Energy Fuels 2010, 24, 2257–2265 Published on Web 11/18/2009
: DOI:10.1021/ef900897a
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Stepwise Structural Characterization of Asphaltenes during Deep Hydroconversion Processes Determined by Atmospheric Pressure Photoionization (APPI) Fourier Transform Ion Cyclotron Resonance (FT-ICR) Mass Spectrometry† Jeremiah M. Purcell,‡ Isabelle Merdrignac,*,§ Ryan P. Rodgers,‡, Alan G. Marshall,‡, Thierry Gauthier,§ and Isabelle Guibard§ )
‡ National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310-4005, §IFP-Lyon, CEDI Rene Navarre, BP3, Vernaison 69390, France, and Department of Chemistry and Biochemistry, Florida State University, 95 Chieftain Way, Tallahassee, Florida 32306
Received August 18, 2009. Revised Manuscript Received October 26, 2009
The compositional analysis (speciation) of heavy oil products is a key step to improve our understanding of hydrotreatment processes and reaction mechanisms. Thus, detailed characterization of polar fractions, such as asphaltenes, should be considered. Here, we employ atmospheric pressure photoionization Fourier transform ion cyclotron mass spectrometry to monitor the evolution of the asphaltene hydrocarbon and sulfur families in deep hydrotreatment processes (fixed and ebullated beds). The results suggest that the complexity of the asphaltenic fractions (in terms of chemical polydispersity) is drastically lowered with increased process severity. In either fixed or ebullated beds, the evolution of the sulfur species is quite similar in class composition, aromaticity (DBE/carbon number ratio), and polycondensation (DBE). The compositional changes are marked by a drastic increase in aromaticity to highly polycondensed dealkylated aromatic structures. Asphaltene disaggregation followed by a dealkylation of the remaining species could be suggested. The proposed scheme would converge toward those previously proposed.
(N, O, and S) and metals.5-7 In solution, they exhibit selfassembly and colloidal behavior, according to solvent, concentration, and temperature.8-11 Once dissolved, they may precipitate during hydroconversion. Asphaltenes are known precursors of coke in acid catalysis and can inhibit catalysts. The deactivation of catalysts thus strongly depends upon the concentration of heavy fraction constituents. Of the numerous analytical techniques for asphaltene characterization, atmospheric pressure photoionization (APPI) coupled to Fourier transform ion cyclotron resonance (FTICR) mass spectrometry provides the most extensive molecular characterization of nonpolar petroleum compounds. FTICR mass spectrometry provides ultra-high mass resolving power, m/Δm50% (in which Δm50% is the mass spectral peak full width at half-maximum peak height) and, correspondingly, high (parts per billion) mass accuracy,12,13 enabling asphaltene speciation at the level of elemental composition.14 Ultra-high resolving power and mass accuracy are required because of the compositional complexity of petroleum
Introduction To enhance the economic value of heavy oils, various upgrading processes have been developed.1,2 Hydroconversion processes, in either fixed or ebullated beds, require catalysts to hydrogenate residue feeds, to remove and accumulate metals (nickel and vanadium), and to desulfurize the feed.3 In refineries, it is often observed that the conversion of feedstocks with similar bulk compositional properties requires different operating conditions to achieve similar products (limited in some cases by flocculation), thereby inducing aging of catalysts. Thus, global (bulk property) analyses may not be sufficient to describe such feeds. The compositional analysis (speciation) of heavy oil products is a key step for understanding hydrogenation processes. The compositional and structural complexity of these oil matrices increases rapidly with the boiling point.4 Resins and asphaltenes constitute the most polar fractions. Asphaltenes, defined by their solubility in toluene and insolubility in a normal paraffinic solvent (e.g., n-heptane), are a heterogeneous mixture, highly disperse in both molecular size and chemical composition, with a high content of heteroatoms
(5) Szewczyk, V.; Behar, F.; Behar, E.; Scarsella, M. Rev. Inst. Fr. Pet. 1996, 51 (4), 575–590. (6) Groenzin, H; Mullins, O. C. Energy Fuels 2000, 14, 677–684. (7) Sheu, E. Y. Energy Fuels 2002, 16 (1), 74–82. (8) Murgich, J.; Merino-Garcia, D.; Andersen, S. I.; del Rio, J. M.; Lira Galeana, C. Langmuir 2002, 18 (23), 9080–9086. (9) Murgich, J. Mol. Simul. 2003, 29, 451–461. (10) Merino-Garcia, D.; Andersen, S. I. Langmuir 2004, 20, 1473– 1480. (11) Merdrignac, I.; Espinat, D. Oil Gas Sci. Technol. 2007, 62 (1), 7–32. (12) Marshall, A. G.; Hendrickson, C. L.; Jackson, G. S. Mass Spectrom. Rev. 1998, 17, 1–35. (13) Marshall, A. G.; Rodgers, R. P. Acc. Chem. Res. 2004, 37, 53–59. (14) Purcell, J. M.; Juyal, P.; Kim, D.-G.; Rodgers, R. P.; Hendrickson, C. L.; Marshall, A. G. Energy Fuels 2007, 21 (5), 2869–2874.
