Comparison of Fractionation Methods for the Structural

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Energy & Fuels 2001, 15, 429-437

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Comparison of Fractionation Methods for the Structural Characterization of Petroleum Residues Isabel Suelves, Carlos A. Islas, Alan A. Herod,* and Rafael Kandiyoti Department of Chemical Engineering and Chemical Technology, Imperial College (University of London), Prince Consort Road, London SW7 2BY, U.K. Received August 15, 2000. Revised Manuscript Received November 20, 2000

Two petroleum residues have been fractionated using solvent (heptane) separation, planar and column chromatography. The residues and the separated fractions have been characterized by size exclusion chromatography (SEC), MALDI (matrix-assisted laser desorption/ionization) mass spectrometry, and by UV-fluorescence spectroscopy (UV-F). MALDI mass spectrometry has indicated both residues to contain material with molecular mass ranges up to 15 000 u. The upper mass ranges indicated by size exclusion chromatography using polystyrene standards were higher; the earliest eluting material from both distillation residues eluted at times corresponding to polystyrene standards of MMs above 1.85 million u. Data from UV-F suggests that the heptane solubility separation method was the most successful for the separation of the largest molecular masssand also probably the most polarsmaterials in these residues. However, all three fractionation methods produced similar trends, showing greater polarity of the fractions to correlate with increasing molecular mass. The shift of maximum intensity of fluorescence toward longer wavelengths (in UV-fluorescence) with increasing molecular size, as indicated by SEC, strongly suggests that the fluorescing molecules are large rather than aggregates of small molecules.

Introduction Economically significant proportions of crude oils processed in refineries end up being classed as distillation residues. When cooled to ambient temperature, these residues are usually sticky solids or very viscous pastessof relatively little commercial value. More detailed structural characterizations appear to be needed, to provide background information and broader perspectives for developing process routes to upgrade these materials. Within this framework, the polar and large molecular mass components of complex hydrocarbon mixtures usually prove to be the more difficult to characterize and to upgrade. However, most available characterization techniques tend to provide information on properties of more abundant materials/fractions of complex mixtures. Fractionation of such mixtures is, therefore, a necessary first step in attempting to obtain a more complete inventory of structural features [e.g., see refs 1-3]. Clearly, the nature of the fractions obtained depends on sample structures as well as the fractionation methods used. Recently we compared the fractionation of a coal tar pitch by solvent solubility and by planar chromatography (PC). Subsequent size exclusion chromatography and UV-fluorescence spectrometry showed that PC-separation enabled a better separation to be achieved with less overlap between the fractions.4 The present paper presents preliminary analytical results obtained following the fractionation of two * Corresponding author. E-mail: [email protected]. (1) Lazaro, M. J.; Domin, M.; Herod, A. A.; Kandiyoti, R. J. Chromatogr. A 1999, 840, 107-115.

petroleum residues by several different fractionation methods, including solvent separation, planar and column chromatography. The samples used were a vacuum resid and an atmospheric pressure distillation residue. Analytical characterizations of the original samples and fractionated materials have been carried out by size exclusion chromatography (SEC), MALDI (matrix-assisted laser desorption/ionization) mass spectrometry and by UV-fluorescence spectroscopy (UV-F). Both SEC and MALDI-MS are techniques useful in estimating molecular mass (MM) distributions of complex mixtures. In recent work, we have shown how SEC elution times of a set of standard compounds with widely differing structural features could be predicted (to within (1 min) by a calibration based on polystyrene standards.5,6 The work was carried out using 1-methyl2-pyrrolidinone (NMP) as eluent and the range of MMs of the standard compounds used in the study extended to slightly below 1100 u. The range of samples included PAHs, azaarenes, and other nitrogen-bearing compounds, several dyes, polars, and numerous oxygenated compounds, yet elution was observed to have taken place with a predominantly size-dependent mechanism. (2) Herod, A. A.; Zhang, S-F.; Carter, D. M.; Domin, M.; Cocksedge, M. J.; Parker, J. E.; Johnson, C. A. F.; John, P.; Smith, G. P.; Johnson, B. R.; Bartle, K. D.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1996, 10, 171. (3) Herod, A. A.; Kandiyoti, R. J. Chromatogr. A 1995, 708, 143160. (4) Lazaro, M-J.; Herod, A. A.; Kandiyoti, R. Fuel 1999, 78, 795801. (5) Lazaro, M. J.; Islas, C. A.; Herod, A. A.; Kandiyoti, R. Energy Fuels 1999, 13, 1212-1222. (6) Herod, A. A.; Lazaro, M-J.; Domin, M.; Islas, C. A.; Kandiyoti, R. Fuel 2000, 79, 323-337.

