Spherical Poly(vinyl imidazole) Brushes Loading Nickel Cations as

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Spherical Poly(vinyl imidazole) Brushes Loading Nickel Cations as Nano Catalysts for Aquathermolysis of Heavy Crude Oil run zou, Jun Xu, Sebastian Kueffner, Julian Becker, Tao Li, Xiang Guan, Xiaowan Zhang, Li Li, Martien Abraham Cohen Stuart, and Xuhong Guo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03964 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019

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Spherical Poly(vinyl imidazole) Brushes Loading Nickel Cations as Nano Catalysts for Aquathermolysis of Heavy Crude Oil Run Zou1, Jun Xu1*, Sebastian Kuffner2, Julian Becker2, Tao Li1, Xiang Guan1, Xiaowan Zhang1, Li Li1, Martien A. Cohen Stuart1, Xuhong Guo1,3*

1State

Key Laboratory of Chemical Engineering, East China University of Science and Technology,

Shanghai 200237, China 2Department

of Process Engineering, Nuremberg Institute of Technology, Nuremberg 90489,

Germany 3Engineering

Research Center of Materials Chemical Engineering of Xinjiang Bingtuan, Shihezi

University, Xinjiang 832000, PR China

*To whom correspondence should be addressed. E-mail: [email protected] (Jun Xu) or [email protected] (Xuhong Guo)

1

ACS Paragon Plus Environment

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Abstract Catalytic aquathermolysis in-situ upgrading and reducing the viscosity of heavy oil in the reservoir remarkably enhances the recovery and is considered as a promising technology. But the low catalytic efficiency and inferior dispersity in both water and oil limit its applications. In the present work, spherical polymer brushes nanocatalysts were synthesized, in which nano TiO2 is the core and poly(vinyl imidazole) (PVI) loading nickel cations are polymer brushes. The chemical characteristics, polymer grafting content, nickel loading content, morphology of as-prepared catalysts was characterized by infrared spectroscopy (IR), thermogravimetric analysis (TGA), inductively coupled plasma emission spectra (ICP-OES), scanning electron microscope (SEM), transmission electron microscope (TEM). The polymerization degree of PVI was analyzed by 1H NMR spectra. The effects of nickel loading content, catalytic conditions and hydrogen donor on the viscosity of heavy oil were studied. The results show that the heavy oil is catalytic cracked by the synthesized catalysts which leads to the reduction of oil viscosity. The viscosity reduction is enhanced by the increase of nickel loading content, catalytic temperature, dosage of catalyst and hydrogen donor. The rheological behaviors in terms of flow curve, thixotropy, viscoelasticity and time dependence of cracked oil were studied. To explore the cracking mechanism, the four compositions of heavy oil before and after aquathermolysis were compared. The extracted asphaltenes and resins were further analyzed by elemental analysis (EL), 1H NMR spectra and 2


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infrared spectroscopy (IR). The organic compounds in reacted water were characterized by gas chromatographic mass spectrometry (GC-MS). It is found that, the content of light saturates is much increased after the aquathermolysis, along with the distinct decrease of resins. From the structure change of resins, such as the decrease of H/C and methylene/methyl ratio and increase of aromaticity and aromaticity condensation, the increased light saturates are due to the dissociation of alkyl side chains in resins. In addition, the aromaticity and aromaticity condensation in asphaltenes are found decreased, which is because of the fragmentation and depolymerization of large aromatics. Meanwhile, the loss of oxygen in both asphaltenes and resins is connected with the phenols found in the reacted water, indicating the breakage of C-O bond and heteroaromatic ring-open reaction in both asphaltenes and resins during the aquathermolysis. Keywords: heavy oil, aquathermolysis, spherical brushes, catalytic cracking

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1. Introduction With the increasing demands for energy consumption, heavy crude oil has attracted more and more interests worldwide. However, the high viscosity of heavy oil often brings difficulties to its exploitation. Various approaches have been developed to solve them, such as thermal recovery,1 chemical recovery2 and microbial recovery.3 Among them, the catalytic aquathermolysis has become one of the most promising technology in recent years.4 During the thermal recovery, not only the viscosity of heavy oil is reduced, but components of the heavy oil are partially cracked into light compounds in the presence of the reservoir minerals. If catalyst is present, the viscosity of heavy oil will be further reduced due to the cleavages of CC, C=C, C-X (where the X=S, N, O) bonds in those components.5 By in-depth analyzing the composition of heavy oil after aquathermolysis, it is found that the involved reactions include pyrolysis,

depolymerization,

hydrogenation,

isomerization,

ring

opening,

oxygenation,

alcoholization, esterification, reconstruction, and so on.6 The products of these reactions includes light saturates and aromatics, which upgrades the quality and improves the flowability of heavy crude oil. Evidently, the presence of good catalyst can significantly enhance the content of light hydrocarbons in cracked products, so high-efficiency and economical rational aquathermolysis catalysts for heavy crude oil are highly demanded.

