Aquathermolysis of Heavy Crude Oil with Amphiphilic Nickel and Iron

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Aquathermolysis of Heavy Crude Oil with Amphiphilic Nickel and Iron Catalysts Jiqian Wang, Lai Liu, Longli Zhang, and Zhaomin Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef502134p • Publication Date (Web): 17 Nov 2014 Downloaded from http://pubs.acs.org on November 19, 2014

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Aquathermolysis of Heavy Crude Oil with Amphiphilic Nickel and Iron Catalysts Jiqian Wang*, Lai Liu, Longli Zhang, Zhaomin Li State Key Laboratory of Heavy Oil Processing, China University of Petroleum, 66 Changjiang West Road, Qingdao, Shandong 266580, China Corresponding Author *Jiqian Wang, Tel: 86-532-86981130, E-mail: [email protected] Postal address: State Key Laboratory of Heavy Oil Processing, China University of Petroleum, 66 Changjiang West Road, Qingdao, Shandong 266580, China

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Abstract: Two amphiphilic catalysts (i.e. metal dodecylbenzenesulfonates, noted as C12BSNi and C12BSFe) were synthesized and characterized by Fourier Transform infrared spectroscopy (FT-IR), element analysis (EA), atomic absorption spectroscopy (AAS) and thermogravimetric (TGA). Their interfacial activities were determined using a surface tensiometer and an interfacial tensiometer. Both catalysts are interfacial active and thermostable enough for heavy oil aquathermolysis. Their performance on heavy oil aquathermolysis was assessed in an autoclave. According to the viscosity reduction results, the synthesized amphiphilic catalysts are more effective than water soluble or oil soluble catalysts, with C12BSNi more efficient than C12BSFe. The average molecular weight, group compositions, and average molecular structure of heavy oil samples were analyzed using EA, FT-IR, and 1H nuclear magnetic resonance (1H NMR) before and after aquathermolysis reaction. And the results show that both catalysts caused the change of molecular structures in heavy oil. The change of asphaltene and resin molecular structures and decrease of their contents are crucially important to the reduction of viscosity. C12BSNi causes more changes of the asphaltene than C12BSFe, while C12BSFe is beneficial to the breakage of CS bonds in asphlatenes and resins.

Key words: amphiphilic catalyst; Nickel; Iron; aquathermolysis; heavy oil;

1. Introduction Heavy oil is an unconventional petroleum resource with high viscosity and density, and even is solid or semisolid at ambient temperature. However, it will be the main petroleum supply in the near future because of its huge reserves along with the rapidly increasing shortage of energy resources. There remains great challenging for the exploitation and transportation of heavy oil


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due to its high viscosity [1]. Many techniques have been developed to exploit heavy oil reservoirs, such as thermal recovery, chemical recovery, microbial recovery [2-6]. While chemical recovery is based on polymers and surfactants, and microbial recovery is based on microbes which degrade some components in heavy oil and excreted biosurfactants, thermal recovery is based on heat input through steam injection. Technologies based on thermal recovery are more effective and economic than those based on chemical or microbial recovery. The steam “Huff and Puff”, or steam stimulation, is a typical process of thermal recovery, and has been widely adopted and proved to be effective on the production of heavy crude oils. During this process, superheated water or steam is injected into the reservoir to reduce heavy oil viscosity. Hyne et al [7] proposed the thermal cracking of some compounds in heavy oil in the presence of superheated water, and named this process as “aquathermolysis” firstly in the 1980s. They did a series of studies on the cracking reactions of heavy oil components, and found that the cracking of saturate, aromatic, resin and asphaltene was not obvious if only superheated water was used for aquathermolysis [8-13]. Therefore, additives such as catalysts, hydrogen donors, and emulsifying agents are injected into a reservoir together with steam or hot water. Emulsifying agents, usually surfactants, improve the emulsification of water and heavy oil. Hydrogen donors, like cylcohexane, methylcyclohexane, or tetralin, provide active hydrogens which could benefit hydrogenation and desulfurization reactions, and stabilize intermediates during catalytic aquathermolysis. These additives play synergetic roles and catalysts are the key factor in heavy oil aquathermolysis. Inspired by Hyne’s work, many researchers have been studying on heavy oil aquathermolysis to reduce its viscosity [14-21]. Clark et al [10] reported the catalytic activity of metal compounds and sand in heavy oil aquathermolysis. They also used thiophene and tetrahydrothiophene as


