Occlusion Phenomena

Article pubs.acs.org/EF

Experimental Studies on the Adsorption/Occlusion Phenomena Inside the Macromolecular Structures of Asphaltenes Jing Zhao,†,‡,§ Zewen Liao,*,† Anna Chrostowska,‡ Qing Liu,∥ Linye Zhang,∥ Alain Graciaa,§ and Patrice Creux§ †

State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China ‡ Equipe de Chimie Physique, Institut des Sciences Analytiques et de Physicochimie pour l’Environnement et les Matériaux IPREM, UMR CNRS 5254, Université de Pau et des Pays de l’Adour, BP 1155, 64013 Pau Cedex, France § Laboratoire des Fluides Complexes, UMR 5150 TOTAL-CNRS-UPPA, BP 1155, 64013 Pau Cedex, France ∥ Geology Scientific Research Institute, Shengli Oilfield Company Limited, Dongying 257015, People’s Republic of China ABSTRACT: Asphaltenes are the polar macromolecules with complex structures in oils, usually existing as aggregates. Inside the macromolecular structures of asphaltenes, some other fractions can be adsorbed/occluded, and the occluded compounds contain some important geochemical information. However, the adsorption/occlusion phenomena of asphaltenes need verification. Thus, this work was aimed to experimentally study the adsorption/occlusion processes inside the asphaltenes, using the deuterated paraffin n-C20D42 as the target compound under the conditions of high temperature/high pressure, with some chloride salts as additives to probe the mechanism of the adsorption/occlusion inside asphaltenes. The results show that the adsorption/occlusion phenomena take place inside asphaltenes during their thermal evolution. Occlusion inside asphaltenes could occur via two paths, namely, the physical process maybe through the polar interactions among the molecules and the chemical process through condensation or polymerization of the molecules, and then some substantial microporous units could develop to adsorb/occlude other fractions. The temperature and the additives have influences on the occlusion taking place inside asphaltenes. Higher temperatures promoted the pyrolysis of asphaltenes, and the additives play an important role in the properties of adsorption/ occlusion inside asphaltenes.

1. INTRODUCTION Asphaltene is defined as a fraction in oils that is insoluble in saturated hydrocarbon solvents (such as n-heptane) but soluble in aromatic hydrocarbon solvents (such as toluene).1 According to this definition, asphaltene is a complex mixture system whose physical and chemical parameters (such as molecular weight, molecular size, solubility, polarity, and elemental composition) cannot be described by single data.2−7 Some researchers reported that asphaltenes are liable to flocculate in oils to form stable aggregates and even aggregate in good solvents, such as toluene.4,8−10 The fractal-type structure of asphaltene aggregates enable them to adsorb or trap other compounds from oils.11 It has been reported that the occluded compounds are protected from the post-depositional processes occurring in oil reservoirs, owing to the structural nature of asphaltenes. Therefore, they contain some important geochemical information,12−14 which can be applied in the research of oil−oil correlation, oil− source rock correlation, sedimentary environment, and secondary alternation in oil reservoirs. Mild chemical oxidation treatment was applied to release the adsorption/occlusion compounds inside asphalenes.1,15−17 Many steps were required to obtain the occluded fraction from the asphaltenes; the amount of the occluded fraction is very small (usually below 0.5% based on the weight of initial asphaltenes); and some results remain problematic for reasonable explanation. That is why the adsorption/occlusion phenomena inside © 2012 American Chemical Society

asphaltenes are not commonly recognized, and thus, more work is needed concerning this topic. It has been reported that inorganic salts have different influences on the pyrolysis of kerogen, and chloride salts may play a negative effect.18 The divalent cations, such as Ca2+, were reported to increase the adhesion force between silica and bitumen.19,20 This work was designed to experimentally study the adsorption/occlusion processes inside asphaltenes under the conditions of high pressure/high temperature, using asphaltenes from one lowmatured crude oil and n-C20D42 as targeted compounds. Mono-, di-, and trichloride salts were applied as additives during the experiments to evaluate the possible role of metal cations on the adsorption/occlusion inside asphaltenes.

