Article pubs.acs.org/EF
Modified Effects of Additives to Petroleum Pitch on the Mesophase Development of the Carbonized Solid Products Bin Lou,† Dong Liu,*,† Ming Li,† Xulian Hou,†,‡ Wenqian Ma,† and Renqing Lv† †
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao, Shandong 266555, People’s Republic of China ‡ China Petroleum Engineering Company, Ltd., Beijing Company, Beijing 100085, People’s Republic of China S Supporting Information *
ABSTRACT: Comparative studies in carbonization behaviors of petroleum pitch (SS70 pitch) and SS70-pitch−additive mixtures were conducted to develop a better understanding of the modified mechanisms of additives. The carbonization of solvent fractions in SS70 pitch was also investigated to reveal the relationships among solvent fractions and the additives. The characterization of resultant solid products by X-ray diffraction and scanning electron microscope analysis showed that samples obtained from co-carbonization of SS70−aromatic oil (P1) and SS70−aromatics enriched fractions of FCC slurry (P2) mixtures respectively had higher stacking layer numbers of mesogens and flatter morphology than those of specimens produced by carbonization of SS70 pitch and SS70−deasphalt oil (P3) mixtures. Adding additives P1 and P2 into SS70 pitch lowered not only the rates of carbonization but also the aromaticity and size of aromatic molecules in toluene insolubles formed in the early stage of carbonization, both jointly contributing to the formation of well-developed mesophase. The asphaltenes of SS70 pitch were prone to forming the poorly developed mesophase due to its high reactivity and also interfered with the carbonized performance of maltenes. Additives P1 and P2 played the “dominant partner effect” on mesophase development by providing physical fluidity of the reaction system and some chemical stability for asphaltenes via their dilution effect and H-transfer reactions.
1. INTRODUCTION In recent years, the petroleum pitch, mainly consisting of polycyclic hydrocarbons with numerous alkyl substitutes,1 has been widely used to prepare advanced carbon materials,2−7 such as lithium battery cathode material, carbon fiber, composite C/C materials, needle coke, and high density activated carbon materials, etc. The carbonaceous mesophase can be obtained by carbonization of petroleum pitch.8−10 But there are thousands of different types of molecules in petroleum pitch to react distinctively, leading to complex chemistry of pitches.11 Due to incompatiblity in carbonizing rates of different fractions, the pyrolysis of petroleum pitch always leads to poor quality of the mesophase in the resultant solid product which is not suitable for manufacturing advanced carbon materials. So modified methods including hydrogenation modification,12,13 catalytic polymerization modification,14,15 and co-carbonization modification16,17 were used to improve the quality of the mesophase pitch from the pyrolysis of the modified petroleum pitch. Co-carbonization modification is regarded as a practical and effective method, which is performed by mixing two kinds of feedstock to improve carbonization properties of feedstock.1 Previous research studies17−19 have shown that the alkyl and hydrogen transfer reactions between pitch and additives are the main factors to promote the mesophase development in resulting cokes. Recently, strong interactions between pitch constituents and polymers during the co-carbonization have been reported.20−24 In these studies, coal-tar pitch has always been used as feedstock and a great deal of attention has also been given to the characterization of the solid products from co-carbonization of feedstock. It is well-known that respective © 2016 American Chemical Society
carbonization behavior of petroleum pitch and coal-tar pitch is very different owing to their distinct constituent molecular structures.25 And the mechanism of co-carbonization of petroleum pitch and petroleum-based additives is not systematically investigated. Besides, there are a few papers26,27 focusing on the relationships between solvent fractions and the additives during the co-carbonization. In this study, the comparison in chemical compositions and structures between resulting solid samples was carried out, and variations in the carbonization rates and molecular structures of toluene insoluble formed in the early stage of carbonization were also investigated to further explore the co-carbonization modification effect. In addition, carbonizing behaviors of solvent fractions of the SS70 pitch were also investigated in order to determine the relationships among maltenic fraction, asphaltenic fraction, and the additives.