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model compounds of heavy oil to study their aquathermolysis in the presence of vanadium and nickel salts [10, 11]. Fan et al [16] systematically studied the effects of minerals on aquathermolysis, and found that both the viscosity and the average molecular weight of heavy crude oil with minerals were reduced in comparison with those without minerals. Hyne et al [7] reported the water-soluble catalyst for heavy oil aquathermolysis. Their results showed that water-soluble nickel and cobalt salts had positive influence on the reduction of crude oil viscosity. Both Hyne’s and Clark’s works demonstrated the catalytic activity of transition metals in heavy crude oil aquathermolysis, especially for the breakage of the C-S bonds of heteroatomic compounds in heavy oil. Wen et al [22] studied the effects of oil-soluble molybdenum oleate on Liaohe heavy oil. They detected CO2, H2S, and C2-C7 light hydrocarbons after reaction, which proved the thermolysis of heavy oils and the decrease of viscosity consequently. Dispersed catalysts, such as nickel, iron, and copper nanoparticles, are also proved to be fairly effective for the aquathermolysis of heavy crude oil, which can drastically reduce the viscosity and degrade the compositions and molecular structure of heavy oil [23-25]. The activity of minerals is quite low, and even negligible when compared with other kinds of catalysts. In addition, it is difficult to inject mineral particles into reservoir in field. Watersoluble or oil-soluble catalysts only dissolve well in one phase during aquathermolysis, which indicates that the catalysis will not be efficient because most of the aquathermolysis reactions take place at the oil/water interface [26]. Therefore, the use of amphiphilic catalysts is expected to improve the catalytic efficiency significantly due to their well distribution at the oil/water interface. Chen et al [19, 20] and Wu et al [27] designed a catalyst composed by a metal cation and an amphiphilic anion. They found the aromatic sulfonic iron and nickel catalysts showed a good performance in heavy oil aquathermolysis according to the reduction of viscosity. Other


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metal cation (Mo6+) also had excellent catalytic activity on heavy crude oil aquathermolysis despite of the different catalytic mechanism. Aromatic sulfonic iron mainly catalyzes the cracking of resin, saturates, and oxygen-containing groups, while aromatic sulfonic molybdenum induces the cracking of asphaltene, aromatic hydrocarbons, and sulfur-containing groups [21, 28]. Katritzky et al studied the reaction of various organic compounds in superheated aqueous media thoroughly and systematicly. These compounds include hydrocarbons, heterocycles, and compounds with oxygen-containing functional groups, nitrogen-containing functional groups, and sulfur-containing funtional groups. They detected the formed products and described the mechnism. Although these works were conducted with pure compounds in a well-defined system, they are very meaningful to understand the chemical mechnism of heavy oil aquathermolysis [15, 29, 30]. It is indicated the property and composition of heavy oil were the essential factor of aquathermolysis activity according to the totally different reaction mechanism of various organic compounds. Although the viscosity reduction ratio could be as high as above 90% in some previous works [19-21, 27], it is noted that the aquathermolysized oil samples were either dewatered by oil-water separation automatically or failed to mention the dewatering process. Water might not be removed completely in these cases and would affect the viscosity measurement. We also notify that the viscosity of these oil samples was usually measured at 50 °C. Since the melting point of microcrystalline wax is usually around 70 °C, the viscosity at 80 °C is much more meaningful for heavy oil. It will help us to focus on asphaltene and resin which are usually regarded as the origin of high viscosity of heavy oil. In this paper, we synthesized two amphiphilic catalysts with different transition metals and investigated their activities on Shengli heavy oil aquathermolysis. The amphiphilic part of the catalysts is dodecylbenzenesulfonic acid (C12BS), and the active