10.1021/ef000183a CCC: $20.00 © 2001 American Chemical Society Published on Web 01/19/2001

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The same polystyrene-based calibration has been applied to coal liquids including fractions of a coal tar pitch. At the shortest elution times, SEC showed the presence of pitch components eluting at times similar to those of a polystyrene standard of 1.85 million u. The structural features and molecular masses of materials eluting at such short times are clearly of interest. Pyrolysis-GC-MS was used to obtain useful correlations between increasing molecular masses of fractions of the same coal tar pitch and the dominant structural features of the fractions;7 a similar study has more recently been carried out with a coal liquefaction extract sample.8 However, in attempting to evaluate molecular mass distributions found by SEC using an entirely independent technique, MALDI-MS has emerged as the most useful technique we have used to date. We have been able to observe quantitative agreement9,10 between SEC and MALDI-MS at MMs up to about 3000 u. At higher masses, the response of MALDI-mass spectrometers (which we have used) does not appear to be quantitative, although the technique has been helpful in confirming the presence of the higher MM-materials observed by SEC. For the pyridine-insoluble fraction of the coal tar pitch, a value of about 94 000 u was found, even when results were evaluated with a relatively conservative statistical method for estimating the upper mass limit of the spectrum.11 Much of the discussion on the upper limit of molecular masses in heavy hydrocarbon liquids appears to have shifted to petroleum derived liquidssa clearly more topical application. However, the substance of the argument regarding the upper mass limit in heavy hydrocarbon liquids and solids appears to have retained much of its earlier resonances. Data from several indirect techniques have been used to argue12,13 that MMs of petroleum asphaltenes range between 500 and 1000 u. By marked contrast, straightforward MALDIMS spectra of two petroleum vacuum resids clearly show14 the presence of material at up to between 10 000 and 20 000 u; analogous data will be presented below. It would appear that much of the uncertainty derives from the interpretation of results from different mass spectrometric techniques. Briefly, mass spectrometric techniques involving evaporation of the sample prior to ionization do not appear to allow larger MMs materials to be volatilized and subsequently ionized. For example, in a recently cited12 early study,15 thermal volatilization between 50 and 300 °C prior to ionization has been used on four progressively heavier fractions of a sample of (7) Herod, A. A.; Islas, C.; Lazaro, M.-J.; Dubau, C.; Carter, J. F.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1999, 13, 201-210. (8) Islas, C. A.; Suelves, I.; Carter, J. F.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2000, 14, 1766-1782. (9) Johnson, B. R.; Bartle, K. D.; Ross, A. B.; Herod, A. A.; Kandiyoti, R.; Larsen, J. W. Fuel 1999, 78, 1659-1664. (10) Islas, C. A. Ph.D. Thesis, University of London (in preparation). (11) Lazaro, M. J.; Herod, A. A.; Domin, M.; Zhuo Y.; Islas, C. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1999, 13, 1401-1412. (12) Winans, R. E.; Hunt, J. E. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 44 (4), pp 725-727. 218th ACS National Meeting, August 22-26, 1999, New Orleans, LA. (13) Groenzin, H.; Mullins, O. C. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 44 (4), pp 728-732. 218th ACS National Meeting, August 22-26, 1999, New Orleans, LA. (14) Deelchand, J-P.; Naqvi, Z.; Dubau, C.; Shearman, J.; Lazaro, M-J.; Herod, A. A.; Read, H.; Kandiyoti, R. J. Chromatogr. A 1999, 830, 397-414. (15) De Canio, S. J.; Nero, V. P.; DeTar, M. M.; Storm, D. A. Fuel 1990, 69, 1233-1236.

