Comparisons between Open Column Chromatography and HPLC

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Comparisons between Open Column Chromatography and HPLC SARA Fractionations in Petroleum C. A. Islas-Flores,† E. Buenrostro-Gonzalez,† and C. Lira-Galeana*,‡ Branches of Molecular Engineering Research and of R&D on Deep Water E&P, Mexican Institute of Petroleum, Eje Central Lazaro Cardenas No. 152, Col. San Bartolo Atepehuacan C.P. 07730, Mexico D.F., Mexico Received August 19, 2004

Two chromatographic methods for the SAR (saturates, aromatics, and resins) separation of crude oil samples are compared. Open column chromatography (OPC) using silica and subsequent elution with heptane, toluene, and toluene/methanol is contrasted with flow reversal (backflushing) high-performance liquid chromatography (HPLC) using an amino-modified silica column with heptane, dichloromethane, and chloroform as elution solvents. The fractions obtained in both methods have been characterized with gel permeation chromatography-THF and FTIR. It is demonstrated that the OPC system is unable to effectively separate the aromatic nonpolar hydrocarbons in crude oil from the resin fraction. The OPC toluene mobile fraction corresponds mostly to resin-type material, but some nonpolar aromatics are present. The toluene/methanol mobile fraction from OPC corresponds only to the most polar part of the resin fraction. Different sample:silica mass ratios from 1:4 to 1:20 were tested for the OPC system. Similar separation was obtained with the different sample:silica mass ratios. Quantitative amounts of resins can be obtained by the 1:4 system by mixing the fraction eluted with toluene and that eluted with toluene/ methanol. Although the HPLC method for SAR gives a sharper separation between the resin and the nonpolar aromatic fractions, for a more expedited separation of large quantities of resins the OPC method can be used. Nonetheless, the material eluting with toluene has to be considered part of the resin fraction.

Introduction Asphaltenes in petroleum are prone to cause several technical difficulties in oil production, refining, and transportation. The formation of thermal coke, the deactivation of catalysts, and the plugging of production pipes1-3 are some operating problems caused by asphaltenes in such processes. Asphaltenes and resins are the petroleum components with the higher aromaticity, polarity, molecular weight, and heteroatom content. The rest of the oil is composed of the nonpolar saturated and aromatic fractions with lower molecular weight.1 Asphaltenes are defined as the crude oil fraction insoluble after addition of n-alkanes (n-pentane, n-heptane, etc).1 Resins are defined as the fraction retained by polar solids such as silica or alumina that elutes with mixtures of polar solvents.1,4,5 * To whom correspondence should be addressed. E-mail: clira@ imp.mx. Phone: +52 (55) 91756507. Fax: +52 (55) 91757225. † Branch of Molecular Engineering Research. ‡ Branch of R&D on Deep Water E&P. (1) Andersen, S. I. and Speight, J. G. Pet. Sci. Technol. 2001 19 (1, 2), 1-34. (2) Garcıá-Hernańdez, F. Ingenierıá Petrolera; 1989. (3) Cha´vez-Alcara´z, J. L.; Lory-Mendoza, A. Revista del Instituto Mexicano del Petro´ leo, XXIII, Enero-Marzo; 1991. (4) Acevedo, S., Escobar, G., Ranaudo, M. A., Pinãte, J., and Amorıń, A. Energy Fuels 1997, 11, 774-778. (5) Leon, O.; Contreras, E.; Rogel, E.; Damblaki, G.; Espidel, J.; Acevedo, S. Energy Fuels 2001, 15, 1028-1032.

