From Coal-Heavy Oil Co-refining Residue to Asphaltene-Based

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From coal-heavy oil co-refining residue to asphaltene-based functional carbon materials Fangfang Qin, Wei Jiang, Guosong Ni, Jiashi Wang, Pingping Zuo, Shijie Qu, and Wenzhong Shen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00003 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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ACS Sustainable Chemistry & Engineering

From coal-heavy oil co-refining residue to asphaltene-based functional carbon materials Fangfang Qina,b, Wei Jianga,b, Guosong Nia,b, Jiashi Wanga,b, Pingping Zuoa, Shijie Qua, Wenzhong Shena* a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese

Academy of Sciences, 27 South Taoyuan Road, Taiyuan, Shanxi, 030001, P.R. China b

University of Chinese Academy of Sciences, No. 19 (A) Yuquan Road, Shijingshan

District, Beijing, 100049, P.R. China E-mail: [email protected] (Fangfang Qin), [email protected] (Wei Jiang), [email protected] (Guosong Ni), [email protected] (Jiashi Wang), [email protected]

(Pingping

Zuo),

[email protected]

(Shijie

Qu),

[email protected] (Wenzhong Shen).

Abstract: Asphaltene-based carbon fiber, needle coke and porous carbon nanosheets were synthesized using the extracted asphaltene from coal-heavy oil co-refining residue. Asphaltene with high average molecular weight and aromaticity is considered proper to be polymerized or crosslinked, and is a high-performing precursor to synthesize carbon materials. It can be transferred into pitch after pyrocondensation polymerization, the resultant asphaltene-based carbon fiber with a tensile strength of 0.92 GPa was obtained via spinning, preoxidization and carbonization. While high graphitized needle coke was formed when asphaltene was thermal polycondensation, delayed coking and carbonization, it provided a facile route to produce needle coke from asphaltene. Porous carbon nanosheet with a high surface area of 1717 m2/g and pore volume of 0.94 cm3/g was synthesized using asphaltene as carbon precursor, melamine as nitrogen resource and molten salt as template, the porous carbon nanosheet showed high CO2 adsorption amount and outstanding adsorption selectivity for CO2-N2 and CO2-CH4. It indicated that asphaltene was an excellent precursor to

*

Corresponding author: Fax: +86-351-4041153, E-mail: [email protected]. 1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

prepare functional carbon materials for its adjustable molecular weight distribution, high aromaticity, controllable thermal condensation and easy graphitization. It provided an effective strategy to tailor solid residue from coal and heavy oil co-refining and developed directional conversion or utilization of asphaltene. Keywords: Coal-heavy oil co-refining residue; Asphaltene, Carbon fiber; Needle coke; Porous carbon nanosheet

Introduction: Coal and heavy oil co-refining as new clean coal conversion technology, combines direct coal liquefaction and heavy oil processing to reduce the operational difficulty of direct coal liquefaction and highly efficient process of heavy oil and residual oil. Coal slurry and heavy oil are catalytic pyrolysis by molybdenum cobalt/ferric oxide and molybdenum nickel/ferric oxide catalysts and transferred into light, medium oil, a small amount of hydrocarbon gas and solid residue (CHRR) under 15-22 MPa and 400-470 oC. Similar to direct coal liquefaction residue, CHRR accounts for 40-50 wt% of the coal and heavy oil and is composed of 10-15 wt% heavy product, 35-50 wt% asphaltene, 15-25 wt% minerals, 15-20 wt% coke and un-reacted coals. The coal liquefaction residue was mainly directly used as feedstock to be gasified for hydrogen production,1 produced lighter oil by catalytic hydrogenation, flexible coking and pyrolysis,2,3 generated hydrogen-rich syngas by gasification or dynacracking,4 improved the tar yield and quality of co-pyrolysis with lignite5 or enhanced combustion reactivity of co-pyrolysis with chars.6 It could be also selected as precursor to synthesize porous carbon for supercapacitor electrode,7 catalysts for methane decomposition.8 Moreover, carbon microfibers could be obtained from coal liquefaction residue through arc-jet plasma.9 In these processes, the mineral matters should be removed after carbonization and activation, it is tedious and complex; moreover, the pore structure is difficult to control and the morphology of resultant pore carbon is irregular. As the most important component of coal liquefaction residue, asphaltene has high average moleluclar weight and aromaticity, because it is consisted of alkylations and 2


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polycyclic fused aromatic hydrocarbons. Thus, it is easy to polymerize or crosslink due to its high carbon contents. Asphaltene with controllable structure and concentrated molecular weight distribution is considered as an ideal precursor to synthesize functional carbon materials. Asphaltene should be separated from coal liquifaction residue to effectively utilize. Various solvents and methods were used to extract and separate asphaltene from direct coal liquefaction residue, such as, cyclohexane, methanol, acetone, carbon disulfide, n-hexane, toluene, benzene, tetrahydrofuran, acetone,10 subcritical water11 and supercritical fluid extraction.12 The extraction temperature, solvent physicochemical property and solvent/solid ratio are the important factors on the separated asphaltene molecular weight distribution and chemical composition. The asphaltene-based carbon materials, such as porous carbon,13-15 carbon foam,16, 17 carbon nanosheets,18, 19 carbon fibers,20, 21 doped carbon materials and versatile composite,22,