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metal parts is Fe3+, and Ni2+ respectively. The viscosity of dehydrated heavy oil samples was determined in this work to eliminate the effect of dilution and emulsification. And the viscosity was measured at 80 °C besides 50 °C to avoid the influence of wax. The changes of SARA (saturate, aromatic, resin and asphaltene) compositions and average molecular structures were also studied to compare the catalytic mechanism of heavy oil aquathermolysis with these two different amphiphilic catalysts. 2. Experimental 2.1. Synthesis and characterization of amphiphilic catalysts 0.1 mol/L nickel sulfate or ferric sulfate solution was added into a three-neck flask and preheated in an oil bath for 15 min at 60 °C. 1 mol/L NaOH aqueous solution was slowly dropped into the solution under constant stirring to keep the pH value at 11 for 20 min, and then the solution was vacuum filtered. The precipitate was collected and washed three times with water, then put into another three-neck flask with a certain amount of C12BS. The temperature was raised and kept at 100 °C for 4~6 h under constant stirring with a magnetic polytetrafluroethylene bar. Finally the product was extracted by chloroform and dried under vacuum at 80 °C for 24 h. All chemicals except dodecylbenzenesulfonic acid (C12BS) were of analytical grade and bought from Sinopharm Chemical Reagent Co. Ltd. C12BS was purchased from Tokyo Chemical Industry Co. Ltd. (TCI) with the purity of 90%. All reagents were used as received. The element contents of as-synthesized catalysts were determined by element analysis (EA, Elementar Vario EL III) and atomic absorption spectroscopy (AAS, ContrAA 700). The catalysts were also characterized by Fourier Transform infrared spectroscopy (FT-IR, Nicolet 6700) after mixed with KBr and compressed into a transparent disc. Thermal stability of the catalysts was


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determined by thermogravimetric analysis (TGA, Beijing Optical Instruments, WCT-1D) coupled with differential thermal analysis (DTA) with the speed of 10 °C/min up to 600 °C under N2 atmosphere. Surface activity of the catalysts was assessed by a surface tensiometer (Krüss Easydyne) through Wilhelmy plate method; and the interfacial tension between amphiphilic catalyst aqueous solution and heavy oil was also determined with a spinning drop interfacial tensiometer (TX 500C). 2.2. Catalytic aquathermolysis of heavy oil Heavy oil from Shengli oilfield China with viscosity of 8957 mPa·s at 80 °C, and 167372 mPa·s at 50 °C was used in the experiments. All the reaction parameters of aquathermolysis experiments were kept the same except for catalysts. A typical experimental procedure is described as follows. 70 g heavy oil, 30 g water, and 0.2 mol/kg catalyst (active metal mole content to oil mass) were added into a 250 ml autoclave. 0.05 wt% surfactant (sodium dodecyl benzene sulfonate) and 2 wt% hydrogen donor (tetralin) were also added to decrease the interfacial tension, and improve the cracking of heavy oil. The initial pressure in autoclave was kept at 3 MPa by N2 charging, and would rise to about 7-8 MPa with the temperature increasing to 250 °C. The temperature was kept at 250 °C for 24 h. The mixture was taken out of the autoclave, and then dehydrated by distillation in a flask below 150 °C until there was no water dropped from the condenser. Over 90% water was separated during dehydration after aquathermolysis. If the loss during operations and the water trapped in the autoclave pipelines were taken into account, the residue water in oil sample was less than 5%. Then the viscosity of heavy oils was measured through a programmable viscometer (Brookfield DV-III) at 80 °C and 50 °C. The torque was kept at around 50% during measurement. Most of the aquathermolysis


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experiments were repeated at least 2 times, and the data were averaged. The viscosity reduction ratio (∆η) of heavy oil is defined as below. ∆η =

η -η × 100% η 0

(1)

0

∆η, η0, and η are viscosity reduction rate, viscosity before reaction, and viscosity after reaction, respectively.

2.3. Oil sample analysis The SARA group composition of heavy oil samples was determined through separation with an alumina column chromatography. The average molecular weight of oil samples was determined through the vapor pressure osmometry (VPO) method in toluene. The element analyses of resin and asphaltene were conducted on an elemental analyzer (Elementar Vario EL III). FT-IR spectra of resin and asphaltene were collected by daubing the sample in CCl4 solution on a KBr disk to form a thin film with proper thickness after the solvent evaporation. 1H NMR spectra of resin and asphaltene were recorded on a Bruker ARX400 spectrometer. CDCl3 was used as the solvent and TMS as the internal chemical shift standard.

