Controllable Synthesis of Highly Graphitizable Pitches from 1

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Controllable synthesis of highly graphitizable pitches from 1methylnaphthalene via closed-system dehydrobromination Haixiao Yang, Hexiang Han, Jitong Wang, Wenming Qiao, and Licheng Ling Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02714 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

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Controllable synthesis of highly graphitizable pitches from 1-methylnaphthalene via closed-system dehydrobromination Haixiao Yang,† Hexiang Han,† Jitong Wang,† Wenming Qiao,†,‡ and Licheng Ling*,†,‡ †

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

Technology, Shanghai 200237, China ‡

Key Laboratory of Specially Functional Polymeric Materials and Related Technology, Ministry

of Education, East China University of Science and Technology, Shanghai 200237, China KEYWORDS: graphitizable pitches, mesophase development, 1-methylnaphthalene, closedsystem dehydrobromination, methyl migration

ABSTRACT: In this paper, novel graphitizable pitches with controllable softening points and methylene-bridged structures were successfully prepared through photobromination of 1methylnaphthalene (1-MNa) followed by closed-system dehydrobromination (CSD). The structures of bromination products and dehydrobromination pitches were determined using GCMS, NMR and LDI-TOF/MS. It was found that the amount of bromine introduced greatly affected the composition of bromination products. 1-MNa brominated at a bromine/1-MNa molar ratio of 0.75 (BMNa-0.75) demonstrated the highest methyl bromination selectivity (Smb), which was selected as the dehydrobromination precursor. After a CSD/polymerization reaction under 200-250 oC, dehydrobromination pitches with softening points of 148-226 oC were acquired. Compared with open-system dehydrobromination (OSD), CSD endowed bromine radical longer life and boosted intramolecular and intermolecular linking, thereby substantially increasing

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softening points and coking values. The polymerization degree of CSD-derived pitches is highly regulable

depending

on

dehydrobromination

temperature

and

the

amount

of

tetrahydronaphthalene (tetralin) introduced. Moreover, methyl migration products (naphthalene, 2-methylnaphthalene,

dimethyl-naphthalene

and

trimethyl-naphthalene)

together

with

hydrogenation products (mainly tetralin) were detected by analyzing the hexane-soluble components (HS). The structural features of CSD-derived pitches contributed to some unique properties such as high polymerization degree accompanied by low aromaticity, continuous molecular weight distribution and complex connection types among subunits including α-α’, α-β’ and β-β’. The semi-coke with 94% anisotropy of coarse flow texture was synthesized at 420 oC under atmospheric pressure. Well-developed graphitic carbons with graphitization degree of 81.40% and ID/IG of 0.12 was obtained from graphitization of dehydrobromination pitches under 2600 oC.

1. INTRODUCTION Carbon materials are generally divided into two categories depending upon whether they are evolved from liquid-phase carbonization or solid-phase carbonization: graphitizable carbon (GC) and non-graphitizable carbon (NGC).1 For graphitic carbons, the graphite-like layers are arranged in parallel array, within which a small number of disordered structures are dispersed randomly. These turbostratic structures could be eliminated via heat-treatment above 2800 oC, resulting in highly ordered stacking of carbon layers.2,3 Owing to its oriented and crystallized carbon structures, GC exhibits higher chemical stability and thermal conductivities as well as better mechanical properties compared with NGC.4 As significant graphitizable carbon precursor, mesophase pitch has drawn growing attention, especially for pitch-based carbon fibers


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of high strength, modulus and thermal conductivity. Mesophase pitch, known as nematic liquid crystal of optical anisotropy, is composed of aromatic oligomers where aromatic sheets are stacked in parallel.5,6 Normally, mesophase pitch could be obtained from thermal condensation of coal tar and petroleum residues or catalytic polymerization of model aromatics. Despite that thermal polymerization exhibits virtues of easy synthesis, low cost and non-corrosive to the equipment, it needs to be held under higher temperature and longer reaction time in order to thermally induce radicals and ensure full growth of mesophase.7,8 Coupling of model aromatics at relatively low temperature can be realized in the presence of HF/BF3 or AlCl3.9-12 However, harsh acidic conditions and difficulties in catalyst separation limit the wide application of these processes. Thus, it is of necessity to further explore new ways of preparing mesophase pitch. In our previous work, bromine has been applied as an inducing agent to construct intermolecular C-C bond and polymeric pitches could be obtained through dehydrobromination of brominated arenes.13 The key to this process lies in selective side-chain bromination induced by visible light irradiation. The relatively low bond energy of methyl C-Br is quite beneficial for milder dehydrobromination (200-270 oC).14,15 However, bromination of 1-MNa using equimolar amounts of bromine is pre-requisite, which adds the cost of materials. Besides, elaborate vacuum distillation of brominated mixtures to obtain single 1-(bromomethyl) naphthalene is indispensable to control structures and properties of the resultant pitches, but the operation cost is significantly increased. Accordingly, how to utilize bromine to effectively and simply polymerize 1-MNa while achieving high feedstock utilization and decreasing operation cost is still a challenge task. Since photobromination has been proven to substantially enhance methyl bromination selectivity, more attention should be focused on improving dehydrobromination process. To date,


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numerous methodologies have been reported for dehydrobromination of alkyl or aryl halides.16-20 Most of these technologies involve the use of metal catalysts including palladium, nickel and iron. However, it is difficult to separate these metals from substrates, which seriously reduces the spinnability of pitches. Direct thermal dehydrobromination has features of convenient operation, essentially free from catalysts or additives and low costs. Thus, this method has already been employed to synthesize polymeric pitches from aryl bromides, but at least one to two molar equivalents of bromine is required due to low polymerization efficiency of this open-system.21 Borojovich et al. investigated the thermal behavior of brominated compounds in closed vessels and found that the presence of HBr may exert a catalytic radical effect on the pyrolysis of aromatics.22 To the best of our knowledge, there are few relevant reports concerning closed system dehydrobromination except for the study mentioned above. Herein, a new approach to prepare methylene-bridged graphitizable pitches from 1-MNa through CSD process is developed. Firstly, 1-MNa was brominated with the assistance of visible light irradiation, basically focusing on the influence of reactants ratio on Smb. Then, brominated aromatics were directly thermal-treated in a well-sealed container to cleave C-Br bond, thus polymerizing 1-MNa monomers. The effect of dehydrobromination mode (closed system or open system), dehydrobromination temperature and the amount of tetralin added (a kind of scavenger for bromine radicals) on the composition, physical properties and mesophase development of the final pitches were examined. Closed-system dehydrobromination mechanism was proposed as well. This facile CSD process significantly improves polymerization efficiency and bromine utilization. The prepared pitches exhibit high carbon yield, good rheology properties and huge potentials to develop 94% coarse flow texture, suggesting the promising application of CSD in the preparation of advanced functional carbon materials.


