Structural Change of Petroleum Asphaltenes and Resins by

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Energy & Fuels 2000, 14, 980-985

Structural Change of Petroleum Asphaltenes and Resins by Hydrodemetallization Hiroyuki Seki* and Fumio Kumata Advanced Catalysts Research Laboratory, Petroleum Energy Center, KSP R&D 1237, 3-2-1, Sakado, Takatsu-ku, Kawasaki, Kanagawa, 213-0012 Japan Received January 19, 2000. Revised Manuscript Received June 8, 2000

Structural changes of Kuwait asphaltenes and resins by hydrodemetallization (HDM) reaction were investigated with laser desorption mass spectrometry, 1H and 13C NMR, and elemental analyses. Molecular weight (MW) for both asphaltenes and resins gradually decreased through HDM reaction. But their MW distributions (MWD) were different from each other. Namely, the MWD of resins became narrow while that of asphaltenes became polydispersed with an increase of HDM temperature. It was found that a steep increase of aromaticity around 400 °C was due to the shortening of alkyl side chains and the decrease of those numbers for resins and asphaltenes, respectively. For asphaltenes, the increase in internal quarternary aromatic carbons and the decrease in external ones were observed up to 410 °C, which indicates the shift of the aromatic skeleton in asphaltenes toward peri-type.

Introduction Many studies with regard to the structure of petroleum asphaltenes have been carried out for several decades.1-10 However, asphaltenes are mixture of a number of compounds and consequently their chemical structure has not been well understood yet. Recent articles have reported that average molecular weight (MW) of asphaltenes are from 700 to 1000 and that their polycondensed aromatic skeleton contains 5-10 benzene rings.7-10 The average MW is an important parameter on molecular size. To determine the average MW of asphaltenes, gel permeation chromatography (GPC), size exclusion chromatography (SEC), and vapor pressure osmometry (VPO) have been, in general, employed. The physical nature (colloidal properties) of asphaltenes, however, often disturbs the determination of average MW with these methods. For example, the average MW measured with GPC or SEC depends on the measuring conditions such as temperature, solvents, and column, ranging from several hundred to over 10 thousand.11-13 * Author to whom correspondence should be addressed. Tel: +8144-812-7487. Fax: +81-44-812-7488. E-mail: [email protected]. (1) Yen, T. F.; Wu, W. H.; Chilingar, G. V. Energy Sources 1984, 7, 203-235. (2) Speight, J. G.; Wernick, D. L.; Gould, K. A.; Overfield, R. E.; Rao, B. M. L.; Savage, D. W. Rev. Inst. Pet. 1985, 40, 51-61. (3) Nali, M.; Calemma, V.; Montanari, L. Org. Mass Spectrom. 1994, 29, 607-614. (4) Storm, D. A.; Sheu, E. Y.; Detar, M. M. Fuel 1993, 72, 977-981. (5) Artok, L.; Su, Y.; Hirose, Y.; Hosokawa, M.; Murata, S.; Nomura, M. Energy Fuels 1993, 7, 287-296. (6) Andersen, S. V. Fuel Sci. Technol. Int. 1995, 13, 579-604. (7) Miller, J. T.; Fisher, R. B.; Thiyagarajan, P.; Winans, R. E.; Hunt, J. E. Energy Fuels 1998, 12, 1290-1298. (8) Calemma, V.; Iwanski, P.; Nali, M.; Scotti, R.; Montanari, L. Energy Fuels 1995, 9, 225-230. (9) El-Mohamed, S.; Achard, M. F.; Hardouin, F.; Gasparoux, H. Fuel 1986, 65, 1501-1504. (10) Groenzin, H.; Mullins, O. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1999, 44, 728-732.