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petroleum crude (a, pentane-solubles; b, heptane-soluble pentane-insolubles; c, cyclohexane-soluble heptane-insolubles; d, cyclohexane-insolubles). Triple quadrupole MS on these samples was reported to have shown progressively decreasing average molecular masses, from 615 u for the pentane-solubles down to 410 u for the cyclohexane-insolubles. Prima facie, such a data series is normally explained in terms of the diminishing volatility of increasingly heavy fractions (going from a to d). A recent report12 has suggested that FIMS, another technique based on evaporation of the sample prior to ionization, produces similar anomalous results. No such difficulties were observed when more appropriate mass spectrometric techniques were used on samples prepared from the fractionation by preparative size exclusion chromatography of a +525 °C boiling Athabasca bitumen distillation residue.16 The MMdistributions of the five fractions were determined by several mass spectrometric techniques: 252Cf-plasma desorption mass spectrometry (PDMS), MALDI-MS, and laser desorption-MS (LD-MS). These are techniques where ionization is thought to take place prior to or concurrently with desorption. The samples were also characterized by SEC (with NMP as eluent) and vapor pressure osmometry. The results showed the largest size molecules concentrated in the early-eluting fractions (as would have been expected). MM-distributions were found to be continuous up to and beyond 15 000 u. VPO, MALDI-MS, and SEC were found to give similar results, although PDMS was helpful in confirming the trends. A discussion of these topics, however brief, would remain incomplete without mentioning the debate on aggregate formation. In our opinion, it remains difficult to establish whether and when it would be legitimate to invoke molecular aggregates to explain the large MMs observed here and in other work. Certainly biochemists, also dealing with wide ranges of natural materials, have no particular inhibitions in discussing the existence of MMs up to or above 300 000 u [e.g., see refs 17-19]. Nor is it always possible to invoke agglomeration due to sample polarity, as giving rise to the appearance of the early eluting “excluded” peaks in SEC-chromatograms: examination of a manifestly nonpolar naphthalene mesophase pitch by SEC has given significant signal for early-eluting material excluded from column porosity.20 Similarly, the addition of LiBr to the NMP used as eluent (thought by the researchers 21-23 to disaggregate the “aggregates” and return the molecules to their appropriate elution times) has been shown to ruin20 the solvent properties of NMP, to promote surface effects, and to cause material to elute well after the permeation limit of the column (elution times of the smallest molecules). (16) Domin, M.; Herod, A. A.; Kandiyoti, R.; Larsen, J. W.; Lazaro, M-J.; Li, S.; Rahimi, P. Energy Fuels 1999, 13, 552-557. (17) Hillenkamp, F.; Keras, M.; Beavis, R. C.; Chait, B. T. Anal. Chem. 1991, 63, 1193A. (18) Galvani, M.; Bordini, E.; Piubelli, C.; Hamdan, M. Rapid Commun. Mass Spectrom. 2000, 14, 18-25. (19) Galvani, M.; Hamdan, M. Rapid Commun. Mass Spectrom. 2000, 14, 721-723. (20) Herod, A. A.; Shearman, J.; Lazaro, M-J.; Johnson, B. R.; Bartle, K. D.; Kandiyoti R. Energy Fuels 1998, 12, 174-182. (21) Masuda, K.; Okuma, O.; Kanaji, M.; Matsumara, T. Fuel 1996, 75, 1065. (22) Mori, S. Anal. Chem. 1983, 55, 2414. (23) Takanohashi, T.; Iino, M.; Nakumara, K. Energy Fuels 1994, 8, 395.

Structural Characterization of Petroleum Residues

It is difficult to show the absence of aggregations particularly when the phenomenon may occur under certain conditions. The probability that induced-dipole/ induced-dipole interactions between two or more nonpolar molecules through London dispersion forces could lead to aggregation cannot be discounted.24 It must be said, however, that the evidence from the behavior of the standard polycyclic aromatics compared with polystyrene standards does not favor aggregation in NMP solution. It seems likely that the solvating power of NMP for aromatics, with interaction between the small polar solvent and a relatively large nonpolar aromatic by dipole/induced dipole attraction forces, could be more important in dilute solutions than the interaction between nonpolar aromatics through the induced dipole/ induced dipole attraction. There is much evidencessome of it circumstantialsfrom our work suggesting that many of the large MM-material we observe are not aggregated. In general, observations by MALDI are thought to be entirely free of aggregation. Recent structural work by pyrolysis-GC-MS has shown that aromatic fragments of the pyridine-insolubles of a coal tar pitch were unable to get through a GC-column, suggesting polycyclic aromatic-fragments of pyrolyzed pitch to have masses above 250-300 u. Nevertheless, in samples of combustion tars and soots, the earliest eluting material corresponds to the void volume of the SEC-column and has been shown25,26 to have cross-sectional diameters between 20 and 50 nm; this is undoubtedly very large. Similar considerations appear to apply to wood tar1 where the material excluded in SEC was found to be concentrated in the fraction immobile in TLC. More direct evidencesas opposed to established consensussappears necessary to explore whether and when aggregates actually form in heavy hydrocarbon liquids. Experimental Section Samples. Petrox. A crude oil residue was obtained from the bottom of an atmospheric-pressure distillation column of the Petrox oil refinery, near Concepcion, Chile; 70% of the oil vaporizes while the rest of the feed is treated with a steam counterflow at 370 °C. The present sample is part of the reduced crude which leaves the bottom of the column at ∼330 °C. About 40% of this bottom fraction ends up as pitch, the involatile residue (boiling point >∼500 °C depending on process conditions) from further distillation, which is processed in a visbreaking unit and used as bunker fuel. Further analytical details of the sample have been given elsewhere;27 the ultimate analysis was C 86.8%, H 12.9%, N 0.42% (wt %). The sample may originate from several mixed crudes depending on the sourcing of supplies to the refinery at the time of collection. ‘‘Sample 1”. A vacuum residue of a South American crude was a pre-refinery sample prepared during pilot crude assay tests (ASTM D 8628) and was thought to have undergone (24) Atkins, P. W.; Physical Chemistry, 3rd ed.; Oxford University Press: U.K. 1986; Chapter 24, p 586. (25) Herod, A. A.; Lazaro, M-J.; Shearman, J.; Card, J.; Jones, A. R.; Kandiyoti, R. Fuel 1999, 78, 861-863. (26) Herod, A. A.; Lazaro, M-J.; Suelves, I.; Dubau, C.; Richaud, R.; Shearman, J.; Card, J.; Jones, A. R.; Domin, M.; Kandiyoti, R. Energy Fuels 2000, 14, 1009-1020. (27) Pindoria, R. V.; Megaritis, A.; Chatzakis, I. N.; Vasanthakumar, L. S.; Lazaro, M. J.; Herod, A. A.; Garcia, X. A.; Gordon, A.; Kandiyoti, R. Fuel 1997, 76, 101-113. (28) Annual Book of ASTM Standards, Vol. 5, 1997.