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To date, diverse catalysts were developed for the aquathermolysis of heavy oil, such as minerals,7,8 water-soluble catalysts,9,10 oil-soluble catalysts11,12 and dispersed catalysts.13-26 Minerals have low catalytic activity which often cut off the carbon-sulfide bonds in sulfur-bearing molecules. 7 Water soluble catalyst, such as NiSO4, which has shown abilities to crack crude oil, is difficult to contact the oil for its hydrophilic properties.9 Oil soluble catalyst, such as Nicarboxylates, 11 although the affinity for oil is enhanced, easily aggregate in water-rich environment. Dispersed catalysts can neither be dissolved in oil nor water, but they are able to be dispersed in both water and oil, and even at their interface where most of the aquathermolysis reactions take place.4 Therefore, they have attracted widespread attention recently. Nanoparticles, such as NiFe2O4 nanoparticles,14 nickel and cobalt nanoparticles15 and solid acid TiO2–ZrO227 are reported for

their

high

catalytic

efficiencies.

Beyond

that,

dodecylbenzenesulfonic nickel23 and toluenesulfonic copper,

amphiphilic 24

molecules,

like

also show respectable catalytic

cracking performances for crude oil. It is worth mention that, solid acid, such as spherical TiO2, which already possesses nice catalytic performances, can be highly improved by grafting polymer chains bearing some transition metals on its acidic surface, which is also called spherical polymer brushes. Poly(vinyl imidazole) (PVI) is capable of coordinating specific cations in aqueous solution.28 By the “graft to” method, PVI can be chemically grafted onto the surface of inorganic 5


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nanoparticle.29 The grafted PVI chains can also increase the affinity of inorganic nanoparticle to oil and serve as space exclusion layer to improve the water dispersibility of nanoparticle. Spherical TiO2 is a conventional carrier to prepare polymer bushes, for its surface being rich in hydroxyl groups.30 Its acidic surface also provides abundant catalytic sites for cracking crude oil.27 After adsorbing transition metal cations in the PVI polymer brushes, the core-shell catalyst possesses two catalytic active centers. One is solid acid TiO2, the other is transition metal cation. Therefore, the catalyst possesses both of the advantages of TiO2 and transition metal cation. In the present work, spherical polymer brushes nanocatalysts, consist of nano TiO2 as core and PVI loading nickel ion as brushes, were fabricated for aquathermolysis of heavy oil. The chemical characteristics, polymer grafting content, nickel loading content, morphology of asprepared catalysts was characterized by FT-IR, TGA, ICP, SEM and TEM. The polymerization degree of PVI was calculated from 1H NMR spectra. The effects of nickel loading content, catalytic conditions and hydrogen donor on the viscosity of heavy oil were studied. Rheological behaviors of heavy oils before and after cracking were also measured. To investigate the change of oil composition after cracked, four group compositions of Xinjiang heavy crude oil were extracted and analyzed. The extracted asphaltenes and resins were further characterized by EL, 1H NMR and IR, and the organic compounds in reacted water were analyzed by GC-MS.

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2. Experimental section 2.1 Materials Vinyl imidazole (VI, 99%), γ-mercaptopropyltrimethoxysilane (MPS, 95%), titanium oxides (Anatase, BET SA=120 m2/g) and azo-bis-isobutyronitrile (AIBN, 99%) were purchased from Aladdin. Nickel chloride (99%) is from Alfa Aesar. Toluene (99%), methanol (anhydrous) and diethyl ether (99%) are from Shanghai LingFeng Company. All chemicals were used without purification. Heavy crude oil, with density and viscosity of 0.9538 g/cm3 (R.T) and 35,000 mPa·s (50°C), is obtained from Xinjiang Oilfield Company. The four group compositions of heavy oil are ca. 16.3% of asphaltenes, 11.5% of resins, 28.4% of aromatics, and 43.8% of saturates (SH/T 0509-2010). The elemental composition is 86.56% of carbon, 10.67% of hydrogen, 0.19% of oxygen, 0.46% of nitrogen, and 2.12% of sulfur (Vario EL III).

