Speciation of Aromatic Compounds in Petroleum Refinery Streams by

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Energy & Fuels 2005, 19, 1566-1573

Speciation of Aromatic Compounds in Petroleum Refinery Streams by Continuous Flow Field Desorption Ionization FT-ICR Mass Spectrometry Tanner M. Schaub, Ryan P. Rodgers, and Alan G. Marshall* Ion Cyclotron Resonance Program, National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310-4005, and Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306

Kuangnan Qian, Larry A. Green, and William N. Olmstead ExxonMobil Research & Engineering Company, Annandale, New Jersey 08801 Received October 24, 2004. Revised Manuscript Received April 9, 2005

We present continuous flow field desorption ionization Fourier transform ion cyclotron resonance mass spectrometry (FD FT-ICR MS) as a method for detailed speciation of aromatic compounds in fractionated crude oils and petroleum refinery process streams. The high efficiency for field desorption ionization of nonpolar molecules combined with the unparalleled mass resolving power and mass accuracy of FT-ICR MS enable unequivocal elemental composition assignment for ∼700-1400 species for each of four samples in the present analysis. On the basis of the elemental composition assignments, we present complete class and type determination for enriched aromatic fractions from refinery process streams, including high-sulfur vacuum gas oil, low-sulfur vacuum gas oil, fluid catalytic cracking bottoms, and a coker vacuum gas oil. These process streams were selected to demonstrate FD FT-ICR MS capability for a variety of compositional scenarios. The data obtained by FD FT-ICR MS are qualitatively consistent with the predicted characteristics of the refinery process streams. Combined with our prior electrospray ionization results, FT-ICR MS analysis now spans petroleum components ranging from metalloporphyrins, basic nitrogens, nonbasic nitrogens, and acids to aromatics and thiophenoaromatics.

Introduction Detailed characterization of petroleum composition provides the basis for molecular management of refining processes.1,2 Although bulk property measurements and chromatographic separations furnish a starting point for the predictive management of refinery operations, elucidation of even the most basic organization of reaction networks in a typical refinery process is possible only through a broad knowledge of feedstock and product composition. That level of detail, however, is beyond the capability of any single analytical method and tools that provide comprehensive description of the species present in complex petrochemical samples are greatly needed. High-resolution mass spectrometry (m/ ∆m50% > 100 000, in which m/∆m50% denotes the mass spectral peak full width at half-maximum peak height) currently offers the most comprehensive speciation of the individual components of these complex mixtures and reveals compositional trends with respect to heteroatom content, degree of unsaturation (number of rings plus double bonds), and carbon number distribution for numerous compound classes and types. * To whom correspondence should be addressed. E-mail: [email protected]. (1) Quann, R. J.; Jaffe, S. B. Ind. Eng. Chem. Res. 1992, 31, 24832497. (2) Jaffe, S. B. Abstracts of Papers, 222nd ACS National Meeting, Chicago, IL, August 26-30, 2001; PETR-032.

To date, mass spectrometry has played a vital role in the analysis of petrochemicals, and some of the earliest applications of mass spectrometry were in the field of petrochemical analysis.3-5 For mass spectrometry, as with other techniques, analytical challenges increase dramatically as the boiling point of a given petroleum product increases not only because of the sharp increase in compositional complexity but also because the constituents of heavy end petroleum are difficult to volatilize without thermal decomposition. The development of electrospray ionization (ESI) has recently provided an effective tool for the ionization of large polar molecules.6 In previous work, we have demonstrated exhaustive characterization of polar components in petroleum by high-field ESI FT-ICR mass spectrometry7 in both positive8,9 and negative ion modes.10-12 To more com(3) Hipple, J. A.; Dralle, H. E. Pet. Refin. 1943, 22, 425-428. (4) Hoover, H., Jr.; Washburn, H. Tech. Publ.sAm. Inst. Min. Metall. Eng. 1940, No. 1205, 7. (5) Fulton, S. C.; Heigl, J. J. Instruments 1947, 20, 35-38. (6) Zhan, D. L.; Fenn, J. B. Int. J. Mass Spectrom. 2000, 194, 197208. (7) Senko, M.; Hendrickson, C.; Emmett, M.; Shi, S.-H.; Marshall, A. J. Am. Soc. Mass Spectrom. 1997, 8, 970-976. (8) Qian, K.; Robbins, W. K.; Hughey, C. A.; Cooper, H. J.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2001, 15, 1505-1511. (9) Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G.; Greaney, M.; Qian, K. Can. J. Chem. 2001, 79, 546-551. (10) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G.; Qian, K.; Robbins, W. K. Org. Geochem. 2002, 33, 743-759.