2. EXPERIMENTAL SECTION 2.1. Sample and Chemicals. Crude oil M4 used in this work was collected from the Shengli oil field, northeast China, with the maturity Ro (vitrinite reflectance, the maturity index commonly used in the field of organic geochemistry) below 0.5% and asphaltene content of ca. 10.66 wt %. The oil was mainly derived from the salty lacustrine deposits, with aquatic organisms as the main origin of the organic matter.21 Inorganic chloride salts used in the experiments (with the purity of NaCl, CaCl2, and AlCl3 as 99.99, 99.9, and 99.985%, respectively) Received: November 11, 2011 Revised: February 17, 2012 Published: February 21, 2012 1746

dx.doi.org/10.1021/ef201758g | Energy Fuels 2012, 26, 1746−1755

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Table 1. Gas Yields in Different Thermo Experiments asphaltenes tempreture sample (mg) (°C) 240-A 240-B 240-C 240-D 240-E 240-F 270-A 270-B 270-C 270-D 270-E 270-F 290-A 290-B 290-C 290-D 290-E 290-F a

202.73 200.39 201.13 201.06 202.83 201.39 202.68 202.53 202.92 203.78 202.01 204.01 203.80 202.48 203.57 201.99 203.03 203.87

240 240 240 240 240 240 270 270 270 270 270 270 290 290 290 290 290 290

additives (mg) /a H2O (49.20) NaCl (17.07) + H2O (49.71) CaCl2 (29.76) + H2O (49.52) AlCl3 (38.45) + H2O (49.42) AlCl3 (34.72) / H2O (47.79) NaCl (16.53) + H2O (48.27) CaCl2 (30.16) + H2O (47.55) AlCl3 (38.15) + H2O (48.15) AlCl3 (40.00) / H2O (48.44) NaCl (16.46) + H2O (48.19) CaCl2 (30.38) + H2O (47.13) AlCl3 (37.32) + H2O (47.74) AlCl3 (36.36)

C1 C2 C3 C2−C5 C1−C5 H2 CO2 H2S (mL/g) (mL/g) (mL/g) (mL/g) (mL/g) (mL/g) (mL/g) (mL/g) Nb 0.00 0.00 N 0.00 0.39 0.27 0.18 0.22 0.26 0.98 0.64 0.62 0.55 0.45 0.56 1.29 28.95

N 0.00 0.00 N 0.00 0.35 0.03 0.02 0.02 0.02 0.22 1.03 0.08 0.08 0.06 0.07 0.43 8.72

N 0.00 0.00 N 0.00 4.72 0.02 0.01 0.01 0.01 0.20 11.34 0.04 0.04 0.04 0.04 0.41 2.05

N 0.00 0.00 N 0.00 12.05 0.09 0.03 0.03 0.04 0.57 33.61 0.13 0.13 0.11 0.12 1.20 11.76

N 0.00 0.00 N 0.00 12.44 0.36 0.20 0.25 0.30 1.55 34.25 0.75 0.69 0.56 0.68 2.50 40.71

N 0.00 0.00 N 0.00 0.00 0.00 0.00 0.00 0.00 0.09 0.26 0.03 0.04 0.04 0.04 0.18 0.21

N 0.04 0.00 N 0.00 0.35 0.88 0.92 1.09 0.85 1.23 0.38 1.12 1.40 1.33 1.16 1.32 0.64

N 0.00 0.00 N 0.00 6.44 0.32 0.22 0.29 0.23 3.71 8.10 0.64 0.68 0.69 0.54 5.39 4.87

inorganic gases (mL/g)

total yield (mL/g)

N 0.04 0.00 N 0.00 6.79 1.20 1.14 1.38 1.07 5.04 8.74 1.79 2.12 2.06 1.74 6.90 5.73

N 0.04 0.00 N 0.00 19.23 1.56 1.34 1.63 1.37 6.59 42.99 2.54 2.81 2.62 2.42 9.40 46.44

/ = without additives. bN = not determined.