2.2 Preparation of nickel-containing spherical polymer brushes catalyst The synthesis route of nickel-containing spherical polymer brushes catalyst is illustrated in Figure 1. Firstly, PVIs with different polymerization degree (f) were synthesized by varying the feeding molar ratio of VI/MPS (1, 20 and 80). The reactant was dissolved in anhydrous methanol, then AIBN as initiator with 2 wt. % of total reactant was added in. The solution was stirred at 70°C 7


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for 48 h in N2 gas atmosphere. After the completion of reaction, excess solvent was removed by vacuum evaporation. Then the concentrate was added into cold diethyl ether, acquired orangeyellow precipitates. The precipitates were filtrated and washed thoroughly with diethyl ether for three times and dried overnight in vacuum at 50°C. Then, excess PVI was added to well-dispersed titanium oxide solution, in which toluene and methanol are solvent. The mixture was stirred vigorously under 100°C for 24 h. After the reaction, the obtained crude product was centrifuged to separate TiO2@PVI (bottom solid) from unreacted PVI (upper liquid). The crude TiO2@PVI was washed with methanol, then centrifuged and ultrasonic dispersed for three times. Pure TiO2@PVI powder was acquired by dried in vacuum oven overnight at 50°C. Finally, TiO2@PVI was dispersed in NiCl2 methanol solution and stirred vigorously under 90°C for 48 h. Then Ni2+ was loaded onto the PVI layer in TiO2@PVI by adsorption. The obtained TiO2@PVI/Ni2+ was purified with methanol by repeated centrifuging and ultrasonic dispersing for three times and dried in vacuum oven overnight at 50°C.

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Figure 1. Synthesis route of nickel-containing spherical polymer brushes catalysts.

2.3 Characterization 1H

NMR. The synthesized PVI polymers with different polymerization degree were

characterized by 1H NMR, performed by Bruker Avance 500 spectrometer at 500 MHz using a solvent of deuterated methanol. Besides, deuterated chloroform was used as solvent for asphaltenes and resins. FT-IR. FT-IR spectroscopy measurements were performed to identify functional groups in the catalysts and aquathermolysis product (asphaltenes and resins) by using the Nicolet 7800 Fourier Transform Infrared Spectrometer with KBr as tableting.

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TGA. TGA was measured to determine the content of polymer in catalysts, performed under air atmosphere by thermogravimetric analyzer (STD Q600, USA). The temperature was ended at 800°C with a heating rate of 10 °C/min. ICP-OES. The nickel content loaded in synthesized catalysts was measured by ICP-OES (Agilent 725, USA). SEM. Morphology of synthesized catalysts was observed by a FEI Nova Nano SEM 450 microscope. TiO2@PVI-80/Ni2+ suspension was dispersed on a silicon wafer and placed in the chamber under the pressure of 120-500 Pa. After dried, a sample was prepared. This sample was observed at 25 kV of acceleration voltage, and images were captured with the backscattering electron signal. TEM. TEM images of catalysts were taken by using a JEM-2100 microscope. The electron accelerating voltage is 200 kV. TiO2@PVI-80/Ni2+ suspension was deposited on a carbon film supported on a standard copper grid and dried as a sample. Aquathermolysis experiments. All the experiments were carried out following this process: First, ca. 20 g of oil is added into a 50 mL high pressure reactor (TGYF-A, Dufu, China), followed by adding in various quantity of water and catalyst. The original pressure in reactor is kept to 2 MPa by aerating N2. The final pressure reaches about 6 MPa as the temperature is increased to 240°C. The aquathermolysis of heavy oil last 24 h. Throughout the reaction, magnetic stirring is 10


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maintained. After the completion of reaction, the water is decanted manually at 90°C. The residual oil is centrifuged at 10000 rad/min for 30 min to separate water from oil before rheological measurements. Rheological measurement. The rheological properties of heavy oil were measured by using the Anton-Paar rotational rheometer (MCR501, Austria). The 25 mm parallel plate geometry and platform was applied. Viscosity was measured at the steady shear rate of 10 s−1 and 50°C. The average value of three parallel measurements was collected. The flow curves were measured at varying share rate from 0.1 to 10 s-1 under 50°C. Thixotropic loops were tested by increasing the shear rate from 0 to 10 s-1 in 5 seconds linearly and decreasing back to 0 s-1 at the same rate under 50°C. Viscoelastic properties were measured by small angular oscillation at 50°C, with logarithmic frequency sweeping from 0.1-100 rad/s. Time dependence of oil samples was measured by the same method as viscosity test. Four group compositions analysis. The four group compositions of various oil samples, including original one, cracking without catalyst, cracking with catalyst, and cracking with both catalyst and hydrogen donor, were determined by using alumina column chromatography. The detailed procedures refer to the standard SH/T 0509-2010. Elemental analysis. The elemental composition of asphaltenes and resins were characterized by elemental analyzer (Vario EL III, Germany). 11