10.1021/ef049734d CCC: $30.25 © 2005 American Chemical Society Published on Web 05/20/2005

Speciation of Aromatic Compounds in Petroleum Refinery Streams

pletely characterize petroleum, which is composed predominantly of nonpolar hydrocarbons and sulfurcontaining hydrocarbons, an analytical technology is needed that can effectively ionize, resolve, and identify nonpolar petroleum molecules at a level similar to that shown by ESI FT-ICR MS. Methods for “soft” ionization of nonpolar petrochemical constituents (i.e., little internal energy is imparted to analytes during the ionization process) include lowvoltage electron impact ionization (EI),13 chemical ionization (CI),14,15 laser desorption ionization (LDI),16 atmospheric pressure chemical ionization and photoionization (APCI/APPI),17,18 matrix-assisted laser desorption (MALDI),19,20 and field desorption/field ionization (FD/FI).21,22 FD/FI is highly attractive because the absence of a matrix and ion formation in vacuo (as opposed to atmospheric techniques) ensure that the resultant spectra are not only free of fragments but also free from interfering species that result from reactions of gas-phase ions with matrix molecules, water vapor, and so forth. These benefits, combined with the highionization efficiency of FD/FI for a variety of nonpolar compounds, make this ionization source well suited to analysis of petrochemical mixtures. FD/FI ionization occurs when analyte molecules are subjected to a strong electric field (∼108 V/cm) located at the tips of carbon microneedle dendrites that comprise the FD/FI emitter. The carbon microneedles are grown on the activated surface of a 10-µm diameter tungsten filament. The ionizing electric field is established by application of a high voltage (-10 kV for positive ion formation) to an electrode located 2 mm from the FD emitter. The distinction between field desorption and field ionization is that for field ionization, gaseous analyte molecules flow past the highelectric field at the microneedle tips, whereas for field desorption, condensed-phase analytes are applied directly to the emitter and are subsequently desorbed by the electric field and thermal assistance. In this way, constituents of heavy end petroleum samples may be analyzed. FD/FI ionization is capable of soft ionization for both saturate and aromatic compounds, generating largely M+• ions.23-29 The technology has also been used as a means to measure molecular weight distributions (11) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2002, 74, 4145-4149. (12) Qian, K.; Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G. Energy Fuels 2001, 15, 492-498. (13) Rodgers, R. P.; White, F. M.; McIntosh, D. G.; Marshall, A. G. Rev. Sci. Instrum. 1998, 69, 2278-2284. (14) Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Energy Fuels 2000, 14, 217-223. (15) Veloski, G. A.; Lynn, R. J.; Sprecher, R. F. Energy Fuels 1997, 11, 137-143. (16) Tanaka, R.; Sato, S.; Takanohashi, T.; Hunt, J. E.; Winans, R. E. Energy Fuels 2004, 18, 1405-1413. (17) Rudzinski, W. E.; Rodriguez, R.; Sassman, S.; Sheedy, M.; Smith, T.; Watkins, L. M. Prepr. Symp.sAm. Chem. Soc., Div. Fuel Chem. 1999, 44, 28-31. (18) Kolakowski, B. M.; Grossert, J. S.; Ramaley, L. J. Am. Soc. Mass Spectrom. 2004, 15, 301-310. (19) Robins, C.; Limbach, P. A. Rapid Commun. Mass Spectrom. 2003, 17, 2839-2845. (20) Limbach, P. A.; Macha, S. F.; Robins, C. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 2000, 45, 561-563. (21) Schaub, T. M.; Hendrickson, C. L.; Qian, K.; Quinn, J. P.; Marshall, A. G. Anal. Chem. 2003, 75, 2172-2176. (22) Schaub, T. M.; Linden, H. B.; Hendrickson, C. L.; Marshall, A. G. Rapid Commun. Mass Spectrom. 2004, 18, 1641-1644. (23) Qian, K.; Edwards, K. E.; Siskin, M. Energy Fuels 2001, 15, 949-954.