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2. EXPERIMENTAL SECTION 2.1 Pitch preparation. 1-MNa (97%, Acros Organics) and liquid bromine (AR, Sinopharm Chemical Reagent Co., Ltd) were used as raw materials without further purification. AR grade nheptane and tetralin was used as solvents for the bromination and dehydrobromination, respectively. The synthesis was started from the photobromination of 1-MNa in the presence of visible light irradiation. The reaction was carried out in a 1000 mL three-necked flask equipped with a reflux condenser. A series of brominated samples were obtained by varying the proportion of bromine/1-MNa. In a typical run, 142.20 g 1-MNa was dissolved in a certain amount of nheptane to adjust the solution to 1 mol/L by vigorous stirring under a N2 gas flow of 100 mL/min. The xenon lamp (PLS-SXE300, Beijing Trusttech Co. Ltd., China) equipped with an ultraviolet cut-off filter was applied to irradiate the reaction mixture (λ ≥ 400 nm). About 120g (0.75 molar equivalent) bromine was added dropwise to stirred solutions over a period of 16 hr. The generated hydrogen bromide was absorbed using saturated sodium hydroxide solution. The brominated mixture was recovered through rotary evaporation and denoted as BMNa-0.75. After the optimal proportion of bromine/1-MNa determined, dehydrobromination was carried out in a 100 mL well-closed polytetrafluoroethylene cylindrical vessel. 60g brominated products were added into the vessel and Argon was blown to replace air for 15 min. Stainless steel shell was used to seal polytetrafluoroethylene vessels. Then dehydrobromination was conducted in a chamber furnace at 200 oC for 12h, 230 oC for 9h and 250 oC for 6h under a nitrogen flow of 100 mL min-1 at a heating rate of 2 oC min-1, respectively. The final pitch was extracted with nhexane to remove low-molecular-weight fractions. The resultant pitches were labeled as P200,


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P230 and P250 respectively. In order to study the effect of dehydrobromination mode on the composition of pitches, 60g brominated products were heat-treated at 230 oC for 9h in a 100 mL three-necked Pyrex glass flask with continuous Argon blowing to remove generated hydrogen bromide. The extracted pitch was named as T230. As for tetralin assisted dehydrobromination, brominated products and tetralin were firstly premixed via supersonic treatment for 10min. Then the mixture was thermal-treated by similar procedures aforementioned. The amount of tetralin added was based on moles of brominated products (the bromination mixture is considered as a unity and relevant calculations are on the basis of average molecular weight of brominated products). The samples were abbreviated as Pxy, where the x and y denote dehydrobromination temperature and molar ratio of tetralin to brominated products. 2.2 Carbonization and graphitization procedure. The feasibility of mesophase formation of as-synthesized pitches was systematically evaluated in this work. Typically, 2 g of sample was placed in a horizontal quartz tube followed by carbonization at 420 oC for 3h with a heating rate of 150 oC/hr under a nitrogen flow of 120 mL/min. Graphitization was carried out in an electric resistance furnace. The sample devolatilized at 800 oC was first heated to 2000 oC (at a heating rate of 10 oC/min from room temperature), then kept heating to 2600 oC at a heating rate of 8 o

C/min and finally held at 2600 oC for 30min. The carbonized and graphitized samples were

named as P230-C and P230-G (taking P230 as an example), respectively. 2.3 Characterization. The categories and distributions of bromination products were determined in a Shimadzu GCMS-QP2010 plus instrument. Column temperature was initially kept at 100 °C for 5 min, then gradually increased to 260 °C at a rate of 7 °C/min and finally


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held for 3 min. The samples were diluted 1:100 (v/v) with toluene, and filtered through a 0.45 µm pore-size polytetrafluoroethylene syringe ﬁlter. 1.0 µL of the diluted sample was injected with a split ratio of 1:30. The calibration curves of each fraction in the bromination products were established for quantitative analysis by using standard substances. The HS fractions of polymeric pitches were also analyzed by GCMS through similar procedures as above. The main difference resided in quantitative methods: for HS fractions of polymeric pitches, quantitative analysis results of each essential component (expressed as area percentage) was obtained from peak area normalization measurement. The softening points of extracted pitches were measured by thermomechanical analysis (DMA2980, America). The samples were initially melted in a quartz crucible and then placed in the chamber with a probe on the surface of the samples exerting a constant force of 0.001N. The samples were heated from ambient to 350 °C at a heating rate of 5 °C/min in inert atmosphere. The elemental analysis of C and H was performed using an Elemental Vario EL III. The residual bromine content was calculated by the difference. The solubility was evaluated by sequential Soxhlet extraction using toluene, pyridine and quinolone as solvents. Toluene-soluble fractions (TS) of the pitches were characterized by 1H NMR on a Bruker Avance 400 spectrometer. The sample was dissolved in CDCl3 using tetramethylsilane (TMS) as an internal standard. The chemical structures of prepared pitches were investigated by solid-state 13C NMR (Bruker AVANCE III 300 WB spectrometer). The molecular weight of pitches in terms of toluene-soluble fractions was analyzed with the 4800 Plus LDI TOF/MS Analyzer (AB Sciex). The pitches were dissolved in toluene to prepare solutions with a concentration of 10 mg/mL for test. The rheological properties of the synthesized pitches were recorded using a parallel plate rheometer (Haake Mars III, Thermo Scientific, Germany) in an N2 atmosphere.