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GC-MS. The organic compounds in reacted water after aquathermolysis were analyzed by an Agilent 7890B GC system and 5977B MSD mass spectroscope. The organic compounds were extracted by CCl4 from reacted water and acquired by solvent evaporation.

3. Results and discussion 3.1 Characterization of PVI polymers The 1H NMR spectra of synthesized PVI polymers is shown in Figure 2. The polymerization degree (f) of PVI polymers is calculated through Eq.1, 27 where A(-C3H3N2) refers to the integral area of three protons in VI unit (6.7-7.4 ppm) and A(-SiCH2) refers to the integral area of two protons in MPS unit (0.3-0.7 ppm). The polymerization degree of PVI polymers calculated by integration of the peak area and corresponding molecular weight are listed in Table 1. From this table, the calculated polymerization degree (f) of PVI is quite close to the feeding ratio.

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Figure 2. 1H NMR spectra of PVI polymers with different polymerization degree (a) PVI-1, (b) PVI-22 and (c) PVI-80. f=

A(C3 H 3 N 2 ) / 3 A( SiCH 2 ) / 2

(1)

Table 1. The polymerization degree of synthesized PVI polymers calculated from 1H NMR by integration of the corresponding peak area. Feeding ratio

A(-SiCH2)

A(-C3H3N2)

f (calculated)

MWb (g/mol)

1

1.0 a

1.0

0.7

289

20

1.0

32.5

21.7

2263

80

1.0

120.0

80.0

7715

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a Integral

b

area of A(-SiCH2) is normalized to 1.0 for all the three PVI polymers;

Molecule weight of PVI calculated from the f (calculated).

3.2 Characterization of catalysts The chemical structure of synthesized catalysts was characterized by FT-IR spectra (Figure 3). From Figure 3a, the stretching vibration of -OH bond on the surface of TiO2 is found at 3418 cm-1. In Figure 3b, the stretching vibrations of the =CH in the ring and -CH2 in the chain of vinyl imidazole group appear at 3108 and 2940 cm−1, respectively. The bending vibration of -CH2 bond is found at 1419 cm-1. The stretching vibration of the C=C and C=N bond in vinyl imidazole group is at 1492 cm−1.29 The characteristic peaks of PVI-80 also appear in Figure 3c, which confirms that PVI-80 is grafted to TiO2 successfully.

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Figure 3. FT-IR spectra of (a) TiO2, (b) PVI-80 and (c) TiO2@PVI-80/Ni2+.

Thermostability of TiO2, TiO2@PVI-1, TiO2@PVI-22 and TiO2@PVI-80 was measured by means of TGA (Figure 4). The weight loss below 200°C is due to the bound water inside the TiO2. Thermolysis of PVI polymer is mainly occurred between 300 to 500°C, resulting in the largest weight loss for catalysts (Fig. 4b-d), which indicates that the PVI chains were not cracked during aquathermolysis. The grafting content of PVI is calculated by subtracting the weight of TiO2 and the water content, which is 5.9%, 9.2% and 10.7% for TiO2@PVI-1, TiO2@PVI-22 and TiO2@PVI-80, respectively. It indicates that the polymer grafting content increases with the increment of polymerization degree. Based on the known molecular weight and grating content of PVI, the grafting density was calculated to be 6.9, 1.4 and 0.5 1/nm2 (calculated by Eq. S1 to Eq. 15


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S7 and the details are listed in Table S1 in Supporting Information), which shows a decline with the increment of polymerization degree.

Figure 4. TGA curves of (a) TiO2, (b) TiO2@PVI-1, (c) TiO2@PVI-22 and (d) TiO2@PVI-80.