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of petroleum products.25,26,30-33 Historically, FI/FD was coupled to sector mass spectrometers; however, that configuration suffered from marginal mass resolving power and slow scan rate. There have been two important recent developments in the resolution of nonpolar petroleum molecules by FD/FI mass spectrometry. First, the coupling of gas chromatography (GC) and supercritical fluid chromatography (SFC) with a field ionization time-of-flight mass spectrometer (TOF) has been successfully demonstrated.34,35 Tandem separation of petroleum molecules by boiling point (or polarity) and separation of the corresponding ions by mass-to-charge ratio (m/z) at moderate mass resolving power (m/∆m50% ) 5000) allows resolution of isobaric species. With that configuration, species not resolvable by mass alone (at 5000 resolving power) are separated by GC retention time. Most notably, species differing in mass by 3.4 mDa (i.e., molecules with elemental compositions that differ by C3 vs SH4) may be identified at masses less than 400 Da. Second, a liquid injection field desorption ionization (LIFDI) source has recently been coupled to a 9.4 T FTICR mass spectrometer.21,22,36 With continuous flow sample introduction,22 numerous FT-ICR data sets may be coadded to enhance signal-to-noise ratio and dynamic range. This capability combined with the ultrahigh resolving power (m/∆m50% > 200 000) and sub-ppm mass accuracy attainable with FT-ICR37 enables elemental composition assignment spanning a wide dynamic range in broad-band petrochemical mass spectra. In this report, we demonstrate the use of continuous flow FD FT-ICR MS to evaluate the composition of enriched aromatic fractions of several petroleum refinery streams. The selected petroleum streams are designed to cover all compositional scenarios: straight distillations with very high and low sulfur content, catalytically cracked products, and thermally cracked products. The field desorption FT-ICR mass spectrometer employed for this analysis is currently the only one of its kind but is available for use at the National HighField FT-ICR Mass Spectrometry Facility at the National High Magnetic Field Laboratory in Tallahassee, Florida. This report complements our previous coverage of petroleum analysis by ultrahigh resolution FT-ICR MS, including metalloporphyrins,9 polar basic nitrogen (24) Qian, K.; Tomczak, D. C.; Rakiewicz, E. F.; Harding, R. H.; Yaluris, G.; Cheng, W.-C.; Zhao, X.; Peters, A. W. Energy Fuels 1997, 11, 596-601. (25) Wang, Z.; Shen, H.; Ding, Z.; Zhang, Y.; Zhang, S. Shiyou Xuebao, Shiyou Jiagong 1996, 12, 76-84. (26) Larsen, B. S.; Fenselau, C. C.; Whitehurst, D. P.; Angelini, M. Anal. Chem. 1986, 58, 1088-1091. (27) Tago, S.; Imai, I.; Sato, K. Sekiyu Gakkaishi 1984, 27, 341347. (28) Schulz, C.; Chowdhury, S. K.; Blum, S. C. Anal. Chem. 1993, 65, 1426-1430. (29) Yoshida, T.; Meakawa, Y. Anal. Chem. 1982, 54, 967-971. (30) Boduszynski, M. M. Energy Fuels 1987, 1, 2-11. (31) Boduszynski, M. M. Energy Fuels 1988, 2, 597-613. (32) Boduszynski, M. M.; Altgelt, K. H. Energy Fuels 1992, 6, 7276. (33) Altgelt, K. H.; Boduszynski, M. M. Energy Fuels 1992, 6, 6872. (34) Qian, K.; Diehl, J. W.; Dechert, G. J.; DiSanzo, F. P. Eur. J. Mass Spectrom. 2004, 10, 187-196. (35) Qian, K.; Dechert, G. J. Anal. Chem. 2002, 74, 3977-3983. (36) Schaub, T. M.; Hendrickson, C. L.; Quinn, J. P.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2005, 77, 1317-1324. (37) Marshall, A. G.; Hendrickson, C. L.; Jackson, G. S. Mass Spectrom. Rev. 1998, 17, 1-35.

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Figure 1. Schematic diagram of the 9.4 T field desorption ionization FT-ICR mass spectrometer, configured for continuous flow sample introduction.

compounds,11 polar nonbasic nitrogen compounds,38 acids,8,10 and now aromatics and thiophenoaromatics. Experimental Methods Chemicals. Aromatic fractions were isolated from samples taken from four ExxonMobil petroleum refinery streams: two straight run vacuum gas oils (288-510 °C or 550-950 °F) with high- and low-sulfur contents (high-sulfur vacuum gas oils (VGO), 7.10% S and low-sulfur VGO, 0.52% S), fluid catalytic cracking “bottoms” (FCC), and a coker vacuum gas oil (Coker VGO). The latter two samples have sulfur contents of 1.22% and 4.88%, respectively, and boiling ranges that are similar to the straight run VGOs. Bulk nitrogen and acid content were not measured for these aromatic fractions. However, the corresponding unfractionated high-sulfur VGO and low-sulfur VGO have nitrogen contents of 2800 and 1800 ppm and total acid numbers (TAN) of 0.08 and 0.12 mg KOH/g, respectively. The procedure used to fractionate the aromatic compounds from these samples is similar to that described in ASTM D2007,39 which separates petroleum into saturate, aromatic, and polar fractions. Instrumentation. All experiments were performed with a home-built 9.4 T field desorption FT-ICR mass spectrometer. This instrument employs a commercial field desorption ionization source including a field desorption probe, control electronics, and emitters (Linden CMS, Leeste, Germany). Sample is delivered from atmospheric pressure through a fused silica capillary to the probe-mounted FD emitter within the vacuum chamber.40 We have previously reported operation of this source in a continuous flow mode22 by supplying dilute sample solution to the heated FD emitter maintained at high electric potential difference relative to the counter electrode (see Figure 1). FD generated positive ions were formed by continuous application (75 nL/min) of dilute sample solution (100 µg/mL in methylene chloride) delivered to the FD emitter by a syringe pump (kdScientific Inc., New Hope, PA) through a 50-cm long, 10-µm i.d. fused silica capillary (Polymicro Technologies, LLC, Phoenix, AZ). The tapered end of the fused silica capillary (vacuum side) just touches the microneedles of the FD emitter, which is heated (15 mA) and kept at high electric field (-10 kV applied to counter electrode). Following a several minute delay for pressure equilibration, the pressure in the source vacuum chamber was 1 × 10-5 Torr. (38) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G.; Walters, C. C.; Qian, K.; Mankiewicz, P. Org. Geochem. 2004, 35, 863-880. (39) Standard Test Method for Characteristic Groups in Rubber Extender and Processing Oils and Other Petroleum-Derived Oils by the Clay-Gel Absorption Chromatographic Method. Annual Book of ASTM Standards; ASTM: West Conshohocken, PA, 2003; ASTM D 2007-03. (40) Linden, H. B. Liquid injection field desorption ionization: A new tool for soft ionization of samples including air-sensitive catalysts and non-polar hydrocarbons. Eur. J. Mass Spectrom. 2004, 10 (4), 459468.