23

have been successfully utilized in water

purification, gas adsorption, electrodes, etc. As carbon-rich resource, it is important to convert the CHRR into various high-value-added products by classification using. Especially for asphaltene, it is highly aromatic and polydisperse mixtures and is easy to form pitch with controllable mesophase features and as excellent candidate precursor for carbon fiber, needle coke and nanosheet porous carbon. As for the components other than asphaltene, it contains amount of mineral and coke; it can be transferred into porous materials by removal of carbon or mineral matter. However, more attention is paid to pyrolysis, gasification and preparation of conventional carbon materials, there are few studies on the classification utilizing of heavy products. So, it is necessary to divide CHRR into different components and develop new route for directional conversion or utilization of each component, and to realize its efficient utilization and high added value. In here, solid residue from coal-heavy oil co-refining was extracted by tetrahydrofuran to receive the soluble and insoluble components. Asphaltene was obtained after the recovery of tetrahydrofuran from soluble constituents; it was transferred into spinnable pitch after pyrocondensation polymerization, and resultant asphaltene-based carbon fiber with a tensile strength of 0.92 GPa was obtained via 3



spinning, preoxidization and carbonization; while high graphitized needle coke with perfect streamline was prepared after delayed coking and carbonization. Nitrogen-doped porous carbon nanosheet with a high surface area of 1717 m2/g and large pore volume of 0.94 cm3/g was synthesized using asphaltene as carbon precursor and melamine as N-doping source in molten salt system, it showed high CO2 adsorption amount and outstanding adsorption selectivity for CO2-N2. It provided an effective strategy to tailor solid residue from coal and heavy oil co-refining and develop directional conversion or utilization of asphaltene.

Experimental Materials CHRR was obtained by coal and heavy oil co-refining under 15-22 MPa and 450-470 oC in coal-oil new technology development company, Shaanxi Yanchang Petroleum (group) Co. LTD., China. The CHRR was first extracted using tetrahydrofuran (THF) at 70 oC, the THF soluble fraction was asphaltene (denoted as ASP) and the THF insoluble fraction was residue (denoted as THF-IS), and the ASP was obtained by reduced pressure distillation and further treated by vacuum drying. The proximate and ultimate analyses of CHRR and THF-IS were listed in Table 1. The complete flow schematic diagram of CHRR classification using was shown as Scheme 1.

Scheme 1. Diagram of CHRR classification utilization.

The chemical reagents of KOH, LiCl, KCl, melamine and THF were provided by Sinopharm Chemical Reagent Co. (China). 4


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Table 1: Primary properties of Test CHRR, THF-IS and Asphaltene

Sample

Proximate analysis (wt%) Md Ad Vdaf 0.28 18.24 44.46 2.69 36.65 18.91

Elemental analysis (wt%) C H N S diff H/Ca CHRR 74.21 4.21 0.54 1.74 0.54 0.681 THF-IS 54.50 2.42 0.35 3.08 0.31 0.533 Asphaltene 92.37 5.45 0.74 0.22 1.22 0.708 Md: moisture content on dry basis; Ad: ash content on dry basis; Vdaf: volatile matter yield on dry-ash-free basis; diff: difference; H/Ca: molar ratio. Preparation of asphaltene-based carbon fiber The ASP was used to prepare spinnable pitch by oxidizing heat-treatments system (Zhaoyang chemical machinery Co. Weihai, China). In a typical process, ASP was subsequently pre-heated to 220 oC and air-blown at a flow of 400 mL/min for 0.5 h to remove volatile components, and further heated to 300 oC at the heating rate of 5 oC/min

and kept for 2, 3 or 4 h, respectively. The obtained pitches were labeled as

ASPA-x, x represented the air-blowing time. The ASPA-x was further thermal treatment at 350 oC under nitrogen-blowing for 2, 3 or 4 h, and designed as ASPAN-y, y was the nitrogen-blowing time. The spinning of resultant pitch was carried out using melt spinning apporach though a monohole spinneret (diameter of 0.2 mm, length/diameter as 3) at a temperature higher of 40-50 oC than its softening point under 0.5-2.0 MPa and a rotating speed of 200-400 m/min. Stabilization of the green fiber was pre-oxidized in air at 250 oC for 2 h with a heating rate of 0.5 oC/min. Finally, the stabilized pitch fibers were further carbonized at 1000 oC for 15 min in an argon atmosphere with a heating rate of 5 oC/min. Preparation of needle coke In a typical process, ASP was pre-heated to 250 oC under air-blowing at a flow of 400 mL/min for 0.5 h to remove volatile components, and it was heated to 400 oC and sustained 4 h in nitrogen atmosphere to generate mesophase pitch (heat rate: 5 oC/min).