† Presented at the 10th International Conference on Petroleum Phase Behavior and Fouling. *To whom correspondence should be addressed. E-mail: isabelle.
[email protected]. (1) Lepage, J. F.; Chatila, S. G.; Davidson, M. Residue and Heavy Oil Processing; Technip: Paris, France, 1992. (2) Gray, M. R. Upgrading Petroleum Residues and Heavy Oils; Marcel Dekker: New York, 1994. (3) Rana, M. S.; S amano, V.; Ancheyta, J.; Diaz, J. A. I. Fuel 2007, 86, 1216–1231. (4) Strausz, O. P.; Lown, E. M. The Chemistry of Alberta Oil Sands, Bitumens and Heavy Oils; Alberta Energy Research Institute (AERI): Calgary, Alberta, Canada, 2003.
r 2009 American Chemical Society
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(e.g., the mass difference between þ H] and [C34H12]þ • is 1.1 mDa).15 The petroleum polar compounds may be identified by electrospray ionization coupled to FTICR mass spectrometry.16,17 That technique is unmatched for the speciation of nonvolatile acidic or basic petroleum species and has been successfully employed to mitigate up- and downstream refinery problems. However, purely hydrocarbon cycloalkane and aromatic species, thiophenes, and furans are inaccessible to electrospray ionization because the molecules are insufficiently basic or acidic to accept or lose a proton. APPI can positively charge these species to produce both radical cations (Mþ •) and protonated molecules ([M þ H]þ). Specifically, APPI ionizes species that can either undergo direct ionization from 10 eV photons (aromatics, such as asphaltenes) or gas-phase proton transfer and charge exchange reactions.18-20 A major advantage of APPI is that it efficiently ionizes many important classes [nonpolar sulfur species and polycyclic aromatic hydrocarbons (PAHs)] that impact the petroleum refinery and its regulation by governmental agencies. APPI FT-ICR mass spectrometry is well-suited to attempt to describe (at a molecular level) the heavy crudes and, thereby, improve our understanding of the chemical transformations and mechanisms of hydrotreatment. Here, we analyze the sulfur-containing components of the most polar fraction (asphaltenes) by APPI FT-ICR mass spectrometry to follow the progression of asphaltene structures during the hydroconversion process. We chose the same upgraded effluent products from an initial feed generated by fixedand ebullated-bed bench units as in complementary prior studies.21-24 We find that asphaltene species first evolve through dissociation of molecular units, then cracking of small polycondensed structures, and finally dealkylation of aliphatic chains. The remaining asphaltene molecules after extensive conversion consist primarily of large polycondensed structures. [C30H14S113C1
Figure 1. Residue hydroconversion scheme. Samples A1, A2, A11, and A22 were hydroconverted under fixed-bed conditions. Samples A1 and A2 (at two different temperatures in the HDM section) were further converted to samples A11 and A22 in the HDS section. Samples B1, B2, and B3 were obtained under ebullated-bed conditions for three different residence periods.