Schaub et al. Design details for the 9.4 T FD FT-ICR mass spectrometer, as well as model compound experiments and methodology that relate to petrochemical analysis, have recently been published.21,22 The mass spectrometer is based on an actively shielded 9.4 T superconducting solenoid magnet that features high-field homogeneity and temporal stability. The FD/FI ionization source is located outside the magnet bore about 1.5 m from the magnetic field center. The vacuum manifold consists of three differentially pumped stages (TW 700 turbomolecular pumps, Leybold Vakuum, Cologne, Germany) and encloses the ion optics shown in Figure 1. Following ionization, ions were transported through a 76mm-long octopole and were then collected by collisional cooling with helium in a 160-mm ion accumulation octopole (all octopoles operated at 2.2 MHz, ∼250 Vp-p). Ions were accumulated for 20 s before ejection from the storage octopole through the magnetic field gradient, via a 1450-mm octopole, to an open cylindrical41 (70-mm diameter, 212-mm long) capacitively coupled42 Penning ion trap. Trapped ions were then cooled with helium collision gas supplied to the Penning trap by opening a solenoid pulsed valve (10 ms pulse, 1 Torr external reservoir pressure). Following a 30-s pump down delay, ions were subjected to broad-band frequency sweep (chirp) excitation (200 Vp-p from 125 kHz to 500 kHz at 150 Hz/µs). The detected signals were digitized (1.2 MHz bandwidth, 4 M time-domain data) and transferred to the control PC, and 50-75 time-domain transients were coadded for each sample and stored for later fast Fourier transformation and frequency-to-mass conversion.43,44 The stored time-domain signal was Hanning-apodized and zero-filled once prior to fast Fourier transformation and generation of the magnitudemode frequency domain spectrum. A modular ICR data system (MIDAS) data station, developed in-house, provides instrument control, data acquisition, and data analysis.45,46 Data Analysis. The data analysis procedure for nonpolar petroleum components is the same as that employed in our previous electrospray ionization FT-ICR reports.47 Within reasonable elemental composition restraints, the high mass accuracy of FT-ICR MS yields unique elemental compositions for species less than ∼400 Da in mass. If two (or more) elemental compositions have masses within (1 ppm, one formula can usually be confirmed/eliminated by the presence/ absence of the corresponding nuclide containing one 13C or one 34 S. Kendrick mass sorting48 facilitates the identification of higher mass species because members of a homologous series differ only by integer multiples of CH2. Therefore, assignment of a single (low mass) member of such a series is sufficient to identify all members of the series. Thus, by recognizing homologous series, one can assign unique elemental compositions for species up to ∼1000 Da, even though sub ppm mass accuracy alone would not suffice for such a determination. In general, petroleum composition is expressed in terms of the chemical formula, CcH2c-zNnOoSs, in which the hydrogen deficiency index, Z, of the molecule is the same for all members (41) Beu, S.; Laude, D. A. Int. J. Mass Spectrom. Ion Processes 1992, 112, 215-230. (42) Beu, S.; Laude, D. A., Jr. Anal. Chem. 1992, 64, 177-180. (43) Ledford, E., Jr.; Rempel, D. L.; Gross, M. L. Anal. Chem. 1984, 56. (44) Shi, S. D.-H.; Drader, J. J.; Freitas, M. A.; Hendrickson, C. L.; Marshall, A. G. Int. J. Mass Spectrom. 2000, 195/196, 591-598. (45) Senko, M. C. J.; Guan S.; Marshall, A. G. Rapid Commun. Mass Spectrom. 1996, 10, 1839-1844. (46) Blakney, G. T.; van der Rest, G.; Johnson, J. R.; Freitas, M. A.; Drader, J. J.; Shi, S. D.-H.; Hendrickson, C. L.; Kelleher, N. L.; Marshall, A. G. Further Improvements to the MIDAS Data Station for FT-ICR Mass Spectrometry. Proceedings of the 49th American Society of Mass Spectrometry Conference on Mass Spectrometry & Allied Topics; American Society of Mass Spectrometry: Chicago, IL, 2001; WPM265. (47) Marshall, A. G.; Rodgers, R. P. Acc. Chem. Res. 2004, 37, 5359. (48) Hughey, C. A.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G.; Qian, K. Anal. Chem. 2001, 73, 4676-4681.

