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their complete characterization remains elusive. In this work, we utilize selective functional

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group derivatization of both n-heptane and asphaltene fractions of crude oil for unprecedented

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identification of chemical functional groups by a combination of high resolution mass

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spectrometry (HR MS),

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paramagnetic resonance (EPR) spectroscopy methods. Chemical tagging with elemental bromine

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suggests that the aromatic protons within the asphaltene fraction are less abundant and more

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sterically hindered than previous models might suggest. An efficient column purification was

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H and

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C nuclear magnetic resonance (NMR), and electron

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utilized to remove paramagnetic metals and allow identification of crude oil functional groups

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via analysis of spectrometry and spectroscopy data combined with chemical reactivity studies.

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* Corresponding Author’s email address: [email protected]

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Introduction. Crude oil is a complex mixture of hydrocarbons with a variety of functional group

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variations, including various hetero-substituents such as oxygen, sulfur, and metals.1 The n-

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heptane-insoluble, toluene-soluble fraction of crude oil is known as the asphaltene fraction,

4

which contains the heaviest components of crude oil, generally with the highest boiling points.2,3

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Asphaltenes contain highly-substituted, polyaromatic rings that stack and aggregate, forming

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sediments that are typically removed during the processing of crude oil to increase the ease of

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handling of the remaining oil fractions.4-6 Because of the complexity of asphaltene samples, it is

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difficult to efficiently prevent premature aggregation. Therefore, a better understanding of the

9

chemical structures present in asphaltene samples could help minimize adverse effects on

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industrial processes. 1-7

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Although significant contributions have been made to elucidate the structure of asphaltenes to

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predict reactivity, there has been comparatively little work done to utilize the reactivity of

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asphaltenes in order to characterize structure.8-9 For instance, analyte tagging with mass deficient

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reagents can be used for functional group identification.10 Elemental bromine (Br2) is a

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particularly useful mass deficient tag for probing the reactivity of asphaltenes as the reactivity of

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Br2 with hydrocarbons is well known.11 Moreover Br2 is well-suited for mass spectrometry (MS)

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analysis of crude-oil fractions as it is an easy-to-introduce reactant (i.e., it is a liquid that is

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miscible with both toluene and n-heptane solutions), provides a distinct isotopic pattern (i.e., two

19

peaks, at roughly 51% and 49% abundance with an ~2 m/z separation), and introduces a mass-

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deficit (i.e., the lower-mass isotope of bromine is 78.9183 amu instead of the nominal 79 amu, a

21

difference of -0.0817 amu).

22

As shown in Scheme 1, there are three main pathways that Br2 can react with organic

23

molecules: 1) addition to an unconjugated double bond, 2) free radical bromination, 3) and


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electrophilic aromatic substitution (EAS).11-15 Asphaltenes lack unconjugated double bonds and

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hence, bromine addition reactions should not occur.2 Free radical bromination requires UV light

3

to generate free radicals and thus can be avoided via an opaque container. Therefore, EAS is

4

expected to be the primary reaction resulting from addition of Br2 to asphaltenes under dark

5

conditions.

6

Scheme 1

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Kendrick mass defect (KMD, or difference from the nominal mass) analysis is a data

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visualization technique in MS for “mass-deficiency-based” compound or hydrocarbon

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classification that is useful for complex samples including crude oil fractions.16-19 Although


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highly useful for observing differences between the mass spectra of complex samples, KMD has

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not been widely used to probe the chemical reactivity of crude oils via mass deficient

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derivatization agents. “Labeling” via Br2 electrophilic aromatic substitution predictably alters

4

analytes in both the mass and mass deficiency dimensions and can move reactive molecules’

5

signals from the congested regions of Kendrick plots.17 Moreover, the distinctive isotopic pattern

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of Br2 makes KMD plots especially useful for visualizing the extent of reactivity of molecules

7

toward Br2 (via number of additions of Br).