Figure 1. Experiment flowchart. were purchased from Alfa Aesar China (Tianjin) Co., Ltd., n-C20D42 and n-C24D50 (with isotopic enrichment of D at 99.22% and purity > 98.7%) were purchased from CND Company, Canada. 2.2. Method. Asphaltenes were precipitated from crude oil by n-hexane, and the details can be found from Liao et al.16,17 The nC6-

asphaltenes and n-C20D42 (10:1, w/w) were dissolved by dichloromethane in beakers, and then, the n-C20D42 solution was completely transferred into the beaker containing the asphaltenes with stirring. After dichloromethane completely volatilized, the mixture was ground into powder for the following experiments. 1747


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The experiment was performed employing gold tubes, which have been well-applied for the kinetic study of kerogen or oil pyrolysis.22−25 Around a 220 mg mixture of asphaltenes and n-C20D42 (see above), some specific chlorides and deionized water were added to gold tubes (8 cm × 6 mm inner diameter), according to Table 1 and Figure 1. The loaded tubes were flushed with argon for 10 min to ensure the complete removal of air and then sealed under an argon atmosphere. These gold tubes were enclosed in the stainless-steel pressure vessels, which were connected together to maintain a uniform pressure among them, and then placed in an oven. The pressure was kept at 20 MPa throughout the whole heating process by pumping water in or out of the vessels. The temperature was first raised from room temperature (around 16.5 °C) to the desired point (240, 270, and 290 °C) and then held for a specific time range (72, 48, and 24 h), respectively. The pressure and temperature deviations from the set values are within 5 MPa and 1 °C, respectively, throughout the whole process. The control experiment was carried out at room temperature and atmosphere pressure without any additives, which were processed along with the products from the gold tube thermo experiments.

The products from the experiments were grouped into the gases, liquids, and residues (Figure 1). Gaseous products: After the experiments, the gold tubes were put into a T-shaped apparatus connected with a gas chromatograph instrument. The gas collection was performed by puncturing the gold tube under vacuum. After about 30 s for gas balance, gases in the gold tubes were analyzed and determined by gas chromatography (GC) directly using an external standard method. Liquid products: After the gases were analyzed, the gold tube was cut into pieces and packed by filter paper, first Soxhlet extracted by n-hexane for 48 h and then by acetone for 72 h. The n-hexane and acetone extracts were evaporated and made weight constant, among which n-C20D42 was quantified through a gas chromatography−flame ionization detector (GC−FID) analyzer by adding another deuterated compound n-C24D50 as an internal standard compound. Residues: The acetone-extracted residues were gathered from the gold tubes and, thus, treated by H2O2/CH3COOH to release the occluded compounds inside the asphaltenes,1 among which possibly

Figure 2. Histogram of the gas yield from different thermo experiments (on the basis of the weight of initial asphaltenes; the details of different groups A−F are listed in Table 2).

Figure 3. Histogram of the amount of n-hexane extracts from the pyrolysates from different thermo experiments (on the basis of the weight of initial asphaltenes; the details of different groups A−F are listed in Table 2).

Table 2. Amounts of Different Fractions from Different Thermo Experiments n-hexane extracts asphaltnenes sample (mg) 240-A 240-B 240-C 240-D 240-E 240-F 270-A 270-B 270-C 270-D 270-E 270-F 290-A 290-B 290-C 290-D 290-E 290-F CEc a

202.73 200.39 201.13 201.06 202.83 201.39 202.68 202.53 202.92 203.78 202.01 204.01 203.8 202.48 203.57 201.99 203.03 203.87 178.09

additives (mg) /b H2O (49.20) NaCl (17.07) + H2O (49.71) CaCl2 (29.76) + H2O (49.52) AlCl3 (38.45) + H2O (49.42) AlCl3 (34.72) / H2O (47.79) NaCl (16.53) + H2O (48.27) CaCl2 (30.16) + H2O (47.55) AlCl3 (38.15) + H2O (48.15) AlCl3 (40.00) / H2O (48.44) NaCl (16.46) + H2O (48.19) CaCl2 (30.38) + H2O (47.13) AlCl3 (37.32) + H2O (47.74) AlCl3 (36.36) /

total amount after amount subtraction of n-C20D42 (mg) (mg) 42.66 47.43 51.91 35.60 54.56 70.30 63.54 64.93 64.04 59.77 92.86 64.18 75.72 67.76 72.91 74.61 106.61 88.45 30.72