Energy & Fuels, Vol. 15, No. 2, 2001 431 relatively little thermal degradation; this sample has been described in considerable detail elsewhere,14 labeled as “Sample 1”. The ultimate analysis was C 87.2%, H 12.6%, N 0.24% (wt %). The sample was obtained from a single crude taken from a drilling site; the location is commercially confidential. Fractionation Methods. Solvent Fractionation. A residue sample was stirred with excess heptane overnight and filtered to give an oil fraction and an asphaltene fraction. The asphaltene fractions were 2.1% and 1.6% of the Petrox and “Sample 1”, respectively, by weight. Planar Chromatography. Procedures for the fractionation of samples by thin-layer chromatography have been presented elsewhere.2,29,30 Briefly, Whatman chromatographic plates (silica gel: K6, 20 × 20 cm preparative plates, thickness 1000 µm) were used with two solvent systems: (a) pyridine followed by acetonitrile, and (b) tetrahydrofuran followed by toluene. Fractions from the preparative plates were examined by UVfluorescence and SEC. Before use, the plates were washed in the more polar solvent, pyridine and dried, to remove contaminants from the coating materials. NMP does not dissolve the residues completely since the aliphatic components are relatively insoluble. However, it appears to dissolve aromatic components and since the detection method is UV absorbance, the aliphatics would not be detected. NMP cannot be readily evaporated from the plate, since the boiling point is over 200 °C at atmospheric pressure. Sample was therefore applied to the plates in the form of partially dissolved slurries in pyridine. Development tanks were equilibrated for half an hour to saturate the vapor phase before insertion of the plates. Sample was applied to the plates on a narrow band at the origin along a 20 cm side by multiple spotting and dried in air before development. The first development in the more polar solvent proceeded for 4 or 5 cm; the plates were removed from the tank and dried before insertion into the second solvent for development for a further 2 or 3 cm beyond the first solvent front; a manual multiple development technique was used to enhance the separation of fractions. After the final drying, bands of silica were scraped from the plates representing (A) material immobile in both pyridine and acetonitrile, (B) material at the pyridine solvent front (not mobile in acetonitrile), and (C) material mobile in both solvents advancing to the second solvent front. Fractions from the second solvent system were (D) immobile in THF and toluene, (E) mobile in THF but not in toluene, and (F) mobile in both solvents. Recovery of the residue fractions from the silica was achieved by solution at room temperature in 1-methyl-2-pyrrolidinone (NMP) assisted by ultrasonic agitation. Recovery of fractions from silica was semipreparative with sample remaining on the substrate. No attempt was made to recover quantitative fractions. Column Chromatograph. Because the planar chromatographic methods produce relatively small quantities of fractions, a method to produce fractions from up to 1 g of sample has been developed, based on the use of silica gel with sequential elution using acetonitrile, pyridine, and NMP. Silica gel (SIGMA), 10 g and 5 g amounts, 15-40 µm particle size, and 60 Å average pore size were weighed separately and heated in a vacuum oven at 200 °C overnight, cooled, and stored. The sample (1 g) as a slurry (4 wt %) in acetonitrile was added to 10 g of silica gel and excess solvent removed by rotary evaporation under vacuum. The coated silica was added to a column (20 cm height × 3 cm i.d.) already containing the other 5 g of the cleaned silica gel. Acetonitrile (75 mL) was added to the column and allowed to elute under gravity; a subsequent volume (75 mL) was eluted using vacuum. Air was drawn through the column to dry residual solvent before pyridine was added. Two volumes (75 mL each) of pyridine were used, one with gravity elution and the second under (29) Herod, A. A.; Kandiyoti, R. J. Chromatogr. A 1995, 708, 143. (30) Herod, A. A.; Kandiyoti, R. J. Planar Chromatogr. 1996, 9, 16.