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The optical textures of carbonized samples were observed with OLYPUMS-BX51 reflected polarized microscope equipped with an adjusted ocular (10ⅹ), an oil immersion objective (50ⅹ) and a 1-λ retarder plate. The samples were embedded in epoxy resin and polished using alumina powder on a cloth pad. Representative pictures of the samples were taken using objective of 20ⅹ magnification. The crystal structures of graphitized products were obtained by X-ray diffraction (XRD) analysis using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ=1.54056 °A). Silicon was used as inner standards to calibrate the (002) peak position. The interlayer spacing (d002), crystalline size along the c-axis (Lc) and degree of graphitization (g) were estimated using the corresponding formulas (Bragg, Alexander and Mering-Maire equations).23 Raman spectra were recorded on Raman spectrometer (Spex 1403) with an argon ion laser at an excitation wavelength of 514.5 nm. The average crystalline size along the a-axis (La) was calculated using the Cançado equation.24 The morphology and microstructure of samples were observed by a transmission electron microscope (TEM, JEOL 2100F) at 200 kV.

3. RESULTS AND DISCUSSION 3.1 Selection of the dehydrobromination precursor. Previously, Ge et al. reported a visible light irradiation assisted bromination of 1-MNa followed by thermal-dehydrobromination method to prepare polymeric pitches.13 In this process, visible light irradiation has turned out to be an effective way to promote methyl bromination. Smb could reach 99.4% when 1-MNa was reacted with equimolar amounts of bromine under visible light. However, the influence of bromine/1-MNa ratio on the composition of bromination products was not presented in the work. In addition, we has discovered that the total amount of reactants strongly affects the selectivity.


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Smb drops to 86.6% when a fivefold scale-up bromination is performed (as shown in Table. 1). So in order to increase Smb and reduce materials cost, varying the proportion of bromine/1-MNa is proposed to regulate the composition of bromination products. To note, the scale-up bromination reactions are carried out as follow: 60 oC, 1mol/L in n-heptane, visible light irradiation. Fig 1(a) displays GC-MS spectra of various bromination products. 0.25, 0.5, 0.75 and 1.0 molar equivalents of bromine are reacted with 1-MNa, and the bromination products are labelled as BMNa-0.25, BMNa-0.5, BMNa-0.75 and BMNa-1.0 respectively. As can be seen from the spectra, five kinds of components are detected with the retention time 7.26 min, 13.69 min, 14.35 min, 18.45 min and 19.38 min. The identification of each component was based on matching of the mass spectra with the National Institute of Standards and Technology (NIST) database. Fig 1(b) illustrates the structures of components in bromination products, including unreacted 1-MNa monomer (A), monobromo-MNa (B, C) and dibromo-MNa (D, E). Calibration curves of each fraction are available in Fig S1. Detailed product distributions under different synthesis conditions are listed in Table 1. It can be found that when bromine amount is lower than 0.5 equiv., almost no dibromo-MNa is produced. But at least 55% 1-MNa remains unsubstituted. With the increase of bromine amount to 0.75 equiv., substantial 1-MNa molecules are monobrominated with Smb increased to 96.6%. Further increase of bromine amount to 1.0 equiv. favours the generation of dibromo-MNa and Smb is reduced to 86.6%. As for the reason, the viscosity gradually increases with more molecules brominated, the diffusion rate of monobrominated derivatives from surface layer to the hydrocarbon bulk is decreased and monobromoMNa would be re-brominated more easily.25 So from the view of methyl selectivity, BMNa-0.75


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is chosen as dehydrobromination precursor. It needs to be pointed out that BMNa-0.75 is directly used for dehydrobromination without further purification such as distillation or extraction.

Figure 1. (a) GC-MS spectra of bromination products obtained from 1-MNa brominated under different bromine addition, (b) Schematic diagram for photobromination of 1-MNa.

Table 1. Component distribution of bromination products obtained from different bromine addition Molar ratio

Mole percent/mol%

Samples

a

Smba /%

(Br2:1-MNa)

A

B

C

D

E

BMNa-0.25

0.25

87.0

0.6

12.4

0

0

95.4

BMNa-0.5

0.50

44.9

2.1

52.3

0

0.7

96.1

BMNa-0.75

0.75

19.8

2.6

74.0

2.6

1.0

96.6

BMNa-1.0

1.00

7.2

6.3

78.2

2.3

6.0

86.6

Smb=100C/(B+C).

3.2 Comparison of OSD- and CSD-derived pitches. After the optimized bromination conditions (molar ratio of bromine/1-MNa is set as 0.75) determined, a novel closed-system dehydrobromination was carried out to polymerize 1-MNa monomers. Table 2 summarizes some physical properties of P230 including softening point, carbon yield, solubility and H/C ratio. T230 prepared through OSD is used for comparison. As can be seen from Table 2, CSD


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substantially increases softening point and carbon yield compared with OSD. Besides, T230 is 100% soluble in toluene while P230 has 51.3 wt % toluene-insoluble fractions, indicating higher polymerization degree of the latter. To further elucidate the difference between the two pitches, both were measured by 13C NMR. The 13C NMR spectra of P230 and T230 are presented in Fig 2, and the distributions of aromatic and aliphatic carbons are listed in Table 3. Both pitches carry a considerable amount of methylene hydrogen, which are consistent with the characteristics of dehydrobromination pitches.13 P230 possesses 34.70% protonated aromatic carbons compared to 47.69% found in T230. In addition, Car3/ Car2 of P230 is 0.49 while this value is 0.30 for T230. All results provide solid evidence that P230 has more condensed structures than T230 and CSD significantly boosts intermolecular crosslinking.26 However, it’s strange that the aromaticity of P230 is lower than T230 (0.83 vs 0.88). Besides, Csar of P230 reaches 13.51%, which is higher than T230. The fa value contradicts the conclusion that P230 is more condensed than T230. Table 2. General physical properties of the synthesized pitches Solubilityd/wt%

Elemental analysis

S.P.a (oC)

CYb (wt%)

C/wt%

H/wt%

H/Cc

TS

TI-PS

PI-QS

QI

P230

218

59.6

88.15

4.49

0.611

48.7

17.7

15.0

18.6

T230

140

28.2

91.29

5.05

0.664

100

0

0

0

Samples

a

Softening point.

b

Carbon yield at 850 oC.

c

Atomic ratio.

d

TS: toluene soluble fraction; TI-PS: toluene insoluble-pyridine soluble fraction; PI-QS: pyridine insoluble-quinoline soluble fraction; QI: quinoline insoluble fraction.