Resins are known to maintain the colloidal stability of crude oil by dispersing the asphaltene fraction in the rest of the nonpolar crude oil constituents. Although the mechanisms causing these interactions seem to be known,1,6 little is known about the impact they have on the overall stability of oil.6 Several authors have proposed the use of model compounds to understand these interactions,5,7,8 but the use of synthetic molecules in such approximations may not be accurate in reproducing the behavior of natural resins as inhibitors.9,10 A number of recent works have been dedicated to the systematic study of resins as precipitation inhibitors,4,9,11-14 and some properties of resins have been proposed as effective to correlate their inhibition behavior. Nonetheless, almost all of these studies have (6) Murgich, J. Pet. Sci. Technol. 2002, 20 (9,10), 983-997. (7) Wiehe, I. A.; Jermansen, T. G. Pet. Sci. Technol. 2003, 21, 527536. (8) Chang, C.-L.; Fogler, H. S. Langmuir 1994, 10, 1749-1757. (9) Gonza´les, G.; Guilherme, B. M.; Neves, S. M.; Saraiva, S. M.; Lucas, E. F.; Dos Anjos de Souza, S. Energy Fuels 2003, 17, 879-886. (10) Porte, G.; Zhou, H.; Lazzeri, V. Langmuir 2003, 19, 40-47. (11) Carnahan, N. F.; Salager, J.-L.; Anton, R.; Davila, A. Energy Fuels 1999, 13, 309-314. (12) Christy, A. A.; Dahl, B.; Kvalheim, O. M. Fuel 1989, 68, 430435. (13) Leon, O.; Contreras, E.; Rogel, E.; Dambakli, G.; Acevedo, S.; Carbognani, L.; Espidel, J. Langmuir 2002, 18, 5106-5112. (14) Sjo¨blom, J.; Sæther, Ø.; Midttun, Ø.; Ese, M.; Urdhal, O.; Førdedal, H. Asphaltene and resin stabilized crude oil emulsions. Structures and Dynamics of Asphaltenes; Plenum: New York, 1998.

10.1021/ef050148+ CCC: $30.25 © 2005 American Chemical Society Published on Web 07/21/2005

Comparing SAR Fractionations in Petroleum

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Figure 1. HPLC SAR chromatograms of two KU deasphalted crude oil samples with a difference of one week between the two fractionations. B ) back-flushing.

Figure 2. GPC chromatograms of HPLC SAR heptane mobile fractions (saturates and aromatics) and chloroform/DCM mobile fractions (resins) of KU deasphalted crude oil.

Table 1. Saturate, Aromatic, Resin, and Asphaltene (SARA) Analysis and API Grade of a KU Petroleum Sample Used in the Present Study

Table 2. Average Molecular Weights (Mn and Mw), Molecular Weight at Maximum Intensity (Mp), and Polydispersity Index (Q)

property/sample saturates aromatics resins asphaltenes °API

KU 18.5% w/w 31.9% w/w 37.9% w/w 11.7% w/w 19

reported the behavior of whole resin samples. It is known that the fractionation of crude oil into asphaltenes, resins, and nonpolar oils only produces broad fractions with average behaviors,1,15-17 and that most of the available characterization techniques tend to provide information on the properties of more abundant material/fractions of complex mixtures. Therefore, the fractionation of resins and asphaltenes is needed to obtain a more complete inventory of structural features,18-21 and as a consequence an improved knowledge of interactions between them. The first step in the search for a better understanding of the chemical phenomenon that maintains asphaltene colloidal stability is the selection of a suitable separation method to obtain resins in quantitative amounts. Chromatography has been widely used to characterize and fractionate petroleum and its derivatives.1,15,22,23 Highperformance liquid chromatography (HPLC) and open column chromatography can be used to separate crude oils into hydrocarbon-type fractions (i.e., saturates, aromatics, and resins).1,15,22-24 However, the complexity of the samples and the lack of an aromatic/resinselective column make impossible a clear distinction (15) Speight, J. G. The chemistry and Technology of Petroleum; Marcel Dekker: New York, 1999. (16) Mullins, O. C.; Sheu, E. Y. Structures and Dynamics of Asphaltenes; Plenum Press: New York, 1998. (17) Sheu, E. Y.; Mullins, O. C. Asphaltenes: Fundamentals and Applications; Plenum Press: New York, 1996. (18) Lazaro, M. J.; Domin, M.; Herod, A. A.; Kandiyoti, R. J. Chromatogr., A 1999, 108-115. (19) 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, 171. (20) Herod, A. A.; Kandiyoti, R. J. Chromatogr., A 1995, 143-160. (21) Suelves, I.; Islas, C. A.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2001, 15, 429-437. (22) Altget, K. H.; Jewell, D. M.; Latham, D. R.; Selucky, M. L. Separation schemes. Chromatography in Petroleum Analysis; Marcel Dekker: New York, 1979; pp 185-214. (23) Barman, B. N.; Cebolla, V. L.; Membrado, L. Crit. Rev. Anal. Chem. 2000, 30 (2,3), 75-120.