Speciation of Aromatic Compounds in Petroleum Refinery Streams

Figure 2. Broad-band continuous flow FD FT-ICR mass spectra of aromatic fractions. Top: a high-sulfur vacuum gas oil. Bottom: a low-sulfur vacuum gas oil. Each spectrum results from coaddition of 75 time-domain signals, each for 20 s external ion accumulation. The total analysis period for each spectrum is approximately 1 h. of a homologous “type” series (i.e., fixed number of rings plus double bonds). Every two-unit decrease in Z-value represents the addition of one ring or double bond. Number-average molecular weight, Mn, and weight-average molecular weight, Mw, are defined as

Mn )

∑N M /∑N i

i

(1)

i

and

Mw )

∑N M /∑N M 2

i

i

i

i

(2)

in which Ni is the relative abundance of ions of mass Mi.

Results and Discussion Recent evaluation of model compounds with our 9.4 T FD FT-ICR mass spectrometer reveals substantial fragmentation of paraffin molecules during external ion accumulation. Thus, we focus here on the aromatic fractions of four petroleum refinery streams. Aromatic molecules yield largely M+• radical cations in FD FTICR mass spectra, as for FD coupled to other mass analyzers. In the following discussion, the relative abundances of ions of various compositions do not necessarily reflect the relative abundances of the corresponding neutrals in the original sample, because ionization efficiencies have not yet been determined or applied for all observed compound classes and types. High- versus Low-Sulfur VGO Aromatics. Broadband continuous flow FD FT-ICR mass spectra of enriched aromatic fractions of high- and low-sulfur vacuum gas oils (VGO) that originate from distillation of corresponding crude oils (typically 288-510 °C) are shown in Figure 2. All ions are singly charged, as evident from the ∼1 Da spacing between monoisotopic compound signals and the corresponding signals from nuclides that have one 12C replaced by 13C. The molecular weight distributions of the two samples are similar and both are centered around ∼400 Da (Mn/Mm ) 383/ 392 and 375/382 for high- and low-sulfur VGO, respectively). These values are consistent with the molecular weight distribution expected for VGO range molecules. The average mass resolving power for the two spectra (m/∆m50% ) 350 000) is sufficient to resolve the 0.0034

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Figure 3. Class analysis for the high- and low-sulfur vacuum gas oil aromatic fractions presented in Figure 2. Inset shows expanded vertical scale.

Da mass doublet (SH4/C3) across the full m/z range of interest. Discrete Fourier transformation of digitized time-domain transient ICR signals yields frequencydomain magnitude spectra for the two samples. Peak frequencies are converted to m/z values by fitting the measured ICR frequencies of 18 members of the alkylbenzene homologous series (which occurs naturally in both samples) to their known exact masses (i.e., “internal” calibration). The alkylbenzenes homologues were chosen as internal calibrants because they are observed at sufficient signal-to-noise ratio throughout the observed m/z range in both samples. The low RMS error for the set of calibration compounds (306 and 136 ppb for high- and low-sulfur VGO, respectively) confers a high degree of confidence to the elemental composition assignments as evident by the low RMS assignment error for all peaks in the spectra (485 and 518 ppb, for high- and low-sulfur VGO, respectively). Compound Classes. In the mass spectra shown in Figure 2, 97-99% of the peaks can be matched to a unique elemental composition. From the elemental composition assignments, complete class (heteroatom content) and type analyses (distribution of Z-value or the number of rings plus double bonds for each class) become possible. Figure 3 shows the class distributions for high- and low-sulfur VGOs. Notably, high-sulfur VGO is the first example of a petrochemical sample, analyzed with our FD FT-ICR instrument, for which the hydrocarbon class (i.e., no heteroatoms) is not the most abundant. The HSVGO sample contains 57% S1 compounds, 25% S2 compounds, and 1% S3 compounds and is, to our knowledge, the first direct observation of S2 or S3 compounds in a VGO sample. Low levels of nitrogen species were also observed for both samples by this analysis. In these aromatic fractions, we suspect that the nitrogen compounds are nonbasic (pyrrolic) types that can coelute with condensed aromatic hydrocarbons during the clay-gel separation procedure. The high-sulfur VGO also contains N1S1 and S1O1 compounds, which are grouped into the “miscellaneous” category in Figure 3. As expected, we observe only low levels of sulfur compounds (∼1%) in the low-sulfur VGO sample. We also observe O1, O2, and O4 compounds for that sample. The O2 and O4 compounds have 16-34 carbon atoms and have relatively low numbers of rings and double