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Figure 2. 13C NMR spectra of (a) P230 and (b) T230, respectively.

Table 3. The distributions of aliphatic and aromatic carbons of P230 and T230 by analyzing solid-state 13C NMR spectra Pitch

Aliphatic (%)

Aromatic (%) Car1,3

Car1,2

Car3/ Car2

fa

CH3a

CH2b

Cchainc

CHard

Car3e

Csarf

Car2g

P230

4.82

4.76

7.74

34.70

11.34

13.51

23.13

0.49

0.83

T230

2.35

1.99

7.43

47.69

6.90

10.74

22.90

0.30

0.88

a

Methyl carbons.

b

All the rest of methylene carbons (CH2 different from Cchain).

c

Bridge/hydroaromatic structures (methylene carbons in α position to two aromatic rings).

d

Protonated aromatic carbons.

e

Pericondensed aromatic carbons.

f

Aromatic carbons joined to aliphatic chains.

g

Catacondensed aromatic carbons, aromatic carbons with aromatic substituents.

Considering the obvious structural differences between P230 and T230 as well as contradictions of aromaticity (fa) with physical parameters, it is of necessity to explore the mechanism of CSD, thus adjusting dehydrobromination conditions to obtain high-quality pitches. Due to the complexity of reaction system and corrosive properties of HBr, it is


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very difficult to acquire related information of intermediates. An effective solution to reveal CSD mechanism is to analyze structures of resultant pitches. GC-MS techniques have been successfully applied to elucidate the compositions of hydrocarbons such as ethylene tar and low temperature coal tar.27,28 One advantage of this technique lies in its exceptional sensitivity so that nearly all relatively light components can be reflected in the spectra. Pitches are complex mixtures consisting of aromatic hydrocarbons having more than three condensed rings and aliphatic hydrocarbons with side chains or naphthenic rings. Taking the upper temperature limit of chromatographic column into consideration (350 oC for DB-5ms, Shimadzu), high-molecular-weight components can’t be measured by GC-MS. Here, HS constituents of dehydrobromination pitches are used for GC-MS analysis. Fig 3(a) compares GC-MS spectra of P230HS and T230HS. As can be seen from the spectra, P230 and T230 present totally different product distributions. T230HS consists of two major components while P230HS is composed of up to ten fractions. The possible structures are summarized in Fig 3(b). Comparing with T230, P230 has more complicated compositions including hydrogenation products (1, 3 and 4), methyl migration products (2, 5, 7 and 9), 1-MNa, α-bromonaphthalene and 1-bromo-4-MNa. As mentioned earlier, there are conflicts between the aromaticity and physical parameters for P230. This strange phenomenon can be explained as follows: for CSD, methyl migration and hydrogenation reaction occur during thermal-treatment of brominated products, so it is reasonably assumed that more methyl groups would be bonded to aromatic nucleus of resultant pitches and the existence of hydrogenation structure further increases the content of


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aliphatic carbons, thereby decreasing the aromaticity. This explanation is also evidenced by the higher amount of Csar in P230.

Figure 3. (a) GC-MS spectra of hexane-soluble fractions of P230 and T230, (b) possible structures of components in P230 and T230.

3.3 Elaboration of closed-system dehydrobromination reaction pathways. In section 3.2, CSD has been proved to polymerize 1-MNa molecules efficiently. Furthermore, methyl migration and hydrogenation products emerges in CSD-derived pitches, which is not found in OSD-derived pitches. However, Leininger et al. investigated thermal-pyrolysis of 1-MNa in a batch reactor and found that demethylation, methyl addition and hydrogenation reactions were observed in the temperature range of 380-450 oC under a constant pressure of 100 atm.29 The study concerning thermal cracking of 1,2,4trimethylbenzene by Fusetti et al. had demonstrated that ipso addition reactions of CH3 and H occurred in the temperature range of 395-450 oC and at pressures of 100 bar.30 So how could methyl migration and hydrogenation reactions happen during CSD process under such low temperature (230 oC) without the assistance of catalysts? In order to make this problem clear, the closed-system dehydrobromination mechanism is proposed in Fig


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4. The reaction is initiated by thermal cleavage of methyl C-Br. The dissociation of C-Br bond in 1BMNa results in 1RMNa and bromine radicals. For OSD, bromine radicals are immediately in combination with hydrogen generated from polymerization of two 1-MNa molecules. For CSD, however, the high HBr concentration at gas-liquid interface in conjunction with the lack of gas-blowing restrains the binding of hydrogen and bromine radicals. Consequently, bromine radicals are preferentially retained instead of incorporating with hydrogen. The reserved bromine radicals would react with adjacent 1MNa or 4B-1MNa due to its strong hydrogen abstraction ability. Normally, a bromine radical interacts with several 1MNa or 4B-1MNa molecules, ultimately producing multiple methyl radicals-naphthalene. Leininger et al. reported that the rate constant of hydrogen abstraction from the aromatic moiety was markedly lower than that from the methyl group.31 So hydrogen abstraction from methyl group dominates (equation 2 and 3 in Fig 4). Meanwhile, the formation of 1RMNa and 4B-1RMNa are accompanied by generation of several free H atoms. The free atomic hydrogens are generally consumed in the following two ways: (1) as illustrated in equation 5, ispo-addition of H atoms yields naphthalene and CH3 radicals. The CH3• collides with 1-methylnaphthalene, hence dimethyl-naphthalene (DMNa) and even a small quantity of trimethyl-naphthalene (TMNa) are formed; apart from addition on 1MNa, the CH3• could undergo addition reactions with naphthalene to produce 2-MNa.29,32 (2) the atomic hydrogens directly attack naphthalene ring, hydrogenating naphthalene (equation 7) and 1-MNa (equation 8) into corresponding products.33 Besides these reactions mentioned above, transformation of 1RMNa into 1MNRa can be realized through radical hydrogen transfer reactions (see equation 4). This radical hydrogen transfer reaction is endothermic and the energy barrier


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is 26.8 kcal/mol, which could be satisfied under such conditions. So at least three kinds of mono-aromatic radicals including 1RMNa, 4B-1RMNa and 1MNRa are incorporated into the oligomers through molecular assembly.