3. Results and discussion 3.1. Characterization of the amphiphilic catalysts The FT-IR spectra of C12BS and two synthesized amphiphilic catalysts with active metal of Ni2+ and Fe3+ are shown in Fig 1. The peak at 907 cm-1 of C12BS is assigned to the stretching vibration of sulfate O-H, and the peaks at 1363, 1177, and 1129cm-1 are assigned to the stretching vibration of sulfate S=O [20]. In spectra of catalysts, peak at 907 cm-1 disappeared, and the position of peaks at 1400-1000cm-1 also slightly changed. The peaks at 1177 and 1363cm-1 shifted to higher wavenumber due to the inductive effect of metallic atom.


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Table 1. Element contents of the synthesized amphiphilic catalysts C (%) c

a

H (%)

a

S (%)

b

O (%)

M (%)

Conm

d

Cont

Conm

Cont

Conm

Cont

Conm

Cont

Conm

Cont

C12BSNi

57.36

60.96

9.08

8.18

8.54

9.03

18.89

13.55

6.13

8.28

C12BSFe

59.75

62.86

8.71

8.44

9.18

9.31

16.09

13.97

5.37

5.42

b

c

O was calculated by subtraction method. M% is the content of metallic element. Conm is the measured element content, and dCont is the stoichiometrically calculated element contents.

To further prove the molecular structure and composition, the element analysis of synthesized catalysts was performed and the results are shown in Table 1. The contents of elements are close to their stoichiometric value. The slight positive deviations of H and O between experimental and theoretical results could be due to the water absorption during the test process. Both FT-IR and element analysis indicate the formation of catalysts as expected.


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Figure 1. FT-IR spectra of dodecylbenzenesulphonic acid (C12BS), nickel dodecylbenzeneslfonate (C12BSNi), iron dodecylbenzenesulfonate (C12BSFe) Simultaneous TGA-DTA results of both amphiphilc catalysts are shown in Figure 2. The obvious weight loss and an exothermic peak appeared after temperature was raised above 400 °C, indicating the thermal decomposition of amphiphilic catalysts. In other words, the catalysts are stable enough during the aquathermolysis of heavy crude oil, which is usually below 300 °C.

Figure 2. TGA (red line) and DTA (black line) of amphiphilic catalysts in nitrogen. (A), C12BSNi; (B), C12BSFe. Surface tension determination shows that both catalysts are able to reduce the water surface tension to approximately 28mN/m at the concentration of 0.03 wt% (Figure 3). And the interfacial tension between water and Shengli heavy oil was also measured with a spinning drop interfacial tensiometer. At the concentration of 0.2 wt%, C12BSNi can reduce the interfacial tension from about 20 mM/m to 1.2 mM/m; and C12BSFe to 7.0 mM/m. The interfacial active


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catalysts will benefit the mixing of water and heavy oil when they are added into heavy oil. Better mixing of water and heavy oil increases the interfacial area, and thus improves the cracking of heavy oil, as most of the cracking reactions take place at the oil/water interface during aquathermolysis.

Figure 3. Surface tension of amphiphilic catalysts. C12BSNi (red line); C12BSFe (black line). 3.2. Viscosity reduction with different nickel catalysts The viscosity reduction effects of amphiphilic catalyst were compared with other types of catalysts, such as water soluble and oil soluble catalyst, at 250 °C and active metal concentration of 0.2 mol/kg. Water soluble catalyst was nickel sulfate (NiSO4•7H2O), and oil soluble catalyst was home-made nickel naphthenate (nickel content 6.3 wt%) , while amphiphilic catalyst was C12BSNi. Other parameters were the same as those described in experimental part 2.2. As shown in Figure 4, the viscosity reduction ratio of amphiphilic catalyst was 48.5%, while for water soluble and oil soluble catalyst were 28.5% and 35.8%, respectively. The viscosity reduction ratio of control oil sample (without catalyst) was 24.4%. Amphiphilic catalyst has the best activity among three catalysts. Oil soluble catalyst is better than water soluble catalyst; and the viscosity reduction ratio of water soluble catalyst is only slightly higher than the control experiment. It is indicated that the catalyst transportation is crucial to heavy oil aquathermolysis, and also confirms that most of the cracking reactions take place at the oil/water interface. Recently, researchers begin to realize the importance of catalyst transportation and distribution,