25.55 30.25 27.91 18.09 35.26 55.36 43.78 42.00 43.65 39.78 72.80 50.14 55.58 47.90 50.84 55.15 86.71 72.34 13.45

acetone extracts

residues

a

percentage after subtraction of n-C20D42 (wt %) 12.60 15.10 13.88 9.00 17.38 27.49 21.60 20.74 21.51 19.52 36.04 24.58 27.27 23.66 24.97 27.30 42.71 35.48 7.55

amount percentagea amount percentagea (mg) (wt %) (mg) (wt %) 4.38 6.25 4.00 5.53 10.78 15.20 3.00 3.00 3.13 4.98 11.80 14.95 3.25 3.38 3.25 4.93 16.78 12.53 5.50

2.16 3.12 1.99 2.75 5.31 7.55 1.48 1.48 1.54 2.44 5.84 7.33 1.59 1.67 1.60 2.44 8.26 6.15 3.09

149.80 167.00 174.10 189.80 184.00 156.20 145.80 148.10 160.80 169.70 110.40 143.30 123.90 119.80 142.70 169.90 97.10 131.40 159.14

73.89 83.34 86.56 94.40 90.72 77.56 71.94 73.12 79.24 83.28 54.65 70.24 60.79 59.17 70.10 84.11 47.83 64.45 89.36

All of the percentages are based on the weight of initial asphaltenes. b/ = without additives. cCE = control experiment. 1748


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present n-C20D42 was quantified through a GC−FID analyzer by adding n-C24D50 as an internal standard. 2.3. Gas Chromatography (GC) and Gas Chromatography− Mass Spectrometry (GC−MS) Analysis. Quantification of gases was performed using an Agilent Technologies 6890N gas chromatograph equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). A Wasson KC5 column (50 m × 0.53 mm inner diameter × 10 μm) was used, with helium as the carrier gas. The interface and GC inlet were set at 300 °C, and the oven temperature was programmed from 70 °C (6 min) to 130 °C at 15 °C/min and then to 180 °C (held for 4 min) at 25 °C/min. Quantification of n-C20D42 in the extracts from the experimental procedures was performed using a Trace GC Ultra gas chromatograph analyzer from Thermo Finnigan Company. A DB-1 column (60 m × 0.32 mm × 0.25 μm) was used, with helium as the carrier gas (1.5 mL/min). The GC oven temperature was programmed from 80 °C (4 min) to 290 °C (held for 20 min) at 4 °C/min. Full-scan GC−MS was performed using a DSQ II MS detector combined with a Trace GC Ultra analyzer. A DB-1 MS column (60 m × 0.32 mm ×0.25 μm) was used, with helium as the carrier gas

(1.5 mL/min). The GC oven temperature was programmed from 80 °C (4 min) to 290 °C (held for 20 min) at 4 °C/min. The mass spectrometer was operated with ionization energy of 70 eV, a source temperature of 155 °C, and a scan range of 50−650 Da.

3. RESULTS AND DISCUSSION 3.1. Gaseous Products from the Thermo Experiments. Table 1 shows the gas yields of each experiment of asphaltenes under different conditions. A higher temperature generally engendered a higher gas yield (Figure 2), which suggests that higher temperatures promoted the thermal cracking of asphaltenes. As the temperature fixed, groups with AlCl3 (groups E and F of Table 1) had a higher gas yield than the others, which suggests that AlCl3 promotes the cracking of asphaltenes, especially true for group F without the addition of H2O. Thermal cracking of asphaltenes is mainly through the radical reaction mechanism; therefore, the reaction was usually

Figure 5. Histogram of the amount of n-C20D42 in n-hexane extracts from the pyrolysates from different thermo experiments (on the basis of the amount of initial n-C20D42; the details of different groups A−F are listed in Table 2, with CE standing for the result of the control experiment).

Figure 4. Histogram of the amount of acetone extracts from the pyrolysates from different thermo experiments (on the basis of the weight of initial asphaltenes; the details of different groups A−F are listed in Table 2).