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Besides MS, other analytical methods, such as NMR and EPR, have been used to interpret the

9

speciation of crude oil fractions.32-39 Typically, the NMR spectra of crude oil fractions contain

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significant line broadening from aggregation and stabilized free radicals that hinder data

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interpretations.20 In addition, the asphaltene fraction may contain paramagnetic metals which can

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cause further line broadening.3 EPR has previously been employed in studying these unpaired

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electrons in crude oil21-23 but has not been used to compare influence of metal ion on crude oil

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NMR.

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Herein, we report results from KMD analysis of the crude oil product mixtures following

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bromination of the n-heptane and asphaltene fractions of West Texas crude oil. Additional details

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on chemical speciation is reported for 13C/1H NMR and EPR data of the crude oil fractions. We

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also demonstrate that a simple column purification dramatically reduces the paramagnetic

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content of the asphaltene fraction and enables NMR spectral analysis to provide characterization

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and quantification of a range of chemical functional groups within an asphaltene sample.

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Experimental. All reagents and solvents were purchased from Sigma-Aldrich Chemical Co., St.

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Louis, MO. Unless otherwise noted, chemicals were used without further purification. Light

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crude oil (considered West Texas Blended oil that is paraffin based) samples from Texas Crude


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(Midland, TX) were purchased for complex mixture analysis. Asphaltene and n-heptane fractions

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of the crude oil were prepared following the IP 143/01 method.24 Briefly, crude oil was mixed

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with n-heptane in a ratio of 1:30 oil:n-heptane by volume. The mixture was refluxed for 60

4

minutes at 150 ○C and then sealed and allowed to cool for 90 minutes in the dark to avoid photo-

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reactivity. Once cooled, the mixture was filtered through a Büchner funnel. The material that

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remained on the filter was washed with 70 ○C n-heptane. The filtrate (hereafter referred to as the

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n-Heptane Fraction) was collected and stored at 0 ○C. Toluene at 70 ○C was then used to dissolve

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the asphaltenes collected on the filter frit into a clean flask (hereafter referred to as the

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Asphaltene Fraction). Aliquots were taken of each asphaltene and n-heptane fractions and

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evaporated to give concentrations of 70 mg⋅mL-1 of asphaltenes in toluene and 250 mg⋅mL-1 of

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solid in the n-heptane fraction.

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A sample from each fraction was diluted with toluene to 0.25 mg⋅mL-1 and brominated (under

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dark conditions in glass amber vials that had been wrapped with aluminum foil to prevent any

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free radical bromination) with 5 µL of elemental bromine (with an approximate mole ratio of

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1:200 asphaltene:Br2, with the rough assumption of 500 g⋅mol-1 for an average asphaltene

16

molecular weight). The resulting mixtures were analyzed by Orbitrap mass spectrometry after 30

17

minutes of reaction time. Additionally, a silica gel, gravity column-separated asphaltene solution

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was generated by passing 10 mg of dried asphaltene sample through a one inch diameter, three

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inch long glass column packed with 3 grams of silica gel (using 0.06-0.20 mm OD silica beads).

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The column was packed by making a toluene and silica gel slurry and quickly adding the

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resulting slurry to the column (using ~2:1 v/v ratio of toluene:silica). The resulting silica-gel

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column was washed with 50 mL of HPLC grade toluene. After loading 1.0 mL of asphaltene

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solution (at 10 g⋅L-1 dissolved toluene), three sequential eluents were used to purify the


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asphaltenes: (a) 50 mL of toluene, (b) 50 mL of methanol, and (c) 50 mL chloroform. The flow

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rate for the column was roughly 1 drop (~0.05 mL) per 2 seconds. All resulting solutions were

3

combined, and the solvents removed via rotovap at ~25 °C. The resulting solid was put under

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vacuum (~5.0x10-3 Torr) for 24 hours to ensure complete solvent removal, yielding ~ 9.0 mg of

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silica-column treated asphaltenes.