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vacuum; the column was dried before NMP was added, with vacuum elution (150 mL). After elution, sample remained on the silica gel and it was necessary to add the gel to boiling NMP to remove more of the sample. Mass balances for the samples were as follows: Petrox: 25.1% mobile in acetonitrile, 63.7% mobile in pyridine, 1.2% mobile in NMP, sum 90%; “Sample 1”: 22.8% mobile in acetonitrile, 67% mobile in pyridine, 2.1% mobile in NMP, sum 92%. Size Exclusion Chromatography. Procedures for size exclusion chromatography have been described previously.20,30-34 SEC using NMP as solvent was carried out using a 5-µm particle size polystyrene/polydivinylbenzene column (“MixedD”; Polymer Laboratories Ltd., Shropshire, U.K.). The porosity range of this column is proprietary information but such that polystyrene MM-standards from 100 u up to 300 000 u are retained by the column and elute with a linear relation between log MM and elution volume or time. Larger MM polystyrene standards up to 2 × 106 u elute at shorter times with a different relation between MM and time and are classed as excluded from the column porosity; a calibration graph has been shown elsewhere.5,6 The residue samples and the fractions recovered from chromatographic separation have been examined by SEC with UV-absorbance detection at 280, 300, 350, 370, and 450 nm at a temperature of 80 °C and a flow rate of 0.5 mL min-1. The detectors were an Applied Biosciences Diode Array detector (supplied by Perkin-Elmer, Beaconsfield, U.K.) with a Perkin-Elmer LC290 variable wavelength detector in series linked to a computer-based data acquisition system. Evidence presented elsewhere5,6,34 has shown that the polystyrene calibration of the column is a good indicator of MM ranges of coal-derived materials, at least for the material retained by the column. Because NMP is not a good solvent for aliphatics and the detectors are UV-absorbance, only the aromatic portion of the present samples can be examined. Samples and standards were dissolved by mixing with NMP, shaking, and holding in an ultrasonic bath for up to 60 min to ensure solution of the highest molecular mass materials; insolubles were removed by filtration using 6 µm filters. In the present work, residues of petroleum have been examined using an SEC column (Mixed-D) of greater separation range than the Mixed-E column, partly because the kerogen extracts were examined using the same Mixed-D column. Also, recent evidence has indicated that the high mass limit of the Mixed-E column has lost some of the high mass material of coal-derived samples, presumably because they were too large for the void volume of the Mixed-E column.35 UV-Fluorescence Spectroscopy. The procedure has been described in detail elsewhere.36 The Perkin-Elmer LS50 luminescence spectrometer was set to scan at 240 nm min-1 with a slit width of 2.5 nm; synchronous spectra were acquired at a constant wavelength difference of 20 nm. A quartz cell with 1 cm path length was used. The spectrometer featured automatic correction for changes in source intensity as a function of wavelength. Emission, excitation, and synchronous spectra of the samples were obtained in NMP; only the latter have been shown. Due to the inevitable uncertainty in the concentrations of samples recovered from the silica gel, the spectra have been presented in peak-normalized mode. Solu(31) Herod, A. A.; Johnson, B. R.; Bartle, K. D.; Carter, D. M.; Cocksedge, M. J.; Domin, M.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1995, 9, 1446. (32) Lazaro, M-J.; Herod, A. A.; Cocksedge, M. J.; Domin, M.; Kandiyoti, R. Fuel 1997, 76, 1225. (33) Herod, A. A.; Zhang, S-F.; Kandiyoti, R.; Johnson, B. R.; Bartle, K. D. Energy Fuels 1996, 10, 743. (34) Johnson, B. R.; Bartle, K. D.; Domin, M.; Herod, A. A.; Kandiyoti, R. Fuel 1998, 77, 933. (35) Begon, V.; Suelves, I.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. In preparation. (36) Li, C-Z.; Wu, F.; Cai, H-Y.; Kandiyoti, R. Energy Fuels 1994, 8, 1039.

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Figure 1. SEC of the unfractionated petroleum vacuum residues in NMP, Mixed-D column (a) Petrox residue, (b) “Sample 1”; curves and wavelengths: 1-280 nm, 2-300 nm, 3-350 nm, 4-370 nm, 5-450 nm; exclusion limit 10.5 min and permeation limit about 23 min. tions were diluted with NMP to avoid self-absorption effects: dilution was increased until the fluorescence signal intensity began to decrease. However, it was necessary to examine the fluorescence from fractions immobile on the plates in relatively concentrated solutions because the fluorescence quantum yields were rather low; in these cases, sample was added until the fluorescence signal was significantly greater than the background fluorescence. MALDI Mass Spectrometry. The instrument used was a Fisons VG TOFSPEC mass spectrometer fitted with a nitrogen UV-laser (337 nm) and a VAX 4000-based data system with OPUS software. The linear TOF was used at an accelerating voltage of 28 kV at maximum laser power with 2-mercaptobenzothiazole (MBT) as matrix. Between 10 and 30 spectra were summed, depending on the stability of the “sweet spot” from which the spectra were generated. Only the whole residues were examined.