Figure 4. Proposed reaction scheme for closed-system dehydrobromination.


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As an excellent radical scavenger, tetralin has been extensively studied in thermal degradation of brominated compounds.22,34 In these researches, tetralin was used to trap and quench Br• to form thermodynamically stable HBr. In order to examine whether the reserved Br• is responsible for distinctions between CSD and OSD, tetralin was introduced to regulate quantity of the reserved Br• during CSD process. GC-MS spectra of samples synthesized through tetralin-assisted dehydrobromination are illustrated in Fig 5. Detailed product distributions are shown in Table 4. The addition of tetralin greatly changes product distributions. For P230, 1-MNa is the dominant fraction. Nevertheless, naphthalene becomes the dominant with the introduction of tetralin. This phenomenon might partly be explained by the following equation:

Furthermore, the percent of 2MNa, DMNa and TMNa is gradually reduced with more tetralin introduced. For P230, the sum of 2MNa, DMNa and TMNa reaches 18.6%. However, the percentage drops to 13.1% after 5% tetralin is added. This value is monotonically reduced to 7.9% with further increase of tetralin amount to 20%. Accordingly, it can be concluded that the reserved bromine radicals result in methyl migration.


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Figure 5. GC-MS spectra of samples obtained from tetralin-assisted CSD in terms of HS fraction.

Table 4. Component distribution of the products obtained from CSD Area percentage / % Samples

Hydrogenation products

Methyl migration products

1

3

4

Total

2

5

7

9

Total

6

8

10

P230

0.8

0.1

0.6

1.5

26.4

3.1

13.9

1.6

45.0

52.6

0.5

0.4

P230-5%

0.9

0.1

0.5

1.5

46.4

2.5

9.4

1.2

59.5

38.4

0.3

0.3

P230-10%

1.7

0.1

0.6

2.4

45.4

2.1

8.6

0.5

56.6

40.4

0.4

0.2

P230-20%

9.3

0.6

0.9

10.8

41.3

1.5

6.2

0.2

49.2

39.1

0.6

0.3

3.4 Characterization of dehydrobromination pitches 3.4.1 Influence of dehydrobromination temperature on physical properties of pitches. General properties of the resultant products obtained from different dehydrobromination temperatures are summarized in Table 5. Results indicate that dehydrobromination temperature has a noticeable effect on softening point and carbon yield of polymeric pitches. The softening points exhibit a sharp rise from 148

o

C to 218

o

C when

dehydrobromination temperature is increased from 200 oC to 230 oC. Further increase of dehydrobromination temperature to 250 oC leads to a gentle rise of softening points from


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218 oC to 226 oC. The carbon yield, H/C ratio and solubility display similar variation trends. These results suggest that there exists a distinct turning point in variation of softening point, carbon yield and H/C ratio between 200 oC and 230 oC. Before the turning point, methyl C-Br cleavage and hydrogen abstraction reactions proceed at a relatively low rate (as indicated by H/C ratio of P200), which results in insufficient incorporation of arenes into the oligomer framework and thus low polymerization degree. Consequently, the softening point, carbon yield and toluene-insoluble fractions of P200 are far below that of P230 and P250. After the turning point, all related reactions illustrated in Fig 5 are favored, which can be verified by product distributions of pitches in Table S1. Simultaneously, the collision of Br• with heavier products is promoted with the increase of temperature. This can partly explain why P250 possesses more residual bromines than P230 and P200. Table 5. General physical properties of the CSD-derived pitches Elemental analysis

Solubility/wt%

S.P. (oC)

CY (wt%)

C/wt%

H/wt%

H/C

TS

TI-PS

PI-QS

QI

P200

148

30.2

90.94

5.22

0.689

83.0

16.6

0.1

0.3

P230

218

59.6

88.15

4.49

0.611

48.7

17.7

15.0

18.6

P250

226

67.2

88.12

4.24

0.577

42.3

12.3

13.6

31.8

Samples

The 1H NMR spectra of extracted pitches are shown in Fig 6 with regards to toluene soluble components. Detailed distributions of the constituent hydrogens are summarized in Table 6.35 T230 carries more Har and higher Har/Hal than those of P230, which is in agreement with the 13C NMR analysis results. Har of CSD-derived pitches decreases while Hal increases with dehydrobromination temperature. As shown in Table S1, methyl migration and hydrogenation products occupy a larger proportion in HS fractions with the


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increase of temperature. It is believed that similar situations are accessible for heavier components. Therefore, increase of dehydrobromination temperature leads to more severe methyl migration and hydrogenation, thus enhancing the content of aliphatic hydrogens. This conclusion can be testified by the variation of peak at 2920 cm-1 in FT-IR spectra (the above peak is assigned to aliphatic C-H stretching, see Fig S3). In addition, response for Hβ and Hγ is found in the pitches due to the existence of hydrogenation structures. Except for hydrogen distribution information, connection mode of basic structural unit in oligomers can also be deduced from 1H NMR spectra. Ge et al. reported that oligomers exhibited a single and definite peaks at 2.7 ppm in the chemical shift range of 2.0-5.2 ppm when 1MNa molecules were exclusively linked in accordance with 1-4’ or 1-5’ positions, and the peak shifted to higher fields with the advent of α-β’ or β-β’connection mode.13 As can be seen from Fig 6, T230 displays a sharp peak at chemical shift of 2.7 ppm, suggesting the dominance of 1-4’ or 1-5’connection mode in OSD-derived pitches. On the contrary, CSD-derived pitches exhibit a broad peak in the chemical shift of 2.0-3.2 ppm, which indicates complicated bonding position including α-α’, α-β’ and β-β’. This is probably due to the radical hydrogen transfer reactions (see equation 4, Fig 4).


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Figure 6. 1H NMR spectra of dehydrobromination pitches.