2. EXPERIMENTAL SECTION 2.1. Feedstocks. SS70 pitch in this study was obtained by crosslinked vacuum residue derived from the Petrochemical Plant of Chinese Petroleum Group. The aromatic oil (P1), aromatic-rich fraction (P2) of FCC slurry, and deasphalt oil (P3) were selected as the additives. The properties of SS70 pitch and the additives are listed in Table 1. In this work, SS70 pitch was also separated into maltene and asphaltene fractions by extraction of petroleum ether with a boiling range from 90 to 120 °C, according to ASTM D6560-2000. 2.2. Carbonization Process. SS70 pitch, its solvent fractions, and the materials comprising SS70 pitch or solvent fractions mixed with Received: August 23, 2015 Revised: January 16, 2016 Published: January 19, 2016 796
DOI: 10.1021/acs.energyfuels.5b01920 Energy Fuels 2016, 30, 796−804
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amount was the point that was enough to fully exhibit the modified effect of the three additives. 2.3. Characterization of Samples. The content of elements, including carbon, hydrogen, sulfur, and nitrogen, of samples was directly determined by the Varil EL-3 element analyzer, while the oxygen content was obtained by difference. 1H nuclear magnetic resonance (1H NMR) spectra were recorded on a BrukerAvance DMX500 spectrometer using deuterated chloroform as solvent and tetramethylsilane (TMS) as internal standard. Fourier transform infrared (FTIR) spectra were obtained on a Nicolet NEXUS 470 FTIR spectrometer. The ortho-substitution index that can reflect the relative size of the aromatic molecules was defined as Ios = abs750/(abs750+ abs815+ abs880), and the aromaticity index was calculated according to the formula Iar = abs3050/(abs3050 + abs2920).28 X-ray diffraction (XRD) tests were performed on an X’Pert Pro MPD X-ray diffractometer with Cu Kα radiation. The mesophase products were examined by XP-4030 polarized microscope (Shanghai Milite Precise Instrument Co. Ltd., China). And 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 represents the special anisotropic structure index. So the OTI value of the isotropic region is 0. According to the report by Eser,29 the anisotropic microstructures were divided into four types: (1) mosaic (M; feature size < 10 μm; OTI value of 1); (2) small domain (SM; feature size of 10−60 μm; OTI value of 5); (3) domain (D; feature size > 10 μm; OTI value of 50); (4) large domain (LD; length size > 60 μm; width > 10 μm; OTI value of 100). The surface morphology of samples was investigated by using a Hitachi S-4800 field emission scanning electron microscope. Nitrogen adsorption measurement was performed on an ASAP 2020 micropore analyzer at liquid N2 temperature. In order to compare the difference in kinetics of each carbonization process, a simple kinetic model for mesophase formation was established according to the following two assumptions: (1) The toluene insolubles (TI) content is used to represent the mesophase content (X) of the solid carbonized products (TI = X); (2) the overall process of mesophase formation follows a first-order reaction (n = 1). Therefore, the TI formation rate is
Table 1. Properties of SS70 Pitch and the Additives items
SS70
P1
P2
P3
C/(wt %) H/(wt %) S/(wt %) N/(wt %) O/(wt %) ∑(S+N+O)/(wt %) (C/H)atom saturates/(wt %) aromatics/(wt %) resins/(wt %) asphaltenes/(wt %)
87.01 9.94 0.59 1.00 1.46 3.05 0.74 15.79 21.33 30.68 32.20
88.83 9.03 0.55 0.63 0.96 2.14 0.82 22.26 62.94 13.70 0
90.12 7.90 0.62 0.87 0.49 1.98 0.95 8.51 82.67 8.82 0
86.84 12.37 0.24 0.22 0.33 0.79 0.58 51.63 39.18 9.19 0
P1, P2, or P3 in different ratios were used as the basic carbonization feedstocks. The carbonizing processes were conducted using a 300 mL autoclave apparatus within temperature range of 400−440 °C for 0− 14 h under the reaction pressure of 6−10 MPa. Nitrogen (99.999% pure) was used to replace the air in the reactor, and then the initial nitrogen pressure was set at 8 MPa before the heating. During the carbonized reactions, the extra pressure was released back to 8 MPa by means of the back-pressure valve when the pressure in the reactor reached to 8.1 MPa. When the carbonization process finished, the pressure was gradually released to the atmospheric pressure and then the reactor was cooled to room temperature. The heating rate was almost constant at ∼5 °C/min in all experiments. When reaching the reaction temperature, the soaking time was recorded as 0 h. The homogeneous