Table 3. Distribution of n-C20D42 in the Different Fractions n-C20D42 from n-hexane extracts sample 240-A 240-B 240-C 240-D 240-E 240-F 270-A 270-B 270-C 270-D 270-E 270-F 290-A 290-B 290-C 290-D 290-E 290-F CEc a

asphaltenes n-C20D42 (mg) (mg) 202.73 200.39 201.13 201.06 202.83 201.39 202.68 202.53 202.92 203.78 202.01 204.01 203.80 202.48 203.57 201.99 203.03 203.87 178.09

20.98 20.74 20.82 20.81 20.99 20.85 20.88 20.87 20.91 20.99 20.81 21.02 21.00 20.86 20.97 20.81 20.92 21.00 17.91

additives (mg) /b H2O (49.20) NaCl (17.07) + H2O (49.71) CaCl2 (29.76) + H2O (49.52) AlCl3 (38.45) + H2O (49.42) AlCl3 (34.72) / H2O (47.79) NaCl (16.53) + H2O (48.27) CaCl2 (30.16) + H2O (47.55) AlCl3 (38.15) + H2O (48.15) AlCl3 (40.00) / H2O (48.44) NaCl (16.46) + H2O (48.19) CaCl2 (30.38) + H2O (47.13) AlCl3 (37.32) + H2O (47.74) AlCl3 (36.36) /

amount relative content (mg) (wt %) 17.12 17.18 23.99 17.51 19.30 14.94 19.76 22.93 20.39 19.99 20.06 14.04 20.14 19.86 22.07 19.46 19.89 16.12 17.27

81.60 82.84 115.23 84.14 91.95 71.65 94.64 109.87 97.51 95.24 96.40 66.79 95.90 95.21 105.25 93.51 95.08 76.76 96.43

n-C20D42 from acetone extracts a

amount relative (μg) contenta (ppm) 7.06 3.98 8.46 13.08 4.58 7.13 6.88 2.71 3.53 15.97 3.59 18.22 3.22 5.65 3.78 8.48 5.58 8.79 10.38

337 192 406 629 218 342 330 130 169 761 173 867 153 271 180 407 267 419 580

n-C20D42 from oxidation products amount (μg)

relative contenta (ppm)

4.79 4.41 4.96 4.89 0.17 1.05 4.64 3.65 5.75 0.15 0.21 0.17 5.46 5.36 4.85 0.13 0.12 2.66 1.29

228 213 238 235 8 50 222 175 275 7 10 8 260 257 231 6 6 127 72

All of the relative contents are based on the amount of initial n-C20D42. b/ = without additives. cCE = control experiment. 1749


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Figure 6. m/z 66 chromatogram of the n-hexane extracts from the pyrolysates from group 290-F.

At the same temperature, similar amounts of acetone extracts for groups A−C indicated that water and NaCl have little effect on the adsorption of asphaltenes, while a higher amount of acetone extracts was found for group D, which indicated that CaCl2 could promote the adsorption of asphaltenes. This may be due to the bridged effect of Ca2+ between asphaltenes,19,20 which could increase the aggregation degree of asphaltenes to contribute to the adsorption. On the other hand, a distinct increase for the acetone extracts in groups E and F indicated that AlCl3 has a distinct promotion on the adsorption of asphaltenens. The amount of acetone extracts in group F was higher than group E at 240 and 270 °C, whereas the amounts are reversed at 290 °C. Considering the amounts of gases and n-hexane extracts, the degree of asphaltene pyrolysis in group F was still higher than that in group E. 3.3.1. Distribution of n-C20D42 in Different Fractions from the Experiments. Distribution of n-C20D42 in n-Hexane Extracts from Pyrolysates of Asphaltenes. n-C20 D42 in n-hexane extracts from the pyrolysates belonged to the free molecules outside of asphaltenes. Table 3 shows the relative amounts of n-C20D42 (on the basis of its initial amount) in n-hexane extracts from pyrolysates. With the same additives, amounts of n-C20D42 in n-hexane extracts at 240 °C were generally lower compared to the groups at 270 and 290 °C (Figure 5). Some values more than 100% in Figure 5 (C-240, B-270, and C-290) may be caused probably by the coelution of some compounds with n-C20D42. At the same temperature, amounts of n-C20D42 in n-hexane extracts in group F were apparently lower than other groups. Some fragments of n-C20D42 were detected in the chromatogram of the n-hexane extracts from the pyrolysates of group 290-F (Figure 6), suggesting the decomposition of n-C20D42 under this condition, which can account for the low recovery of n-C20D42 in group F. 3.3.2. Distribution of n-C20D42 in Acetone Extracts from the Pyrolysates of Asphaltenes. n-C20D42 in acetone extracts is taken as adsorbed n-C20D42 in asphaltenes.16 The amount of adsorbed n-C20D42 in group D was more than other groups under the same temperature, which may be related to the connection of Ca2+ among asphaltene molecules (Figure 7). There were similar amounts of n-C20D42 in acetone extracts for groups A−C, which were lower than the control experiment (Table 3 and Figure 7). Maybe some compounds produced by asphaltene pyrolysis compete with n-C20D42 during the adsorption, thus leading to their lower amounts compared to the control experiment. It seems that the univalent cation could not