Wiehe14 reported that VPO measurement with o-dichlorobenzene at 130 °C was reliable because asphaltene micelles disassociated under such conditions. On the other hand, laser desorption mass spectrometry (LD-MS) has recently been applied to obtain MW of asphaltenes,5,7,15 bitumen,16 and coal extracts,17 because the interference due to colloidal properties of asphaltenes is negligible. Domin and co-workers16 demonstrated that VPO and LD-MS were qualitatively consistent and more reliable than other methods. Lazaro et al.17 investigated the effect of instrument-related parameters such as ion-exchange voltage and laser power on mass spectra of coal-derived liquids and pointed out that higher laser power was required to detect high MW species. Thus, LD-MS could be the most suitable and reliable technique to determine the average MW of asphaltenes if measuring conditions are optimized. From the standpoint of an efficient refinery operation, the catalyst life in resid hydrotreating is a key issue. There are two main causes for the catalyst deactivations metal deposits and coke formation.18-20 It is well-known that asphaltenes are coke precursors and deactivate the catalysts in resid hydrotreatings. To examine the change (11) Moschopedis, S. E.; Speight, J. G. Fuel 1976, 55, 187-192. (12) Nomura, M.; Kidena, K.; Murata, S.; Su, Y.; Artok, L. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1999, 44, 755-758. (13) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker Inc.: New York, 1991; Chapter 11. (14) Wiehe, I. A. Ind. Eng. Chem. Res. 1992, 31, 530-536. (15) Yang, M. G.; Eser, S. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1999, 44, 768-772. (16) Domin, M.; Herod, A.; Kandiyoti, R.; Larsen, J. W.; Lazaro, M.J.; Li, S.; Rahimi, P. Energy Fuels 1999, 13, 552-557. (17) Lazaro, M.-J.; Herod, A. A.; Cocksedge, M.; Domin, M.; Kandiyoti, R. Fuel 1997, 76, 1225-1233. (18) Thakur, D. S.; Thomas, M. G. Appl. Catal. 1985, 15, 197-225. (19) Quann, R. J.; Ware, R. A.; Hung, C.-W.; Wei, J. Adv. Chem. Eng. 1988, 14, 95-259. (20) Tamm, P. W.; Harnsberger, H. F.; Bridge, A. G. Ind. Eng. Chem. Process Des. Dev. 1981, 20, 262-273.

10.1021/ef000009m CCC: $19.00 © 2000 American Chemical Society Published on Web 07/25/2000

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Table 1. Properties of KW-AR density, g/cm3 @15 °C viscosity, mm2/s @50 °C CCR, wt % sulfur, wt % nitrogen, wt ppm Ni, wt ppm V, wt ppm

0.9898 2410 12.1 4.45 2710 20 61

of asphaltene structure by hydrotreatings, therefore, is significant to understand the mechanism of catalyst deactivation by coke. In our previous study,21 we examined the effect of hydrodemetallization (HDM) operating conditions such as temperature and LHSV on the life of hydrodesulfurization (HDS) catalysts in the two-stage process (HDM/ HDS). Consequently, we found the following: (1) the HDS catalysts were deactivated mainly by coke, (2) asphaltene quality rather than quantity played a key role in coke deactivation, (3) asphaltene aromaticity was closely correlated to the catalyst deactivation, and (4) effect of substances soluble in light gas oil (LGO), socalled soft coke, on the catalyst deactivation was not negligible under certain HDM reaction conditions. Thus, it is interesting to examine how asphaltene structures (quality) are changed by HDM reaction. In the present study, average structural parameters of asphaltenes were determined, especially paying attention to the aromatic skeleton, to understand the structural change and to obtain basic data for the determination of a certain parameter which reflects the deactivation of HDS catalysts. In addition, resins were also characterized since some parts of resins might act as the soft coke. Through such characterization, we obtained new information on the change in the shape of polycondensed aromatic skeleton. Experimental Section Kuwait atmospheric residue (KW-AR) was used as feed of HDM reaction and the properties are summarized in Table 1. HDM catalysts, which contain MoO3 (4.2 wt %) on alumina support, are commercially available and supplied by a Japanese catalyst company. Their surface area and pore volume measured with a N2 adsorption-desorption method are 200 m2/g and 0.85 mL/g, respectively. The pore diameter of HDM catalysts is distributed from 6.0 to 40.0 nm showing a single peak at 13.0 nm. The HDM catalysts were used in the form of extrudate (1/16 in.). The HDM reactions were performed with 300 mL of the catalysts in a bench-scale fixed bed reactor. Pre-sulfiding treatment to activate the HDM catalysts was conducted at 300 °C for 24 h with di-tert-butyl-disulfide spiked in LGO (total S ) 3.0 wt %). To prepare several HDM product oils, the HDM temperature was varied from 370 to 430 °C. The HDM product oils were collected after the HDM catalysts were stabilized at each HDM temperature. Other reaction conditions were constant: pressure was 14 MPa, LHSV was 0.5 h-1, and H2/ oil was 2000 scfb. The separation method of asphaltenes and resins is described in Figure 1. The bottom fraction (380 °C+) of HDM product oils (10 g) and n-heptane (300 mL) were refluxed for 2 h, followed by cooling to room temperature. Then, the mixture was filtered using a membrane filter with a pore diameter of 0.8 µm. The residue was thoroughly washed with n-heptane in a Soxhlet apparatus and thereafter Soxhletextracted with toluene to separate it into solubles (asphaltene) (21) Seki, H.; Kumata, F. Stud. Surf. Sci. Cat. 1999, 126, 357-364.