Mn

Mw

Mp

Q

HPLC C7 mobile fractions

sample

269 274

814 825

755 762

3.0 3.0

HPLC resins

307 276

1939 1965

1789 1599

6.3 7.1

OPC C7 mobile fractions

179 175

540 526

507 494

3.0 3.0

OPC toluene mobile fractions

472 471

1522 1519

1268 1265

3.2 3.2

OPC toluene/methanol mobile fractions

614 614

2171 2178

2220 2166

3.5 3.5

between the aromatic and resin fractions, and systemdependent fractions are usually obtained.24,25 The use of alternative methods for resin separation include precipitation with propane,25 but resins separated using this method only correspond to part of the total resin. Therefore, two basic problems have to be addressed at this point: (1) the lack of a specific separation obtained with open column systems and (2) the small amounts obtained using HPLC. The use of HPLC with aminomodified silica columns has been reported as a suitable and fast method for the separation of deasphalted crudes into well-defined nonpolar hydrocarbons and resins.24,26 Preparative HPLC can be used to fractionate complex hydrocarbon mixtures;21,27 nevertheless, the amounts separated using this technique, even in the case of preparative HPLC, are very small. In this paper we report the results of a comparative study of two chromatographic methods for separating the resin fraction of deasphalted crude oils. The open column chromatography (OPC) method was applied using three different sample:silica ratios to establish the effect of this ratio on the quality of the separation. The fractions obtained by this method were characterized and compared with those obtained by the HPLC method to assess the differences between both methods. It is shown that the OPC method has limitations to ac(24) Aske, Narve, Kallevik, Harald, and Sjo¨blom, J. Energy Fuels 2001, 15 (5), 1304-1312. (25) Goual, L.; Firoozabadi, A. AIChE J. 2002, 48, 2646. (26) Bollet, C.; Escalier, J. C.; Souteyrand, C.; Caude, M.; Rosset, R. J. Chromatogr. 1981, 289. (27) Lazaro, M.-J.; Islas, C. A.; Herod, A. A.; Kandiyoti, R. Energy Fuels 1999, 13, 1212-1222.

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Figure 3. FTIR spectra of HPLC saturate and aromatic (a, top) n-heptane mobile fractions and the resin (b, bottom) chloroform/ DCM mobile fractions.

complish a good separation of the resins from the nonpolar aromatics in comparison to the HPLC method. However, the OPC method could be useful as a preparative method to obtain bulk quantities of resins in a short period of time for further analyses. Experimental Section Samples. The crude oil sample was provided by PEMEX E&P. The sample corresponds to a Ku-Maloob-Zaap Mayan crude oil (KU). Table 1 shows the properties of the whole crude oil sample. The crude sample was deasphalted following the precipitation procedure using n-heptane reported by Buenrostro-Gonzalez et al.;28 briefly, the whole crude sample is mixed with n-heptane on a 40:1 (mL of heptane:g of oil) basis, shaken for 3 h, and allowed to rest for 16 h. The slurry of crude in n-heptane was then filtered using a vacuum through a (28) Buenrostro-Gonzalez, E.; Andersen, S. I.; Garcia-Martinez, J. A.; Lira-Galeana, C. Energy Fuels 2002, 16, 732-741.