supported on alumina, designed to enhance asphaltene disaggregation. The first HDM section is mainly dedicated to remove most of the metals but also some of the sulfur. In a second set of experiments, each of the demetallized effluents was then processed in the HDS section (second section) at 380 °C but with a slightly different contact period (effluents A11 and A22). The HDS catalyst is a mesoporous CoMo catalyst supported on alumina, adapted to deep sulfur removal of smaller molecules and also to complete the removal of metals. Furthermore, the residue was hydrotreated in an ebullatedbed bench unit consisting of two reactors (R1 and R2). Experiments were performed at a constant but higher temperature (T = 427 °C) than for the FB, and the residue conversion level was adjusted by modifying the contact time. Three different effluents (B1, B2, and B3) were then collected (see Figure 1). The NiMo catalyst loaded in each reactor was supported on alumina. Conditions are summarized in Table 1, and detailed experimental conditions are described elsewhere.21,25 Asphaltenes (AsC7) from the feed residue and resulting effluents were extracted according to a method derived from the norm (NF T60-115),26 with n-heptane at 80 °C at an oil/ þ heptane ratio of 1:50. Residue conversion (X540 °C ) was determined by combining the bench unit mass balance and the simulated distillation analyses of the effluents as follows: " # þ Mf xf 540 °C -Me 540 °Cþ X ðwt %Þ ¼ 100 ð1Þ þ Mf xf 540 °C
Experimental Section Sample Preparation. An Iraqi vacuum residue 540þ (boiling point above 540 °C) was hydrotreated in benchtop units under fixed-bed (FB) and ebullated-bed (EB) conditions, representative of industrial operations (Figure 1 and Table 1).22 The fixedbed hydrotreatment process is composed of two sections connected in series, the hydrodemetallization (HDM section) and the hydrodesulfurization (HDS section). The operating conditions of each are described elsewhere.24 A first set of experiments was carried out with the HDM catalyst by varying the temperature (T = 380 and 400 °C) to produce effluents A1 and A2. The catalyst for effluents A1 and A2 is a macroporous NiMo catalyst
in which Mf represents the mass of the feed, Me represents the mass of the 540 °Cþ liquid effluent, and xf represents the weight fraction of 540 °Cþ. Asphaltene conversion (XAsC7) was then calculated as follows: " # Mf xf AsC7 -Me xe AsC7 AsC7 X ð2Þ ðwt %Þ ¼ 100 Mf xf AsC7
(15) Purcell, J. M.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2006, 78 (16), 5906–5912. (16) Rodgers, R.; Schaub, T.; Marshall, A. Anal. Chem. 2005, 77 (1), 20A–27A. (17) Rodgers, R. P.; Klein, G. C.; Stanford, L. A.; Kim, S.; Marshall, A. G. Prepr.-Am. Chem. Soc., Div. Pet. Chem. 2004, 228, U654–U654. (18) Revel’skii, I. A.; Yashin, Y. S.; Kurochkin, V. K.; Kostyanovskii, R. G. Zavod. Lab. 1991, 57 (3), 1–4. (19) Robb, D. B.; Blades, M. W. J. Am. Soc. Mass Spectrom. 2005, 16 (8), 1275–1290. (20) Syage, J. A.; Evans, M. D.; Hanold, K. A. Am. Lab. 2000, 32 (24), 24–29. (21) Merdrignac, I.; Quoineaud, A. A.; Gauthier, T. Energy Fuels 2006, 20, 2028–2036. (22) Gauthier, T.; Danial-Fortain, P.; Merdrignac, I.; Guibard, I.; Quoineaud, A. A. Catal. Today 2008, 130, 429–438. (23) Favre, A. Ph.D. Thesis, 1984. (24) Le Lannic, K.; Guibard, I.; Merdrignac, I. Pet. Sci. Technol. 2007, 25, 169–186.
in which Mf represents the mass of the feed, Me represents the mass of the 540 °Cþ liquid effluent, and xAsC7 is the asphaltene recovery of either the feed or the effluent. Specifically, the asphaltene conversion (XAsC7) equals the amount converted (rate of product formation) relative to the concentration of residue (asphaltene) in the feed. Elemental analyses of the asphaltenes are reported in Table 2. All asphaltene samples were diluted to the same concentration (1 mg/mL in toluene) and analyzed by negative- and positive-ion APPI FT-ICR mass spectrometry. (25) Gauthier, T.; Heraud, J. P.; Kressmann, S.; Verstraete, J. Chem. Eng. Sci. 2007, 62, 5409–5417. (26) American Society for Testing and Materials (ASTM). ASTM D6560 (IP 143). Annual Book of ASTM Standards; ASTM: West Conshohocken, PA, 2009.
