Structural Modification of Petroleum Pitch Induced by Oxidation

Article pubs.acs.org/EF

Structural Modification of Petroleum Pitch Induced by Oxidation Treatment and Its Relevance to Carbonization Behaviors Bin Lou,† Dong Liu,*,† Yajing Duan,‡ Xulian Hou,§ Yadong Zhang,† Zhiheng Li,† Zhaowen Wang,∥ and Ming Li*,⊥ †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao, Shandong 266580, People’s Republic of China ‡ Patent Examination Cooperation Center of The Patent Office, SIPO, Henan, Zhengzhou, Henan 450002, People’s Republic of China § China Petroleum Engineering Company, Ltd., Beijing Company, Beijing 100085, People’s Republic of China ∥ Dongying Municipal Environmental Protection Bureau, Dongying, Shandong 257091, People’s Republic of China ⊥ College of Chemical Engineering, Qingdao University of Science & Technology, Qingdao, Shandong 266042, People’s Republic of China S Supporting Information *

ABSTRACT: Structural modifications of petroleum pitch and its subfractions induced by mild air-blowing modification have been monitored by elemental analysis, average molecular weight, Fourier transform infrared spectrometer (FTIR), 1H nuclear magnetic resonance (1H NMR), and carbon residue. Combined with the investigation into carbonization behaviors of asprepared oxidized pitch, the relationship between oxidized modification and carbonization behaviors has also been discussed. With the air blowing treatment process, average molecular weight and oxygen content grow rapidly in the petroleum etherinsoluble fraction of the air-blown pitch in comparison with those of corresponding petroleum ether-soluble fractions, which gradually enlarges the distinction in carbonization reactivity and mutual solubility between these two subfractions. As a result, although the yield of carbonized residue is dramatically increased from 36.1 wt % to 64.5 wt % during the direct thermal carbonization process, the air blowing treatment produces an adverse effect on mesophase development of oxidized pitches, leading to the optical texture index (OTI) significantly decreasing from 35.9 to 0.6. While adding the hydrogen-donor aromatic oil (HAO) into the air-blown pitches is able to effectively improve the mutual compatibility among the solvent subfractions via H-transfer reactions and dilution effect, consequently contributing to mesophase development during the co-carbonization process accompanied by increasing OTI value of mesophase residues to about 65, except 33.6 of the PP25-HAO mixture, simultaneously the co-carbonization process still maintains the relatively high carbonized residue yield between 42.1 and 61.8 wt %. Hence, a possible combined process including air blowing modification and subsequent co-carbonization process can obtain mesophase products with high residue yield and excellent anisotropic texture by controlling the degree of oxidation and selecting compatible co-carbonization additives.

1. INTRODUCTION

higher softening points with simultaneously lower carbon residues than highly aromatic coal tar pitches. In the last decades, compared with the thermal process,5 adding elemental sulfur6 or oxygen blowing,7,8 the air blowing treatment featuring a low-cost and simple procedure9−12 is most commonly used to effectively promote the carbon residue of pitch.13,14 Machnikowskia et al.15,16 reported that the influence of air-blowing treatment on the optical texture in the subsequent carbonization reactions relies on the properties of starting substances and oxidized conditions. In most cases, however, the air-blown treatment degrades the mesophase development of modified pitch.17 That is to say, in order to develop a suitable “on demand” modification of molecular compositions during the oxidized process, it is necessary to select both suitable feedstock and the appropriate oxidized

Commercial petroleum pitches and coal-tar pitches have been extensively used to manufacture various carbon materials. Generally, commercial pitches can meet the requirements of traditional application. However, because of their low carbon yield and poor graphitization ability, the properties of commercial pitches are not suitable for more special and modern applications,1−4 such as carbon fibers, needle coke, or C/C composites. In order to satisfy the requirements as precursor of an advanced carbon material, the modified treatments are usually utilized and the objective of pitch modification are generally defined as follows: (1) maintaining or promoting the mesophase development; (2) improving fluidity when the mesophase products fused; (3) increasing carbon residues. As compared to coal tar pitches, petroleum pitches are more aliphatic, whose molecules are featured with aliphatic and aromatic constituents forming a cobweb structure. Owing to such molecular structure, petroleum pitches possess © 2017 American Chemical Society