Then, the obtained mesophase pitch was swept twice using N2 and the

feedstock was carbonized at 430 oC for 2 h and 500 oC for 1 h. In order to remove liquid products and the toluene-soluble, the solid product was washed by toluene and

5



named as semi-coke. Finally, the semi-coke was further carbonized at 900 oC and 1500 °C for 1 h with a heating rate of 10 °C/min, respectively. Syntheses of porous carbon nanosheet and CO2 adsorption Oxidation of asphaltene: A 10 M HNO3 (200 mL) was used to oxidize the asphaltene (20 g) under reflux at 80 °C for 12 h. The obtained yellow substance was washed to neutral with water, and then dried for 24 h, finally designed as ASP(O). Molten salt of LiCl and KCl (molar ratio: 41/59; melting point: 352 oC) was selected as template to synthesize porous carbon nanosheet. ASP(O), melamine and LiCl/KCl molten salt (mass ratio of 1:1:9) were fully milled and evenly mixed, the mixture was continuously heated and kept for 2 h at 150 oC, 400 oC and 600 oC under N2 flow (heat rate: 3 oC/min), respectively. The obtained sample was washed by deionized water to remove LiCl/KCl molten salt and dried at 120 oC, the resultant sample was named as ASP(O)-M. Finally, the ASP(O)-M was mixed with KOH (mass ratio of 1:1), and further heated to 800 oC at a heat rate of 5 oC/min for 1 h under N2 flow. The sample was noted as ACN. The HCl (1M) and distilled water were used to wash and remove the redundant metal residue in ACN. The detailed process was depicted in Scheme S1. Characterization In this paper, various characterization instruments were adopted to characterize the properties of carbon fiber, needle coke and nitrogen-doping porous carbon nanosheet. Such as elemental analyzer,

13C

NMR spectrometer apparatus, constant-wavelength

synchronous fluorescence spectromety, thermal mechanical analysis apparatus, universal strength tester apparatus, transmission electron microscopy instrument and many other characterization methods, and the detailed characterization instruments and operations were presented in supporting information (Detailed characterization procedure).

Results and discussion Asphaltene-based carbon fiber

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The extracted asphaltene from CHRR is a high-performing precursor for the synthesis of functional carbon materials because it contains plenty of aromatic hydrocarbon fraction and low ash content (less than 50 ppm). It typically consists of polycyclic aromatic hydrocarbons with softening point of 70-75 °C, it is easy to thermal condensation to promote softening point and adjust its fluidity and aromatic compounds composition, this endows its as potential candidate to synthesize spinning pitch for carbon fiber. High softening point and good rheological property are the primary demands for spinnable pitch, the former is for a higher oxidation temperature in stabilization process, and the later is for spinnability in melt spinning. In generally, high softening point can be achieved by degradation of the rheological property. Therefore, it is necessary to investigate the thermal condensation behavior of asphaltene to synthesize spinnable pitch. Air-blowing is an effective way for increasing the softening point due to its easier cross-link reaction.24 The asphaltene was thermal treated under air-blowing at 300 oC for 2 h, 3 h and 4 h, the softening points of resultant pitches markedly increased and reached up to 175 oC, 186 oC and 192 oC, and the resultant pitches yields were 77.35%, 74.24% and 72.46%, respectively. The primary elemental compositions of parent asphaltene and resultant pitches were listed in Table 2. Obviously, H contents of all resultant pitches were lower than that of parent asphaltene, while O contents were higher than that of parent asphaltene. This suggested that more oxygen cross-linked structures were formed by oxygen radicals and dehydrocondensation reaction during thermal polymerization under air-blowing. The oxygen radical mainly attacked the hydrogen atom instead of the carbon atom or the nitrogen atom, resulting in oxygen content increased and hydrogen content decreased. In generally, the reduction of hydrogen content significantly degraded the spinning performance of resultant pitch. As spinnable pitch, the higher its softening point is, the less difficulty of green fiber oxidization is. The softening point of ASPA-x series from asphaltene by air-blowing was markedly increased, but it was still lower than 200 oC. So, it is necessary to remove some small molecules and further thermal polymerization. The ASPA-4 was 7



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treated under nitrogen-blowing at 350 oC for 2 h, 3 h and 4 h and the softening points of the final samples were increased to 217 oC, 223 oC and 225 oC, respectively. The thermal melt behaviors of asphaltene, ASPA-4 and ASPAN-4 were observed by thermo-stage microscope with under N2 flow (heat rate: 10 oC/min). Their thermal melt behavior images were shown in Figure S1. All samples displayed excellent liquidity and homogeneity above softening points; moreover, there were no any insoluble. Table 2: Elemental analysis of asphaltene and resultant pitches with different conditions

Sample

Elemental composition (wt.%)