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Table 1. Main Operating Conditions and Performances Obtained for Experiments in Fixed- and Ebullated-Bed Pilot Plantsa ebullated bed fixed bed
condition 1
HDM section feed residue
samples T (°C) contactþ time (h) X540 °C (%) XAsC7 (%) AC7 (wt%) a
A1
A2
380 2 12 40 7.9
13.2
HDS section A11
400 5 43 82 2.4
380 5 52 80 2.7
R1
A22 380 4 57 95 0.7
condition 2 R2
B1
B2
427 3.3 56 63 7.9
427 3.3 74 86 4.8
R2 B3 427 5 85 89 3.7
Details are described elsewhere.21,22,24
Table 2. Bulk Elemental Analyses for Asphaltenes n-Heptane-Extracted from the Feed Residue and the Effluents Obtained under Fixed- and Ebullated-Bed Conditions AsC7 samples
feed
A1
C (wt %) H (wt %) N (wt %) O (wt %) S (wt %) H/C
82.06 7.12 1.08 1.03 8.12 1.04
83.37 7.10 1.16 0.71 7.09 1.02
A2
A11
A22
B1
B2
87.65 85.41 89.79 86.14 89.18 6.50 6.84 6.20 6.39 5.68 1.21 1.05 0.90 1.31 1.28 0.91 0.65 0.91 0.88 0.99 3.22 4.98 1.69 5.50 2.45 0.89 0.96 0.83 0.89 0.76
Table 3. Number of Assigned Elemental Compositions and RootMean-Square Difference between Experimentally Measured Mass and the Exact Mass for the Elemental Composition (Assigned to That Mass Spectral Peak)a
B3
elemental assignments
90.48 5.30 1.18 0.97 1.38 0.70
fixed bed
APPI ionisation source. The APPI source was supplied by Thermo Fisher Scientific (San Jose, CA). The vaporized analyte gas stream flows orthogonal to both the mass spectrometer inlet (heated metal capillary) and the krypton vacuum UV lamp that produces 10 eV photons. The source is mounted to an adapter between the first differentially pumped stage of the 9.4 T FT-ICR mass spectrometer through a heated metal capillary. The metal capillary (750 μm inner diameter) is resistively heated (3-4 A direct current). The source-adapter apparatus construction provides a closed area such that the nebulizer gas (CO2) provides an inert atmosphere with a slight positive pressure. A Harvard stainless-steel syringe (8 mL) and syringe pump deliver solution to the heated nebulizer of the APPI source. In the APPI source, the solvent flow rate is 50-100 μL/min, the nebulizer heater is operated at 380 °C with carbon dioxide sheath gas at 550 kPa, and the auxiliary gas port is plugged. 9.4 T FT-ICR MS. All experiments were performed with a custom-built FT-ICR mass spectrometer equipped with a passively shielded Oxford 9.4 T superconducting magnet27,28 and controlled by a modular ICR data system.29,30 Ions produced by the external APPI source traverse the heated metal capillary to the first stage of vacuum pumping into a skimmer region. The skimmer provides a conductance limit to the second differentially pumped stage where the ions enter the first radio frequency (rf)-only octopole and are accumulated (1-5 s)31 before transfer through a quadrupole (not operated in mass-resolving mode) into a second rf-only octopole where they are collisionally cooled (50 ms) with helium before transfer through an rf-only octopole to a 10 cm diameter and 30 cm long open cylindrical
ebullated bed
asphaltenes
positive ion (rms error)b
negative ion (rms error)b
feed A1 A11 A2 A22 B1 B2 B3
5624 (423) 5905 (432) 2881 (232) 4040 (303) 2971 (296) 4608 (452) 3564 (278) 1823 (262)
3268 (282) 4939 (319) 5812 (450) 6792 (410) 4589 (441) 5466 (470) 4128 (384) 2997 (434)
a
The asphaltene sample designations are as in Figure 1. b rms mass error reported in ppb.
Penning ion trap. The octopole ion guides (1.6 mm titanium rods, 4.8 mm inner diameter) were typically operated between 1.5 and 2.0 MHz and 190 < Vp-p < 240 V rf amplitude. Broadband frequency-sweep excitation (∼90-600 kHz at a sweep rate of 150 Hz/μs and 190 V peak-peak amplitude) accelerates the ions to a detectable cyclotron orbital radius. Ion cyclotron resonant frequencies were detected from the current induced on two opposed electrodes of the ICR trap. Multiple (100-200) time-domain acquisitions were summed for each sample, Hanning-apodized, and zero-filled once before fast Fourier transform and magnitude calculation.32 Negative-ion data was collected with similar parameters and appropriate polarity changes. All observed ions were singly charged on the basis of the unit m/z separation between 12 Cn and 13C112Cn-1 isotopic variants of the same elemental composition.32 Therefore, mass spectral peak positions are reported in daltons rather than as m/z.