Received: May 9, 2017 Revised: August 21, 2017 Published: August 25, 2017 9052

DOI: 10.1021/acs.energyfuels.7b01314 Energy Fuels 2017, 31, 9052−9066

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Energy & Fuels process. Normally, the feedstock is far from achieving this a series of requirements for molecular composition. So the structural modification induced only by the air/oxygen blowing process is not effective for achieving improvement of mesophase development and promotion of carbon residues. In order to improve the mesophase development during the carbonization and fused fluidity of the resultant mesophase pitch, hydrogenation modification,18,19 catalytic polymerization modification,20,21 and co-carbonization modification22,23 were also developed. Mochida24 has reported a representative process of hydrogenation modification: naphthenic groups are introduced into the aromatic molecules of coal tar pitch with the aid of a hydrogen donor solvent like tetrahydroquinoline or catalyst, respectively; and then some of the introduced naphthenic groups survive in the resultant mesophase pitch. However, the high cost of aromatic hydrogenation limits its wide application. As far as I know, the aim of almost all hydrogenation treatment focuses on improving the mesophase development. The published reports concerning increasing the pitch yield through hydrogenation are rare. Co-carbonzation modification is regarded as a practical and effective method, which is performed by mixing two kinds of feedstock to improve carbonation properties of each feedstock. With respect to the co-carbonization mechanism, the concept of “Eutectic Effect” reported by Marsh and Mochida25,26 is commonly accepted, specially including an additive as a seed crystal to contribute to rapid mesophase formation, regulating the carbonization rate and mesogen configuration by alkyl and/or hydrogen transfer reactions during the co-carbonization process, as well as excellent dissolution ability of additive to decrease the viscosity of the reaction intermediate. So the selection of matched additive is crucial according to the defects in the carbonization behaviors of the other feedstock. At present, it is gradually accepted that only the single modified method has difficulty achieving the three objectives above. To produce pitch with excellent carbonization properties, several kinds of modification treatment should be successively applied to feedstocks.27 However, as we know, few attempts appear to have been made to achieve the compatible combination of different modification treatments. In this paper, several oxidized pitches with different softening points were prepared from a commercial petroleum pitch by air blowing. The properties of the as-treated pitches and their solvent fractions as well as mesophase development during both direct carbonization and co-carbonization processes were investigated to further understand the relationship between the oxidized treatment and their carbonization behaviors, aiming to disclose a combined process that can promote both carbon yield and mesophase development.

Table 1. Properties of Co-Carbonization Additive HAO item

HAO

ρ/g·cm−3 nD20 Mw

0.9944 1.5608 383.2 Elemental Analysis

C/wt % H/wt % S/wt % N/wt % O/wt %

87.93 9.93 0.55 0.58 1.00 n-d-M Method

C/H CA/wt % CN/wt % CP/wt %

0.74 40.4 34.6 25.0 SARA Analysis

saturates/wt % aromatics/wt % resins/wt % asphaltenes/wt %

22.26 62.94 13.70 0

the colloidal stability of the pitches but disperses their highly reactive species. Besides, according to the structural parameters calculated by the n-d-M method, it is seen that the percent of aromatic carbon (CA) of HAO is as high as 40.4 wt %, accompanied by 34.6 wt % of naphthenic carbon (CN) and only 25.0 wt % of alkyl carbon (CP), implying that constituted molecules of HAO contain not only high aromaticity but also abundant naphthenic structure attached to the aromatic ring. In this paper, the pitches were also separated into two kinds of solvent subfractions including petroleum ether-soluble (PES) and petroleum ether-insoluble (PEI) fractions by extraction of petroleum ether with a boiling range from 90 to 120 °C, according to ASTM D6560−2000. 2.2. Air Blowing Treatment. According to the optimal results of air blowing treatment (seen in Tables S1and S2 in the Supporting Information), the petroleum pitch was treated at temperature of 280 °C under an air flow of 160 L kg−1 h−1 and constant 200/min stirring speed in a 1 L flask (seen in Figure 1). The PP14 pitch, PP20 pitch, and PP25 pitch were prepared by air blowing treatment for 14, 20, and 25 h, respectively. Before air blowing treatment, the weight mflask of empty 1 L threeneck flask and the gross weight of the flask and parent pitch mflask+parent pitch are obtained by using AGF-4000 elctronic balance (made by A & D company, Japan). Likewise, when the blowing time is

2. EXPERIMENTAL SECTION 2.1. Feedstock. A commercial petroleum pitch as the parent pitch was obtained from Shengli Petrochemical Plant of Chinese Petroleum Group. The modified pitches derived through the air blowing treatment were used as feedstock in the subsequent carbonization process. And the hydrogen-donor aromatic oil (HAO) from naphthenic base crude oil was selected as the additive of cocarbonization. The properties of HAO are listed in Table 1. The relatively high C/H atomic ratio 0.74 of HAO indicates that it has relative high aromaticity that contributes to the mutual solubility with the colloidal system of the pitch-HAO mixtures.28 Likewise, the SARA analysis of the HAO shows that the predominant fraction was the aromatics (62.94 wt %), and the saturates as well as resins are only 22.26 and 13.69 wt %, also suggesting that adding HAO rarely destroys

Figure 1. Experimental autoclave for oxidized cross-linking of the pitch. 9053


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Energy & Fuels reached, the gross weight of the flask and oxidized pitch mflask+oxidative pitch is also measured. Therefore, the oxidized pitch yield in Table 3 is calculated as follows mflask + oxidative pitch − m flask Oxidized Pitch Yield/wt% = × 100% mflask + parent pitch − mflask

the anisotropic optical texture. So the optical texture index (OTI) used to evaluate the mesophase development was calculated according to the following formula: OTI = Σf i × OTIi, where f i is the proportion of different anisotropic structures and OTIi, representing the special anisotropic structure index suggested by Ester,30,31 is summarized in Table 2.