Table 6. Structural parameters calculated from 1H NMR spectra of dehydrobromination pitches 1

H NMR

Samples Har

Hα

Hβ

Hγ

HCH2

Har/Hal

T230

0.787

0.213

0

0

0.111

3.69

P200

0.666

0.306

0.021

0.007

0.129

1.99

P230

0.644

0.315

0.035

0.006

0.091

1.81

P250

0.638

0.318

0.038

0.006

0.076

1.76

Har: aromatic hydrogens, 9.5-6 ppm. Hα: alpha aliphatic hydrogens, 5.2-2.1 ppm. Hβ: beta aliphatic hydrogens, 2.1-1.1 ppm. Hγ: gamma aliphatic hydrogens, 1.1-0.5 ppm. HCH2: aliphatic hydrogens in methylene α to two aromatic rings, 5.2-4.0 ppm. Har/Hal: hydrogen aromaticity index.


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Fig 7 illustrates LDI-TOF/MS spectra of the pitches in terms of toluene soluble fractions. T230 exhibits major peaks at 282, 422, 562, 702 and 842 m/z, which are assigned to dimers, trimers, tetramers, pentamers and hexamers respectively. Some weak peaks emerge around main peaks in T230, which are attributed to slight radical hydrogen transfer during OSD (a small number of free-state H atoms would be generated inevitably). Different from T230, CSD-derived pitches exhibit several major peaks in various oligomers (dimer to octamer). Take trimer as an example, peaks at 408, 422, 436, 450 and 464 m/z are distributed periodically with intervals of 14, implying extensive methyl migration during CSD process. Besides, considerable weak peaks continuously distributed between the major peaks are available, which indicates a deeper radical hydrogen transfer. The average molecular weight (AMW) of P230 and T230 is 660 and 556 respectively, suggesting the dramatic effect of CSD on molecular evolution. Increasing dehydrobromination temperature from 200 oC to 250 oC apparently enhance AMW of CSD-derived pitches from 573 to 694. Interestingly, the LDI-TOF/MS spectra of TI-PS components in P230 (see Fig S4) shows that P230TI-PS is mainly composed of dimer to octamer, indicating the formation of TI-PS fractions is originated from intramolecular crosslinking and peri-condensation rather than chain growth and elongation.


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Figure 7. LDI-TOF/MS spectrums of as-prepared pitches. (a) Whole spectrum for each sample; (b) Spectrum for the variation of representative trimer in each sample.

The capability of mesophase formation and development is one of the most critical index to evaluate the performance of pitches. In order to investigate the liquid-phase carbonization behavior of dehydrobromination pitches, samples are subject to heattreatment at 420

o

C for 3h and the carbonized products are observed by optical

microscopy. Fig 8 shows optical micrographs of the carbonized products obtained from different precursors. T230-C is totally isotropic and no anisotropic texture is observed. Contrary to T230-C, P230-C shows mosaic structures containing a great number of discrete microspheres (less than 30 µm in diameter) and coalesced mesophase. The distinctions in optical texture of P230-C and T230-C are derived from structural difference of precursors. On one hand, substantial methylene hydrogens in T230 would be transformed into naphthenic structures through dehydrogenation under elevated temperature, the naphthenic structures significantly reduce thermal reactivity of the system via H-transfer reactions. However, the enrichment of alkyl chains in P230


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compensates for the thermal reactivity to a certain extent. On the other hand, T230 is relatively homogeneous in molecular constitution (100% soluble in toluene) while P230 has 18.6 wt% QI. These QI particles in P230 serve as nucleating centers, which would accelerate the formation of mesophase textures. Furthermore, the dehydrobromination temperature has a significant influence on the optical texture of carbonized products. The optical texture of P200-C is fine-grained mosaic accompanied by plenty of tiny microspheres (less than 1.5 µm in diameter) while P250-C exhibits coarse-grained mosaic without obvious coalescence of spheres (less than 12 µm in diameter).36,37 As mentioned earlier, the aliphatic hydrogen content of the pitches obeys the order P250 > P230 > P200, resulting in diverse thermal reactivity during carbonization. P200 with the least amount of side-chains is supposed to manifest the lowest thermal reactivity. Consequently, the nucleation rate is reduced and further development of mesophase is hindered. In contrary, P250, which possesses the highest content of aliphatic chains, undergoes violent cracking of side-chains during carbonization. Although excessive aliphatic chains improve thermal reactivity and promote the formation of mesophase, the drastic elimination of alkyl groups sharply increases the viscosity, which will restrict the coalescence of mesophase spheres and contribute to the formation of coarse-grained mosaic textures. Accordingly, P230 with appropriate compositions and moderate thermal reactivity shows potentials to develop mesophase with larger unit dimensions.


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Fig 8. Optical micrographs of semi-cokes. (a) T230-C, (b) P200-C, (c) P230-C and (d) P250-C.

3.4.2 Influence of tetralin addition amount on physical properties of pitches. Despite P230 demonstrates its advantages on mesophase development compared with P200 and P250, P230-C presents mosaic texture, which is probably due to high QI content. One solution to improve the optical texture of obtained semi-cokes is to shorten reaction time in order to suppress excessive polymerization. However, this solution is at the expense of reducing yield and increasing residual bromine content. Another approach is to in situ control hydrogen abstraction reaction by introducing bromine free radical scavenger. Table 4 has proven that the addition of tetralin could effectively limit methyl migration reactions. Therefore, investigation of effect of tetralin amount on physical properties of resultant pitches is carried out. Table 7 summarizes physical properties of resultant


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Page 26 of 38

pitches prepared through tetralin-assisted CSD. The softening points, carbon yield and residual bromine content gradually decrease with the increase of tetralin amount. Furthermore, QI are reduced monotonously with the increase of tetralin amount, especially for P230-20% with 0.7% QI. All these indicates that adjusting the amount of tetralin added could control the properties of obtained pitches with purposes. Table 7. General physical properties of pitches prepared via tetralin-assisted CSD Elemental analysis

Solubility/wt%

S.P. (oC)

CY (wt%)