0.45 µm membrane. The precipitate was washed with heptane several times until no color was observed in the washing solvent. The heptane-soluble fraction of the crude oil was the deasphalted sample used in the present study. Reagents. HPLC-grade heptane and chloroform were provided by Aldrich, HPLC-grade toluene (Tol), acetone, dichloromethane (DCM), and cyclohexane were provided by Burdick and Jackson, HPLC-grade methanol (MeOH) was provided by Mallinckrodt, and HPLC-grade tetrahydrofuran (THF) was provided by J. T. Baker. Silica gel 60 for chromatography, with a 70-230 mesh filter, was provided by Merck. Instruments and Methods. Open Column Chromatography. The silica gel was activated overnight at 120 °C under vacuum. A 1 g sample of deasphalted oil was weighed and dissolved in 40 mL of DCM. The solution was then placed in a 500 mL round-bottom flask with the activated silica gel. Three sample:silica mass ratios were prepared: 1:4, 1:10, and 1:20. The slurry was then placed in a rotary evaporator to remove the DCM and to adsorb the sample onto the silica in a homogeneous way. A 5 g sample of clean activated silica was



Table 3. FTIR Spectral Analyses of the HPLC SAR and OPC Fractions functional groups NH, OH stretch CH stretch, aliphatic CH3 CH2 carbonyl stretch of aldehyde ketone and H-bonded acid conjugated CdC and aromatic CdC C-CH3 and methylene, asymmetric C-CH3, asymmetric HC-N (possible) sulfoxides aromatic bending H-C one H atom on ring two adjacent H atoms on ring four adjacent H atoms on ring

OPC toluene CC toluene/methanol wavenumber HPLC heptane HPLC OPC heptane mobile fractions interval (cm-1) mobile fractions resins mobile fractions mobile fractions 3500-3200 2950-2850

1.00

0.15 1.00

1.00

0.17 1.00

0.21 1.00

2920-2830 1720-1680

0.87 0.13

0.85 0.20

0.92 0.07

0.91 0.34

0.90 0.35

1600 1465 1375 1285 1030 870

0.07 0.58 0.38 0.14 0.14 0.07

0.22 0.61 0.42 0.34 0.35 0.17

0.08 0.62 0.42 0.14 0.11 0.08

0.29 0.71 0.53 0.31 0.30 0.17

0.34 0.68 0.50 0.30 0.43 0.09

810 743

0.11 0.14

0.28 0.19

0.12 0.16

0.22 0.21

0.11 0.16

placed in a 40 × 3 cm glass column. The silica adsorbed with the sample was then placed at the top of the clean silica into the glass column. A 1 g sample of clean activated silica gel was then placed on top of the adsorbed silica to act as a buffer for the solvent flow. The three silica layers were carefully prepared to have horizontal ends at the bottom and at the top. The top of the column was gently filled with heptane. Approximately 300 mL of heptane was passed through the column by gravity until no color was observed in the effluent from the column, and the heptane mobile fractions were collected. Heptane was passed until the top clean silica was dry at the top, and then the top of the column was gently filled with toluene. The heptane mobile fractions were collected until the toluene front reached the bottom of the column. The collection flask was then changed, and the toluene mobile fractions were collected. Approximately 300 mL of toluene was passed though the column, until no color was observed in the effluent. The same procedure was followed for a 30:70 v/v solution of toluene/methanol. A 300 mL sample of the mixture was passed through the column. The remaining silica was recovered and dried overnight at 70 °C under vacuum. This silica was allowed to calcinate (air atmosphere, 400 °C, overnight), but no quantitative residues were measured. HPLC Saturates, Aromatics, and Resins (SAR). This fractionation method uses a Waters 600 multisolvent delivery system, a Waters 717 plus autosampler, a Vichi Valco Instruments six-way automated valve for flow reversal or backflushing, and a Waters 996 photodiode array detector. The column used was an amino-modified silica Waters Spehrisorb Si0NH2 20 mm × 250 mm preparative column with a flow rate of 8 mL/min. A 200 µL sample of the deasphalted oil was injected into a heptane flow. The heptane flow was kept for 31 min, and the effluent from the column was collected in a 2 L flask. The solvent was then changed to DCM, and the system was back-flushed, i.e., the direction of the flow was