Results and Discussion Table 1 illustrates that asphaltene conversion (XAsC7), achieved in under either FBþ or EB conditions, is higher than residue conversion (X540 °C ). The results suggest that asphaltene reactivity is higher than the overall residue under the described test conditions. Furthermore, the residue conversion was higher in EB than in FB and ascribed to a higher thermal cracking severity in EB and with less pronounced catalytic contribution. Asphaltenes from all samples have been extracted and analyzed by APPI FT-ICR MS. Table 3 summarizes the negative- and positive-ion mode APPI FT-ICR MS results. The elemental composition assignments represent all spectral
(27) Senko, M. W.; Hendrickson, C. L.; Pasa-Tolic, L.; Marto, J. A.; White, F. M.; Guan, S.; Marshall, A. G. Rapid Commun. Mass Spectrom. 1996, 10 (14), 1824–1828. (28) Hakansson, K.; Chalmers, M. J.; Quinn, J. P.; McFarland, M. A.; Hendrickson, C. L.; Marshall, A. G. Anal. Chem. 2003, 75 (13), 3256– 3262. (29) Senko, M. W.; Canterbury, J. D.; Guan, S.; Marshall, A. G. Rapid Commun. Mass Spectrom. 1996, 10, 1839–1844. (30) Blakney, G. T.; Chalmers, M. J.; Lam, T. T.; Emmett, M. R.; Hendrickson, C. L.; Marshall, A. G. Proceedings of the 51st Conference on Mass Spectrometry and Allied Topics, American Society for Mass Spectrometry, Montreal, Quebec, Canada, 2003. (31) Senko, M. W.; Hendrickson, C. L.; Emmett, M. R.; Shi, S. D. H.; Marshall, A. G. J. Am. Soc. Mass Spectrom. 1997, 8, 970–976.
(32) Senko, M. W.; Beu, S. C.; McLafferty, F. W. J. Am. Soc. Mass Spectrom. 1995, 6, 52–56.
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Figure 2. Heteroatom class distribution obtained for the asphaltene feed sample, from positive-ion APPI FT-ICR MS. Ion abundances are scaled relative to the most abundant class for that sample, and the relative abundances for all members of a given class are then summed (to yield, e.g., the total S1 content).
Figure 3. Heteroatom class distributions for feed and A1 (fixed bed, HDM section, 380 °C) asphaltenes, from positive-ion APPI FT-ICR MS. Ion abundances are scaled the same as for Figure 2.
peaks for which a unique molecular formula could be assigned on the basis of accurate mass measurement combined with homologous class, type (DBE), and alkylation series, including isotopic contributions from 13C and 34S. The root-meansquare (rms) mass error for all samples is m/z > 2000), limits the detailed
compositional analysis to the center of the molecular-weight distribution. Thus, compositional trends highlighted are limited accordingly. Fixed-Bed Asphaltene Analysis, HDM Section. For each positive-ion APPI FT-ICR mass spectrum, spectral peak assignments were sorted by class, e.g., hydrocarbon (HC), heteroatoms, NnOoSs, aromaticity (DBE = number of rings plus double bonds to carbon), and number of carbons. The feed asphaltene exhibits a heteroatom-rich class distribution (Figure 2). For the more abundant classes (>1% relative abundance, top of Figure 2), 15 of the 17 classes contain one or more sulfur atoms. Compounds without heteroatoms (HC class) are present only in low abundance. These results are in accordance with bulk elemental analyses (Table 2), for which sulfur is the major heteroatom (>8 wt %), followed by nitrogen (1 wt %). Samples A1 and A2 are the effluents produced under FB conditions at two different temperatures (380 and 400 °C)
(33) Kim, S.; Rodgers, R. P.; Marshall, A. G. Int. J. Mass Spectrom. 2006, 251, 260–265.
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(Table 2) based on bulk measurements. The relationship between the two class distributions (feed and A1 asphaltenes, Figure 3) suggests that the loss of one sulfur atom could occur during the conversion process. Thus, a molecular class that contains Sx in the feed could be converted to a Sx-1 class in the AsC7 A1 sample. A class comparison between A1 (380 °C) and A2 (400 °C) asphaltenes is displayed in Figure 4. The class composition of AsC7 A2 is less complex than that for AsC7 A1, indicating that the asphaltenes seem to be less polydisperse in terms of chemical structures. The predominance of the HC as well as the N classes is accentuated in A2 relative to A1 asphaltenes.