Figure 1. Separation procedure of asphaltenes and resins. Table 2. Assignments of 1H and NMR chemical shift range (ppm from TMS) 1H

NMR 0.5-4.5 6.0-9.5 13C NMR 14.6-16.9, 8.5-21.0 110-129 123-126 128-136 137-150 150-165

13C

Chemical Shifts in

assignment aliphatic hydrogen aromatic hydrogen methyl carbon attached to aromatic ring unsubstituted aromatic carbon internal quarternary aromatic carbon external quarternary aromatic carbon alkyl-substituted aromatic carbon (CH3 excluded) carbons attached to heteroatoms

and insolubles (sludge). Resins were separated from the n-heptane solubles (maltene) by column chromatography as final elution materials. LD-MS measurements were carried out on a Thermoquest Co., Ltd. Vision 2000 Spectrometer using angiotensin as a calibration standard. Samples were dissolved in toluene (1 and 10 mg/mL for asphaltenes and resins, respectively) and 1 µL of the solution was dropped on the laser target, then dried in air. The LD-MS spectrum of one sample was obtained as an average of 20 spectra at different spots. From our experience, there exists a range of laser power in which almost the same average molecular weight (MW) and MW distribution can be obtained: if the laser power is low, high MW materials are hardly detectable; if high, decomposition of the sample takes place. The suitable laser power, therefore, was selected in such a range. We did not use matrix since the addition of matrix, for example 2,5-dihydroxylbenzoic acid, showed no effect on the MW and MW distribution. 1H and 13C NMR spectra were recorded with a JEOL JNM LA400. For 1H NMR measurements, measuring solution was prepared by dissolving 60 mg of samples in 6 mL of deuteriochloroform, and the measurement was repeated 32 times. For 13C NMR measurements, relaxation agent, chromium trisacetylacetonate (Cr(acac)3, 0.02 M) was added to obtain quantitative 13C NMR spectra. The measurements were performed for 20000 scans in a gated proton decoupled mode. The assignments of chemical shift range for 1H and 13C NMR spectra were made according to the literature22-25 and are listed in Table 2. Elemental analyses of samples were conducted on a Yanako MT-5 Analyzer with a combustion method. Structural parameters for asphaltenes and resins were determined with the data from LD-MS, 1H and 13C NMR, and elemental analyses, referring to the method by other workers23-25 (Table 3). (22) Gillet, S.; Rubini, P.; Delpuech, J. J.; Escalier, J. C.; Valentin, P. Fuel 1981, 60, 221-225. (23) Dickinson, E. M. Fuel 1980, 59, 290-294. (24) Rongbao, L.; Zengmin, S.; Bailing, L. Fuel 1988, 67, 565-569. (25) Ali, M. F.; Saleem, M. Arab. J. Sci. Eng. 1994, 19, 319-333.