(1)

Loss of Feed/wt% = 100% − oxidized pitch yield

Table 2. Classification of Mesophase Microstructures and Their OTI Values

(2)

From formulas 1 and 2, the amount of oxygen atoms involving in generating the constituent molecules of oxidized pitch is not deducted. 2.3. Carbonization Process. The carbonizing processes were carried out using a 300 mL autoclave apparatus (seen in Figure 2). The

microstructural types

feature size/μm

OTI value

mosaic small domain domain large domain

>10 10−60 >60 length >60, width >10

1 5 50 100

3. RESULTS AND DISCUSSION 3.1. Structural Modification of Oxidized Pitches. 3.1.1. Properties Analysis of Oxidized Pitches. The variations in softening points, yields of oxidized residues, elemental compositions, carbon residues, and iodine values of parent and treated pitches are summarized in Table 3. By omparison of the Table 3. Variations in Basic Properties of Oxidized Pitches item Softening point/°C Oxidized pitch yield/wt % C/H Carbon residue/wt % Iodine value/mmol·g−1

Figure 2. Equipment used for carbonization. detailed description of the operating procedure, experimental design, and repeatability of carbonization experiments can be found in Section 2 and Section 3 of the Supporting Information. In order to discuss the structural modification and its relevance to subsequent carbonization, a simple variable method was adopted to select the optimal conditions including reaction temperature, reaction pressure, soaking time, and amount of HAO added in terms of mesophase content in carbonaceous mesophase products. The corresponding results are discussed in Sections 3.2 and 3.3. 2.4. Characterization. Softening points (SP) used to characterize the oxidized degree of derived pitch were determined using the ring and ball method according to ASTM D3461 standards. In order to monitor the variations in structural composition during air blowing, the content of elements including carbon, hydrogen, sulfur, and nitrogen in samples was directly determined by the Varil EL-3 elemental analyzer, while the oxygen content was obtained by difference; 1H nuclear magnetic resonance (1H NMR) spectra which were recorded on a BrukerAvance DMX500 spectrometer, using deuterated chloroform as solvent and tetramethylsilane (TMS) as internal standard, as well as the Fourier transform infrared (FTIR) spectra with an average of 32 scans and a resolution of 2 cm−1 in the region of 4000−500 cm−1 which were determined on a Nicolet Magna-750 FTIR spectrometer were also used. Besides, the crosslinking extent in the pitches was estimated from pitch iodine uptake29 and carbon residues of samples are determined according to ASTM Method D189-05. The average molecule weight (Mw) of the solvent fraction of the pitch was determined by KNAUER K-7000 vapor pressure osmometry (VPO) to distinguish the varied amplitude of subfractions of oxidized pitch. The content and shape of the mesophase structure as important indicators to evaluate the quality of carbonized solid residues were characterized as follows: the mesophase products were mounted in epoxy resin, polished, and examined by XP-4030 polarizing microscope (Shanghai Milite Precise Instrument Co. Ltd., China). To a considerable level, the properties of mesophase products are determined by their crystalline structure which is mainly reflected by

C/wt % H/wt % S/wt % N/wt % O/wt %

parent pitch

PP14 pitch

PP20 pitch

PP25 pitch

50 113 -93.7 0.67 0.70 21.6 43.5 3.984 3.698 Elemental Analysis 86.96 86.99 10.80 10.32 0.68 0.46 0.87 1.03 0.69 1.20

136 91.8 0.73 48.4 3.231

158 90.5 0.74 54.6 3.043

87.01 9.94 0.59 1.00 1.46

86.67 9.72 0.76 1.04 1.81

properties from parent pitch to PP25 pitch, the oxidized treatment results in vaporization loss of less than 10% for the parent pitch but significant growth of softening point to 158 °C, accompanied by gradually reducing iodine value from 3.984 to 3.043 nmol·g−1. Besides, it is observed that the carbon residue of the derived pitch dramatically increases from 21.6 wt % to 54.6 wt %. These findings clearly indicate that the air blowing regarded as the effective process to promote the carbon residue of pitch is mainly due to the reactions such as aromatization and cross-linking among the components,32 and not just to the removal of volatile compounds. Furthermore, as the oxidized reaction proceeds, the hydrogen content of the derived pitches shows a progressive decrease but increase in C/ H atom ratio; meanwhile the oxygen content slightly increases from 0.69 wt % for parent pitch to 1.81 wt % for PP25 pitch. These results all suggest that air blowing can induce the dehydrogenative polymerization of pitch constituents and most oxygen is not incorporated in the aromatic molecules, but is eliminated as water.33,34 To obtain better insight into structural changes during the oxidation treatment, FTIR and 1H NMR analysis was also conducted. 3.1.2. FTIR and 1H NMR Characterization of Oxidized Pitches. The FTIR spectra and 1H NMR analysis of the original pitch and modified pitches are presented in Figure 3 and Table 9054