and are driven by catalytic mechanisms, whereas reactions under EB conditions are thermally driven. Table 2 reveals that the total sulfur content in AsC7 A1 is still very high (7.09 versus 8.12 wt % in the AsC7 feed) and asphaltene conversion is moderate (XAsCþ7 = 40%). The 540þ conversion is comparatively low (X540 °C = 12%) (Table 1). However, a large reduction in sulfur is noted (3.22 versus 8.12 wt % in the AsC7 feed) for the AsC7 A2 sample. Unsurprisingly, the asphaltene, as well as the 540þ residue conversion, for A2 is much higher than for the A1 sample. The feed and A1 asphaltene class distributions are compared in Figure 3. The AsC7 A1 distribution is still highly polydisperse in terms of the number of heteroatom classes, although the HC relative abundance has increased relative to the AsC7 feed. The results confirm the high percent of sulfurcontaining compounds remaining in the AsC7 A1 effluent
Table 4. DBE and Carbon Number/DBE Ratio for Some Examples of Model Moleculesa
Figure 4. Heteroatom class distributions for samples A1 (fixed bed, HDM section, 380 °C) and A2 (fixed bed, HDM section, 400 °C) asphaltenes based on positive-ion APPI FT-ICR MS. Abundance scaling is the same as in Figure 2.
a
DBT, dibenzothiophene; Me-DBT, methyldibenzothiophene.
Figure 5. Isoabundance-contoured plots of DBE versus the carbon number for S1, S2, S3, and HC classes from the feed asphaltene and its A1 (fixed bed, HDM section, 380 °C) and A2 (fixed bed, HDM section, 400 °C) asphaltene products based on positive-ion APPI FT-ICR MS. Dashed reference lines facilitate class-class comparisons.
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Figure 6. Heteroatom class distributions for reaction products A11 (fixed bed, HDS section from A1) and A22 (fixed bed, HDS section from A2) asphaltenes based on positive-ion APPI FT-ICR MS. Abundance scaling is the same as in Figure 2.
The prominent class changes between the A1 and A2 asphaltene samples (a 20 °C difference in reaction temperature) suggest that thermal reactions predominate under A2 conditions, and thus, cracking mechanisms occur preferentially. On the basis of these results, a reaction threshold seems to be crossed between 380 and 400 °C. Figure 5 is an isoabundance-contoured plot of double bond equivalents (DBE) versus the carbon number for the HC and sulfur classes in the feed and A1 and A2 asphaltene samples. The plot represents all of the detected species for a given class displayed in a single image. The spectral peak heights from which each image is constructed are scaled relative to the highest spectral magnitude peak in each mass spectrum; relative abundances between classes are represented in bar-type graphs (Figure 3), whereas relative ion abundance within a single class is depicted as DBE versus carbon number images. The DBE versus carbon number images for feed and A1 and A2 asphaltene samples (Figure 5) each show a progressive shift to a lower carbon number and DBE with an increasing S atom number. The addition of a sulfur atom does not result in an increase of 2 in DBE, as would be expected by the addition of a complete thiophenic ring to an existing aromatic core. Thus, the compounds containing 1-3 sulfur atoms evidently preferentially form more aromatic (condensed) but less alkylated structures. Also, the aromaticity of the sulfur-containing classes is large, as evidenced by the carbon number/DBE ratios = 1.5 and 1.37 for the S1 and S3 families. The DBE = 25-35 range indicates high aromaticity with little alkyl substitution. For comparison, Table 4 shows DBE and carbon number/DBE values for some model compounds. On the basis of those standards, sulfur is likely present in thiophenic form. If a molecular structure is projected from the molecular formula and the requirement for high aromaticity, then the fraction of aromatic carbon is high (∼75%) relative to bulk asphaltenes (∼50%). If there are sequential loss sulfur atoms (Figure 3), then S3 species in the AsC7 feed should convert to S2 species in the AsC7 A1 sample and so forth. In this case, one might expect the decrease of one DBE (loss of one sulfur atom from an aromatic core) and a decrease of the carbon number
Figure 7. DBE versus carbon number images for the HC classes from A11 and A22 asphaltenes (fixed bed, HDS section from A1 and A2).
(hydrotreatment mechanism). Instead, the comparison between feed and A1 asphaltene samples illustrated in Figure 5 shows that the A1 S2 species slightly exhibit a higher DBE (DBE = 3-4) than for the feed S3 species as well as a slightly higher carbon number (∼6 carbons). This observation is general for each displayed family. These results highlight the complexity of the reaction mechanisms (dehydrogenation of naphthenic rings must also be considered at such temperatures). Alternatively, the converted product compounds may not be present in the asphaltene fraction but in other fractions (i.e., the maltenes) or may result from lower molecular-weight asphaltenic species cracked from highmolecular-weight (>1000 Da) species. Figure 5 shows that the progression of S1, S2 and S3 classes in asphaltenes for increased processing temperature (A1-A2, 380-400 °C) is similar to the progression from feed to conversion at 380 °C (feed-A1). As expected, a higher reaction temperature increases the extent of sulfur removal, without increasing the aromaticity and polycondensation of the remaining sulfur species (as reflected by the carbon number/DBE ratio and DBE). The produced asphaltene effluent (A2) is less polydisperse, and its complexity is less pronounced than from a lower temperature (A1, 380 °C), because of preferential depletion of classes containing one or 2262
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Figure 8. Heteroatom class distributions for the feed and B1, B2, and B3 asphaltene samples (ebullated-bed conversion). Abundance scaling is the same as in Figure 2.