By means of ICP spectra, the nickel loading content of TiO2@PVI-1/Ni2+, TiO2@PVI-22/Ni2+ and TiO2@PVI-80/Ni2+ is measured, which is 1.13%, 2.69% and 5.26%, respectively. It shows that those catalysts possessing PVI with higher polymer degree are capable of loading more nickel ions. The morphology of TiO2@PVI-80/Ni2+ catalyst was observed by both SEM and TEM (Figure 5). From SEM image (Figure 5a), the catalyst is found possessing a nearly spherical shape. The average particle size of catalyst was calculated by statistic software, which is 84 nm. Observed by 16


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TEM (Figure 5b), the catalyst shows an indistinct spherical shape. However, the core-shell structure is not able to be found, due to the collapse of polymer brushes. The water dispersibility of catalyst was investigated by counting the settling time (Figure S1). It shows that the TiO2@PVI-80/Ni2+ solution keeps good uniformity and stability in water in 40 min. A slight settling is not found until 60 min later. By contrast, the bare TiO2 settles down from water within 20 min. It indicates that the grafting of PVI can inhibit the settling of TiO2, or improve the dispersion stability of catalysts in water.

Figure 5. (a) SEM image, and particle size calculated by statistic software (ImageJ); (b) TEM images of TiO2@PVI-80/Ni2+ catalyst.

3.3 Catalytic aquathermolysis 3.3.1 Effect of nickel loading content 17


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The catalytic efficiency of catalysts with varying nickel loading content was investigated by measuring the viscosity of heavy oil after aquathermolysis (Figure 6). From this figure, it is found that, in the presence of 0.6 wt. % of bare TiO2, 240°C, 7:3 of O/W ratio and 24 h of reaction time, the viscosity of heavy oil is reduced by 65%, which is changed from 35,000 to 12,500 mPa·s. Once grafted by PVI, the reduction of viscosity is declined from 65% (bare TiO2) to 55% (TiO2@PVI80). In the presence of catalysts, the viscosity of heavy oil is reduced much. Apparently, it is found that the reduction of viscosity is increased with the polymerization degree of PVI. According to the Donnan effect, the interlayer of polymer brushes can adsorb more ions than the outer layer. Thus, the longer is the polymer brushes, the higher is the quantity of adsorbed nickel cations. The ICP spectra have also identified that the nickel in TiO2@PVI-80 is higher than the other two. In the presence of TiO2@PVI-80/Ni2+, the reduction of viscosity reaches 83.5%. Therefore, the grafted PVI polymer brushes only slightly reduce the catalytic efficiency by covering few active sites on the surface of TiO2. After loading nickels into the polymer brushes, the viscosity reduction is increased by 28.5%. In the presence of dual catalytic center, the catalytic efficiency of polymer brushes catalysts is promoted remarkably.

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Figure 6. Effects of various catalysts on viscosity reduction (240°C, 0.6 wt. % of catalyst dosage, 7:3 of O/W ratio and 24 h of reaction time). HO: original heavy oil; TiO2: bare TiO2; Brushes: TiO2@PVI-80; CA-1: TiO2@PVI-1/Ni2+; CA-22: TiO2@PVI-22/Ni2+; CA-80: TiO2@PVI-80/Ni2+.

3.3.2 Effect of reaction parameters The effects of catalytic conditions, including temperature, catalyst dosage and O/W ratio, on the catalytic efficiency were investigated by single-factor and orthogonal experiments. Effect of single factors including individual reaction temperature, catalyst dosage and O/W ratio on the viscosity reduction was shown in Figure 7. In Figure 7a, the viscosity reduction increases from 50 to 84% with the increase of temperature from 210 to 250°C. At low temperatures, the reduction of viscosity increases rapidly with the increment of temperature. When the 19


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temperature exceeds 240°C, the change of viscosity reduction is not evident with the increase of temperature. Similar tendency is also found in the effect of catalyst dosage on the reduction of viscosity (Figure 7b). When the catalyst dosage increasing from 0 to 0.4 wt. %, the reduction of viscosity increases from 53 to 79%. But the viscosity reduction only increases 6% when the catalyst dosage increases from 0.4 wt. % to 0.8 wt. %. The effect of O/W ratio on the reduction of viscosity is quite different from the former two parameters (Figure 7c). When the O/W ratio is high, such as 9:1, the viscosity reduction is low (60%). Obviously, the shortage of water cannot provide enough opportunities for the catalysts in contact with oil phase, since the catalysts are dispersed in water phase. Therefore, with the decrease of O/W ratio from 9:1 to 7:3, the viscosity reduction rate increases up to 83.5%. However, the viscosity reduction decreases when O/W ratio exceeds 7:3, which indicates that excess water hinders the catalysts access to the oil phase in turn. Orthogonal experiments were performed to find the optimal reaction conditions. Three levels of temperature, catalyst dosage and O/W ratio are chosen and listed in Table S2. The results of orthogonal experiments are shown in Table S3. From this table, the optimal reaction conditions are found as follows: 240°C, 0.6 wt. % of catalyst dosage and 7:3 of O/W ratio. Under these conditions, the viscosity of heavy oil is reduced by 83.5%. In addition, the influencing factors on the reduction of viscosity are in this order: temperature > catalyst dosage > O/W ratio. Consequently, both

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temperature and catalyst dosage, which is relatively easy to be adjusted, are often applied to acquire an optimal catalytic efficiency.