C/wt%

H/wt%

H/C

TS

TI-PS

PI-QS

QI

P230-5%

209

54.2

88.31

4.57

0.621

56.2

15.4

14.1

14.3

P230-10%

203

45.3

89.43

4.86

0.652

67.4

13.6

11.4

7.6

P230-20%

186

25.6

91.73

5.27

0.689

90.5

6.9

1.9

0.7

Samples

Fig 9 shows optical micrographs of semi-coke obtained from various precursors. It can be found that the amount of tetralin strongly affects optical textures of resultant semi-coke. The optical texture of P230-5%-C is mixture of fine-grained mosaic and fine fibrous mesophase. Some small islands of isotropic matrix are wholly surrounded by mesophase and the isotropic droplets also contain a small number of minute spherules. Increase of tetralin amount to 10% contributes to the improvement in the optical texture of obtained semi-cokes. Compared with P230-5%-C, P230-10%-C exhibits better-developed mesophase texture, where large domains and discrete spheres coexist. Although the introduction of tetralin eases the thermal reactivity and improves optical textures, considerable number of isotropic droplets are entirely enclosed by mesophase even in P230-10%-C. Intense cracking of aliphatic side-chains during carbonization dramatically increases the viscosity. The system could not provide decent viscosity for mesogenic molecules to shift, align and form well-organized optical texture. So further increase of


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tetralin amount to 20% is carried out in an attempt to maintain the viscosity at a moderate level. Results indicate that the optical texture of P230-20%-C is completely separated mesophase spherules (up to 90 µm in diameter) and no evident coalescence of adjacent spheres is observed. The low degree of mesophase development for P230-20%-C should be attributed to the combined effect including low abundance of oligomers possessing multi-alkyl substituents and a certain number of naphthenic structures (as shown in Fig S5). Besides, the contour of the spheres in Fig 9(c) is precisely circular. Brooks and Taylor reported that the presence of fine insoluble particles restrains symmetric growth of spheres.1 P230-20% with only 2.6 wt% PI fractions could ensure the uniform growth of spherules in all directions during liquid-phase carbonization. Extending the residence time to 6h promotes full growth and complete coalescence of mesophase spherules to produce semi-coke with 94% anisotropy of coarse flow texture.


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Page 28 of 38

Fig 9. Optical micrographs of semi-cokes. (a) P230-5%-C, (b) P230-10%-C, (c) P230-20%-C and (d) P230-20%-C-L (P230-20% heat-treated at 420 oC for 6h).

3.5 Rheological properties of dehydrobromination pitches. The processing ability of pitches, spinnability in particular, is closely related with flow properties of such materials. In order to better understand the processibility of dehydrobromination pitches, rheological behavior of as-prepared pitches is investigated on the HAAKE Rotational Rheometer. Fig 10(a) shows the viscosity versus temperature curves of P230 and P250. The initial viscosity of P250 is about 60000 Pa•s, which is higher than that of P230 (20000 Pa•s), P200 (686 Pa•s, see Fig S6) and T230 (346 Pa•s as indicated by Fig S6). The polymerization degree follows the order P250 > P230 > P200 > T230 according to initial viscosities of samples. The apparent viscosity of dehydrobromination pitches exhibits a


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sharp decrease with increasing temperature and finally approaches a constant value. Such strong dependence of viscosity decrease on temperature is probably due to the flexible methylene-bridged structures, which reduces the friction resistance of component molecules during thermal movements. Moreover, the viscosity of P230 under 300 oC is merely 534 Pa•s. Pitches with such low viscosity and narrow molecular weight distribution are excellent precursors for preparation of carbon fibers, which enables spinning process smooth and stable. The influence of shear rate on melt rheology of dehydrobromination pitches at constant temperature is also quantitatively studied. Fig 10(b) illustrates the viscosity-shear rate correlation curves of P230 at 250 oC and 270 oC, respectively. The two curves exhibit similar variation trends of viscosity with increasing shear rate. The apparent viscosity steadily decreases with increasing the shear rate from 0.01 s-1 to 100 s-1, presenting the typical shear-thinning behavior. The high degree of shear-thinning for P230 implies that the increase of shear rate alters the arrangement of molecular chain from complex three-dimensional network to oriented alignment. Notably, no obvious plateau region is observed for P230 which largely results from the kink of aliphatic chains.26,38

Fig 10. Rheological behavior of dehydrobromination pitches. (a) Apparent viscosity vs. temperature curves of P230 and P250; (b) The shear rate and apparent viscosity co-relation curves of P230 under 250 oC and 270 oC.


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Page 30 of 38

3.6 Graphitization properties of dehydrobromination pitches. Fig 11(a) shows XRD profiles of graphitized samples. All samples exhibit sharp peaks of (002) planes, which correspond to the stacks of aromatic molecules.39 The intensity of (002) peak increases with the amount of tetralin added. The d002 value of P230-G is 0.3434 nm, whereas it decreases to 0.3370 nm for P2330-20%-G. The g value of P230-20%-G is 81.40%, which is very close to P230-10%-G (80.20%) and higher than P230-G (6.98%). Fig 11(b) records Raman spectra of graphitized samples. All samples show three major peaks at around 1356 (D band), 1581 (G band) and 2720 cm-1 (G’ band), which are assigned to E2g mode of graphite, the breathing mode of κ-point phonons of A1g symmetry and the second order of the D peak respectively.40,41 The intensity ratio of D band and G band (ID/IG) is regarded as disordered degree. The ID/IG of P230-G, P230-10%-G and P230-20%-G are 0.38, 0.24 and 0.12, respectively, indicating that the addition of tetralin contributes to the formation of highly organized structures. Except for XRD and Raman techniques, the graphitization properties of obtained products could be measured by high-resolution transmission electron microscopy (HR-TEM) instrument. Fig 11(c) and (d) demonstrates TEM images of P230-G and P230-20%-G. Ordered lamellar structures (Region ⅹ) and turbostratic structures (Region ⅹ) were both found in P230-G, whereas P230-20%-G shows highly graphitized layer structures with the lattice spacing of 0.3337 nm (see enlarged area in Fig 11(d)).42 These differences in morphologies are in accordance with analysis results from XRD and Raman. The turbostratic structures in P230-G are originated from incompatibility between thermal reactivity and viscosities. Violent cracking of side-chains and polycondensation of component molecules precipitously enhance the viscosity. In addition, plenty of QI particles in P230 accelerate the formation


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of mesophase spheres and therefore reduce the mobility of system. In consequence, carbonization of P230 was conducted in a chemical environment of poor liquidity, thus limiting ordered arrangements of aromatic sheets. The calculated crystal parameters (d002, ID/IG, La and Lc) are summarized in Table 8. All results indicate that highly ordered graphitic carbons with g of 81.40% and ID/IG of 0.12 could be obtained through graphitization of P230-20%.