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The distribution of hydrogens among various chemical groupings in the pitches and some suggestive average structural parameters calculated by the modified Brown-Ladner method are listed in Table 4. In comparison with the distribution of constituent hydrogens in the starting pitch, the air blowing treatment obviously raises the content of aromatic hydrogen Har, followed by decrease in Hα and Hβ contents, suggesting that oxygen attack occurs preferentially at the methyl or methylene sites of the side chains.32 In addition, it can also be deduced that at the initial stage of air blowing treatment, the generation of the aromatic macromolecules by dehydrogenation of the naphthenic group, dealkylation of alkyl group, and successive cross-linking by phenyl−phenyl or methylene bridge among the aromatic constituents34 are primarily responsible for the growth of the softening point in the resultant pitch, which meanwhile leads to a rise in the aromaticity index fa and decrease in the condensation index HAU/CA as well as the index of alkyl substitution to aromatic rings σ (shown in Table 4). When the cross-linking degree of the derived pitch further deepens, combined with analysis of FTIR spectra, the intermolecular cross-linking by introducing the oxygencontaining functional groups like CO and C−O−C as the bridge bond occurs to a relatively larger extent; such excessive cross-linking reactions always generate a three-dimensional, cross-linked molecular configuration prior to the mesophase state.35 3.2. Direct Carbonization Treatment. The direct carbonization behaviors of pitches as well as their PES and PEI fractions, especially in mesophase development and carbonized yield, were evaluated, and the influence of oxidation treatment on direct-carbonization behaviors was expected to be clarified in terms of mutual compatibility of solvent fractions of pitches. 3.2.1. Direct Carbonization Behaviors of Oxidized Pitches. The variations in mesophase content of carbonized residues obtained by direct carbonization at different temperature and soaking time are displayed in Figure 4. The similar change trend of mesophase content with growth of reaction temperatures during the direct carbonization of every pitch can be clearly observed, as presented in Figure 4a. At the low reaction temperature, mesophase formation and development are restricted because of the slow carbonization rate and high viscosity of reaction intermediate. When the reaction temperature increases up to a point slightly higher than the temperature of initial decomposition, not too many chemical bonds in polycyclic aromatic molecules are in the excited state. Meanwhile this temperature can ensure that most polycyclic aromatic molecules react in fluid liquid phase and provide good fluidity of liquid intermediates. Thus, planar aromatic macromolecules with relatively homogeneous molecular structure are generated through the relatively unitary reaction direction of liquid carbonization induced by a certain angle of collision between adjacent molecules. So properly promoting temperature contributes to the generation of those uniform planar macromolecules, parallel stacking of planar macromolecules to form mesophase, and subsequent coalescence into mesophase structures of large size. However, as we know, the carbonized process conducted at excessively high reaction temperature contributes to rapid formation of polymerized molecules that have usually lost molecular planarity and induces a fast rise in the viscosity of medium, which significantly inhibits the formation and development of mesophase.22 As a result, mesophase content increases first and then decreases with the growth of reaction temperature, and the optimal temperature

Figure 3. FTIR spectra: (a) parent pitch; (b) PP14 pitch; (c) PP20 pitch; (d) PP25 pitch.

4, respectively. There are some distinct differences among these characterization results, which contribute to obtaining detailed Table 4. Distribution of Hydrogens among Different Functionalities in the Pitches weight/% sample

Har

Hα

Hβ

Hγ

fa

σ

HAU/CA

parent pitch PP14 pitch PP20 pitch PP25 pitch

4.0 5.8 8.0 8.4

13.1 12.8 12.6 12.2

63.1 61.6 59.5 59.4

19.8 19.8 19.9 20.0

0.28 0.33 0.38 0.38

0.62 0.52 0.44 0.42

0.57 0.53 0.51 0.51

information about the oxidized mechanism at different periods of air blowing treatment. According to Figure 3, with the softening point of the pitch increasing, one significant difference is that the absorption peaks of the aromatic CC stretching near 1600 cm−1 and the aromatic C−H out-of-plane bending in the region of 700−900 cm−1 becomes more and more evident, which indicates that reactions of dealkylation, cyclo-aromatization, and condensation of aromatic rings are induced during the air blowing treatment, contributing to the growth of the softening point of the derived pitch. Moreover, two new absorption bands near 3350 cm−1 attributed to O−H stretching and 1693 cm−1 assigned to CO stretching are seen in the spectra of both the PP20 pitch and PP25 pitch. Another new band at 1108.9 cm−1, which is assigned to phenoxy or ether stretching,34 is significantly intensified only in the PP25 pattern. It shows that the rise of the softening point in the PP20 and PP25 pitch, to a larger extent, is attributed to the cross-linking of pitch components by introducing the oxygen functional groups. 9055


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Figure 4. Influence of temperature and soaking time for mesophase content in direct-carbonized residues: (a) variations in mesophase content with different temperatures; (b) variations in mesophase content with different soaking time; (c) variations in mesophase content with different reaction pressure.

for every pitch is 410 °C in terms of mesophase content, as shown in Figure 4a. Because constituent molecules of feed stock need to undergo thermal pyrolysis, aromatization, and condensation reactions, ultimately to achieve the molecular weight and necessary configuration to generate mesogen molecule. These mesogens leave the parent isotropic phase by the process of “selfassembly”, to form mesophase structure that can be recognized in polarizing microscope until its size reaches 0.5 μm. Therefore, except the PP25 pitch, the induction period of mesophase formation can be clearly observed in the range between 0 and 2 h, seen in Figure 4b. Owing to high molecular weight and a relatively large amount of oxygen-containing functional groups caused by excessive cross-linked reactions among constituent molecules of feed stock during the air blowing treatment, the phase separation is greatly accelerated during the direct carbonization of the PP25 pitch, showing unobvious induction period. After drastic increase of

mesophase content from 2 to 6 h, the mesophase content turned to mild variations for the pitch. So the reaction time of 6 h is selected as optimal soaking time for direct carbonization of every pitch. From Figure 4c, it can be observed that with reaction pressure increasing from 1 to 4 MPa, the mesophase content gradually grows, indicating that the high reaction pressure retains more light components in the reaction intermediate and lowers the increased rate of viscosity with the carbonization reaction proceeding. While reaction pressure is above 4 MPa, too much light component is trapped in liquid intermediate and then hinders the stacking of mesogens and coalescence between mesophase microstructures, causing the mesophase content to decrease slightly. Therefore, the optimal reaction pressure is selected as 4 MPa. At their respective optimal reaction conditions of direct carbonization, namely, under the condition of 410 °C, 4 MPa, for 6 h, the yields of carbonaceous mesophase and their 9056


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Figure 5. Variations in (a) optical microstructures distributions and OTI value as well as (b) yields of derived mesophase pitches obtained by direct carbonization of original and oxidized pitches.