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Figure 7. (a) Effect of reaction temperature (0.6 wt. % of catalyst dosage, 7:3 of O/W ratio and 24 h of reaction time), (b) catalyst dosage (240°C, 7:3 of O/W ratio and 24 h of reaction time) and (c) O/W ratio (240°C, 0.6 wt. % of catalyst dosage and 24 h of reaction time) on viscosity reduction.

3.3.3 Effect of hydrogen donor Hydrogen donor often improves the cracking efficiency of catalyst. Here, tetralin is chosen as the hydrogen donor with the dosage of 3 wt. % of oil. The influence of hydrogen donor in the presence of catalyst on the catalytic efficiency was studied (Figure 8). At the optimal conditions, in the absence of hydrogen donor, the reduction of viscosity is 83.5%. In the presence of hydrogen donor, the reduction of viscosity is increased to 90.0%. This proves that the hydrogen donor helps the catalyst catalytic cracking of heavy oil, but the reduction of viscosity is not much improved.

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Figure 8. Effect of hydrogen donor (3 wt. % of oil) on viscosity reduction (240°C, 0.6 wt. % of catalyst dosage, 7:3 of O/W ratio and 24 h of reaction time). HO: original heavy oil; CA: cracking with catalyst; CA+HD: cracking with catalyst and hydrogen donor.

3.3.4 Rheological behaviors of cracked heavy oils To explore of the effect of aquathermolysis on the rheological behaviors of heavy oil, the flow pattern, thixotropy, viscoelasticity and time dependence of cracked oil were measured (Figure 9). From the flow pattern tests (Figure 9a), it is found that, the original heavy oil exhibits shearthinning behavior of pseudoplastic fluid. After aquathermolysis, all of the oils present a nearly Newtonian fluid character, whether or not there is a catalyst. From the thixotropic measurements (Figure 9b), the original heavy oil shows relatively high structural strength, due to the large area of 23


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thixotropic loop, also does the cracked oil without catalyst. However, in the presence of catalyst or/and hydrogen donor, the upper and lower lines of thixotropic loops almost overlap, indicating the irreversible deformation of cracked oil. The loss module (G”) of oils is shown in Figure 9c. It is found that the G” enhances with the increment of angular frequency, showing that the inner network of oil resists against the increasing external force. In the presence of catalyst or/and hydrogen donor, the G” of heavy oil is decreased, which indicates the improved flowability of heavy oil. The time dependency of oils is also studied (Figure 9d). It takes time for the original heavy oil to reach a stable viscosity. Differently, in the absence and presence of catalyst or/and hydrogen donor, the viscosity is maintained at a low and steady value from the beginning. It indicates that the internal structure of cracked crude oil is damaged and the crude oil becomes easy to flow.

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Figure 9. Rheological behaviors of oil samples. (a) Flow curve; (b) Thixotropic loop; (c) Loss modulus; (d) Time dependence. HO: original heavy oil (HO); NO-CA: cracking without catalyst; CA: cracking with catalyst; CA+HD: cracking with both catalyst and hydrogen donor.

3.3.5 Analysis of aquathermolysis product 3.3.5.1 Four group compositions Four group compositions are measured to analyze the composition changes of the oil samples, including original heavy oil (HO), cracking without catalyst (NO-CA), cracking with catalyst (CA) 25