Figure 4. HPLC SAR chromatograms of OPC-1:4 KU mobile fractions in n-heptane, in toluene, and in toluene/methanol.

inverted, to recover the resin fraction (the polar fraction) trapped in the inlet of the column in a complete and faster way. The DCM flow was kept for 4 min, and then the solvent was changed to chloroform. The chloroform flow was kept for 3 min. The DCM and chloroform mobile fractions were collected in the same flask as a single resin fraction. The system was then regenerated by slowly changing the solvent from chloroform to heptane, and the back-flushing was stopped; i.e., the flow was changed to the normal direction. The application of the HPLC SAR method to characterize the chemical composition of crude oils and related material in general is based on the combination of bonded phase columns that prevent irreversible adsorption of the polar fractions, and a sequence of two kinds of solvents: the first is a nonpolar solvent (generally hexane or heptane) that elutes the nonpolar hydrocarbon fractions of the sample, i.e., saturates and aromatics, and the second is a polar solvent (generally dichloromethane, chloroform, or trichloromethane) that elutes the polar fraction or resins. The reversal flow or back-flushing technique is applied to reduce the analysis time, avoiding the spreading of the resin fraction along the column. This method is well suited when the separation of the saturated, aromatic, and resin fractions from a sample has to be achieved in a minimum time and with high reproducibility.24,26 HPLC-Cyclohexane/DCM/Acetone Analysis. The heptane mobile fractions and the resins obtained by the HPLC SAR method as well as the toluene and toluene/MeOH mobile fractions obtained from the OPC method were analyzed using a Waters amino-modified silica Spherisorb SiONH2 4.7 mm × 300 mm analytical column, with a flow rate of 2 mL/min and an injection volume of 30 µL. The samples were injected into DCM solutions. The solvent sequence used in the present study was as follows: Cyclohexane was passed through for 3.5 min, and the solvent was changed to 50% cyclohexane, 50% DCM and kept for 3.5 min. This was followed by a change to 100% DCM, which was maintained for 3.5 min. The solvent was then changed to 100% acetone and kept for 3.5 min. The solvent was finally changed to DCM and slowly changed to cyclohexane for 2 min. Gel Permeation Chromatography (GPC). The chromatographic system is the same that was described before, except for the type of column and the reversal flow valve that is not required in GPC. In this case the column used was a Polystyrene Waters Styragel HR 4E THF, 7.8 mm × 300 mm. The flow rate of the eluent (THF) was fixed in 1 mL/min. The column was calibrated using a set of polystyrene standards from 111000 to 376 Da with a correlation coefficient of 0.9979 (data not shown). Transmittance FTIR. The instrument used was a Nicolet 710 spectrophotometer operated at a setting of 32 scans at a resolution of 4 cm-1. The samples were prepared with the filmspreading technique.

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Figure 5. Flowchart of chromatographic separation and analysis by HPLC SAR of the OPC fractions.

Figure 6. HPLC SAR chromatograms of the OPC-1:4 KU mobile fraction in n-heptane at 250 and 350 nm.

Figure 7. HPLC SAR chromatograms of the OPC-1:4 KU mobile fraction in toluene at 250 and 350 nm.