Figure 6. Optical microscopic images of mesophase pitch obtained under optimal conditions during the direct carbonization of (a) parent pitch; (b) PP14 pitch; (c) PP20 pitch; and (d) PP25 pitch, respectively.

mesophase microstructures consisting of mosaic, small domain, domain, and large domain were quantitatively evaluated and results are shown in Figure 5. Therefore, the representative optical textures are also presented in Figure 6.

As shown in Figure 5a, the growth of oxidized degree of treated pitches is found to reduce the mesophase content of the carbonaceous residues, from 82.7% of parent pitch to 24.7% of PP25 pitch; by further observing the distributions of mesophase 9057


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Figure 7. Possible scheme of oxidized reactions and carbonization reaction at the early stage.

Figure 8. Variations in the content of PES and PEI fractions derived from parent and oxidized pitches.

microstructures, it is also seen that the percentage of large domain and domain gradually declines, finally not existing in the resultant mesophase products from direct carbonization of PP20 pitch and PP25 pitch. Additionally, the decrease in the OTI value explicitly shows that the oxidized pitches generate more and more poorly developed mesophase with the rise in their softening points, also implying that the air blowing treatment produced adverse effects on the formation and development of mesophase during the subsequent directcarbonizing processes. Figure 6 displays the optical microscopic images of carbonaceous mesophase derived from direct carbonization under the optimal conditions of 410 °C, 4 MPa, for 6 h. The brightness in Figure 6 represents optical anisotropy or, in other word, mesophase structures. In Figure 6a,b, the relatively large sized mesophase structure such as large domain (length >60, width >10) and domain (size >60) can be observed. However, mesophase structure content decreases and large domain mesophase structure disappears in the carbonaceous mesophase obtained from direct carbonization of PP20 pitch (shown in Figure 6c), and in Figure 6d representing the optical image of carbonaceous mesophase from direct carbon-

ization of PP25, the mesophase structure content is continuously reduced and only relatively small-sized mesophase structure like mosaic and small mosaic can be visually seen. Those findings also demonstrate that air blowing treatment contributes to the formation of inferior mesophase during the subsequent direct-carbonizing processes. Nevertheless, according to variations in the yield of carbonized residues showed in Figure 5b, the increase of oxidation degree effectively serves the growth of carbonized residue yield from 44.6 wt % to 64.5 wt % based on air-blown pitch or from 41.8 wt % to 58.4 wt % based on parent pitch. Just like the scheme oxidized and carbonization reactions shown in Figure 7, because the higher numbers of small-sized molecules in the feed which are formerly carried out of the reaction system during the carbonization transform into crosslinked molecules and further form more stable PAHs molecules, and those molecules more readily remain in carbonized residues, contributing to the increase of the yields of carbonized residues. However, because of the aromatization of naphthenic structures and formation of oxygen-containing functional groups during the air blowing process, at the early 9058


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Figure 9. Variations in (a) C/H atomic ratio, (b) O/C atomic ratio, and (c) molecular weight Mw of oxidized pitch obtained at different air blowing times.

mesophase development is obviously restricted, indicating that the relationship between average molecular structure and mesophase development excessively ignores interactions between subfractions but mutual compatibility between solvent fractions of pitch, namely, the carbonization rates and mutual solubility, which plays a critical role in the mesophase development as for the carbonization of oxidized pitches.35,39 3.2.2. Relationship between Mutual Compatibility of Subfractions and Direct Carbonization Performance of Pitches. In this section the structural changes of PES and PEI fractions in oxidized pitches and their carbonization performance were also examined to make an attempt to illustrate the relationship between oxidized degree of air-blown pitches and their direct carbonization performance. 3.2.2.1. Variations in Mutual Compatibility between Solvent Fractions with Oxidized Treatment Deepening. The variations in respective content of solvent fractions derived from parent and oxidized pitches are presented in Figure 8. The oxidized treatment substantially increases the content of PEI

stage of carbonization, not only is the carbonization reactivity enhanced due to low thermal stability of oxygen-containing functional groups, but also the lack of naphthenic hydrogens neutralizes active free radicals, resulting in rapidly increasing viscosity of the reaction medium and then hinder the formation and coalescence of mesophase structures. In addition, the biphenyl-type structure induced by dissociation of −CO− or −O−C−O− influences the planarity of PAH molecules due to the free rotation of the C−C single bond to reduce the steric hindrance between aromatic rings.36,37 As a result, the mesophase formation and development of as-prepared pitch are restricted by air blowing treatment. According to the characterization of average molecular structure discussed in section 3.1, the oxidation treatment has promoted aromaticity and reduced the long aliphatic sidechains accompanied by no incorporation of oxygen atoms, which is generally thought to be beneficial to mesophase development during carbonization.38,39 However, based on the above direct carbonization performance of oxidized pitches, the 9059


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Figure 10. FT-IR spectra of solvent fractions: (a) PES of PP14; (b) PES of PP20; (c) PES of PP25; (d) PEI of PP14; (e) PEI of PP20; (f) PEI of PP25.