C/Ha

C

H

O

N

S

Asphaltene

92.41

5.29

1.33

0.73

0.24

1.46

ASPA-2

92.12

4.61

2.30

0.72

0.21

1.66

ASPA-3

92.27

4.22

2.59

0.72

0.20

1.82

ASPA-4

92.15

4.30

2.65

0.71

0.19

1.79

ASPAN-2

92.28

4.14

2.49

0.91

0.18

1.86

ASPAN-3

93.69

4.03

1.18

0.93

0.17

1.94

ASPAN-4

93.72

4.01

1.16

0.94

0.17

1.95

a

Atomic ratio of C and H

In general, there are intimate relationship between spinning property and the range of molecular weight distribution. The wider the molecular weight distribution is, the poorer the spinning property is. To compare the components evolution of asphaltene and resultant pitches derived from different conditions, the Q-TOF Mass spectrum and Gel Permeation Chromatography were taken to investigate the molecular weights distributions and components, the related results were shown in Figure 1 and Figure S2. The asphaltene showed a wider molecular distribution from 120 to 800 Da; after air-blowing at 300 oC, the related content of molecular weight less than 250 Da was decreased with blowing time, and the molecular weight distribution was concentrated between 250-600 Da; the small molecules content was further decreased and more large molecules were formed by nitrogen-blowing at 350 oC.

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Figure 1. The Q-TOF-MS spectra of the asphaltene (a); ASPA-2 (b), ASPA-3 (c), ASPA-4 (d), ASPAN-2 (e) and ASPAN-4 (f).

The thermal condensation could obviously improve the molecular weight of asphaltene by air-blowing, but the molecular weight distribution of resultant pitch was wider and the light components content was relatively higher. So, it is necessary to remove the light components, increase average molecular weight and concentrate molecular weight distribution for spinnable pitch. Then, the resultant pitch was further thermal treatment at 350 oC by nitrogen-blowing to realize following two objectives, one was promoted the removal of unstable oxygen-containing functional groups generated during air-blowing, and the asphaltene molecules were further cross-linked and the molecular weight was increased; the other was promoted the moderate 9



escaping of light components by nitrogen carrying off, improved the average molecular weight and relatively concentrated molecular weight distribution of the resultant pitch. As a result, the resultant pitch with a moderate increased softening point, relative concentrated molecular weight distribution and enhanced molecular cross-linking degree was obtained. It would benefit the stability of pitch spinning and reduce the difficulty of green fiber pre-oxidation process. FT-IR spectra of asphaltene, ASPA-4 and ASPAN-4 were shown in Figure S3 to investigate the changes of chemical groups. The characteristic peaks at 3045, 1600, 810, and 750 cm-1 could be ascribed as aromatic structure, and the peaks at 2920, 1440 and 1390 cm-1 were the characteristic bonds of aliphatic structure. The peaks at 1680, 1700, 1730 and 1350 cm-1 corresponding to carbonyl, carboxyl, ester bond and ether bond were appeared, which indicated that more oxygen-containing functional groups had been introduced into ASPA-4 by air-blowing. While these absorbance bonds were decreased after nitrogen-blowing at 350 oC due to decomposition of the introduced oxygen-containing groups. To investigate the thermal stability of synthesized spinning pitches, the weight loss behavior of ASP, ASPA-4 and ASPAN-4 were examined under N2 from ambient to 800 oC, the thermogravimetric curves were shown in Figure S4. The weight loss of asphaltene was mainly occurred in the range of 200 oC-450 oC for pyrolysis and volatilization of light components, the final weight loss reached nearly 73% at 800 oC. A prominent weight loss of ASPA-4 (nearly 21%) and ASPAN-4 (nearly 25%) appeared at the ranges of 350 oC to 500 oC could be attributed to pyrolysis of polycyclic aromatic compounds. The crosslinked ring structure formed by the condensation of asphaltenes during decomposition and pyrolysis resulted in a gradual weight loss between 500 oC and 800 oC, and the weight losses of ASPA-4 and ASPAN-4 were 37 % and 39 %, respectively. This suggested that thermal stabilities of ASPA-4 and ASPAN-4 were improved due to the thermal polycondensation of asphaltene under oxygen cross-linking and further polymerization. Chemical structures and aromaticity of the resultant pitches were determined by 13C NMR and the spectra were shown in Figure S5. Different types of carbon atoms could 10


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be ascertained by their chemical shifts appeared in all samples, and they could be ascribed as: terminal -CH3 in alkyl chain and -CH3 to aromatic ring appeared at 0-25ppm, -CH2 groups were situated at 25-51 ppm, methoxy group, -C-O bonds in alcohols and ether group located at 51-93 ppm, Ar-H emerged at 129-148 ppm, Ar-C ranged at 93-129 ppm, and carbonyl carbon atoms were seated at 171-235 ppm. 13C NMR analysis revealed the structure information of the asphaltene, ASPA-4 and ASPAN-4 in detail. During asphaltene thermal polymerization under air-blowing, the -CH3 and -CH2 groups were decreased and the -C-O group was increased due to the dehydrogenization and oxygen cross-linking reaction; aliphatic carbon content increased, while the fa decreased. However, the fa of ASPAN-4 markedly increased after ASPA nitrogen-blowing. The different kinds of carbon distributions of samples were listed in Table 3. The methyl- and methylene-hydrogen were decreased in asphaltene, ASPA-4 and ASPAN-4; this was attributed to the oxidization by air-blowing and thermal polycondensation under nitrogen-blowing. During air-blowing, oxygen not only reacted with hydrogen to form water but also resulted in intermolecular cross-linking of asphaltene, so, ASPA-4 had more R-O groups; while the cross-linked asphaltene molecules would further be polycondensated and decomposed forming some oxygen-containing groups under nitrogen-blowing at 350 oC, therefore, the hydrogen and oxygen contents of ASPAN-4 were lower than that of ASPA-4. The fa of asphaltene, ASPA-4 and ASPAN-4 were 0.65, 0.74 and 0.75, which suggested that the aromaticity of the resultant pitches increased by air-blowing at 300 oC and further nitrogen-blowing at 350 oC. Table 3: Distributions of aromatic and aliphatic components of asphaltene, ASPA-4 and ASPAN-4 obtained from 13C NMR data