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Sample Preparation for ICP MS Analysis: For ICP-MS analysis, pre- and post-column samples

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were prepared from non-brominated asphaltene solutions using an “emulsion breaking” method

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adapted from Cassella, R. J., et al.25 In brief, 1 mg of asphaltenes (for both the pre- and post-

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column ICP-MS samples) was dissolved in 10 mL toluene. Two mL of acidic Triton-X114

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solution (with preparation described by Cassella, R. J., et al.)25 was added to both 1 mg

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asphaltenes in 10 mL toluene solutions and vigorously shaken for 30 minutes, then placed on a

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roller mixer for 60 minutes. No true emulsion formed in either solution (as the toluene and

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aqueous fractions were observed to separate after ~1 minute of no mixing). The two mixtures

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were centrifuged at 10,000 RPM for 10 minutes. Each solution’s acidic, aqueous subnatant was

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carefully removed via pipette and analyzed without dilution using ICP-MS (details discussed

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below).

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Instrumentation. For KMD plots, mass spectrometry (MS) measurements in positive-ion mode

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were carried out using an Orbitrap Discovery mass spectrometer equipped with an atmospheric

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pressure photoionization (APPI) source (ThermoFisher Scientific, Waltham, MA). Sample

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solutions were infused at a rate of 5 µl⋅min-1 with nitrogen as the sheath gas. The optimized APPI

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conditions were as follows: ion spray voltage, 5 V; capillary temperature, 275°C; tube lens

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voltage, 110 V.


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For metal analysis, an Agilent Technologies (Santa Clara, CA) 7900 ICP-MS was used in

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positive-ion mode and a flow of 9.5 L⋅minute-1 Ar gas. The RF power was set to 1550 W and the

3

nebulizer pump to 0.10 rps. An internal standard solution of 7Li,

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concurrently to the samples (with one of the nebulizer pump lines). Analogous to other

5

experiments reported here, method blanks were used in between the pre- and post-column

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samples to avoid carry over.

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89

Y,

The 1H, Distortionless Enhancement by Polarization Transfer (DEPT)

205

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Tl was carried

C, and

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C NMR

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spectra were recorded on an Ascend 600 MHz NMR spectrophotometer (Bruker, MA, USA) at

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298 K and referenced to chloroform (7.26 ppm for 1H and 77.23 ppm for 13C, respectively). The

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spectral widths for the 1H and

11

respectively. The averaged number of scans for proton and carbon NMR spectra were 32 (Figure

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3) and 10,000 (Figure 4), respectively. The excitation pulse widths for the 1H and 13C NMR were

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12 µs and 10 µs, respectively. The microwave frequency was 9.85 GHz.

14 15

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C NMR analyses were ~20.03 ppm and ~240.05 ppm,

EPR spectra were recorded on a EMX Plus spectrometer (Bruker, MA, USA). The center field was 3200 G with a sweep width of 6000 G and sweep time of 60 seconds for 20 averaged scans.

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Results and Discussion. CH2-based Kendrick mass defect (KMD) plots were generated by

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redefining the masses of the observed analytes in the original mass spectrum from C to CH2

18

given by Equation 1; these Kendrick mass (KM) values were used to form the x-axis of the

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KMD plot.17 KMD (the difference between nominal mass and KM) is plotted as a function of the

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KM. Sizes of the points in the KMD plot of Figure 1 are correlated to relative abundances of

21

peaks. : Kendrick mass = observed mass x

nominal mass of CH2 exact mass of CH2


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Figure 1: KMD plot of the unreacted (blue) and brominated (red) n-heptane fraction from 300 to 400 Kendrick mass units. Doublet peaks associated with Br-R species, as described in the text, are highlighted in the inset black box. The two points circled in red highlight a representative bromination event. The red, green, and black arrows point to observed KM and KMD shifts with the substitution of an H for a NH, O, and CH2, respectively.

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from two separate mass spectra are overlaid in Figure 1. For clarity, only the zoomed-in area

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between 300 and 400 Kendrick mass units is shown. The full KMD plot of each sample is

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included in the supporting information as well as a similar “zoomed in” crude oil figure (S1-4).