SS70−additivies mixtures were prepared by mixing the additive and SS70 pitch at the certain ratio under the temperature of 150 °C and stirring at 600 rpm for 1 h, and the whole mixing process was under the flow of nitrogen (99.999% pure). A simple variable method was adopted to select the optimal conditions to discuss the modified effect, and these selected optimal conditions were aimed at reflecting the fully modified effect of the additives. As is known to all, the large, planar mesogens are derived from an appropriate degree of thermal cracking and condensation of feedstock molecules and hardly form at low temperature, but at high temperature the violent carbonization reactions easily cause quick growth in the viscosity of the reaction medium, which can hinder mesogens stacking and coalescence between the anisotropic structures; therefore, each feedstock had its own optimal reaction temperature. With increasing of the reaction time, the mesophase development successively undergoes the generation of mesophase spheres, growth of those spheres, and subsequent coalescence into bulk mesophase; as a result, in the process of selecting conditions, the reaction time first was set at 10 h, which provided enough time for mesophase development. Appropriate growth of reaction pressure contributes to lowering the viscosity of the carbonized medium, whereas excessive high pressure forces many light components into the carbonzied system and then hinders the coalesence between mesophase structures. But on the basis of our previous research the pressure causes relatively little impact on the mesophase formation and development when the pressure was within our experimental range. So, in our study, the optimal reaction temperature was first determined and then the influence of the addition amount was investigated on the premise that the soaking time (10 h in our experiments) was enough for mesophase formation and development, followed by the selection of the pressure. The corresponding results (seen in Tables S1−S4 of the Supporting Information) showed that the operation conditions comprised of 410 °C, 6 h, and 8 MPa were chosen for direct carbonization of SS70 pitch and co-carbonization of SS70−P3 mixtures while the conditions including 420 °C, 6 h, and 8 MPa were applied for co-carbonization of SS70−P1 and SS70−P2 mixtures; the mesophase content and OTI value in the solid carbonized products were basically unchanged when the addition amount exceeded 20 wt %, which demonstrates that the 20 wt % addition
dX = k(1 − X ) dt
(1)
where k is the rate constant. The boundary condition for this equation is X = X*
at t = 0 h
(2)
where X* refers to the TI formed during heating to the hold temperature. Integration of eq 1 yields
ln(1 − X ) = ln(1 − X *) − kt
(3)
According to eq 3 plots of ln(l − X) versus the soaking time (t) should give straight lines.
3. RESULTS AND DISCUSSION 3.1. Properties of Feedstocks. 3.1.1. Elemental and SARA Compositions of Feedstocks. The elemental and SARA compositions of materials are given in Table 1. The highest atom C/H ratio was for additive P2, followed by additive P1, SS70 pitch, and additive P3 in order. The low C/H ratios of additive P3 and SS70 pitch are consistent with their highly paraffinic nature, while the high C/H ratios of additives P1 and P2 can be explained by their high aromaticity degree. It should be noted that SS70 pitch constituted more heteroatoms (S, N, and O) than those of other feedstocks, which can deteriorate the optical textures of the resultant cokes.30 A major difference in distributions of SARA among these staring materials was that only SS70 pitch contained 32.20 wt % asphaltenes. And the largest percentage of saturates (51.63 wt 797
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samples produced by carbonization of every feedstock at respective optimal conditions is listed in Table 3. In this work, the resultant solid products obtained by direct carbonization of SS70 pitch at 410 °C, co-carbonization of SS70-P3 mixtures at 410 °C, and co-cabonization of SS70−P1 and SS70−P2 mixtures at 420 °C were labeled as MP, MP-P3, MP-P1, and MP-P2, respectively. It should be noted that additives P1, P2, and P3 generated no mesophase when they carbonized alone at their respective optimal reaction condition of co-carbonization. The optical microstructures of the MP sample were primarily isotropic region, small domain and domain structures, and no large domain