Samples

Aliphatic (%)

Aromatic (%)

fa

-CH3

-CH2

R-O

Ar-H

Ar-C

Asphaltene

8.09

14.32

12.98

21.62

42.99

0.65

ASPA-4

5.76

4.70

15.14

25.58

48.82

0.74

ASPAN-4

5.24

4.50

14.81

23.48

51.98

0.75

11



The ASPAN-4 was successfully spun into fibers by melt spinning apporach at the temperature of 40-50 oC above its softening point. ASPAN-4 could be continuously spun for 50 minutes at a rotating speed of 300 m/min, and the diameter of obtained green fiber was 14 μm. While ASPA-4 could also be spun for 30 minutes at a rotating speed of 200 m/min, the diameter of obtained green fiber was 20 μm; it was frequently broken at rotating speed higher than 250 m/min. This indicated that ASPAN-4 had a good spinnability due to its higher softening point, concentrated molecular weight distribution and better rheological property.

Figure 2. SEM images of samples, green fibers of ASPA (a) and ASPAN (b), oxidized pitch fibers of ASPA (c) and ASPAN (d), asphaltene-based carbon fibers of ASPA (e) and ASPAN (f).

In generally, the mechanical properties of the pitch-based carbon fibers are enhanced with increasing stabilization temperature. The stabilization of pitch green

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fiber was a time-consuming process and determined the performance of pitch-based carbon fiber. Successful stabilization of pitch green fiber ensured that the fiber morphology and structure could not be destroyed during carbonization process so as to endow the resultant pitch-based carbon fiber with a higher tensile strength. The weight loss curve at a certain heating rate in air could reflect the reaction of the pitch molecules with air, thus providing a reference for the stabilization process of green fiber. The obtained ASPAN-4 green fiber was heated from 25 oC to 220 oC with 3 oC/min

and stabilized at 280 oC for 2h with under air flow rate of 400 mL/min (heat

rate: 0.5 oC/min). The stabilized fibers were heated to 1000 oC at in helium and kept for 10 min (heat rate: 5 oC/min). The surface morphologies of the carbonized fibers obtained from ASPAN-4 were observed by the SEM images, as shown in Figure 2. The average diameters of the carbon fibers were 12 μm with a smooth surface. The tensile strength of them ranged from 600-1100 MPa and the average tensile strength was over 800 MPa; this index met the standard of general property pitch carbon fiber. Compared with pitch carbon fiber from coal pitch and petroleum pitch, asphaltene-based spinnable pitch showed a facile synthesis route (air-blowing followed nitrogen-blowing), relatively concentrated molecular distribution and satisfactory resultant carbon fiber yield (35 wt% and 70 wt% based on CHRR and asphaltene, respectively).

Needle coke As specific type of coke, Needle coke endows developed microcrystalline structure and possesses high level of graphite; it is a vital resource to produce graphite electrodes using for electric arc steel furnaces. The superior needle coke should be equipped with higher anisotropy, larger crystallite size and larger crystal region. So, as to realize superior needle coke, two continuous processes are desired, i.e. the coalescence and formation of mesophase, mesophase rearranging during solidification. The formation of mesophase is determined by aromatic compounds in precursor, while the gas evolution during the solidification phase is facilitated by saturated 13



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compound.25 It has been demonstrated that asphaltenes have the ability to be highly reactive and increase the rate of carbonization; it may form an isotropic mosaic to generate coke, resulting in a high coefficient of thermal expansion value.26,

27

The

asphaltene from CHRR with high aromatic content and branch side with some saturate groups is an excellent candidate to prepare needle coke. When asphaltene was heated under nitrogen atmosphere, thermal-cracking, thermal-polymerization, condensation and dealkylation reactions were occurred. The polymerized aromatic molecules formed liquid crystal, and then the adjacent liquid crystals further coalesced. In a certain period of time, as the temperature increased, the liquid crystal was further converted to a semi-coke by delaying coking due to the high reactivity of the aromatic structure at high temperatures.

Figure 3. The dominating optical textures of semi-coke and needle coke derived from asphaltene, (a) 430 oC for 2 h, (b) 500 oC for 1 h, (c) carbonization at 900 oC for 1h and (d) carbonization at 1500 oC for 1h.