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Each diagonal line defines various polyaromatic hydrocarbons with decreasing saturation, with

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the lower abundance chains containing various hetero substitutions. Brominated species (Br-R)

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are identifiable by apparent “doublet” peaks, due to the near equal natural abundance of 79Br and

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81

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The two points circled in red are a representation of a bromination showing the change in mass

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and mass defect from a specific point. The red, green, and black arrows show the shift in a KMD

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plot from a NH, O, and CH2 replacing of an H, respectively. In the n-heptane fraction, only

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single brominations (highlighted by the black box) are observed. Smaller aromatic species are

The KMD plots of the pre- (blue) and post-bromination (red) n-heptane fractions generated

Br isotopes with double and triple brominations seen as “triplets” and “quartets”, respectively.


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less likely to undergo substitution reactions, which agrees with previous studies that indicate that

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the aromatic species in the n-heptane fraction are smaller than that of asphaltenes.26-27 Bromine

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substitution does not occur until ~ 0.11 mass deficiency (corresponding to loss of ~16 protons

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from a pure CH2 chain, this is due to Br2 substitution occurrence only on larger aromatic systems

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(in the absence of a catalyst or UV-light). In addition, the majority of brominations in the n-

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heptane fraction occur from non-hetero substituted species.

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Figure 2: KMD plot of the unreacted (blue) and brominated (red) asphaltene fraction from 250 to 400 Kendrick mass units. Doublet peaks associated with Br-R species, as described in the text, located in the boxed region.

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Pre- and post-bromination KMD analysis was also performed on the asphaltene fraction,

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Figure 2. The asphaltene fraction (relative to n-heptane fraction) undergoes more extensive

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brominations. The Br-R species in the asphaltene fraction displayed mostly single and some Page Plus 10 of 25 ACS Paragon Environment

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double substitutions (no asphaltene species was observed to brominate more than three times).

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Substitution occurs at the same mass deficiency (~0.11) as the n-heptane fraction. The majority

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of brominations come from non-hetero substituted polyaromatics, but some arise from hetero-

4

substituted species. The lack of substitution from hetero substituted asphaltenes is presumed to

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be due to electron withdrawing nature of these functional groups. To better understand the extent of asphaltene bromination, benzo-[a]-pyrene (BP, 252.31g⋅mol-

6 7

1

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compared to other asphaltenes. Although BP has twelve potential bromination sites, based on the

9

interpretation of the KMD plots (S5), only 1st, 2nd, and 3rd brominations were observed; this level

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of bromination for BP (up to three, as shown in Scheme 2, in any position) is higher than the

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number of brominations observed for other asphaltenes in the asphaltene fraction (especially for

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asphaltenes of similar masses). In a separate mass spectrometry experiment, it was also

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confirmed that BP did not react with asphaltene and it remained intact within the asphaltene

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mixture. The lack of triple brominations of the asphaltenes may be attributed to a deactivation of

15

the ring system, recruitment into larger aggregates, steric hindrance, or general lack of aromatic

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protons. Two other possible explanations are a lack of available bromine atoms for substitution

17

and slow Br2 EAS reaction kinetics. However, the ~200:1 mole ratio of Br2:asphaltenes

18

(discussed in the Experimental section) means that there should be enough Br atoms to substitute

19

more than 3 aromatic protons on each asphaltene. Additionally, the benzo[a]pyrene brominated

20

multiple times (i.e., up to 3 times) given the same reaction time and conditions (i.e., 30 minutes

21

with Br2 in dark conditions, with a toluene solvent, present in a solution of other asphaltenes) as

22

the asphaltene fraction. Therefore, it appears (although is not certain) that the substitution is not

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kinetically limited. NMR analysis was performed to verify the specific reason that higher

) was added as an internal standard to the asphaltene samples and its brominations were

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numbers of brominations were not observed (as NMR might provide evidence for steric

2

hinderance, lack of available aromatic protons, etc.).