Figure 7. Histogram of the amount of n-C20D42 in acetone extracts from the pyrolysates from different thermo experiments (on the basis of the amount of initial n-C20D42; the details of different groups A−F are listed in Table 2, with CE standing for the result of the control experiment).

depressed by water and chloride salts. However, as a strong Lewis acid, AlCl3 can catalyze the pyrolysis of asphaltenes, especially evident for group F without water (Table 1). 3.2.1. Different Fractions from the Products of Thermo Experiments. n-Hexane Extracts from the Pyrolysates of Thermo Experiments. n-Hexane extracts from the pyrolysates stand for the free maltenes from the experiments. Under the same additive conditions, the amount of n-hexane extracts generally increased via the increasing temperature (Table 2 and Figure 3). The amount of n-hexane extracts in groups B and C were nearly the same as group A, whereas lower for group D. On the other hand, those in groups E and F were much more than extracts in other groups, which suggests that AlCl3 is helpful to the pyrolysis of asphaltenes. In combination with the gas yield and the amount of n-hexane extracts, the pyrolysis degree of asphaltenes in group F was higher than that in group E, and this tendency became more obvious via the increasing temperature. However, NaCl and CaCl2 have no distinct effect on the pyrolysis of asphaltenes. 3.2.2. Acetone Extracts from the Pyrolysates of Thermo Experiments. Acetone extracts were supposed as the adsorbed compounds of asphaltenes.16,17 The relative amounts of acetone extracts from the pyrolysates of thermo experiments (listed in Table 2) decreased generally with the increasing temperature under the same additive conditions (Figure 4). 1750


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Figure 8. Chromatograms of the n-hexane eluents from the oxidation products of pyrolysis residues from group 290-F. The degraded material sample (DMS; same for Figures 10−13) code indicates the n-hexane eluents of DM fractions through a SiO2/Al2O3 column, and DM is the n-hexane solubles of the oxidation products. The numbers in the figure represent the carbon numbers.

suggests that the targeted compound indeed occluded inside asphaltenes during the experiments. The amounts of n-C20D42 occluded inside asphaltenes were generally small (Table 3 and Figure 9). More n-C20D42 was detected in group C, as well as for groups A and B; the low amounts of n-C20D42 occluded in the groups with AlCl3 may be related to its decomposition during the thermo experiments. n-C20D42 was also detected occluded in the control experiment (without the thermal process), whose amount was among the values of thermo experiments (Figure 9). In comparison to the control experiment, the higher amounts of n-C20D42 in groups A−C suggest that a high temperature is helpful for the occlusion of targeted molecules inside asphaltenes. Results from the control experiment suggest that occlusion could take place through the physical process; on the other hand, occlusion during the thermo experiments may be via both physical and chemical processes. However, considering the decomposition of n-C20D42 and the competitive occlusion for other compounds, it is difficult to evaluate the main path of occlusion during the thermo experiments. 3.4. Characteristics of Compounds Occluded Inside Asphaltenes. Figure 10 shows the chromatogram of the hydrocarbons released from the acetone-extracted asphaltenes by H2O2/CH3COOH treatment for the control experiment, which is characterized by a complete series of n-alkanes with an obvious even/odd predominance dominated by n-C16, n-C18, and n-C20 alkanes, and a series of n-alk-1-enes with even-numbered carbons were paired with the corresponding n-alkanes. Some

Figure 9. Histogram of the amount of n-C20D42 in the oxidation products from the pyrolysates of different thermo experiments (on the basis of the amount of initial n-C20D42; the details of different groups A−F are listed in Table 2, with CE standing for the result of the control experiment).

promote the aggregation of asphaltenes, while the multivalent cation could act as a binder to combine asphaltene molecules to make adsorption easier (Figure 7).19,20 3.3.3. Distribution of n-C20D42 in the Oxidized Products of Acetone-Extracted Pyrolysates. n-C20D42 released by H2O2/ CH3COOH treatment belonged to the occluded compounds inside the macromolecular structures of asphaltenes according to the previous reports.1,16,17,26 Detection of n-C20D42 in the oxidized products of the acetone-extracted pyrolysates (Figure 8)