Results and Discussion Figure 1 shows two chromatograms for the fractionation of the KU deasphalted crude oil sample obtained by the HPLC SAR method. The chromatograms correspond to two different injections. Both chromatograms have very small differences, which certifies the reproducibility of the fractionation method. Two well-defined peak populations are observed. The first population corresponds to the heptane mobile fractions that include the nonpolar hydrocarbons of the deasphalted crude oil, i.e., saturated and aromatic fractions. The second population, which follows the sharp cut observed at around 31 min (which is marked with B and is due to the back-flushing), corresponds to mobile fractions in DCM and chloroform, i.e., the resin fractions. The molecular weight distributions of the two fractions obtained from HPLC SAR and the three fractions obtained by OPC were determined by GPC. Figure 2 shows the GPC chromatograms of the fractions obtained

Figure 8. HPLC SAR chromatograms of the OPC-1:4 KU mobile fraction in toluene/methanol at 250 and 350 nm.

from HPLC SAR. The results of two separate analyses are tabulated in Table 2. The table shows the number average molecular weight (Mn), the weight average molecular weight (Mw), the molecular weight at maximum intensity (Mp), and the polydispersity index (Q) calculated for the fractions obtained from both techniques. The resin fraction exhibits higher average molecular weights (Mn ≈ 300, Mw ≈ 1940) and molecular weight at maximum intensity (Mp ≈ 1600) in comparison with the saturated and aromatic fractions (HPLC SAR heptane mobiles) with Mn ≈ 270, Mw ≈ 810, and Mp ≈ 760. These results followed the expected trend; the resin fractions have compounds with bigger molecular weights and wider structural differences among them. It is important to mention that average molecular weights obtained by the GPC-THF technique for this type of sample cannot be considered as the real molecular weights of the samples, because the polystyrene standards used for the calibration curve have limitations to represent the elution behavior of the complex polyaromatic hydrocarbon samples.27 Nevertheless, this technique can be used to study trends. The IR analyses of the heptane mobile fractions and the resin fraction from HPLC of the KU crude oil are shown in parts a and b, respectively, of Figure 3. In Table 3 the intensity analysis of these spectra is shown in the third and fourth columns. The basis for the normalization was the minimum percent transmittance of the band between 2950 and 2850 cm-1. As expected, the two fractions have very different chemical structures. The spectrum of the heptane mobile fractions shows a very low conjugation of aromatic rings (band at 1600 cm-1 very suppressed). It also shows a high content of C-CH3 bands at 1465 and 1375 cm-1, which together with the low intensity of protons attached to aromatic rings (870, 810, and 743 cm-1) and the high



Figure 9. HPLC SAR chromatograms of OPC KU mobile fractions in n-heptane obtained with 1:4, 1:10, and 1:20 sample:silica ratios.

Figure 10. HPLC SAR chromatograms of OPC KU mobile fractions in toluene obtained with 1:4, 1:10, and 1:20 sample: silica ratios.

Figure 11. HPLC SAR chromatograms of OPC KU mobile fractions in toluene/methanol with 1:4, 1:10, and 1:20 sample: silica ratios.

values of aliphatic compounds, indicates the presence of small aromatic ring clusters with alkyl tails in aromatic substitution. An important feature that can be observed is the presence of an oxygen-type compound band at 1720-1680 cm-1. This band is an indication that some heteroatom content, possibly naphthenic acids, passes through the column even with heptane as the mobile phase. The resins show the presence of nitrogen-type groups (bands at 3500-3200 cm-1, N-H,

Figure 12. GPC chromatograms of OPC n-heptane, toluene, and toluene/methanol mobile fractions of a KU deasphalted crude oil sample.