Figure 11. Variations in optical microstructure distributions and OTI value of carbonaceous mesophase obtained at the temperature of 410 °C, reaction pressure of 4 MPa, for 6 h during the direct carbonization of (a) PES fractions and (b) PEI fractions.

cm−1, and phenoxy or ether stretching peak at 1030 cm−1 are seen in spectra of PEI derived from PP20 and PP25 pitch, which indicates that oxygen-containing groups are prone to concentrate in PEI fractions. Furthermore, the relative absorption intensity Abs1700/Abs1600 and Abs1030/Abs1600 of PEI fraction of PP25 pitch is 0.82 and 0.84, higher than 0.64 and 0.67 of the PP20 pitch’s PEI fraction. The analysis of FTIR spectra also indicates, to some extent, that more and more oxygen is involved in generation of aromatic molecules of PEI fraction, which is also consistent with the trend of the O/C atomic ratio. Generally, petroleum pitch is regarded as a colloidal system in which the PES fraction plays a part in dispersing the PEI fraction and avoiding its coagulation.28 However, the growing differences manifest that the relatively heavy components of the pitch are prone to be oxidized and then cross-linked into PEI fractions with larger aromatic rings, fewer alkyl groups, and greater number of bridge-bonded oxygen functional groups, leaving the light portion of components slightly changed.27,35

fraction in the derived pitch from 5.9 wt % of the parent pitch to 40.5 wt % of PP25 pitch at the expense of the PES fraction from 94.1 wt % to 59.5 wt %. The C/H and O/C atomic ratios together with average molecular weight Mw with the prolonged air blowing are shown in Figure 9. As expected, the PES fraction contained the lower C/H atomic ratio, lower oxygen content, and higher Mw than those of its corresponding PEI fraction. It is noteworthy that there are more and more obvious gaps in the increase of C/H, O/C, and average molecular weight Mw between the PEI fraction and PES fraction with the growth of softening point of the pitch (or air blowing time). Figure 10 shows the FTIR spectra of solvent fractions derived from PP14, PP20, and PP25. Through the comparison among the FTIR spectra of PES, it is found that almost all peaks lie around 2850−3100 cm−1, 1600 cm−1, 1455 cm−1, 1380 cm−1, and 700−900 cm−1 and no obvious characteristic peaks of oxygen-containing functional groups appear. The O− H stretching peaks at 3350 cm−1, CO stretching peak at 1700 9060


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Figure 12. Optical microscopic images of carbonaceous mesophase obtained from the conditions of 410 °C, 4 MPa, and 6 h during the direct carbonization of solvent fractions: (a1) and (a2) PES and PEI fraction of parent pitch, (b1) and (b2) PES and PEI fraction of PP14 pitch, (c1) and (c2) PES and PEI fraction of PP20 pitch, (d1) and (d2) PES and PEI fraction of PP25 pitch, respectively.

3.2.2.2. Influence of Mutual Compatibility of Subfractions on Direct Carbonization Performance. The optical microstructure distribution and OTI value of carbonaceous mesophase obtained from direct carbonization of PES fractions

Those results imply that during the air blowing treatment the discriminative oxidation reactions continually occur, leading to poorer and poorer compatibility in carbonization rate and mutual solubility. 9061


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Figure 13. Variations of mesophase content of co-carbonized solid residues obtained under (a) different temperatures; (b) different soaking time; (c) different amounts of HAO additions; (d) different reaction pressure.

and PEI fractions at 410 °C, 4 MPa, for 6 h are given in Figure 11. For each pitch, the optical texture of mesophase residues obtained from PES fractions is evidently superior to that from PEI fractions. In addition, it can also be observed that the mesophase content, optical microstructural distributions, and OTI values in the mesophase residues derived from direct carbonization of PES fractions exhibit relatively mild variation. In contrast, during the direct carbonization of PEI fractions, the obvious and gradual decrease in mesophase content and OTI value occurs with growth of softening point of the oxidized pitch. The results are in good agreement with optical microscopic images shown in Figure 12. Via cross-comparison optical texture of mesophase residues produced from directcarbonization of pitches (described in Figures 5 and 6) and their solvent fractions (described in Figures 8 and 9), it can be concluded that with the growth of oxidized degree of pitches, mesophase development during the direct carbonization of PEI fractions is greatly inhibited and apparently affects the mesophase evolutions of PES fraction during the carbonization of pitch.

In conclusion, because there are discriminative reactions between the solvent fractions during the air blowing treatment, which enlarges the gap of the solubility parameter and carbonization rates between PES fractions and PEI fractions leading to reduction in mutual solubility; during the carbonization process, PEI fractions regarded as highly reactive species because of abundant high-molecular-weight polycyclic aromatics and a high content of heterocycles41,42 are prone to selfcondensation to form macromolecules at the initial reaction stage, which further increase the solubility difference. Owing to the driving force of the surface energy minimum, those derived macromolecules precipitate from the matrix resulting in formation of isotropic or small-sized anisotropic microstructures in the following carbonization process, which also reduces the fluidity of the medium and then interferes with the ability of subgenerated planar molecules to diffuse into positions of alignment.40 Consequently, mesophase formation and development are restricted due to poorer and poorer mutual compatibility between PES fraction and PEI fraction with the deepening degree of oxidation. The carbonized residue yield is effectively promoted, resulting from increase in both 9062


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Figure 14. Distributions of optical microstructures, OTI value, and yield of co-carbonized residues derived under the condition of 420 °C, 4 MPa, 20 wt %of HAO, and 6 h during the co-carbonization process: (a) optical microstructures and OTI; (b) co-carbonized residue yield.