Results and Discussion Features of Resid Samples Prior to Fractionation. SEC profiles of the Petrox residue and of “Sample 1” with detectors set at at five UV-absorption wavelengths are shown in Figure 1a and 1b, respectively. The profiles were similar, with intensity of UV absorbance intensities progressively diminishing from 280 to 450 nm; absorbances at 450 nm were very small. The trend suggests the presence of smaller polynuclear aromatic ring systems than ordinarily encountered in coal-derived materials. The material excluded from the column porosity and eluting between 8 and 11 min shows some differences, with a greater proportion of the Petrox residue appear-

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Figure 2. Synchronous UV-fluorescence spectra of the unfractionated Sample 1 (curve 1) and Petrox (curve 2) residues. Spectra are height normalized.

ing in this region compared to the chromatograms of “Sample 1”. It may be noted that the SEC column used in the present study had a different operating range, compared to the column used in previous work.14 The proportion of excluded material observed when using the coarser (5 µm) particle size “Mixed-D” column (Polymer Labs Ltd.) was considerably smaller than that observed when using the finer grained (3 µm) Mixed-E column. The mass range of this material can only be defined in comparison to the behavior of the polystyrene standards used as calibrants.5 The exclusion limit at about 11 min would correspond to a polystyrene mass of about 2 × 106 u while polystyrene of mass 2 × 10 u eluted at about 9 min. The large material of the samples behaves in elution time in a manner similar to polystyrenes of mass between 2 × 105and 2 × 106 u, but we have not established that the largest molecules of the petroleum residues have these masses. We believe they are large molecules and not aggregates of small molecules because of the difficulty of getting then into solution and because of the evidence presented below in a discussion on aggregates. Within the retained region (15 to 22 min), the position of the maximum shifted to shorter elution times with increasing detection wavelength, indicating that the larger aromatic clusters which absorb at longer wavelengths are found in larger molecules, on average, than those which absorb at 280 nm. However, UV-fluorescence spectra of the two resids (Figure 2) showed peaks at similar wavelengths, suggesting that structural differences inferred from SEC were likely to be quantitative rather than qualitative, with possibly similar types of aromatic structures in both samples. We have noted elsewhere5 that the column capacity for elution times shorter than the exclusion limit appears to be much less than in the retained region and in the present work, the solutions examined were deliberately diluted to avoid overloading the excluded region. As observed in the chromatograms of Figure 1, dilution of the samples appears to lead to observing no significant signal between the excluded and retained regions, suggesting that the excluded material may be significantly different from the retained material. The SEC profiles of the whole samples show a range of material in the retained region which corresponds to a mass range between 100 and 10 000 u in comparison with polystyrene standards. The MALDI mass spectrum

Figure 3. MALDI-mass spectra of the unfractionated vacuum residues using MBT matrix (a) the Petrox residue and (b) “Sample 1” residue.

would not be expected to generate signal for the excluded material, if it was of the mass indicated by the polystyrene calibration and of the order of millions of mass units, since the spectrometer has an upper limit of m/z 400 000. In addition, the problem of detecting ions near the upper limit of the mass scale in the presence of a greater quantity of smaller masses, means that the high mass ions would not be visible. The MALDI mass spectra of the two vacuum residues are shown in Figure 3 up to m/z 20 000; both spectra appear to have reached the local baseline by m/z 15 000. Previous work with “Sample 1” and MALDI-MS using R-cyano-3-hydroxycinnamic acid and [2-(4-hydroxyphenylazo)]benzoic acid as matrixes showed similar profiles,14 with some evidence of residual signal at m/z 10 000 possibly extending to m/z 20 000; however, this low intensity signal would not conflict seriously with the indication by SEC calibration of an upper limit to the retained material of about 15 000. More work is needed to obtain spectra from both the whole residues and the less mobile/soluble fractions. SEC Characterization of Resid Fractions. As outlined above, the two residue samples were fractionated using solvent separation, planar and column chromatography. Figure 4 presents area-normalized SEC chromatograms of samples obtained by fractionation of the Petrox sample; for simplicity, only the 350 nm chromatograms have been shown. Analogous diagrams for the separated fractions of ‘Sample 1” have been presented in Figure 5.

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Figure 4. SEC profiles at 350 nm, area-normalized, of the fractions of the Petrox residue (a) TLC fractions A immobile in pyridine, B mobile in pyridine, and C mobile in acetonitrile and pyridine; (b) TLC fractions D immobile in THF, E mobile in THF, and F mobile in toluene and THF; (c) column chromatography fractions soluble in NMP (curve 1), soluble in pyridine (curve 2), and soluble in acetonitrile (curve 3); (d) heptane-insolubles (curve 1) and -solubles (curve 2).

Figure 5. SEC profiles at 350 nm, area-normalized, of the fractions of the “Sample 1” residue (a) TLC fractions A immobile in pyridine, B mobile in pyridine, and C mobile in acetonitrile and pyridine; (b) TLC fractions D immobile in THF, E mobile in THF, and F mobile in toluene and THF; (c) column chromatography fractions soluble in NMP (curve 1), soluble in pyridine (curve 2), and soluble in acetonitrile (curve 3); (d) heptane-insolubles (curve 1) and -solubles (curve 2).