and 1285 cm-1, HC-N) and a higher content of oxygen species (bands at 3500-3200 cm-1, O-H, and 17201680 cm-1, CdO). The strong band at 1600 cm-1 indicates the presence of polyaromatic compounds in the resin fraction. The intensity of the bands at 870, 810, and 743 cm-1 in comparison to those observed in the spectrum of the HPCL SAR heptane mobile fractions indicates a lower substitution degree in the resin fraction. According to Yen and Erdman,29 the predominance of these three bands indicates kata-condensated polyaromatic structures. The comparison between the two spectra indicates the higher degree of condensation of the aromatic compounds and a higher content of heteroatoms (polar compounds) in the resin fraction. To get an idea of the distribution of the resins and the nonpolar hydrocarbons among the fractions obtained by the OPC method (as described in the Experimental Section), and to be able to compare this method with the HPLC SAR method, the three fractions obtained with a 1:4 sample:silica ratio were analyzed using HPLC SAR. Figure 4 shows that HPLC SAR chromatograms of the heptane mobile fraction from OPC correspond to the material that elutes with the heptane mobile frac(29) Yen, T. F.; Erdeman, J. G. ACS Meeting Preprints; Division of Petroleum Chemistry, American Chemical Society: Washington, DC, 1962.

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Figure 13. FTIR spectra of OPC (a, top) n-heptane, (b, middle) toluene, and (c, bottom) toluene/methanol mobile fractions of a deasphalted KU crude oil sample.

tion in HPLC SAR, i.e., saturated and nonpolar aromatic compounds. The toluene mobile fraction in OPC corresponds to a mixture of polar material like the resin fraction separated by HPLC SAR with a small amount

of nonpolar hydrocarbons, i.e., aromatic compounds. This implies that the toluene mobile fraction from OPC is mostly composed of polar compounds. The toluene/ methanol mobile fraction from OPC corresponds exclu-



Figure 14. HPLC-cyclohexane/DCM/acetone analyses of the OPC and HPLC SAR n-heptane mobile fraction of a deasphalted KU crude oil sample.

sively to resins as expected. Figure 5 depicts this analysis. Figures 6-8 show the chromatograms of the three fractions from OPC shown in Figure 4 at two different wavelengths. The smaller intensity at higher wavelengths in Figure 6 indicates that the heptane mobile fraction corresponds mostly to aromatic compounds with a single aromatic ring.1 The peak at around 39 min indicates a small contamination of resin-like material in this fraction sample. Figure 7 shows again that the toluene mobile fraction from OPC corresponds mostly to resin-like compounds, with single aromatic ring and polyaromatic compounds. Finally, Figure 8 shows that the mobile fraction in toluene/methanol is composed only of resins and that, according to the similar intensities at different wavelengths, the compounds in this sample have a wide spectrum of compounds with aromatic clusters of different sizes. As stated in the Introduction, one of the purposes of these studies was to establish a fractionation scheme to obtain larger amounts of fractions in a single run. To establish this method, different sample-to-silica ratios were used in the obtention of the fractions in the OPC system. Figures 9-11 show the results of the analyses for the three fractions from OPC with different sample-to-silica ratios. The differences observed between the chromatograms for each sample-to-silica ratio are not too big. These results allow us to conclude that the silica can allocate a large amount of sample during the fractionation and a bulk separation method can be used safely at least up to a 1:4 sample:silica ratio. Figure 12 shows the GPC-THF analyses of a set of OPC fractions separated with a 1:10 sample:silica ratio. The average molecular weights and the peak at maximum intensity of these analyses are presented in Table 2. It can be observed that the most polar fraction (toluene/methanol mobile fraction) exhibits higher average and maximum peak molecular weights, following the expected trend; i.e., the most polar are also the larger components in the petroleum derivatives.21 The FTIR analyses of the OPC fractions are presented in Figure 13; the band intensity analyses are shown in Table 3. Figure 13a depicts the FTIR spectra of the