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Table 3. Determination of Structural Parameters for Average Molecule symbols

definition (determination or calculation)

MW Car Cal Cus Cs

average molecular weight (by LD-MS) total aromatic carbons (by 13C NMR) total aliphatic carbons (by 13C NMR) unsubstituted aromatic carbons (by 1H NMR) alkyl-substituted aromatic carbons except CH3 (by 13C NMR) methyl-substituted aromatic carbons (by 13C NMR) alkyl-substituted aromatic carbons (Csub ) Cs + CCH3) peripheral aromatic carbons (Cp ) Cus + Csub) carbons attached to heteroatoms (by 13C NMR) internal quarternary aromatic carbons (Cint ) 6 + Car - 2Cp) external quarternary aromatic carbons Cext ) Car - (Cp + Ch + Cint)) aromaticity (fa ) Car/(Car + Cal)) aromatic ring number (Ra ) (Car - Cp)/2 + 1) average length of alkyl side chains (n ) Cal/Csub) condensation index (CI ) (Cint + Cext)/Car)

CCH3 Csub Cp Ch Cint Cext fa Ra n CI

Table 4. Catalyst Activities and Fraction Yields for KW-AR and HDM product oils HDM temperature, °C KW-AR catalyst activities, % HDS HDM fractions, wt % 380 °Csaturate aromatic resins Lossa 380 °C+ saturate aromatic resins asphaltenes sludge

370

390

410

430

43.4 63.0

58.2 71.6

70.6 91.4

82.2 99.9

2.7 0.3

3.8 0.4

1.8

3.9

4.6 0.6 0.1 7.7

7.6 1.1 0.2 19.2

46.4 35.6 11.6 1.6

48.5 32.5 9.8 1.1

51.2 27.7 7.1 1.0

48.3 18.4 4.6 0.3 0.3

Figure 2. LD-MS spectra of original and hydrotreated resins.

42.2 39.2 14.4 4.2

a Light fraction evaporated with solvents after column separation.

Results and Discussion Catalyst activities and fraction yields of KW-AR and HDM product oils are listed in Table 4. The degree of HDM was varied from 63 to ca. 100% and an approximately linear correlation was observed between HDM and deasphaltene, in agreement with the general tendency.26,27 The resin yield gradually decreased with an increase of HDM temperature, while asphaltene yield drastically decreased up to 370 °C. It should be noted that sludge formation was significant at 430 °C. Figure 2 shows LD-MS spectra for original and hydrotreated resins. The spectrum of the original resins was broad, tailing to ca. 3500 m/z, and two peaks were observed around 400 and 1200 m/z. The resins treated at 370 °C showed a spectrum similar to the original one. When HDM temperature further increased, the spectra became narrow and the peak shifted to a lower mass. Interestingly, the asphaltenes showed a different tendency with respect to the change of MW distribution (Figure 3). The LD-MS spectra for the original asphaltenes and those treated under 390 °C were broad, while the spectra for the asphaltenes treated above 410 °C (26) Koinuma, Y.; Kushiyama, S.; Kobayashi, S.; Aizawa, R.; Inoue, K.; Shimizu, Y. Sekiyu Gakkaishi 1988, 31, 250-257. (27) Asaoka, S.; Nakata, S.; Shiroto, Y.; Takeuchi, C. Ind. Eng. Chem. Process Des. Dev. 1983, 22, 242-248.

Figure 3. LD-MS spectra of original and hydrotreated asphaltenes.

became polydispersed, depicting two peaks at 600 and 1100 m/z. The cause for the appearance of two peaks at high HDM temperature is not clear. Two possible explanations, however, could be proposed. One is that either of the two peaks is derived from asphaltene fraction newly formed during HDM reactions. Another is that the two peaks are attributable to compounds with one aromatic skeleton and those with two aromatic skeletons, respectively. Figure 4 described the variations of number average MW (Mn) with HDM temperature.