The polarized light optical microscopy deemed to be the best way to identify and characterize the morphology of thermal coke was chosen to characterize the derived semi-coke from asphaltenes. In this process, the optical texture of the polished cross section of samples is a key indicator. The corresponding polarized light micrographs of samples were compiled in Figure 3, which represented the dominating optical 14


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textures of semi-cokes from asphaltene at 430 oC for 2 h and 500 oC for 1 h before and after thermal treatment under nitrogen. More flow domain textures were formed and the trend of needle shape was increased with thermal treatment temperature. The excellent streamline structure was appeared when semi-coke was carbonized at 1500 oC

for 1 h. The XRD patterns of semi-coke carbonized at 900 oC and 1500 oC were shown

in Figure S6. The as-obtained sample at 900 oC showed a good organized carbon with certain graphitization degree, while diffraction intensity appeared between 10 and 20° was attributable to the disorderly arrangement of carbon and the transition state analogue of transition from amorphous carbon to graphitized carbon. The XRD diffraction pattern of the sample obtained at 1500 oC contained distinct diffraction peaks located at 2θ of 23°-27° and 42°-44°, which represented the (002) and (101) plane signals of graphitized carbon, this suggested that needle coke derived from CHRR was easy graphitized at related low temperature. This is mainly related to the structure of asphaltenes. The basic structural unit of asphaltene is based on the polyaromatic ring, and has high aromaticity and carbon content, so that it is easily graphitized. Compared with that reported in documents, needle coke with high graphitization degree could be prepared using asphaltene in a facile route,28 and its yields reached 25 wt% and 50 wt% based on CHRR and asphaltene.

Porous carbon nanosheet Porous carbon plays important role in catalyst support, adsorbent, and energy storage because of its developed pore structure.29,

30

Due to their ultrahigh ratio of

surface-to-volume, nano-scale porous carbon nanosheets showed excellent mass transportation, fast adsorption and desorption properties.31 The sp2-hybridized carbon species and polycyclic aromatic hydrocarbons richen in asphaltene endows it as a promising candidate for the preparation of highly graphitized porous carbon.32 The sample ASP-KOH with monolith morphology displayed in Figure S7 was obtained using asphaltene as precursor. This suggested that asphaltene without nitric 15



acid oxidation had good thermoplastic property and was prone to agglomeration, so, it was difficult to control and design the nanosheet structure of resultant porous carbon. In here, asphaltene was first oxidized using nitric acid to inhibit its thermoplasticity and improve its surface polarity. The morphologies of the ASP(O)-M and ACN achieved by SEM and TEM images were shown in Figure 4 and Figure S8. The ASP(O)-M obviously showed sheet structure with thickness nearly 33 nm (Figure 4 (a)). After KOH activation, the porous carbon nanosheet was distorted and irregular, and its thickness was decreased to about 12 nm, this was mainly caused by the reaction between activating agent and carbon matrix at 800 oC. The yields of ACN was about 18 wt% based on asphaltene. The HRTEM images in Figure S8 illustrated the formation of porous carbon nanosheet of ASP(O)-M and ACN. Some graphitized-like ribbons appeared in ACN, it was caused by the high aromaticity and prone graphitization of carbon precursor at high carbonization and activation temperature.

Figure 4. The SEM images of ASP(O)-M (a) and ACN (b, c).

Figure 5 showed the N2 adsorption-desorption isotherms and pore size distributions of ACN and ASP(O)-M. The primary structure parameters and elemental compositions of ACN and ASP(O)-M were presented in Table 4. The ACN displayed type I adsorption isotherm, and its BET specific surface area was 1717 m2/g; while that was only 327 m2/g for ASP(O)-M. The pore size distributions of ACN and ASP(O)-M calculated by nonlocal density functional theory were demonstrated in Figure 5 (b). The ACN displayed a high pore volume of 0.94 cm3/g and retained both small micropores (concentrated at 0.74 and 1.74 nm) and mesopores (at 2.5-3.0 nm). While ASP(O)-M showed a less pore volumel of 0.22 cm3/g and micropore locating at 16


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Page 17 of 27

0.74 nm, this indicated that oxidized asphaltene mixing with LiCl/KCl melt salt could form nanosheet structure of resulted porous carbon, and KOH activation could promote the pore structure developing. 700

0.25

(a)

600 500

Pore volume ( cm /g)

ACN

400 300 200

ASP(O)-M

100 0 0.0

0.2

0.4

0.6

(b)

0.20

ACN

3

3

Volume adsorbed ( cm /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8

1.0

0.15 ASP(O)-M 0.10 0.05 0.00

1

2

P/P0

3

4

5

6

7

8

Pore size (nm)

Figure 5. a) N2 adsorption isotherms and b) NLDFT pore size distributions of ASP(O)-M and ACN. Table 4 Chemical compositions and structure parameters of ASP(O)-M and ACN Elemental composition (wt%)a*

Structure parameter

Samples

C

H

N

O

SBET m2/g b*

Vtotal cm3/g c*

VMicro cm3/g

ASP(O)-M

89.47

0.55

5.17

4.81

327

0.43

0.21

ACN

92.28

0.45

2.71

4.56

1717

0.94

0.70

(a) Data from elemental analysis, b) Specific surface area from multiple BET method; c) Total pore volume at P/P0=0.99).