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especially in porous media during enhanced oil recovery [31]. Polymer (xanthan) and ionic surfactant (CTAB) were adopted to move the metal nanoparticle catalyst to oil/water interface through electrostatic interactions [32, 33]. Fortunately, amphiphilic catalysts can move to oil/water interface automatically through hydrophobic-hydrophilic interactions like surfactants.

Figure 4. Viscosity reduction ratios with different type nickel catalysts (viscosity measured at 80 °C).

3.3. Viscosity and SARA compositions of aquathermolyzed heavy oil with amphiphilic catalysts Heavy oil properties, including the viscosity, SARA compositions, and average molecular weight of SARA components, were analyzed before and after aquathermolysis with amphiphilic catalysts. According to the results in Table 2 and Figure 5, the viscosity reduction ratios at 80 °C were 48.5% with C12BSNi and 45.3% with C12BSFe after catalytic aquathermolysis, while 61.5% and 61.2% at 50 °C. The viscosity reduction ratios at 80 °C were much lower than those at 50 °C, which also confirmed the effect of microcrystalline wax on heavy oil viscosity. Since asphaltene and resin with large molecular weight are the primary cause of high viscosity in heavy oil [34], the viscosity in this paper was determined at 80 °C unless otherwise specified. The viscosity


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1 2 3 reduction ratio at 80 °C was only 24.4% in the control experiment. These results prove that our 4 5 amphiphilic catalysts can drastically reduce the viscosity of heavy oil, and C12BSNi is more 6 7 8 effective than C12BSFe. 9 10 Table 2. Viscosity and SARA compositions of heavy oil samples 11 12 13 Heavy oils Saturate Aromatic Resin Asphaltene 14 15 a b c Sample Viscosity MW Content MW Content MW Content MW Content MW 16 (g/mol) (wt%) (g/mol) (wt%) (g/mol) (wt%) (g/mol) (wt%) (g/mol) (mPa · s) 17 18 oil before 8957 710 21.82 561 26.58 708 46.97 1454 5.59 10031 19 aquathermolysis 20 21 oil catalyzed by 4614 642 33.44 454 29.86 634 33.64 1420 4.95 8293 22 C12BSNi 23 oil catalyzed by 4902 683 31.87 495 29.02 679 35.99 1440 5.49 8764 24 C12BSFe 25 a,Viscosity was measured at 80 °C; b, wt% stands for the weight percent of SARA; c, g/mol stands for the unit 26 of average molecular weight of SARA 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 Figure 5. Viscosity reduction ratios with amphiphilic catalysts (viscosity measured at 50 °C and 44 80 °C). 45 46 The SARA analysis shows that asphaltene content decreased 0.64% and 0.10%, after 47 48 aquathermolysis with C12BSNi and C12BSFe, while resin decreased 13.33% and 10.98% 49 50 51 respectively. In contrast, the contents of saturate and aromatic of both oil samples increased 52 53 about 10% and 3% which suggests that resin and asphaltene were partly cracked into saturation 54 55 56 57 58 59 60 ACS Paragon Plus Environment

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and aromatic under the catalysis of metal dodecylbenzenesulfonate. Just like viscosity reduction ratio, C12BSNi is more effective than C12BSFe in decreasing asphaltene and resin contents. From Table 2, the average molecular weights of heavy oil, saturates, aromatics and asphaltenes decreased significantly after aquathermolysis, but the resin molecular weight only decreased slightly in comparison with the original oil samples. It is known that part of resin cracks into saturate or aromatic, and meanwhile asphaltene cracks into resin during thermolysis [35]. The molecular weight of new resin from asphaltene might be larger than that of original resin, while the original resin weight became smaller after cracking. Thus, the overall average molecular weight only changed slightly although the contents of resin decreased largely. The above results show that these two amphiphilic catalysts can reduce the viscosity of Shengli heavy oil. And C12BSNi is more active than C12BSFe. The changes of SARA components show that the decreases of asphaltene and resin contents directly lead to the viscosity reduction. Thus it is necessary to study the molecular structure of resin and asphaltene after aquathermolysis reaction.