Structural Change of Petroleum Asphaltenes/Resins

Figure 4. Variation of number average molecular weight (Mn) of asphaltenes and resins by HDM reaction.

Figure 5. Variation of aromaticity (fa) of asphaltenes and resins by HDM reaction.

It was observed that the Mn for resins and asphaltenes decreased with HDM temperature. In Figure 5, aromaticities (fa) of resins and asphaltenes are plotted against the HDM temperature. The fa of asphaltenes was almost unchanged up to 400 °C and then steeply increased. Resins showed a similar tendency. These observations suggest that quite different changes in chemical structure of resins and asphaltenes took place above 400 °C. Koinuma et al.26 have reported that cleavage of the alkyl side chain mainly occurred without affecting the aromatic skeleton for asphaltenes below 400 °C. In our previous study,21 we found that the fa of asphaltenes in HDM product oils was a good index for the deactivation of downstream HDS catalysts. It is, therefore, interesting to interpret the variation of fa. Variations of average aromatic ring number (Ra) for asphaltenes and resins are shown in Figure 6. The Ra of asphaltenes was almost constant (about 19 aromatic rings) up to 410 °C, then slightly decreased at 430 °C, suggesting that hydrogenated polycondensed aromatics were converted to other fractions and that those remaining unchanged are still in asphaltene. Although the Ra of resins was considerably low compared to that of asphaltenes, the variation with HDM temperature was similar to that of asphaltenes. These findings indicate that the steep increase of fa is mainly due to the change of the alkyl portion rather than that of aromatic portion.

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Figure 6. Variation of average aromatic ring number (Ra) of asphaltenes and resins by HDM reaction.

Figure 7. Variation of average length of alkyl side chain (n) for asphaltenes and resins by HDM reaction.

Figure 8. Variation of the number of alkyl side chain (Csub) for asphaltenes and resins.

Figures 7 and 8 describe the changes of average length (n) and the number (Csub) of alkyl side chain with HDM temperature, respectively. Resins and asphaltenes behaved in a different manner; for resins, a decrease of n and rather constant Csub were observed. In contrast, asphaltenes showed little change of n and a remarkable decrease of Csub. These observations can lead to a conclusion that the steep increase of fa appearing at 400

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Figure 11. methods.

Variation of Cint determined with different

Figure 9. Variation of internal quarternary aromatic carbons (Cint) for asphaltenes and resins by HDM reaction.

Figure 12. Variation of the shape of aromatic skeleton for asphaltenes and resins by HDM reaction.

Figure 10. Variation of external quarternary aromatic carbons (Cext) for asphaltenes and resins by HDM reaction.

°C results from the shortening of alkyl side chains and the decrease of those number for resins and asphaltenes, respectively. At higher HDM temperature than 400 °C, dehydrogenation of cycloparaffins may partly contribute to the increase of fa. Quarternary aromatic carbons give information on the shape of aromatic skeleton. Internal quarternary aromatic carbons (Cint) calculated with the formula23,25 in Table 3 and external ones (Cext) are plotted against HDM temperature in Figures 9 and 10, respectively. For resins, the Cint monotonically decreased and the Cext remained almost constant. These observations suggest that aromatics of resins are not highly condensed and that hydrogenation of the aromatic ring easily loses the Cint without affecting Cext. According to the classification of aromatics by Yen et al.,1 the aromatic skeleton of resins is rectangular and peropyrene is an example. For asphaltenes, the Cint first increased from 11 to 17 followed by a sudden decrease, meanwhile the Cext showed an opposite tendency, indicating that the aromatics of asphaltenes are quite different from those of