Figure 15. Optical microscopic images of the resultant products derived under the condition of 420 °C, 4 MPa, 20 wt % of HAO, and 6 h during the co-carbonization of the feeds: (a) parent pitch; (b) PP14 pitch; (c) PP20 pitch; (d) PP25 pitch.

molecular weight that reduces the content of light component and the content of oxygen-containing functions that is favorable to intermolecular condensation. 3.3. Co-Carbonization Treatment. Although air blowing is an effective method to promote the yield of carbonized residue, mesophase development is obviously restricted during the successive direct carbonization. In view of this, co-

carbonization treatment was employed, expecting to maintain the yield of carbonized residues and improve the mesophase development. 3.3.1. Co-Carbonization Behaviors of pitch-HAO Mixtures. The influence of temperature, soaking time, and amount of HAO additions for mesophase content in co-carbonized residues is presented in Figure 13. It can be clearly seen that 9063


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Energy & Fuels Table 5. 1H NMR Characterization and Oxygen Content Variations in Co-Carbonization Feed weight/%

a

sample

Har

Hα

Hβ

Hγ

fa

σ

HAU/CA

Oa/wt %

parent pitch-HAO PP14-HAO PP20-HAO PP25-HAO

6.6 8.0 9.8 10.1

15.3 15.1 14.9 14.6

58.1 56.9 55.3 55.2

20.0 20.0 20.1 20.1

0.32 0.35 0.38 0.39

0.54 0.49 0.43 0.42

0.66 0.63 0.61 0.60

0.75 1.16 1.37 1.65

Refers to calculated oxygen content by formula as follows: O/wt % = Opitch × 80% + OHAO × 20%.

characterization and oxygen content in the co-carbonization feed are presented in Table 5. It can be found that the oxygen content in PP14-HAO, PP20-HAO, and PP25-HAO mixtures decreases in contrast with growth in parent pitch-HAO, but the amplitude of all variations is very slight in comparison with oxygen increments induced by air blowing with prolonged blowing time (shown in Table 3), implying that adding HAO in the co-carbonization feed hardly causes an obvious influence on the oxygen content of mixtures. Additionally, based on crosscomparison between Table 4 and Table 5, the increase in Har and decrease in Hα and Hβ content in accordance with increase in fa and reduction in both σ and HAU/CA clearly shows the mixtures become less paraffinic, suggesting the aromatic hydrogen-transfer reactions may be promoted during cocarbonization. Combined with the distinct high naphthenic hydrogen (HN) content shown in Table 1, indicating that the abundant naphthenic structures attach to aromatic rings in constituent molecules of HAO, it can also be assumed that naphthenic hydrogen-transfer reactions largely occur. Based on the above discussions, the mesophase improvement during cocarbonization is mainly attributed to the aromatic and naphthenic hydrogen-transfer reactions, not the dilution effect of oxygen-containing functional groups. In order to further illustrate the role of HAO during the cocarbonization process, the distribution of solvent fractions of intermediates prepared from carbonization at the soaking time of 2 h and their respective optimal temperature are listed in Table 6. It should be pointed out that all feedstock including

changing trends of mesophase content induced by different temperature, soaking time, or reaction pressure during cocarbonization are almost the same as that of the direct carbonization process shown in Figure 4. As for the amount of HAO addition, it can be clearly observed from Figure 13c that when the addition of HAO exceeds 20 wt % of the pitch, the mesophase content of co-carbonized solid samples basically no longer changes, which demonstrates that 20 wt % addition was the sufficient amount to fully exhibit the modified effect of HAO. Hence, the optimal reaction condition of co-carbonization is a temperature of 420 °C, reaction pressure of 4 MPa, HAO addition of 20 wt %, and soaking time of 6 h for each pitch−HAO mixture. According to distributions of optical microstructures and OTI value of co-carbonized mesophase residues obtained at their optimal reaction condition described in Figure 14a, the domain and large domain microstructures were prevailing in the optical textures of all the resultant mesophase products prepared from co-carbonization with HAO. In comparison with direct carbonization (shown in Figure 5a), the co-carbonization of each pitch−HAO mixture markedly increases the mesophase content and OTI values of the carbonaceous residues, strongly implying that adding the HAO facilitates the mesophase formation and development. Nevertheless, compared with the co-carbonization process of PP25 pitch-HAO mixtures, the parent pitch-HAO mixture, PP14-HAO mixture, and PP20-HAO mixture can generate the mesophase residues with high OTI value, and the dominant microstructure is the domain type, indicating that the cocarbonization modified effect for the oxidized pitches with low degree of cross-linking like PP14 pitch and PP20 pitch is much better than that of the excessively cross-linked pitch such as PP25. Likewise, from the observation of the optical microscopic image of co-carbonized solid residues shown in Figure 15, it can be seen that the dominant mesophase structure of cocarbonized residues shown in Figure 15a,b is the large domain, although the main mesophase structure is still large domain in co-carbonized residues of PP20-HAO mixtures shown in Figure 15c. However, domain, small mosaic, and mosaic structures became obvious in co-carbonized residues of PP25-HAO mixtures shown in Figure 15d; there are less large domain while domain, small domain, and mosaic become more distinct. This finding is also in agreement with the results in Figure 14. Besides, in terms of carbonized yield presented in Figure 14b, only a slight decrease of about 0.8−3.0 wt % is caused by the co-carbonization process compared with that of direct carbonization (shown in Figure 5b). This finding suggests that combination of air blowing treatment and co-carbonization process could be potentially applied in achieving both high carbonized yields and superior mesophase structure in resultant mesophase products. 3.3.2. Effect of Oxidized Degree of Pitches on Modified Effect of HAO during Co-Carbonization. The 1H NMR