Figure 4a presents the chromatograms of fractions separated by planar chromatography using pyridine and acetonitrile as solvents, showing a clear shift to shorter elution times (larger apparent MMs) with decreasing mobility on the chromatographic plate. Figure 4b shows the analogous profiles for the planar chromatographic separation using toluene and THF. Results from the two

separations differ sharply: although the retained peaks (15-22 min) in Figure 4a show a clear shift to longer times (smaller apparent MMs) with increasing mobility on the plate, the separation by toluene and THF appear to have given rise to fractions with similar peak structuresswith only peak intensities changing with changes in PC-mobility; the three fractions were found

Structural Characterization of Petroleum Residues

to cover the same elution range in the retained region (similar to profile 3 in Figure 1a). While no excluded material could be observed in the toluene mobile fraction (Fraction F; Figure 4b), some was apparent in the THF mobile fraction (Fraction E), but most of the excluded material remained in the fraction immobile in THF (Fraction D). Figure 4c presents SEC (350 nm) chromatograms of samples prepared by the column chromatography fractionation of the Petrox sample. Comparing with samples from planar chromatographic separation, the material mobile in NMP appears to contain extremely large molecules, probably corresponding to the earliest eluting part of the excluded material in Figure 4a. This material constitutes less than 1.2% of the Petrox residue; we would not expect to observe its presence in the SEC of the resid itself (Figure 1a) because of its low concentration in the un-fractionated sample. Of the NMP mobile (heaviest) fraction obtained in column chromatography, even the part resolved by the SEC column appears to contain material eluting at shorter times compared to fractions from the PC-separation. The fraction eluted in pyridine (in column chromatography) showed material excluded from the SEC column porosity at somewhat longer elution times than the pyridine-mobile fraction in the planar chromatographic separation. The differences observed between planar and column chromatography when using pyridine and acetonitrile as solvents appear to reflect the different silica gels used in each case. The PC-plate was used as received, apart from solvent washing; the silica used in the separation column was heat activated, but eluents were not dried and the activation of the column was probably degraded by water in the solvents. Figure 4d presents the SEC chromatograms of the heptane-soluble/-insoluble fractions of the Petrox sample, obtained using a fractionation method usual for separating oils (plus “resins”) from asphaltenes. In comparing the quality of the separation with the previous two methods, PC-separation using toluene and THF appears to be the least efficient and will not be discussed further. It may be observed that the peaks due to material excluded from column porosity in Figure 4a and 4d both occurred at around 9 min. Similarly the retained (resolved) peak for the heptane-insolubles in Figure 4d and the retained peak for the pyridine-immobile Fraction A and the pyridine-mobile (but acetonitrile-immobile) Fraction B in Figure 4a all occurred at around 17-18 min. By contrast, a far sharper separation appears to have been obtained by column chromatography, with significant differentiation between the three fractions in molecular mass ranges of the resolved peaks. The significant concentration of material with shorter elution times in the NMP mobile fraction opens new prospects for the structural characterization of the most intractable materials in these complex mixtures. Analogous data for “Sample 1” (Figure 1b and Figure 5a-d) show largely similar results. The increased proportion of excluded material of fraction A compared with fraction B (Figure 5a) is similar to the increased proportion of the excluded material of the NMP mobile compared with that of the pyridine mobile fractions in Figure 5c. The separation of the three fractions in column chromatography did not

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appear to be as sharp as in the case of the Petrox sample. This may be observed from the similarities between the relative positions of the retained material peaks of fractions A, B,l and C in Figure 5a and those of Figure 5c; the main difference was the absence of excluded material in the acetonitrile-solubles (Figure 5c) compared with Fraction C from planar chromatography (Figure 5a). As in the case of the Petrox residue, the planar chromatographic fractions obtained by using THF and toluene fractions (Figure 5b) showed little variation in base width or intensity maximum in the retained region. However, in the excluded region (8-11 min), fractions E and F both showed detectable signal while in the Petrox fractions (Figure 4b) only a small peak was evident for fraction E and no signal was observed for fraction F. It seems that the THF and toluene system is not as effective in planar chromatography in separating vacuum residues as the acetonitrile and pyridine system. Figure 5d presents area normalized SEC profiles for the heptane-soluble and -insoluble fractions of “Sample 1”. The heptane-solubles showed very little material under the excluded peak while the excluded peak for heptane-insolubles was similar to that for the pyridinemobile fraction in Figure 5c. Relative intensities of the profiles differ from those in Figure 4d for the Petrox fractionation but the similarities of the fractionations are evident. The SEC results for the different sets of fractions are consistent in that the fractions insoluble or immobile in a particular solvent show an increase in molecular size compared with the soluble or mobile fraction. This behavior is expected for a molecular size distribution of a polymer such as polystyrene where increasing insolubility in a solvent follows increasing molecular mass. In the present samples, the changes in molecular structure with increasing size or molecular mass are not known and may include an increase of polarity, but the trends observed in Figures 4 and 5 follow the trends expected for samples of very wide molecular mass range. Characterization of the Resid Fractions by UVFluorescence. Synchronous UV-fluorescence spectra of the Petrox fractions and those for “Sample 1” have been presented in Figure 6 and Figure 7, respectively. A brief overview on the interpretation of UV-fluorescence spectra of coal-derived liquids has been presented elsewhere.37 The criteria used for the evaluation of UV-F spectra for the present samples remain largely unchanged. Considering first fractions separated from the Petrox residue by the different methods: the UV-fluorescence spectra for the least mobile fractions from the two PC separations (i.e., toluene/THF and acetonitrile/pyridine) and that from column chromatography, namely Fractions A, D and the NMP-solubles (Figures 6a-c, respectively) all showed similar shifts to longer wavelengths with a prominent peak at around 470 nm in each profile and with a tail-off of intensity toward 700 nm. As expected, the sequences of curves shown in Figures 6a-c were found to recede toward shorter wavelengths, following the order of mobility of the samples. This (37) Begon, V.; Megaritis, A.; Lazaro, M.-J.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Fuel 1998, 77, 1261-1272.