resins. Namely, the aromatics of asphaltenes are highly condensed and peri-type aromatics are at least contained. Furthermore, the above observation clearly implies that the aromatic skeleton in asphaltenes tends to be peri-type polycondensed aromatics by HDM reaction up to 410 °C. It is interesting, combined with the change of Ra, that the shape of the aromatic skeleton in asphaltenes varies by HDM reaction without changing the average aromatic ring number. To confirm the change of polycondensed aromatics in asphaltenes, we further estimated the Cint with an another method; the Cus obtained from 1H NMR was subtracted from the sum of Cus and Cint obtained from 13C NMR. The result is shown in Figure 11. It is obvious that the change of Cint with HDM temperature is independent of the determination methods. The difference in the Cint values probably comes from the over-estimation in the direct determination with NMR; in the 13C NMR spectrum, the range from 110 to 129 ppm (Cus + Cint) is to some extent overlapped with that of Cext (128-136 ppm).22 It could be useful to compare the aromatic skeleton of asphaltenes with model compounds. Figure 12 illustrates a relationship between condensation index (CI) and aromatic carbon number (Car) for model compounds. The CI is a structural parameter influenced by the shape of aromatics as well as the Car and characterizes the feature of the aromatic skeleton and the extent of condensation of aromatic rings.24 As model compounds, two extremely different kinds of aromatics in shape, i.e.,

Structural Change of Petroleum Asphaltenes/Resins

cata-type and peri-type, were selected. Here, cata-type aromatics include those which do not contain any Cint and they all are on the same line in this figure, although only straight cata-type aromatics are drawn. In addition, the change of aromatic skeleton for Arabian light asphaltenes (AL) was also presented as reference.28 It can be observed that asphaltene skeleton for KW shifts toward peri-type, then finally reverts to cata-type with an increase of HDM temperature (direction of arrow). The first change to peri-type for KW could be explained by the difference in hydrogenation reactivity between cata-type and peri-type aromatics; cata-type aromatics are more likely to be hydrogenated to become other fractions, resulting in a relative increase of peri-type aromatics in the remaining asphaltenes. On the other hand, the CI for AL asphaltenes stays almost unchanged, followed by a drop at the highest HDM temperature (420 °C). The insensitivity at lower HDM temperature suggests that AL asphaltenes consist of similar aromatic skeletons in shape. The change to catatype direction at the highest HDM temperature is a common feature for KW and AL asphaltenes. This must be related to the formation of sludge; aromatics rich in peri-type selectively converted to sludge, remaining aromatics rich in cata-type in asphaltenes. In fact, a considerable amount of sludge was observed only at the highest HDS temperature for both KW and AL. For resins, although the change of aromatics in shape was not significant since the aromatic skeleton was much smaller than that of asphaltenes, it seemed to change to rather cata-type by HDM reaction. (28) Seki, H. Unpublished data.

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In our previous study,21 we obtained the result that the fouling rate of HDS catalysts in subsequent process was remarkably increased if the HDM temperature was above 400 °C. Combined with the present results, one can presume that a decrease of steric hindrance by the loss of alkyl chains in asphaltenes at HDM temperature above 400 °C facilitates the asphaltene adsorption on the HDS catalysts, leading to the catalyst deactivation. Furthermore, the decrease of asphaltene size is also considered to partly contribute the HDS catalyst deactivation since the pore size of HDS catalyst is smaller than that of HDM one. Conclusions The structural change of asphaltene and resins by HDM reactions using KW-AR was investigated. From LD-MS measurements, the MW of both asphaltenes and resins decreased with HDM temperature, showing an opposite change in MW distribution. It was found that the increase of aromaticity for resins and asphaltenes was due to the shortening of alkyl side chains and the decrease of those numbers, respectively. Relating to the shape of polycondensed aromatics, the aromatic skeleton in asphaltenes shifted toward peri-type up to 410 °C of HDM temperature. Acknowledgment. This work has been carried out as a research project of Petroleum Energy Center with subsidy of the Ministry of International Trade and Industry. The authors are grateful to Drs. H. Iki and K. Hayasaka of Nippon Mitsubishi Oil Co. Ltd. for their kind help in NMR measurements and to Professor M. Iino of Tohoku University for useful discussion on asphaltene structure. EF000009M