The FT-IR spectra of original asphaltene, ASP(O), ASP(O)-M and ACN were shown in Figure S9. Among them, the absorption peak at 3437 cm-1 was belonged to the N-H stretching vibration and O-H stretching vibration; the characteristic peak at 2918 cm-1 and 2852 cm-1 was the -CH2 stretching vibration. The peak located at 3045 cm-1 was assigned to the C-H stretching vibration of the aromatic ring skeleton, while the peak at 1610 cm-1 and 1450 cm-1 is the C=C stretching vibration of aromatic ring skeleton. These characteristic peaks were almost disappeared in the samples of ASP(O) and ASP(O)-M, indicating that the nitric acid oxidation and molten salt process had a great influence on the asphaltene aromatic ring skeleton structure. The sample ASP(O) had strong characteristic peaks at 1535 cm-1 and 1344 cm-1, which were caused by the stretching vibration of -NO2 and the stretching vibration of C-N 17



bond. While the peak at 1726 cm-1 was derived from the C=O stretching vibration of the aldehyde group. All of these characteristic peaks indicated that the ASP(O) contained a large amount of oxygen- and nitrogen-containing groups after HNO3 oxidation. These groups were significantly reduced in ASP(O)-M and ACN, but still had certain reservations, which would be favorable for CO2 adsorption. To verify the detailed surface chemical composition, XPS was further conducted. As shown in Figure S10, the complex C 1s spectra of ASP(O)-M and ACN were consisted of three peaks locating at 284.75 eV, 286.2 eV and 289 eV, which represented -C=C-, C-OH/C-O-C/C-O-R and -COOH configurations. The O 1s spectrum of ASP(O)-M was fitted into four peaks (Figure S10 (c)). The peaks at 530.8 eV, 532.15 eV, 533.5 eV and 534.1 eV were attached to C=O (carboxyl), C=O (ester, amides), C-O-C (ether oxygen) and COH/COOH/N-O-C, respectively.33 Compared with ASP(O)-M, there was no characteristic peak at 533.5 eV belonging to C-O-C (ether oxygen) in O 1s spectra of ACN (Figure S10 (d)). The N 1s spectrum of ASP(O)-M was also divided into four peaks (Figure S10 (e)), representing pyridinicnitrogen (N-6, 398.1 eV), pyrrolic- nitrogen (N-5, 400 eV), pyridine-N-oxide ( 402.6 eV) and oxidized nitrogen (NOx, N-O-C, 404.5 eV), respectively. As for ACN, no characteristic peak appeared at 404.5 eV that belonged to oxidized nitrogen (N-O-C/NOx) in N 1s spectra (Figure S10 (f)). The oxygen-containing groups and nitrogen-containing functional groups were decreased significantly, which was mainly caused by KOH activation at high temperature.34 This result was consistent with the elemental analysis result. The above results indicated the presence of oxygen- and nitrogen-containing groups in this carbon nanosheet using asphaltene as precursor. Its adsorption behaviour of CO2 were emerged in Figure 6, the CO2 uptakes of ACN and ASP(O)-M were 3.88 mmol/g and 2.20 mmol/g (at 25 oC). The FT-IR result illustrated that there was little nitrogen-containing groups on ACN surface, so, the high CO2 adsorption by ACN was mainly caused by physical adsorption. It revealed that CO2 adsorption rate and capacity were most related to the high surface area and pore volume of adsorbents. The micropores in carbon materials are the main active sites of physical adsorption 18


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


for CO2, ACN showed much larger surface area and higher pore volume than that of ASP(O)-M, but the difference of CO2 uptake was relatively less (3.88 mmol/g vs. 2.20 mmol/g). The nitrogen-containing chemical groups of ASP(O)-M was nearly 2 times of ACN (5.17 wt% vs. 2.71 wt%), and it had more oxidized nitrogen (N-O-C/NOx) species, so, a higher CO2 adsorption by ASP(O)-M appeared. Besides, the nitrogen groups locating on external surface and mesoporous wall of ASP(O)-M might interact with CO2. Given the above factors, CO2 adsorption was depended on he microporous structure and surface nitrogen groups. Compared with ACN, ASP(O)-M showed a better CO2 adsorption ability although it had a less surface area and lower microporous volume, this was mainly caused by the extra nitrogen groups in it. In order to investigate the reversibility of CO2 adsorption, the recycle performance of ACN was provided in Figure S11. Obviously, the CO2 adsorption amount was almost no attenuation after 10 runs, indicating its excellent cycling performance. 4

Amount adsorption (mmol/g)

Page 19 of 27

CO2 on ACN

3

CO2 on ASP(O)-M

2

1

N2 on ASP(O)-M N2 on ACN

0 0.0

0.2

0.4

0.6

0.8

1.0

P/P0

Figure 6. CO2 and N2 adsorption isotherms of ASP(O)-M and ACN.