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Figure 4. Hydrocarbon Z-distributions (CcH2c-ZNnOoSs) for the high- and low-sulfur aromatic fractions. This analysis, based on elemental composition assignment, is possible only with high-resolution, high-mass accuracy FT-ICR MS.

bonds (Z-values of 0, -2, and -4). The O2 and O4 compounds are suggestive of naphthenic acids and diacids, whereas the O1 compounds are likely benzofurans. The detection of acids is unexpected because those compounds should be removed by the separation procedure. They may result from fragmentation or reaction during the ionization or ion accumulation process. Odd Z-value (even-electron) species indicate fragmentation or protonation and are often observed for several classes in our analyses. However, those species are always present in low relative abundance and their abundances track those of the even Z-value series. Hydrocarbon Types. Type analysis, expressed as a Z distribution, is illustrated for the hydrocarbon class in Figure 4. In contrast to the cracked products (discussed below), Figure 4 shows only low levels of olefins in the low-sulfur VGO and no detectable olefins in high-sulfur VGO. The low-sulfur VGO olefins may result from fragmentation of alkyl or olefin-substituted aromatics during analysis. This result is not surprising, because crude oils typically contain very few (if any) detectable olefins. Furthermore, vacuum distillation normally does not crack petroleum molecules. For both samples, pure hydrocarbons appearing as one to five ring aromatics and naphthenoaromatics were observed. Mass spectrometry cannot completely distinguish aromatics from naphthenoaromatics (e.g., tetracyclobenzenes and naphthalenes share the same Z value of -12). The Z distribution pattern is most informative for the highsulfur VGO, peaking at condensed aromatic core structures, such as benzene, fluorene, and chrysene in Figure 4. This pattern is less evident for the low-sulfur VGO sample, which shows predominantly monoaromatics with a gradual decrease in abundance with expansion of the aromatic or naphthenic core structure. Sulfur Compound Types. Favorable core structures by the successive addition of aromatic rings (as opposed to naphthenic rings) were also found for monosulfur compounds in both samples (e.g., thiophenes, benzothiophenes, and dibenzothiophenes for high-sulfur VGO S1 compounds in Figure 5). The most abundant S1 compound types are benzothiophenes and dibenzothiophenes. We also observed low levels of Z ) 0 and Z ) -2 sulfides for the high-sulfur VGO sample. Favorable core structures are found for high-sulfur VGO S2 and S3 compounds (Figure 5). From our analysis, it

Schaub et al.

Figure 5. Z-distributions for the S1-S3 classes observed in the high-sulfur vacuum gas oil. Favorable core structures are observed in increments of benzene rings. The core structures shown are for illustrative purposes only.

Figure 6. Broad-band continuous flow FD FT-ICR mass spectra of aromatic fractions. Top: catalytic cracking bottoms. Bottom: a coker vacuum gas oil. Both were collected in a manner analogous to that for the high- and low-sulfur VGO (Figure 2). The center of mass for each of these spectra is significantly lower than for the straight run vacuum gas oils.

appears that sulfur incorporation is accomplished predominantly by the addition of thiophenic rings. For example, we observe favorable core structures that correspond to the addition of sulfur from benzothiophenes (-10 S) to thiophenobenzothiophenes (-14 S2) to dithiophenobenzothiophenes (-18 S3). Similarly, we observe favorable addition of sulfur from dibenzothiophenes (-16 S) to thiophenodibenzothiophenes (-20 S2) to dithiophenodibenzothiophenes (-24 S3). The lowsulfur VGO contains a small amount of sulfur-containing molecules (mostly benzothiophenes) with Z values range from -8 to -16. N1 Types. The Z-values for N1 compounds in the highsulfur VGO range from -15 to -25. Those compounds are likely three- and four-ring nonbasic nitrogen species (or neutral nitrogen species), such as carbazoles (-15 N), naphthenocarbazoles (-17 N), and benzocarbazoles (-21 N). Only two N1 series were observed in the lowsulfur VGO (Z ) -15 and - 17). Fluid Catalytic Cracking Bottoms and Coker VGO Aromatic Fractions. Figure 6 shows broad-band FD FT-ICR mass spectra of aromatic fractions from fluid catalytic cracking bottoms and a coker vacuum gas oil.