3.4. Molecular structure of resin and asphaltene FT-IR is a useful method to characterize the aliphatic side chain length (expressed in the ratio of nCH2/nCH3) of asphaltene and resin through Eq. (2) [35], where nCH2 and nCH3 are the number of methylene and methyl. A1460 and A1380 are the IR absorption at 1460cm-1 and 1380cm1

. The FT-IR spectra are shown in Figure 6 and the results are listed in Table 3. The ratios

nCH2/nCH3 of both asphaltenes and resins after aquathermolysis were smaller than those of the original ones except for asphaltene-Fe. It is indicated that parts of the side chains were cracked during aquathermolysis. C12BSNi showed the best activity to crack the aliphatic chains of asphaltene. As to asphaltene-Fe, the larger ratio nCH2/nCH3 seemed to conflict with the


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decreased molecular weight. It will be discussed in the following part combined with the elemental analysis results and molecular structure from 1H-NMR.

nCH 2 / nCH 3 = 2.93 A1460 / A1380 − 3.70

(2)

Figure 6. IR spectra of resins and asphaltenes, (a), original resin or asphaltene; (b), resin-Ni or asphaltene-Ni; (c), resin-Fe or asphaltene-Fe

Table 3. nCH2/nCH3 of asphaltenes and resins aquathermolysized with amphiphilic catalysts Sample

nCH2/nCH3

Sample

nCH2/nCH3

asphaltene

0.212

resin

0.644

asphaltene-Ni

0.137

resin-Ni

0.451

asphaltene-Fe

0.235

resin-Fe

0.236

a

a, Asphaltene-Ni stands for the asphaltene from heavy oil aquathermolysized with Ni catalyst, and by analogy to other asphaltenes and resins.

Molecular structure of resin and asphaltene was calculated through an improved BrownLadner method from the results of element analysis, average molecular weight, and 1H-NMR


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[36-38]. The 1H-NMR spectra of resins and asphaltenes are shown in Figure A1 and Figure A2 in the supplementary data. And the ratios of different types of hydrogen are shown in Table 4. Aromaticity (fA) and aromatic condensation index (HAU/CA) of asphaltenes and resins are calculated from the 1H NMR results with equation (3) and (4) [39], in which CT and HT represent the total amounts of carbon and hydrogen, HT=HA+Hα+Hβ+Hγ, and CT/HT is the ratio of carbon to hydrogen. The value of CT/HT for the saturated content is assumed to be 2. In Eqs (4), the higher HAU/CA ratio, the lower aromaticity condensation is.

fA=

CT / HT − ( Hα + Hβ + Hγ / 2 HT ) CT / HT

HAU HA / HT + Hα / 2 HT = CA CT / HT − ( Hα + Hβ + Hγ / 2 HT )

(3)

(4)

After aquathermolysis catalyzed by C12BSFe, the sulfur contents in asphaltene and resin were reduced, which means that breakage of C-S bonds easily occurred with C12BSFe. Clark studied the activities of all the group VIIIB metal aqueous salts with thiophene and tetrahydrothiophene as model compounds, and also found Fe was active to break C-S bonds [13]. However, the fA and

HAU/CA of asphaltene-Fe and resin-Fe didn’t show noticeable change, suggesting the moderate cracking reactions of asphaltene and resin, and no condensation of aromatic rings. In the case of C12BSNi, the data in Table 4 show that HA and Hα of asphaltene from heavy oil after aquathermolysis are bigger, and Hβ and Hγ are smaller than those of asphaltene before treatment. The aromaticity fA and aromatic condensation index HAU/CA are larger than those of crude asphaltene too. The slight increase of fA is attributed to the rupture of alkyl side chain to the aromatic and naphthenic ring of asphaltenes under the catalysis of C12BSNi, and the increase in HAU/CA might be assigned to the ring-opening or depolymerization of aromatic species, in