The adsorption ability of ACN and ASP(O)-M for N2 were also tested. As shown in Figure 6, the N2 adsorption ability were much weaker than that of CO2 at same condition, achieving a maximum of 0.23 mmol/g and 0.22 mmol/g for ACN and ASP(O)-M at ambient pressure, respectively. The increment of SBET and Vtotal of ACN by KOH activation was attributed to the contribution of its microporosity, which was more facile for CO2 adsorption. Thus, at equal pressure for both CO2 and N2, the amounts of CO2 adsorbed was 16.86 and 10 times than that of N2 on ACN and ASP(O)-M. The CO2 breakthrough curves from CO2-N2 and CO2-CH4 for ACN was 19



tested to evaluate its separation performance, and the separation results were shown in Figure 7. It illustrated that ACN could dynamically separate CO2 from CO2-N2 (15/85, V/V) at 25 oC. The calculated CO2 uptake was upto 1.20 mmol/g by ACN. Meanwhile, the CO2 could also be dynamically separated from CO2-CH4 stream (10/90, V/V) by ACN at 25 oC. The calculated CO2 uptake reached to 0.63 mmol/g by ACN. These data were almost identical with the pure CO2 adsorption values at partial pressures of 0.13 and 0.10 bar, indicating that ACN had extremely selective for adsorbing CO2 over N2 or CH4. This suggested that porous carbon nanosheet derived from asphaltene was a promising candidate separation material for CO2.

(a)

1.0

1.0

0.8

0.8

0.6

0.6

F/F0

F/F0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

(b)

0.4

0.4

CO2

CO2 N2

0.2 0.0

0

2

4

6

8

10

CH4

0.2 0.0

12

0

2

4

6

8

10

12

14

Time (min)

Time (min)

Figure 7. Breakthrough curves of CO2 and N2 (a), CO2 and CH4 (b) on ACN under atmosphere pressure at 25 oC.

Conclusion Asphaltene, as the main composition of coal-heavy oil co-refining residue, is highly aromatic and polydisperse mixtures and is easy to form pitch with controllable mesophase features and as excellent candidate precursor for carbon fiber, needle coke and porous carbon nanosheet. Its chemical composition and structure could be adjusted by solvent extraction process. Spinnable pitch property could be controlled by asphaltene thermal condensation under air-blowing in combination with nitrogen-blowing, the resultant asphaltene-based carbon fiber with a diameter of 12 μm and tensile strength of 0.92 GPa were obtained via spinning, stabilization and carbonization. When asphaltene was heated under nitrogen atmosphere, thermal cracking, thermal polymerization, condensation and dealkylation reactions were 20


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occurred and it further transferred into semi-coke because of the high reactivity of aromatic structure at high temperature by delayed coking. The resultant needle coke with high graphitized degree and excellent streamline structure was appeared when semi-coke was carbonized at 1500 oC for 1 h. In addition, nitrogen-doped porous carbon nanosheet with a thickness of 12 nm, high specific surface area of 1717 m2/g and pore volume of 0.94 cm3/g was synthesized using asphaltene as carbon precursor, molten salt as template and KOH as activating agent. The synergistic effect of high special surface-to-volume ratio, faster mass-transport advantages and larger expanded plane layer resulted in a high CO2 adsorption ability and outstanding adsorption selectivity for CO2-N2 and CO2-CH4. This work revealed a viable utilization strategy of coal-heavy oil co-refining residue and asphaltene as a promising precursor to produce asphaltene-based carbon fiber, needle coke and novel nanosheet carbon structures for CO2 separation. This provided a sustainable principle, and concurrently attained the efficient utilization of different components of coal-heavy oil co-refining residue and develop new route for directional conversion or utilization of each component, and to realize its efficient utilization and high added value.

Acknowledgment This work is supported by National Key Research and Development Program of China (2016YFE0203500), National Science Foundation of China (U1510122) and autonomous research project of SKLCC (2018BWZ002).

Supporting Information Detailed characterization procedure, scheme of asphaltene-based porous carbon nanosheet synthesis, optical images, Gel Permeation Chromatography chromatograms, thermogravimetric curves,

13C-NMR

spectra, XRD patterns, FT-IR spectra,

SEM/HRTEM images, XPS spectra and CO2 recycle performance of samples.

Reference 21



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For Table of Contents Use Only

Asphaltenes with controllable structure and concentrated molecular weight distribution derived from coal-heavy oil co-refining residue could be utilized as high-performing precursor to prepare asphaltene-based carbon fiber, needle coke and porous carbon nanosheet due to its high average moleluclar weight and aromaticity.

26


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Asphaltenes with controllable structure and concentrated molecular weight distribution derived from coalheavy oil co-refining residue could be utilized as high-performing precursor to prepare asphaltene-based carbon fiber, needle coke and porous carbon nanosheet due to its high average moleluclar weight and aromaticity. 325x125mm (120 x 120 DPI)


From Coal-Heavy Oil Co-refining Residue to Asphaltene-Based

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