Speciation of Aromatic Compounds in Petroleum Refinery Streams

Figure 7. Hydrocarbon Z-distributions for catalytic cracking bottoms and coker VGO aromatic fractions. The assignment of mono-, di-, and triolefins is unambiguous because all alkanes, including cyclic alkanes, were removed during the fractionation process.

Both spectra were calibrated in the same fashion as for the high- and low-sulfur VGO aromatics, with similar results as illustrated by the low RMS assignment errors reported in Figure 6. The molecular weight distribution for the catalytic cracking bottoms (Mn/Mm ) 257/260) is significantly lower than that of the coker VGO (Mn/ Mm ) 324/339), because catalytic cracking is a more severe alkyl cleavage process than thermal cracking. Both molecular weight distributions are shifted to lower mass than the straight run high- and low-sulfur VGO aromatic fractions despite similar boiling ranges for all samples. The shift to lower mass is consistent with an increased abundance of large aromatic species observed in the cracked samples. Compound Classes. The classes for these two samples were similar to those observed for the high- and lowsulfur VGO samples and include hydrocarbons, sulfur compounds, and low levels of oxygen and N1 compounds. In contrast to the straight run VGO samples, however, N1S1, S1O1, and N2O2 species were not observed for the catalytic cracking bottoms or coker VGO samples. The hydrocarbon contents of the latter samples are similar (71% and 74%, respectively) and include substantially higher amounts of olefins than the low-sulfur VGO sample. Hydrocarbon Types. Figure 7 shows the Z distribution for the hydrocarbon class of the catalytic cracking bottoms or coker VGO aromatic fractions. Olefins predominate in the latter, whereas the former exhibits mainly three- and four-ring aromatics with short alkyl attachment. The assignments of Z ) 0, -2, and -4 as olefins and cyclic olefins are unambiguous because the saturate components, including cyclics, have been removed from these samples by the fractionation procedure. The total relative abundances for the olefins were, respectively, 15.4% and 27.3% for the catalytic cracking bottoms and coker VGO samples. This trend is reasonable, as thermal cracking typically generates more olefins than does catalytic cracking. For aromatic hydrocarbons, catalytic cracking appears to generate mostly three-, four-, and five-ring condensed aromatic compoundsssignificantly different from that of straight run VGOs. The Z-distribution of the coker VGO is similar to that of the straight run VGOs because of reduced dealkylation known for that process.

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Figure 8. Monosulfur Z-distributions for catalytic cracking bottoms and coker VGO aromatic fractions.

Sulfur Types. As shown in Figure 8, we observed predominantly one to four ring aromatic sulfur compounds for coker VGO whereas catalytic cracking bottoms shows predominantly three- and four-ring aromatic sulfurs, suggesting that the catalytic cracking process does not efficiently remove condensed aromatic sulfur compounds. The sulfur compound type distributions closely track those of the hydrocarbons. Figure 8 also shows preferential stability in the thermal cracking process for the lowest Z-value compounds for each subclass of aromatic sulfur, such as thiophenes (-4 S), benzothiophenes (-10 S), dibenzothiophenes (-16 S), and benzodibenzothiophenes (-22 S). Oxygen and Nitrogen Species. For the catalytic cracking bottoms and the coker VGO, low levels of oxygen and nitrogen were observed as was the case for the straight run VGOs. The catalytic cracking bottoms O1 class (not shown) contains compounds in two separate Z-value regions, one distribution at Z ) -1 to -6 and another at Z ) -18 to -26, in contrast to the coker VGO O1 species that appear through the range from Z ) 0 to -26. For both samples, the furans (-4 O) are the most abundant series. N1 compounds in both samples were far less abundant than that found in straight run VGOs as expected. For each of the cracked samples, N1 compounds were far less abundant than that for the high-sulfur VGO. The most abundant N1 type, carbazoles (Z ) -15), was the most abundant for all four samples reported. Continuous Flow FD FT-ICR MS versus Elemental Sulfur Analysis. Sulfur content (as weight percent) can be calculated from FD FT-ICR MS data from

Swt% ) [Σ(31.791 × siNi)/Σ(NiMi)] × 100

(3)

in which si is the number of sulfur atoms per molecule; Mi is the molecular weight of the ith species (whether sulfur-containing or not); the sum is over all elemental compositions containing at least one sulfur atom; and the relative abundance, Ni, is with respect to all species in the mass spectrum. If the response factors (i.e., relative ionization efficiencies) of individual species were known, Ni would correspond to the mole percent of the ith (neutral) species in the original condensed-phase sample. Because response factors are not available for most of the compound types observed in our analysis, we calculated the sulfur contents from eq 3 (for the four samples discussed above) for assumed equal response