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which the rigid structure of asphaltene turned to be looser than that of asphaltene from crude heavy oil. For the resins catalyzed by both catalysts, the results show that HT/CT, fA, and HAU/CA are almost the same comparing with crude resin. This is consistent with the previous results that the contents of resin decrease evidently but their molecular structure and average weight remain almost the same. This suggests that parts of asphaltene have cracked into resin during the aquathermolysis, and asphaltene is more easily to crack the side chains than resin. Although the original resin cracked and led to lower fA and higher HAU/CA, the new resin from asphaltene has relatively high fA and low HAU/CA. As a whole, the fA and HAU/CA are similar to those of the original resin.

Table 4. Element contents and molecular structure of resins and asphaltenes sample

asphaltene

C (wt/%)

a

asphaltene -Ni

asphaltene-Fe

resin

resin -Ni

resin -Fe

81.13

81.15

80.60

85.20

85.02

85.04

H (wt/%)

8.49

8.43

8.32

10.04

9.93

10.06

S (wt/%)

2.46

2.51

2.20

1.54

1.44

1.41

N (wt/%)

1.98

1.90

1.95

1.49

1.53

1.52

NH/NC

1.25

1.24

1.23

1.40

1.39

1.41

HA(%)

10.54

12.57

9.36

7.25

6.53

6.32

Hα(%)

12.72

33.07

9.65

15.96

16.52

16.34

Hβ(%)

58.01

38.96

56.53

60.40

62.03

62.51

Hγ(%)

18.73

15.40

24.45

16.39

14.92

14.82

HAU/CA

0.48

0.78

0.40

0.61

0.59

0.60

RT

106.58

86.20

97.53

13.78

14.04

13.82

RA

74.56

63.90

64.70

8.52

8.31

8.18

RN

32.02

22.30

32.83

5.27

5.74

5.64

CT

678.18

560.81

588.65

103.23

100.61

102.05

CA

300.24

257.60

260.80

36.06

35.23

34.72

CN

128.08

89.22

131.31

21.07

22.95

22.56

b c

d

e


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CP

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249.86

214.00

196.54

46.10

42.43

44.76

f

fA

0.44

0.46

0.44

0.35

0.35

0.34

fN

0.19

0.16

0.22

0.20

0.23

0.22

0.45 0.42 0.44 a, Asphaltene-Ni stands for the asphaltene from heavy oil aquathermolysized with Ni catalyst, and by analogy to other asphaltenes and resins. b, NH/NC is the atomic ratio of hydrogen to carbon. c, HA refers to aromatic hydrogen. Hα stands for aliphatic hydrogen on Cα to aromatic rings. Hβ stands for the CH2, CH hydrogen on Cβ and carbons beyond Cβ, CH3 hydrogen on Cβ, and CH2, CH hydrogen on alkanes. Hγ stands for the CH3 hydrogen on Cγ and beyond Cγ, and CH3 hydrogen on alkanes. d, RT, RA, and RN are total ring number, aromatic ring number, and naphthenic ring number respectively. e, CT, CA, CN, and CP are total carbon number, aromatic carbon number, naphthenic carbon number, and paraffinic carbon number respectively. f, fA, fN, and fP represent ratio of aromatic carbon, naphthenic carbon, and paraffinic carbon respectively. fP

0.37

0.38

0.33

Table 4 shows the detailed molecular structure of asphaltenes and resins from the 1H NMR analysis. The general trend is that the molecular structure of asphaltene is easier to be changed than that of resin. All the parameters of asphaltene, RT, RA, RN, CT, CA, CS, CN and CP, decrease after aquathermolysis. The changes of RT and CT of asphaltene indicate the occurrence of cracking and ring-opening reactions. By comparing the changes of RA and RN, we find that C12BSNi tends to crack both naphthenic and aromatic rings of asphaltene, and leads to HAU/CA increase. At the same time, according to CA, CS, CN and CP, the numbers of CS and CN after reaction with C12BSNi reduce more than those with C12BSFe. These changes further illustrate that the alkyl side chains and naphthenic rings are easily cracked by C12BSNi. All these changes of molecular structure of asphaltene benefit the viscosity reduction and partially improve the quality of heavy oil. Since C12BSFe doesn’t crack naphthenic rings, the RN and CN of asphlateneFe are bigger that those of original asphaltene, corresponding with FT-IR results. Other active metal ions, such as Mo6+ and Cu2+ were also compared with Fe3+ by Chen et al. Their results showed that aromatic sulfonic molybdenum and copper led to more changes on the asphaltenes, aromatic hydrocarbons, and sulfur-containing groups, and they were suitable for the aquathermolysis of heavy oil with high asphaltene content [21, 40-42]. As to our two amphiphilic catalysts, C12BSFe is suitable for heavy oil with high sulfur and rather low