mobile fraction in heptane. Similarly to the heptane mobile fractions in HPLC SAR, this spectrum shows a small content of heteroatomic compounds (bands at 3500-3200, 1720-1680, 1285, and 1030 cm-1) and a low aromatic conjugation (band at 1600 cm-1). The low content of aromatic-attached protons (bands at 870, 810, and 743 cm-1) and the high aliphatic content indicate that this fraction is mainly composed of saturates and short alkyl and some naphthenic compounds attached to single aromatic ring compounds. The spectra of the OPC toluene and toluene/methanol mobile fractions are depicted in parts b and c, respectively, of Figure 13. These two spectra show only small differences, indicating the similarity between these two fractions. The content of heteroatomic compounds (bands at 35003200, 1720-1680, 1285, and 1030 cm-1) is high in both spectra with a higher content of sulfoxides in the mobile fraction in toluene/methanol. Both fractions show aromatic conjugation (band at 1600 cm-1), but it is a little higher in the toluene/methanol mobile fractions. The bands at 870, 810, and 743 cm-1 are noticeable in these fractions. However, for the toluene mobile fractions the predominant bands are those corresponding to two and four adjacent protons, whereas the predominant band in the toluene/methanol fraction is that for four adjacent protons. Both fractions seem to have kata-condensed aromatic structures,29 but the toluene mobile fractions are probably more substituted. These results indicate that the mobile fraction in toluene/methanol is higher in aromatic conjugation and sulfur and naphthenic content. To further study the behavior of the two fractionation methods, the fractions obtained with OPC and HPLC SAR were characterized by the HPLC-cyclohexane/ DCM/acetone analysis, which uses a series of increasingpolarity eluents, as described in the Experimental Section, to obtain fractions of different polarities. Figure 14 shows the results of these analyses for the heptane mobile fractions from HPLC SAR and OPC. The first peak corresponds to the nonpolar material that is eluted using cyclohexane. Both fractions contain some apparently polar material eluting at 7.5 with the mixture 50% DCM/50% cyclohexane, but the proportion of polar

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The toluene/methanol mobile fraction shows a higher intensity of the fourth peak, supression of the fifth peak, and a total lack of the first three peaks, indicating the higher polarity of this fraction. Conclusions

Figure 15. HPLC-cyclohexane/DCM/acetone analyses of OPC toluene, OPC toluene/methanol, and HPLC dichloromethane/DCM mobile fractions of a deasphalted KU crude oil sample.

material is larger for the OPC fraction. Almost no material eluted at higher times, indicating the lack of more polar compounds. Figure 15 shows the same analyses for the resins obtained via HPLC SAR and for the toluene and toluene/methanol mobile fractions from OPC. The chromatogram of the resin sample from HPLC SAR shows five well-defined peaks with increasing elution times. The OPC toluene mobile fraction shows the same peaks observed with the HPLC fraction, just in different proportions. The toluene mobile fraction shows predominance of the first and the second peaks in comparison with the HPLC resins; also the fourth and fifth peaks are suppressed for the toluene mobile fraction. This indicates that some fractionation of the resins is already achieved during the OPC separation.

The traditional analytical separation system for petroleum using activated silica and n-heptane, toluene, and toluene/methanol as eluting solvents fails to separate the aromatic fractions from the resins with low polarity. A considerable amount of aromatic polar compounds, which are usually considered part of the resin fraction are carried by toluene together with nonpolar aromatics. The toluene/methanol mobile fraction from OPC corresponds only to the most polar part of the resin fraction. A certain degree of fractionation of the resin fraction is achieved by OPC. No remarkable differences in the OPC behavior were measured between 1:20 and 1:4 sample:silica ratios during the separation of the resin fraction. This allowed us to define an OPC method that separates quantitative amounts of the whole resin fraction from deasphalted crude oil by mixing the toluene and the toluene/methanol mobile fractions. This method can be used to separate enough resin fractions for further study. Acknowledgment. This research was supported by the Branch of Molecular Engineering Research of the Mexican Institute of Petroleum (IMP) (Project No. FIES 98-86-I). C.A.I.-F. thanks the IMP for a fellowship. We also thank Miss V. Aguilar-Iris from IMP for valuable IR spectral measurements. EF050148+

Comparisons between Open Column Chromatography and HPLC

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