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Figure 9. Bulk sulfur contents for the aromatic fractions of the four refinery process stream samples, determined by continuous flow FD FT-ICR MS (ions) and an elemental sulfur assay (neutrals).

factors. Those results were compared with bulk sulfur content determined by an elemental sulfur assay at the ExxonMobil laboratory (Figure 9). Given the equal response factor assumption, the two data sets are in reasonable agreement. Furthermore, although a natural concentration dynamic range of at least 106 can be expected for petrochemical mixtures, the dynamic range of ∼103 shown for these FT-ICR analyses nonetheless appears to provide a representative and highly detailed view of bulk sample composition. Qualitative agreement between these two methods may be improved by inclusion of FD FT-ICR measurements at various emitter temperatures (thereby increasing the number of observed compounds by extension of the detected m/z range) and by the inclusion of FD response factors when they become available. Multidimensional Mass Spectral Imaging. Plots of Z-value distribution versus carbon number (with relative abundance represented by color in the third

Schaub et al.

dimension) for the hydrocarbon class of the refinery process stream samples are shown in Figure 10. These images provide compact visualization of the entire hydrocarbon distribution to expose broad trends in the data. For example, for the straight run VGO samples, an increase in Z-value (less negative) is accompanied by a marked shift to higher carbon number, consistent with lower molecular weights for highly condensed aromatics relative to less condensed aromatics of the same boiling point. The carbon number distributions for the high-sulfur VGO are fairly continuous with maxima in the 25-29 carbon number range, whereas the lowsulfur VGO carbon number distributions are more localized. This observation illustrates a different allocation of structurally similar aromatic hydrocarbons between the two samples, perhaps an expression of the geologic formation processes of the parent crude oils. Generally, the catalytic cracking bottoms carbon number distributions are quite narrow. For example, mono-olefins in that sample (Z ) 0) have a carbon number distribution from 14 to 26 carbons, whereas the same distributions for the coker VGO range from 14 to 36 carbons. Both distributions show preferential stability for C21 mono-olefins. Carbon number distributions for cyclic olefins (Z ) -2, -4) are similar to those for the mono-olefins with respect to the carbon number ranges. Aromatic hydrocarbons (Z ) -6 to -30) in the catalytically cracked sample occur in narrow carbon number distributions and at low carbon number relative to those found in the coker VGO sample. For example, monoaromatic hydrocarbons (Z ) -6) in the catalytic cracking bottoms were observed from 14 to 27 carbons, whereas the coker VGO sample contains 14-37 carbon species, of which the 24 and 25 carbon species are most abundant. Similarly, Figure 11 shows contour plots (Z-value vs carbon number, with relative abundance in color) for the monosulfur compounds in the four samples. For all

Figure 10. Contour plots of Z-value versus carbon number for hydrocarbons found in the four refinery process stream samples. The color-coding denotes relative abundance and is scaled relative to the most abundant compound in each plot.

Speciation of Aromatic Compounds in Petroleum Refinery Streams

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Figure 11. Contour plots of Z-value versus carbon number for monosulfur compounds found in the four refinery process stream samples. The color-coding denotes relative abundance and is scaled relative to the most abundant compound in each plot.

samples, some similarity is observed between each monosulfur plot and the corresponding hydrocarbon plot in Figure 10. The high-sulfur VGO contains mainly di-, tri, and tetra-aromatic sulfur compounds (e.g., Z ) -10, -16, and -26) with carbon number ranges of 18-34, 19-29, and 21-24, respectively. The low-sulfur VGO contains largely mono- and diaromatic sulfur compounds. The coker VGO contains mono-, di-, tri-, and tetra-aromatic sulfur compounds with carbon number distributions that shift to lower carbon number with decreasing (i.e., more negative) Z-value. The catalytically cracked sample contains mainly tri- and tetraaromatics with very short alkyl substitution. In summary, we have demonstrated continuous flow FD FT-ICR MS as an effective and powerful method for detailed speciation of the nonpolar composition of fractionated refinery process streams. Unambiguous elemental composition assignment for constituents of these sample types is possible only with high-resolution, high-mass accuracy FT-ICR MS. This analysis enables

class and type determination as well as carbon number distribution for each type for previously inaccessible molecular classes. Until now, speciation of sulfur compounds in petroleum mixtures has been deficient despite the critical importance of their determination for refinery processes because of the significant impact of those compounds on the quality of produced products and the direction of residual material to appropriate downstream treatment. Finally, our results indicate that continuous flow FD FT-ICR mass analysis provides detailed chemical speciation that is in qualitative agreement with bulk petrochemical composition. Acknowledgment. This work was supported by ExxonMobil Research and Engineering, NJ, the National Science Foundation (CHE-99-09502), Florida State University, and the National High Magnetic Field Laboratory in Tallahassee, FL. EF049734D