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Scheme 2

4

5 6 7 8

Additionally, to better understand the structural makeup of asphaltenes and characterize their

9

functional groups, the NMR spectra of fractionated crude oil samples (i.e., toluene/asphaltene

10

and n-heptane/non-asphaltene fractions) were compared. The initial intent was to investigate the

11

relative abundances of aromatic protons present in the asphaltene by obtaining ratios of aromatic

12

to non-aromatic protons, a rough measure of alkyl vs. aromatic content.28-30 However, as

13

previously reported,28-29 because of the significant line broadening observed in 1H NMR spectra

14

of asphaltenes (presumably because of potential aggregation and/or presence of paramagnetic

15

organic radicals and transition metal ions in crude oil samples), NMR spectra provide limited

16

information for complete structural characterization. It is well documented that asphaltenes

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contain a variety of organic radicals & paramagnetic metal ions, such as V, Ni, and Fe species.3,

18

21-23

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2.00 (where g is the “g-tensor” of EPR)31, attributable to organic radicals, and a broader signal at

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g = 2.29, attributable to paramagnetic metal ions, similar to previous characterizations of

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asphaltene.23, 32-33

As seen in Figure 3, EPR spectra of the asphaltene fraction shows a sharp absorbance at g =


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To remove the paramagnetic metal ion species detected by EPR, a short column purification on

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silica gel was performed, that resulted in dramatically improved 1H NMR spectral resolution, as

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seen in Figure 3. This sample clean-up resulted in a loss of ~ 10% of asphaltene mass (from 10.0

4

mg pre-column to 9.0 mg recovered post-column). ICP-MS analysis was performed on the

5

asphaltene fractions pre- and post-column and showed a 95% reduction in all paramagnetic

6

signals studied, including a 98% reduction of Fe, the most abundant paramagnetic species. Thus,

7

the 10% loss in mass is attributed primarily to paramagnetic and other metal-ion-containing

8

species. For further information, including results from ICP-MS analyses of the pre- and post-

9

column asphaltenes, the reader is referred to S6. To determine if sample complexity was retained

10

post-column, mass spectra were re-acquired for the post-column fraction and did not show any

11

loss in complexity (see S7 and S8).

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Figure 3: EPR (left) and 1H NMR (right) of pre-column clean-up (top) and post-column cleanup (bottom) asphaltenes in a 10 mg⋅mL-1 CDCl3 solution. Labels in lower right 1H NMR show assignments of peaks to a) aldehyde or carboxylic acid, b) aromatic (also solvent CDCl3) c) phenolic, d) OH/NH, e) amine substituted CH2 and CH3, f) carbonyl- and benzyl-substituted CH3, g) alkyl branched CH2 and CH, and h) terminal CH3 protons. Page Plus 13 of 25 ACS Paragon Environment

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The post-column 1H NMR spectra show dramatically improved and well resolved peaks

2

that are attributable to a variety of functional groups present in the asphaltene mixture. The

3

relatively narrow line-widths of the NMR spectra (of cleaned-samples) are likely due to loss of

4

paramagnetic species and reduced aggregation (with loss of only ~10% mass based on the

5

weighing of samples before and after the column clean-up procedure).

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As assigned in Figure 3 (post-column NMR), a small peak near 10 ppm (labeled as “a”)

7

indicates the presence of aldehydes or carboxylic acids. Well defined aromatic peaks (labeled

8

“b”) are observed between 7-8 ppm; the lack of peaks between 8-9 ppm in Figure 3 suggest that

9

few N substituted pyridine like heterocycles are present. This is not surprising as the studied

10

asphaltene species between 8–9 ppm would most likely be bound to metals and thus lost in the

11

column (contributing to the observed 10% mass loss after the clean-up). The peak at ~ 5.3 ppm

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(labeled as “c”) is indicative of phenolic alcohols. This region is also attributable to

13

unconjugated double bonds, but the absence of Br2 addition in mass spectra (i.e., addition of two

14

bromines across a double bond) discounts this possibility. Non-aromatic NH and OH peaks

15

between 3-5 ppm (labeled as “d”) are in much lower abundance but observed. NR2 (where R is

16

any alkane) protons are also present (labeled “e”). A high abundance of carbonyl and benzylic

17

protons are present (labeled as “f”), indicating that there are likely many alkyl groups attached to

18

the aromatic core. Lastly, the branching CH and CH2 species (labeled as “g”) and terminal CH3

19

protons are observed between 0-2 ppm (labeled as “h”). Most of these protons are branching,

20

instead of the terminal CH3, meaning the aromatic cores within asphaltenes are highly

21

substituted.