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and cracking with both catalyst and hydrogen donor (CA+HD), which is are listed in Table 2. From this table, it is found that, the aquathermolysis of heavy oil reduces the content of both asphaltenes and resins, whatever in the absence or presence of catalyst or/and hydrogen donor. In the presence of catalyst, the content of asphaltenes and resins is decreased compared with that in the absence of catalyst. Furthermore, with the addition of hydrogen donor, the content of asphaltenes and resins is further decreased, and the decrease of resins content is much higher than that of asphaltenes (from 16.3% to 14% for asphaltenes, and 11.5% to 5.9% for resins). It shows that the aquathermolysis of heavy oil occurs mainly in asphaltenes and resins, and mostly in resins. The hydrothermal cracking of heavy oil can be promoted by catalyst and hydrogen donor. It is also found that the content of cracking products, aromatics and saturates, is increased in the presence of catalyst or/and hydrogen donor. In the presence of catalyst and hydrogen donor, the content of both aromatics and saturates is higher than in the presence of catalyst and no catalyst. It is worth mentioning that the increase of aromatics content is not obvious (43.8% to 50.6%). Differently, the content of saturates increases with the addition of catalysts, catalysts and hydrogen donor sequentially (from 43.8% to 46.7%, 49.6% and 50.6%). It indicates that the cracking parts of the asphaltenes and resins mainly occur on the alkyl side chain. While the linking group or alkyl chain between aromatic hydrocarbons is rarely broken, or else the aromatics will be increased much.

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Table 2. Four group composition of different oil samples. Fraction (%) Sample Asphaltenes

Resins

Aromatics

Saturates

HO

16.3

11.5

28.4

43.8

NO-CA

14.8

8.4

30.1

46.7

CA

14.5

6.9

29.0

49.6

CA+HD

14.0

5.9

29.5

50.6

3.3.5.2 Elemental analysis of asphaltenes and resins The elemental composition (C, H, O, N, and S) and molar H/C ratio (NH/NC) of asphaltenes (Table 3) and resins (Table 4) in different oil samples was investigated by means of elemental analysis. From Table 3, the NH/NC of asphaltenes shows no obvious change, which suggests that the aquathermolysis of heavy oil does not involve the hydrogenation, even in the presence of hydrogen donor. Nevertheless, from Table 4 , it is found that the NH/NC of resins is decreased obviously. Most likely, some small molecule fragments with high NH/NC in resins are dissociated and transferred into light components, and it results in the decrease of NH/NC in remained molecules.24,25 From both of the tables, it is easily to found that, the nitrogen content of asphaltenes does not apparently change, and that of resins is increased much after aquathermolysis. The sulfur content 27


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of asphaltenes is increased in the presence of catalyst and catalyst with hydrogen donor, but that of resins is decreased whenever aquathermolysis happens. Since there is no cracking product containing sulfur and nitrogen being found, the relative content of sulfur (in asphaltenes) and nitrogen (in resins) could be increased. Interestingly, the oxygen content of both asphaltenes and resins is decreased much, especially in the presence of catalyst and hydrogen donor. By GC-MS, considerable quantity of phenols is found in the reacted water, indicating that the oxygen in asphaltenes and resins enters the water in the form of phenols after cracking (Supporting Information, in Table S4 and Figure S4).

Table 3. Elemental analysis of asphaltenes from different oil samples. Sample

Elemental content (%)

NH/NC

C

H

O

N

S

HO

84.72

7.09

2.91

1.32

3.96

1.00

NO-CA

85.29

7.12

2.31

1.29

3.99

1.00

CA

86.13

7.38

0.91

1.31

4.27

1.02

CA+HD

86.47

7.43

0.46

1.35

4.29

1.03

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Table 4. Elemental analysis of resins from different oil samples. Elemental content (%)

NH/NC

Sample C

H

O

N

S

HO

83.36

9.56

2.30

0.97

3.80

1.38

NO-CA

84.15

9.43

1.78

1.39

3.25

1.34

CA

85.40

9.11

0.97

1.30

3.22

1.28

CA+HD

85.75

8.63

0.76

1.49

3.37

1.21

3.3.4.3 1H NMR of asphaltenes and resins The aromaticity index (fA, the ratio of aromatic carbon number to total carbon number) and aromaticity condensation index (HAU/CA) of asphaltenes (Table 5) and resins (Table 6) in different oil samples are analyzed from corresponding 1H NMR spectra (Figure S2 and S3). They are calculated by Eq. S8 and S9 in Supporting Information, respectively. In general, the larger is the HAU/CA value, the lower is the aromaticity condensation. From Table 5, the fA is decreased and HAU/CA is increased in the presence of catalysts or/and hydrogen donor. Considering the unchanged NH/NC and decreased oxygen in asphaltenes, it is most likely that the decrease of aromaticity and aromaticity condensation is due to the fragmentation of large aromatic compounds14, ring opening of heterocyclic aromatic groups and depolymerization of the closed aromatic system.6 From Table 6, on the contrary, the fA of resins is increased and HAU/CA is decreased in the presence of catalysts 29


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or/and hydrogen donor. Apparently, the increase of aromaticity and aromaticity condensation means components with higher NH/NC, such as saturated alkyl side chains or naphthenic rings, are cracked into the light components22, which is verified by the decrease of NH/NC found in elemental analysis and increase of saturates found from group compositions. In addition, the molar ratio of methylene and methyl group in resins revealed by IR spectra31 (Figure S5 and Table S5) is decreased from 1.1 to 0.71, which also verifies the dissociation of alkyl side chain from resins.