Figure 9. DBE versus carbon number images for the sulfur S1 class from A1 and A2 (fixed bed, 380 and 400 °C) and B1, B2, and B3 (ebullated bed) asphaltene samples.
more sulfur atoms. The desulfurization obtained at 400 °C may also be accompanied by increased asphaltene disaggregation that reduces the diffusional resistance,34 thereby enhancing the accessibility of the sulfur species to the macroporous catalyst. In the mass spectral results, disaggregation would also affect the class relative abundances, because disaggregation renders observable species that are not observable (because of high mass) in the aggregated sample. Fixed-Bed Asphaltene Analysis, HDS Section. Both A1 and A2 effluents were subsequently hydrotreated under the HDS conditions to yield A11 and A22 effluents. The temperature was fixed in both cases (380 °C), but the contact
periods were slightly different (5 versus 4 h; see Table 1). Because of the nature and specificity of the catalyst, deeper sulfur removal of small molecules from samples A1 and A2 is expected. The asphaltene conversions significantly increase in both cases: from 40 to 80% (A1 versus A11) and from 82 to 95% (A2 versus A22) (Table 1). Higher residue and asphalþ tene conversions (X540 °C = 57 versus 52%, and XAsC7 = 95 versus 80%) are reached when conditions in the HDM section are more severe (high temperature and long contact period). Desulfurization is, however, effective for A1A11 asphaltenes (HDS = 29%) relative to A2-A22 (47%) (Table 2). The heteroatom relative abundance of A11 and A22 asphaltenes appeared to be nominally identical in either chemical class or relative distribution (Figure 6). The complexity and polydispersity are much reduced for the HDS section relative to the HDM section.
(34) Ferreira, C.; Marques, J.; Tayakout, M.; Guibard, I.; Lemos, F.; Toulhoat, H.; Ribeiro, R. Chem. Eng. Sci. 2009, in press.
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Considering the similar class distributions in Figure 6 for AsC7 A11 and A22, it is difficult to account for their sulfur content discrepancies (Table 2). One possibility is that the reaction pathways for A1 and A2 lead to similar remaining asphaltenic sulfur species. The converted species are preferentially found in other fractions [maltenes of either resid (boiling point above 540 °C) or lighter boiling cut fractions (boiling point below 540 °C)]. DBE versus carbon number images for the HC species from AsC7 A11 and A22 (Figure 7) expose major differences in aromaticity distributions (carbon number/DBE) relative to AsC7 A1 and A2 (Figure 5). However, on average, A11 is slightly more aromatic than A1, for which the abundance maximum is centered at ∼47 carbons and DBE = 34 for A1 versus carbon number = 44 and DBE = 32 for A11. The aromaticity increase is mainly due to a change in the distribution of the species in compositional space toward higher DBE values. The change is most pronounced in sample A22 that shows a distinct shift toward the planar aromatic limit (i.e., the maximum DBE for a planar PAH). In fact, the most abundant species fall directly on the planar aromatic limit (dashed line on the right of Figure 7). That limit represents the boundary between a planar carbon structure and a bowl-shaped (nonplanar) structure (e.g., coronene versus corannulene). This shift is mainly due to a large decrease in the carbon number (45 for A2 versus 37 for A22). Dealkylation of HC species is virtually complete for species on or in close compositional proximity to the line for sample A22. These observations support those of previous studies,35 in which the loss of alkane chains results in a reduction in the size of the fused ring systems as well as a reduction in the molecular size for the asphaltene fraction. The residual asphaltenes consist of more polycondensed structures. Ebullated-Bed Asphaltene Analysis. Three effluents (B1, B2, and B3) were obtained from the second set of experiments performed under EB conditions at high temperature (427 °C) for three residence periods (Figure 1 and Table 1). In comparison to the fixed-bed process, the 540þ residue (boiling point above 540 °C) can thereby be converted in EB to a very high extent (up to 85% in these experiments). However, the asphaltene conversions (XAsC7) remain in the same range as for the fixed bed (60-90%). As for the FB process, the three effluents from the EB process show a significant decrease in sulfur-containing species, increase in the HC class, and reduced class complexity (Figure 8), a phenomenon even more accentuated with an increased contact period. DBE versus carbon number images for the asphaltene S1 class (Figure 9) show similar aromaticity (carbon number/DBE ratio) and polycondensation (DBE) for both FB and EB processes, suggesting that the residual S1 species characterized in the asphaltenes seem not to contain products formed during the conversion. The reason is that, with different conditions, catalyst, and process, we expect that the reaction mechanisms (and thus the products) would be different. If FB and EB show the same DBE and C number ranges for the remnant species, the converted species must fall out of the asphaltenes and into another fraction. The converted products may thus be found in maltenes of either residue (boiling point above 540 °C) or lighter boiling cut fractions (boiling point below 540 °C).