22

Using peak integration of the assignments above yield estimates of proton content

23

percentages provided in Table 1 below; aliphatic -CH1/2/3 peaks make up the majority of all


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observed 1H signals, at ~65%. The quantification of aromatic protons, ~ 10%, is complicated by

2

subtraction of the solvent CDCl3 peak, but in good agreement with previous estimates.34-38 The

3

lack of aromatic protons coupled with the extensive branching found in the NMR analysis

4

correlates well with our mass spectrometry results and KMD tagging studies. The lack of

5

multiple brominations is likely caused by the steric hindrance of the branched alkanes or

6

deactivation of the aromatic core by electron withdrawing functional groups.

7

Table 1. Relative speciation of protons by NMR peak integration

8 9 10

Peak assignment a %b -C(O)H 0.21 Ar-H 10.24 Ar-OH 3.02 -OH, -NHx 4.13 N-CH2/3 7.47 C(O)CH2/3, Ar-CH3 9.84 -CH2, -CH 40.97 -CH3 24.12 a as per references 36-42 b by integration of area under assigned 1H NMR peaks, as percentage of total.

11 12 13 14 15

Figure 4. 13C NMR of column purified asphaltene sample. Labels show assignments of peaks to a) aldehyde or carboxylic acid, b) ester, c) aromatic d) solvent (CDCl3), e) N- or O- substituted, f) branched CH2 and CH, and g) terminal CH3 carbons.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

The 13C NMR spectrum of column-purified asphaltene sample is shown in Figure 4; this

2

cleaned-up asphaltene also shows enhanced spectral resolution not attainable for unpurified

3

samples. As above, downfield peaks between 160-200 ppm can be attributed to the presence of

4

aldehyde, carboxylic acid, and ester carbons (labeled as “a” and “b” in Figure 4). Similar to the

5

1

6

as “c”); this suggests a lack of heteroatom substitutions within the aromatic cores. Various other

7

non-aromatic carbons attached to amine and ether functional groups above 50 ppm are also

8

observed in

9

species seen in the 1H NMR of Figure 3. Similar to the 1H NMR, more branching carbons, region

10

“f” in Figure 4, are observed than terminal CH3 carbons, region “g”. In addition, DEPT-135

11

carbon experiments were performed (S9-12) and results agree with MS, 1H NMR, and 13C NMR

12

data. These DEPT-NMR scans assist in identifying many quaternary carbons (carbons with no

13

protons attached) in the alkane region (labeled as “f” in Figure 4), from 15-60 ppm (e.g., Figure

14

4 and S10), suggesting extensive alkyl branching. Only a few such peaks are apparent in the

15

aromatic region (“c” in Figure 4); due to poor spin-lattice relaxation of these absorbances, we

16

cannot assign all quaternary carbon functional groups in the mixture (S7-10).39-44 It is interesting

17

to note that the crude oil and its n-heptane fraction are more similar than its asphaltene fraction

18

(see 1H NMR data in S13-14); this is expected for light crude oils (such as Texas Crude) that do

19

not contain large asphaltene fractions.

H spectrum shown in Figure 3, the aromatic species are not as de-shielded as possible (labeled

13

C NMR (including peak labeled as “e” in Figure 4), consistent with the NR2


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Energy & Fuels

1 2

Figure 5. New proposed structure of asphaltenes with more steric hinderance and fewer EAS

3

sites.

4

The lack of both aromatic protons (conformed by NMR results) and multiple

5

brominations (confirmed by MS results) from the analyzed asphaltenes did not follow the

6

conventional model45 used for asphaltenes. Hence, our mass spectrometry, Br2 tagging, and

7

NMR results of cleaned asphaltene samples suggest that a more appropriate model asphaltene

8

structure should contain more substituted aromatic positions and increased branching as shown

9

in the model structure presented in Figure 5.