Table 5. The aromaticity (fA) and aromaticity condensation (HAU/CA) of asphaltenes in different oil samples. Structure parameters

HO

NO-CA

CA

CA+HD

HA (6.0-9.0)

7.61

7.67

9.71

11.29

Hα (2.0-4.0)

12.27

11.24

23.12

12.1

Hβ (1.0-2.0)

49.09

47.94

52.66

65.37

Hγ (0.5-1.0)

15.87

15.17

16.72

14.69

fA

0.61

0.63

0.52

0.52

HAU/CA

0.09

0.08

0.14

0.12

Types of proton (%)

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Table 6. The aromaticity (fA) and aromaticity condensation (HAU/CA) of resins in different oil samples. Structure parameters

HO

NO-CA

CA

CA+HD

HA (6.0-9.0)

11.89

9.95

6.77

10.38

Hα (2.0-4.0)

16.78

11.56

12.97

13.59

Hβ (1.0-2.0)

51.61

66.93

65.44

60.20

Hγ (0.5-1.0)

23.10

14.69

15.69

15.91

fA

0.37

0.40

0.40

0.46

HAU/CA

0.13

0.10

0.09

0.11

Types of proton (%)

4. Conclusions In order to acquire high-efficiency catalysts for the aquathermolysis of heavy crude oil, spherical polymer brushes nanocatalysts with nano TiO2 as core and PVI loading nickel cations as brushes were synthesized. The appearance of characteristic peaks of vinyl imidazole group in IR spectra reveals the successful grafting of PVI polymers on TiO2. The polymerization degree of grafted PVIs is increased with the increase of monomer dosage revealed by 1H NMR. Analyzed by ICP, the loading capacity of nickels is enhanced with the increment of polymer degree of grafted PVI. The single-factor experiments show that the reduction of viscosity increases rapidly with the increment of temperature, but the change is not evident when temperature exceeds 240°C. Similarly, 31


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no apparent viscosity reduction is found when the catalyst dosage is over 0.4 wt. %. Differently, the reduction of viscosity is increased when the O/W ratio decreases from 9:1 to 7:3, but decreased as the O/W ratio further decreases to 5:5. The orthogonal experiments indicate that the key influencing factors is the reaction temperature and catalyst dosage. From the rheological measurements, in the presence of catalyst or/and hydrogen donor, the oils show typical Newtonian fluid behaviors with weaker structural strength and better flowability after aquathermolysis. From the results of group compositions, the content of saturates increase remarkably, but the content of asphaltenes and resins are decreased after cracking. Also, the elemental analysis shows that the H/C of asphaltenes changes less, but the H/C of resins decreases much, indicating the increased saturates are mainly from resins. Revealed by 1H NMR analysis, the aromaticity and aromaticity condensation of asphaltenes are decreased. Combined with the oxygen in asphaltenes also being decreased and phenols being found in reacted water, the asphaltenes undergo large aromatics fragmentation, heteroaromatics ring opening and depolymerization. Both the aromaticity and aromaticity condensation of resins are increased, together with the decrease of the molar ratio of methylene and methyl group, indicating the rupture of alkyl side chain. Consequently, the spherical polymer brushes nanocatalysts have great potential to be the high-efficiency catalyst for the aquathermolysis of heavy crude oil.

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5. Supporting Information Grafting density of PVIs on TiO2; Water dispersibility test; Orthogonal experiments; 1H NMR spectra of asphaltenes and resins; GC-MS analysis of organic compounds in reacted water; IR spectra of asphaltenes and resins.

6. Acknowledgement Financial support by National Natural Science and Foundation of China (51761135128, 51003028), PetroChina Innovation Foundation (2016D-5007-0211), the Open Project of State Key Laboratory of Shihezi University (2016BTRC004), and 111 Project Grant (B08021) are gratefully acknowledged. The authors also thank Petrochina Liaohe Oilfield Company for affording oil samples and technological supports.

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Spherical Poly(vinyl imidazole) Brushes Loading Nickel Cations as

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