Table 6. Distributions of Solvent Fractions in Samples Prepared by Carbonization at 2 h, 4 MPa weight% feedstock

TS

TI-PS

PI

parent pitch parent pitch-HAO PP14 pitch PP14 pitch-HAO PP20 Pitch PP20 Pitch-HAO PP25 Pitch PP25 Pitch-HAO

68.7 85.8 60.2 68.2 50.1 54.3 40.8 39.9

27.2 14.2 33.2 31.8 39.6 45.7 40.5 54.7

4.1 0 6.6 0 10.3 0 18.7 5.4

pitches and their mixtures with HAO dissolves well in toluene, and no toluene insoluble (TI) fractions were generated when the HAO are carbonized alone at reaction pressure of 4 MPa and the temperature of 420 °C for 2 h. Therefore, it can be concluded that TI is mainly derived from reactions of reactive species in PEI fractions. From Table 5, unlike the direct carbonization process, the pyridine insoluble (PI) fractions no longer formed during the co-carbonization process of parent pitch-HAO mixtures, PP14-HAO mixtures, and PP20-HAO mixtures; and the resultant content of PI substantially 9064


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Energy & Fuels

components still fails to be transformed into the planar macromolecules with the low viscosity of reaction medium during the co-carbonization process.

decreased from 18.7 wt % to 5.4 wt %. These variations show that adding HAO can lower the reaction rate of highly reactive species during the co-carbonization process. Combined with structural analysis of HAO shown in Table 1, the high aromaticity and naphthenic structure attached to aromatic rings are the structural features of HAO. So it can be deduced that the additive HAO, as a part of PES fraction in the pitch-HAO mixtures, can induce partial hydrogenation and condensation without extensive dehydrogenation of highly reactive species by means of HN-transfer reactions and provide relatively similar solubility parameters with the free radical generated from constituent molecules of PEI fraction to achieve dispersion of highly reactive species or its free radical.38,40−42 So these polycondensed molecules formed at the initial stage of carbonization exhibit good mutual solubility with the matrix and then participate in the formation of a well-developed mesophase in the subsequent process. In addition to improving the formation of mesogens, both HN-transfer reactions and dilution effect can increase the fluidity of the carbonization medium, which is also a critical factor to generate an extensively developed mesophase during the co-carbonization. However, the three-dimensional molecular conformations, predominantly involved in petroleum ether-insoluble fractions of PP25 pitch, were substantially generated, mainly by introducing the appreciable amount of oxygen-containing functional groups. Thus, adding HAO still fails to contribute to the bridge bond cleavage and then be restructured into the planar macromolecules with the low viscosity of reaction medium.16 Hence, the degree of cross-linking of the treated pitch should be controlled so as to obtain both excellent mesophase and high carbon yield of carbonaceous mesophase during the co-carbonization process. To sum up, the additive HAO improves the mutual compatibility of carbonization between the solvent subfractions, that is, by providing the necessary physical fluidity of the system and possibly some chemical stability via the dilution effect and hydrogen transfer reactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b01314. Selection of conditions during the air blowing treatment; detailed description of carbonization experimental procedure and experimental design (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Telephone/Fax: +86-053286984629. *E-mail: [email protected]. Telephone: 15063083161. ORCID

Dong Liu: 0000-0003-1184-3989 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Fundamental Research Funds for the Central Universities (15CX05009A), Shandong Provincial Key Project of Research and Development, China (2016GGX102017) and Shandong Provincial Natural Science Foundation, China (ZR2015BM003).

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REFERENCES

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4. CONCLUSION Pitches with different degrees of oxidation produced carbonized residues with different mesophase development during the direct carbonization and co-carbonization with hydrogen-donor aromatic oil (HAO), which could be explained from variations in the mutual compatibility of solvent fractions during the carbonization as well as macromolecular structure induced by the cross-linking treatment. Despite the significant rise in carbon residue, the air blowing treatment for the pitch exhibited an adverse effect on the mesophase development during the subsequent direct carbonization of the treated pitches, which is ascribed to enlargement in the difference in reactivity and mutual solubility of PES and PEI fractions generated during the cross-linking treatment. Also, during the early stage of subsequent carbonization, the loss of molecular planarity in mesogens such as formation of biphenyl structure induced by dissociation of oxygencontaining functional groups restricts the formation and development of mesophase. The hydrogen-donor aromatic oil (HAO) as a portion of PES fraction of the pitch-HAO mixtures effectively improves the mutual compatibility of the solvent fractions via H-transfer reactions and dilution effects during the co-carbonization process. However, the three-dimensional molecular conformations in PP25 pitch caused by excessive cross-linking among the 9065


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Structural Modification of Petroleum Pitch Induced by Oxidation

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