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Figure 6. Synchronous UV-fluorescence spectra of the Petrox residue fractions; overlaid spectra are height normalized. (a) A, B, and C from TLC in acetonitrile and pyridine; (b) D, E, and F from TLC in THF and toluene; (c) column chromatography fractions mobile in NMP (curve 1), pyridine (curve 2), and acetonitrile (curve 3); (d) heptane-insolubles (curve 1) and -solubles (curve 2).

Figure 7. Synchronous UV-fluorescence spectra of the “Sample 1” residue fractions; overlaid spectra are height normalized. (a) A, B, and C from TLC in acetonitrile and pyridine; (b) D, E, and F from TLC in THF and toluene; (c) column chromatography fractions mobile in NMP (curve 1), pyridine (curve 2), and acetonitrile (curve 3); (d) heptane-insolubles (curve 1) and -solubles (curve 2).

ordering strongly suggests a diminishing sequence of polynuclear aromatic ring system sizes with increasing mobility of the sample. Figure 6d shows the spectra for heptane-insolubles with a peak at 470 nm, followed by a tailing off of intensity toward 700 nm, similar to samples from planar and column chromatography. The peak at 470 nm was not visible in the spectrum of the whole sample, Figure 2. The minor peak at 400 nm observed in the spectrum of the heptane-soluble fraction was also detected in all the fractions separated by planar chromatography and

column chromatography. One common feature of the spectra in Figures 6a-d is the occurrence of signal at short wavelengths (ca. 300 nm) from all fractions except the heptane-insolubles. Analogous UV-F data for “Sample 1” (Figures 7a-d) largely display features similar to the Petrox fractions. It may be noted that the sequence of spectra shown in ref 14 for this sample (ref 14, Figure 5f) is different from the one presented here in Figure 7a (acetonitrile/ pyridine). Care was taken to confirm the present findings; the internal consistency of results presented in

Structural Characterization of Petroleum Residues

Figures 3-7 of the present work strongly suggest that these results show the correct sequence. Once again, the peak observed at 400 nm in the “whole” sample was observed in all fractions except the heptane-insolubles while all fractions except the heptane-insolubles showed signal at shorter wavelengths. The maximum intensity peak of the heptane-insolubles, 470 nm, was also observed in the intractable fractions, A, D and NMP mobiles but only as a minor peak on the tailing-off of intensity toward 700 nm. Our data thus show that the three fractionation methods agree reasonably well in producing fractions which concentrate the excluded material into the increasingly immobile/insoluble fractions, but that heptane solubility may be better at isolating the heaviest material from petroleum distillation residues. It appears from these results that the separation of petroleum residue components with smaller/larger sized PNA ring systems was best achieved using separation by heptane solubility rather than fractionation on silica using acetonitrile, pyridine, toluene, THF, and NMP. This finding contrasts with our observations from the fractionation of coal tar pitch.4 The Problem of Aggregates. A major problem remaining is the question of whether the early eluting signal in SEC consists of aggregates of small molecules or actually corresponds to large molecules. The evidence presented here favors large molecules rather than aggregates for the following reasons. (a) In NMP solution, standard polycyclic aromatic compounds behave as distinct molecules, in terms of elution relative to polystyrene standards, irrespective of their polarity or other structural features. (b) The MALDI mass spectra of the material retained (i.e., resolved) by the SEC column indicate a similar range of masses as the polystyrene calibration. The available evidence suggests that the laser desorption of highly polar known molecules such as proteins and peptides, as well as polycyclic aromatic compounds produces mainly the molecule ion rather than cluster ions, which show only relatively low intensities. (c) The synchronous UV-F spectra indicate that the larger MM-materials are structurally different from the smaller MM-fractions; increasing the concentration of solute for small standard aromatics, a move favoring agglomeration, reduces the fluorescence intensity rather than shifting the maximum. Therefore the red-shifted spectra of Figures 6 and 7 for the increasingly immobile or insoluble fractions indicate an increase in size of the aromatic system, rather than an increase of agglomeration. (d) In previous work, fractions collected from the SEC column were re-injected as a dilute solution; the observation of an unchanged elution time5 with no production of later eluting (apparently smaller MM) material, indicates that increased dilution did not produce any level of dis-aggregation. Also, filtration of a soot to isolate material >20 nm, followed by re-solution and refiltration,26 produced no