10

Conclusion: A combination of bromine tagging, spectrometric and spectroscopic analyses, and

11

sample purifications were used to investigate the functional group speciation of asphaltene and

12

n-heptane fractions derived from Texas Crude oil. Kendrick mass defect (KMD) plots of the

13

direct-infusion orbitrap data help to visualize the differences in reactivity between asphaltenes

14

and crude oil; both MS-tagging and NMR data suggest that aromatic protons are not freely

15

available for electrophilic aromatic substitution. The observed reactivity patterns do not match

16

current models of asphaltenes that show freely available aromatic protons. In other words,


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

structures containing available aromatic protons would be expected to undergo multiple

2

electrophilic aromatic substitutions, but our Br-tagging experiments do not support this view.

3

Additionally, it was shown that simple silica gel column purification removes the

4

majority of paramagnetic species in asphaltene. Post-column NMR analysis allowed

5

identification and relative quantification of many distinct chemical functionalities never

6

previously characterized in asphaltenes. The diminished line broadening after removal of metal

7

ion contaminants makes paramagnatism the more likely cause of the decreased resolution of

8

NMR, not just the difficulty of asphaltene molecules tumbling. Moreover, our combined MS and

9

NMR analyses suggest greater steric hindrance around aromatic protons in asphaltenes than

10

previously proposed. Our preliminary NMR spectra simulations confirm that the proposed

11

asphaltene model structure, shown in Figure 5, better resembles the experimentally observed

12

NMR spectra of asphaltene samples than previous models. However, because asphaltene samples

13

contain hundreds of structures that contribute to the observed NMR spectra appearances, more

14

detailed comparisons between one- and two-dimensional studies will be needed for potential

15

deconvolution of NMR signals. We predict that the ability to probe functional group details of

16

macromolecules present in complex mixtures, such as crude oil samples and biological fluids,

17

will address existing limitations with complex sample analyses and benefit multiple areas of

18

science.

19 20


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Energy & Fuels

1

Supporting Information.

2

Fourteen additional figures (all as a single power point file) including additional KMD plots (S1-

3

S5, S7-S8), ICP-MS data (S6), and NMR spectra (S9-S14) are provided as Supporting

4

Information. The following contains a summary of figure captions for S1-S14:

5 6 7 8 9 10 11 12 13 14 15

S1: KMD plot of pre- (blue) and post- (red) brominated crude oil (from Texas Crude) between 200-1000 KMU. These plots indicate that bromination patterns of crude oil (S1) and its nheptane fraction (S2) are similar. This is also confirmed by the NMR experiments shown in S13 and S14. Depending on the availability of aromatic protons, bromination patterns may be dissimilar for different crude oil types as heavier crude oil samples should exhibit increased bromination. Light crude oil samples, such as those from TEXAS CRUDE have a lower abundance of asphaltene and hence bromination patterns of n-heptane fraction and crude oil are more similar than asphaltene fraction and crude oil. Also, it is interesting that Kendrick plots from these three samples (crude oil and its n-heptane and asphaltene fractions) are quite similar before the Br2 addition; however, their post Br2 reacted Kendrick plots are dissimilar and allow for differentiation between the three samples.

16 17 18 19

S2: KMD plot of pre- (blue) and post- (red) brominated n-heptane fraction (from Texas Crude) between 200-1000 KMU. The similarity in bromination patterns between the n-heptane and the original crude oil was expected, as the light crude oil analyzed is known to not have a large asphaltene fraction.

20 21 22

S3: KMD plot of pre- (blue) and post- (red) brominated asphaltene fraction (from Texas Crude) between 200-1000 KMU. The difference in bromination patterns highlights the difference in reactivity between the crude oil and the asphaltene fraction.

23 24 25

S4: KMD plot of the unreacted (blue) and brominated (red) crude oil sample from 300 to 400 Kendrick mass units. This zoomed in area from S1 helps make clear the differences between the crude oil KMD plot and the asphaltene KMD plot.

26 27 28 29 30 31 32

S5: KMD of benzo-[a]-pyrene (BP) spiked asphaltene tracking the brominations of the spike, shown within triangles. Due to the decreased electron density in the ring upon a bromine substitution, we suspected that after the initial bromination, electrophilic aromatic substitution would be hindered and no additional brumation of BP would take place. However, as shown in S5, BP reacts with Br2 to yield up to 3 bromine substitutions. Because BP brominated more than the majority of observed asphaltenes in the sample, there are likely less available aromatic protons in asphaltene molecules than previously presumed.