Figure 10. Total ion current (TIC) chromatograms of the hydrocarbons released from residues oxidized by H2O2/CH3COOH in the control experiment. The numbers in the figure represent the carbon number. 1751


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Figure 11. Chromatograms of the hydrocarbons released from residues oxidized by H2O2/CH3COOH in the groups of thermo temperature 240 °C. The numbers in the figure represent the carbon number.

branched hydrocarbons were detected occluded inside asphaltenes for the control experiment, which belonged to the fraction occluded inside asphaltenes at one earlier stage of oil generation. On the other hand, distributions of the compounds occluded inside asphaltenes for the thermo experiments were different from those of the control experiment (Figures 11−13). Similar distributions were found for groups A−C, characterized by n-alkanes with an obvious even/odd predominance. A series of n-alk-1-enes with even-numbered carbons paired with n-alkanes

in groups A and B but are absent in group C. The chlorinated hydrocarbons were readily detected for groups C−F, which were formed with chloride salts during the experiments. Monoethylalkanes were considered as the cyanobacterial input,27 and the even-numbered ethylalkanes were reported originated from eubacterial organisms, possibly cyanobacteria living in fresh or brackish water environments.28 The series of monoethylalkanes with even-numbered carbons detected as the occluded compounds inside asphaltenes verified the point that 1752


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Detection of n-C20D42 in the oxidation products from both the thermo experiments and the control experiment suggests that occlusion could occur by two ways. One is through chemical processes, e.g., polymerization or condensation of the functional groups in the asphaltene molecules, and then micropores developed, inside which some other molecules are trapped. The other is that occlusion can take place through physical processes, possibly by the strong polar−polar interactions, such as hydrogen bonding, between asphaltene molecules, and then, inside the networks, some other compounds are trapped. With respect to the pyrolysis decomposition of targeted compounds and the competitive occlusion for

the crude oil used in this work was mainly derived from aquatic organisms.21 3.5. Possible Mechanism of Adsorption/Occlusion inside Asphaltenes. Detection of n-C20D42 in the acetone extracts and oxidative products from asphaltenes shows that the adsorption/occlusion did occur inside asphaltenes. The adsorption could develop in asphaltenes without any additives during the experiments. CaCl2 and AlCl3 can promote the adsorption inside asphaltenes but not NaCl. It is proposed that the multivalent cations as a binder promote the aggregation of asphaltenes.19,20 1753


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other molecules during the thermo experiments, it is difficult to evaluate the relative importance of each.

inside the macromolecular structures of asphaltenes in oil reservoirs. The addition of multivalent metal chlorides makes adsorption easier to occur, but it is difficult to evaluate its influence on the occlusion inside asphaltenes, because of the decomposition of targeted compounds during the thermo experiments and the competitive occlusion for other molecules. Occlusion inside asphaltenes may proceed via both physical and chemical ways, e.g., through the polar−polar interaction and the polymerization or condensation of the functional groups in asphaltenes. Characteristics of the occluded fraction released

4. CONCLUSION This work experimentally studied the adsorption/occlusion phenomena inside asphaltenes, using the deuterated compound n-C20D42 under the conditions of high temperature/high pressure. Detection of n-C20D42 from the oxidation products of acetone-extracted pyrolysates from all of the thermo experiments indicates that some other molecules can be occluded 1754


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from asphaltenes are distinctly different from the maltenes, which indicated that the occluded compounds belonged to the earlier products from kerogen to oil and then preserved inside asphaltenes, and thus bear important geochemical information. This point is significant for the geochemical studies of oil reservoirs, especially for the oil reservoirs that suffered from serious biodegradation, in which much geochemical information was destroyed; therefore, the occluded compounds inside asphaltenes were important targets for the related studies.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are very grateful to Prof. Jinzhong Liu for technical assistance during the thermo experiments. Prof. Pingan Peng from SKLOG supplied helpful discussion. This work has been financially supported by the National Key Science Projects Program of China (2011ZX05005-001 and 2011ZX05008-002-11).

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Occlusion Phenomena

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