Fig 11. (a) XRD diffraction patterns of P2330-G, P230-10%-G and P230-20%-G; (b) Raman spectra of P230G, P230-10%-G and P230-20%-G; TEM images of graphitized samples P230-G (c) and P230-20%-G (d).

Table 8. Crystal parameters of graphitized samples calculated from XRD patterns and Raman spectra Samples

d002 (nm)

g (%)

ID/IG

La (nm)

Lc (nm)

P230-G

0.3434

6.98

0.38

44.3

97.1

P230-10%-G

0.3371

80.20

0.24

70.1

123.5


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P230-20%-G

0.3370

81.40

0.12

Page 32 of 38

140.2

124.3

4. CONCLUSION In conclusion, we have developed a novel synthetic process to prepare methylenebridged graphitizable pitches from 1-MNa by a two-stage method of visible light irradiation assisted bromination and subsequent CSD. The amount of bromine introduced was found to be key factors for determining the constitution of bromination products. BMNa-0.75 with the highest Smb is chosen as raw materials for dehydrobromination without additional treatment, which reduces the cost of raw materials and operations. After a facile CSD reaction and n-hexane extraction, dehydrobromination pitches with various softening points and coking values are obtained. By analyzing HS components, methyl migration products (naphthalene, 2-MNa, DMNa and TMNa) and hydrogenation products (THNa, 5M-THNa and 1M-THNa) are detected, which are attributed to the retentive bromine radicals. The introduction of tetralin effectively regulated the quantity of reserved Br•, resulting in pitches with diverse physical properties and capacity of mesophase development. The optical texture of semi-coke from P230 is fine-grained mosaics while P230-20%-C exhibits coarse flow texture. The as-prepared pitches display excellent fluidity and typical shear-thinning rheological behavior. Analysis results from XRD, Raman and TEM indicate that highly graphitizable pitches could be synthesized through closed-system dehydrobromination process.

AUTHOR INFORMATION Corresponding Author


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*Telephone/Fax: +86-021-64252924. E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (U1710252, U13032015), the Young Elite Scientists Sponsorship Program by CAST (2017QNRC001), the Fundamental Research Funds for the Central Universities (222201817001) and the Shanghai Rising Star Program (17QB1401700). REFERENCES (1) Brooks, J. D.; Taylor, G. H. The formation of graphitizing carbons from the liquid phase. Carbon 1965, 3(2), 185-193. (2) Kim, D. W.; Kil, H. S.; Kim, J.; Mochida, I.; Nakabayashi, K.; Rhee, C. K.; Miyawaki, J.; Yoon, S. H. Highly graphitized carbon from non-graphitizable raw material and its formation mechanism based on domain theory. Carbon 2017, 121, 301–308. (3) Lee, J. S.; Kim, Y. K.; Hwang, J. Y.; Joh, H. I.; Park, C. R.; Lee, S. Carbon nanosheets by the graphenization of ungraphitizable isotropic pitch molecules. Carbon 2017, 121, 479–489. (4) Ishii, T.; Kaburagi, Y.; Yoshida, A.; Hishiyama, Y.; Oka, H.; Setoyama, N.; Ozaki, J. ichi; Kyotani, T. Analyses of trace amounts of edge sites in natural graphite, synthetic graphite and high-temperature treated coke for the understanding of their carbon molecular structures. Carbon 2017, 125, 146–155. (5) Mochida, I.; Korai, Y.; Ku, C. H.; Watanabe, F.; Sakai, Y. Chemistry of synthesis, structure, preparation and application of aromatic-derived mesophase pitch. Carbon 2000, 38 (2), 305–328.


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(6) Mochida, I.; Yoon, S. H.; Korai, Y. Mesoscopic structure and properties of liquid crystalline mesophase pitch and its transformation into carbon fiber. Chem. Rec. 2002, 2 (2), 81–101. (7) Liu, D.; Lou, B.; Li, M.; Qu, F.; Yu, R.; Yang, Y.; Wu, C. Study on the preparation of mesophase pitch from modified naphthenic vacuum residue by direct thermal treatment. Energy & Fuels 2016, 30 (6), 4609–4618. (8) Liu, D.; Li, M.; Qu, F.; Yu, R.; Lou, B.; Wu, C.; Niu, J.; Chang, G. Investigation on preparation of mesophase pitch by the cocarbonization of naphthenic pitch and polystyrene. Energy & Fuels 2016, 30 (3), 2066–2075. (9) Mochida, I.; Shimizu, K.; Korai, Y.; Otsuka, H.; Sakai, Y.; Fujiyama, S. Preparation of mesophase pitch from aromatic hydrocarbons by the aid of HF BF3. Carbon 1990, 28 (2-3), 311–319. (10) Korai, Y.; Nakamura, M.; Mochida, I.; Sakai, Y.; Fujiyama, S. Mesophase pitches prepared from methylnaphthalene by the aid of HF BF3. Carbon 1991, 29 (4-5), 561–567. (11) Mochida, I.; Shimizu, K.; Korai, Y.; Sakai, Y.; Fujiyama, S.; Toshima, H.; Hono, T. Mesophase pitch catalytically prepared from anthracene with HF/BF3. Carbon 1992, 30 (1), 55– 61. (12) Mochida, I.; Kudo, K.; Fukuda, N.; Takeshita, K.; Takahashi, R. Carbonization of pitchesIV: Carbonization of polycyclic aromatic hydrocarbons under the presence of aluminum chloride catalyst. Carbon 1974, 13(2), 135–139. (13) Ge, C.; Yang, H.; Miyawaki, J.; Mochida, I.; Yoon, S. H.; Qiao, W.; Long, D.; Ling, L. Synthesis and characterization of high-softening-point methylene-bridged pitches by visible light irradiation assisted free-radical bromination. Carbon 2015, 95, 780–788.


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Controllable Synthesis of Highly Graphitizable Pitches from 1

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