33 34 35 36 37

S6: Table of ICP-MS results. Paramagnetic metals are highlighted in blue-grey. The metal identity is found in column 1; the counts (in arbitrary units, AU) are found in the blank (S6 column 2); wash (S6 column 3); pre-column (S6 column 4); and post-column (S6 column 5). Percent reductions (by percent) is listed in column 8. The total reduction of all paramagnetic metal signal is 95% with much of the loss being Fe at 98% reduction.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 3

S7: KMD of pre- (blue) and post-(red) column asphaltenes. Comparison of these samples shows that the complexity of the sample is retained after the column clean-up. The column was used to remove the metal ions for the NMR shown in Figure 3.

4 5 6 7 8 9 10 11

S8: KMD of Post Column Brominated (blue) and Non-Brominated (Red) asphaltenes. Three differences were noted in the post-column asphaltenes in comparison to the pre-column asphaltenes (S3): 1) a general decrease in relative abundance of MS peaks below 700 Kendrick mass (with mass deficiency values between 0.0 to 0.5), 2) an increase in relative abundance of MS peaks above 700 Kendrick mass (with mass deficiency values between -0.5 to 0), and 3) an increased relative abundance of mass deficiency (0.25–0.5) between 400 and 700 Kendrick mass. We believe that the shift in relative-abundance of the peaks in these locations may be due to a reduction in competing reactants with the added Br2.

12 13 14

S9: DEPT 135 13C NMR of the columned asphaltene sample (will lack all quaternary carbons). a) aromatic protons, b) N- Or O- substituted carbons such as alcohols or amines, c) CH (down) and CH2 (up) carbon chains, d) terminal CH3.

15 16 17

S10: Zoomed regions from 115 ppm to 145 ppm (for comparison of aromatic regions) of 13C NMR and DEPT-135 13C scans to highlight quaternary carbons (a clear example of which at ~ 137 ppm is labeled).

18 19 20

S11: Aliphatic region of 13 C and DEPT-135 13 C to highlight quaternary carbons (expanded regions from ~32-38 ppm showing lower intensity peaks within the two ovals are presented in S12).

21 22

S12: Expanded regions of S9 13C NMR and DEPT-135 13C NMR spectra. Very low intensity peaks in ~32-38 ppm region indicative of quaternary carbon examples are observed in 13C NMR.

23 24 25

S13: 1H NMR of the n-heptane fraction. Inset: zoomed in area from 6.5-9.0 ppm. This NMR is not of a columned sample but shows the lack of aromatic protons in comparison to the asphaltene fraction.

26 27 28

S14: H NMR of crude oil. Inset: zoomed in area from 6.5-9.1 ppm shows the region where NMR signals for the solvent peak and aromatic protons are present. The NMR from S13 and S14 are very similar, which was expected as explained in S1.

1

29 30 31

Author Contributions

32

The manuscript was written through contributions of all authors. All authors have given approval

33

to the final version of the manuscript.

34


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Energy & Fuels

1 2

Acknowledgment/Funding Sources

3

The authors would like to acknowledge the financial support provided by the National Science

4

Foundation (NSF) (NSF-IDBR Award DBI-1455668).

5 6 7 8 9 10 11 12 13 14 15 16 17

APPI: Atmospheric Pressure Photoionization DEPT: Distortionless Enhancement by Polarization Transfer EAS: Electrophilic Aromatic Substitution EPR: Electron Paramagnetic Resonance HR MS: High Resolution Mass Spectrometry KM: Kendrick Mass MS: Mass Spectrometry NMR: Nuclear Magnetic Resonance UV: Ultraviolet BP: Benzo-[a-]pyrene

18

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19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

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ABBREVIATIONS


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For Table of Content Only 3 4 5 6 7


Reactivities of Aromatic Protons in Crude Oil Fractions toward Br2

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