Figure 10. DBE versus carbon number plot images for the HC class from samples B1, B2, and B3 (ebullated bed) asphaltenes. Structures a, b, and c (bottom) are representative stable structures corresponding to highly abundant species for sample B3. Proposed structures maximize the amount of aromatic sextet carbon (shown in red) in each PAH.36
Figure 10 shows DBE versus carbon number images for B1, B2, and B3 asphaltene products. Aromaticity increases with an increased residence time period, as confirmed by the elemental analysis H/C ratio (Table 2). A distinct shift toward the planar aromatic limit is observed again, mainly because of a large decrease in the carbon number. For the most extreme B3 processed effluent, a dealkylation of HC species is essentially complete; namely, the most abundant species fall on the planar aromatic limit. The constancy in the DBE value suggests that no molecular condensation occurs. However, because of the compositional complexity and broad range of DBE and carbon numbers in the HC class, proportional assignment of dealkylation versus aromatization contribution in naphthenic rings (both of which could account for the bare PAH species at the planar aromatic limit) cannot be assigned. The proposed structures (the bottom of Figure 10) represent single isomers of stable aromatic molecules that correspond to most abundant ions observed by MS. Interestingly, the three most abundant species match the proposed elemental compositions of theoretically determined stable aromatic cores proposed by Ruiz-Morales and co-workers.36 The remaining asphaltenes in B3 constitute a mixture of highly polycondensed dealkylated aromatic molecules and support the conclusions of prior complementary studies.21,22 Conclusions Various bulk property analytical techniques can provide clues to asphaltene desulfurization but no molecular level detail. In contrast, APPI FT-ICR MS provides detailed molecular characterization of such polydisperse mixtures. Here, we have assigned and sorted the molecular formulas for asphaltene HC and sulfur families, making it possible to follow their evolution during deep hydrotreatment processes. We find that the chemical polydispersity of the asphaltenic fractions is drastically lowered with an increasing extent of desulfurization. Under fixed-bed conditions, the simplification is even more enhanced than the experimental conditions and the temperature and contact period have increased. Under either fixed- or ebullated-bed conditions, the product
(35) Buch, L.; Groenzin, H.; Buenrostro-Gonzalez, E.; Andersen, S. I.; Lira-Galenana, C.; Mullins, O. C. Fuel 2003, 82 (9), 1075–1084.
(36) Ruiz-Morales, Y J. Phys. Chem. A 2002, 106 (46), 11283–11308.
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: DOI:10.1021/ef900897a
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sulfur species exhibit quite similar class composition, aromaticity (carbon number/DBE ratio), and polycondensation (DBE). The evolution of the HC species results in a drastic increase in aromaticity, culiminating in highly polycondensed dealkylated aromatic structures. The same structure evolution seems to be obtained for both the fixed- and ebullated-bed processes. Asphaltene disaggregation (allowing for better access of sulfur/heteroatoms species to the catalyst active sites) followed by dealkylation of the remaining species is possible. The present results are consistent with and converge toward previously proposed mechanisms.21,22,24 This work highlights the difficulty of elucidating definitive desulfurization mechanisms based solely on the characteriza-
tion of asphaltenic fractions remaining after hydrotreatment. The maltene fraction must also be considered, with detailed characterization of all fractions from the different produced cuts (gasoline, gas oil, vacuum distillate, and residue). A balance including the major species before and after conversion could then provide a basis for the model of molecular reconstruction. Acknowledgment. This work was supported by the National Science Foundation (NSF) Division of Materials Research through DMR-06-54118 and the state of Florida. The authors thank J. Ponthus and J. Verstraete for their helpful discussions.
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