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Energy & Fuels

asphaltene content, while C12BSNi is suitable for naphthene-base heavy oil. In other word, the matching between catalyst active metal type and heavy oil property is crucial to the application of catalytic aquathermolysis technology in oilfield. From the results and discussions mentioned above, the structures of asphaltene had been changed more than those of resin during the catalytic aquathermolysis. This may be due to the fact that asphaltene has longer alkyl branch sides, more layer structures and larger average molecular weight than resin. Under the catalysis of amphiphilic metal dodecylbenzenesulfonates, most of reactions occurred at the interface of O/W. Moreover, asphaltene has natural interfacial activities [43], it spontaneously moves to the interface. Comparing the changes analyzed by EA and 1H-NMR, two catalysts have different behaviors during aquathermolysis, and all the performances of catalysts are related to certain structures in the asphaltene [44]. According to Rosales [45] and Machín’s [46] theoretical modeling results with a model asphaltene molecule using a quantum mechanics method, both Ni and Fe are able to form metal-asphaltene complex and decrease C-N, C-S, C-C bond energies in asphaltene. Nickel is more active to activate carbon-heteroatom bonds in the presence of water, which corresponds with our experimental results. The effects of aquathermolysis catalyzed by C12BSNi are better than that catalyzed by C12BSFe with the consideration of viscosity reduction. The kinetic analysis of aquathermolysis also shows nickel catalyst reduces the activation energy of H2S generation by about 50% [47]. Ni and Fe, usually in their sulfides, are often used as catalysts in heavy oil hydrotreatment, too. It is also generally thought that Ni is more active than Fe [48, 49]. When hydrocracking of petroleum asphaltene was studied alone, Ni is much more suitable for selective conversion of asphaltene into maltene [50, 51]. The catalytic reaction mechanism of aquathermolysis might be similar to the presence of H2O and H-donor. The catalyst activates hydrogens from H-donor and H2O, and


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transfers the active hydrogen to asphlatene and resin to initiate the cracking reaction. Water also provides a polar circumstance for hydrogen transfer [52, 53]. However, it is very difficult to elucidate the molecular mechanism of aquathermolysis for the sake of heavy oil’s complexity at present. More works are being carried out in our research to prove the mechanism.

4. Conclusion In summary, the synthesized amphiphilic catalysts C12BSNi and C12BSFe are interfacial active and thermostable enough and they can be applied to the aquathermolysis of heavy oil. Through comparison of the changes in viscosity reduction, average molecular weight and the contents of SARA of catalyzed heavy oil samples, the results show that C12BSNi is more effective than C12BSFe on viscosity reduction of Shengli heavy oil. According to the molecular structure of resin and asphaltene analyzed by EA, FT-IR, and 1H NMR, it is clear that the changes of asphaltene structures are crucially important during the aquathermolysis of heavy oil. Furthermore, C12BSNi is beneficial to the cracking of naphthenic rings, whereas the breakage of C-S bonds occurs easier under the catalysis of C12BSFe. Therefore, we can choose proper catalyst or combine them according to the chemical properties of different heavy oils.

Acknowledgements We thank the National Natural Science Foundation of China (under grant 20906105), and Natural Science Foundation of Shandong Province, China (under grant 210ZRE28056).

Supporting Information Available The 1H-NMR spectra of asphaltenes and resins are shown as Figure A1 and Figure A2 in supporting information. This information is available free of charge via the Internet at http://pubs.acs.org/.

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Aquathermolysis of Heavy Crude Oil with Amphiphilic Nickel and Iron

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