generated (seen in Table 3 and Figure 1a). This poorly developed mesophase is attributed to the high reactivity of SS70 pitch containing substantial large polycondensed aromatic molecules attached with long alkyl chains. Comparing with optical microstructures of MP sample, the percentages of isotropic region in MP-P1 and MP-P2 samples obviously reduced accompanied by significant increase in the percent of domain and large domain structures (seen in Table 3 and Figure 1b,c). These variations clearly manifest that the additives with high aromaticity, especially for the additive containing abundant aromatic molecules attached to the naphthenic structures such as P2, can promote mesophase formation and development in carbonaceous mesophase products. Nevertheless, the optical textures of MP-P3 sample showed an increase in the percentage of the isotropic region and mosaic at the expense of small domain and domain, which implied that the mesophase formation and development was restricted when SS70 pitch co-carbonized with additive P3 (seen in Table 3 and Figure 1d). Considering structural characteristics of the three additives, it can be concluded that during the co-carbonization with P1 or P2, aromatic and naphthenic hydrogen transfer reactions that can lower the co-carbonizing rate.31 Besides, the modified effectiveness of additive P1 or P2 can also be ascribed to the dilution of rapidly carbonizing species in SS70 pitch and their free radicals, and then increase the solubilization of polycondensed aromatic molecules produced during the reactions. The dilution effect and H-transfer reactions maintain the fluidity of the reaction medium and contribute to development of extensive mesophase in a relatively slow and orderly progression.32 Instead, additive P3, due to its relatively high paraffinic nature, reduces the colloidal stability of SS70
%) was for additive P3, while the aromatics in additives P1 and P2 were the most abundant, 62.94 and 82.76 wt %, respectively. Therefore, it can be concluded that P1 and P2 have good compatibility with the SS70 due to their high degree of aromaticity in contrast to P3 prone to cause the reduction of the collide stability of SS70 pitch owing to its paraffinic nature. 3.1.2. 1H NMR Analysis. The distributions of constituent hydrogens in feedstocks are given in Table 2. The proportions Table 2. 1H NMR Analysis and Structural Parameters Calculated by B−L Method weight/% material
Har
Hα
HN
Hβ
Hγ
fa
HAU/CA
SS70 P1 P2 P3
8.0 16.8 32.4 6.7
12.6 24.2 35.0 6.0
9.5 18.6 6.2 8.5
50.0 19.7 18.3 56.6
19.9 20.7 8.1 22.2
0.38 0.49 0.64 0.20
0.51 0.72 0.82 0.85
of aromatic hydrogen (Har) in feedstocks varied in a range from 32.4 wt % (additive P2) to 6.7 wt % (additive P3), clearly distinguishing additives P1 and P2 from SS70 pitch and additive P3. Moreover, the percentage of HN in additive P2 was 18.6 wt %, far more than the 9.5 wt % of SS70 pitch, 6.2 wt % of additive P2, and 8.5 wt % of additive P3. Another distinct difference was seen in the Hα, Hβ, and Hγ contents. SS70 pitch and additive P3 contained the highest content of Hβ to Hα and Hγ, indicating that their constituent compounds have long alkyl side chains, while additives P1 and P2 have more Hα to Hβ and Hγ, which suggested that high concentrations of methyl substitutes and/or α-CH2 groups to the aromatic rings. Table 2 further showed that the aromaticity degree ( fa) value of SS70 pitch was only 0.38 but its condensation index HAU/CA value was a minimum (0.51). It suggests that the SS70 pitch contains large molecules consisting of condensed aromatic ring systems attached with long alkyl chains. In contrast, additives P1 and P2, which both possessed relatively high fa and HAU/CA values, comprise an abundant relatively small size of aromatic compounds with short side chains. Additive P3 with minimal fa value but maximal HAU/CA value contains substantial small aromatic molecules connected with long-side-chain substituents. 3.2. Characterization of Carbonized Solid Products. 3.2.1. Optical Textures of Solid Products. The quantitative evaluation of optical microstructures in the resultant solid
Table 3. Optical Microstructures of the Solid Products at 8 MPa, 6 h optical microstructures T/°C
I
M
SD
D
LD
mesophase content/(vol %)
OTI
SS70 pitch
410
84.9 ± 2.3
45.07 ± 0.77
SS70-P3 mixture
410
0 0 0 50.3 48.2 52.7 25.3 28.0 24.5 0 0 0
62.07 ± 1.28
420
30.1 33.6 28.5 21.8 23.1 19.8 36.2 34.1 38.8 19.4 21.4 17.3
93.0 ± 2.1
SS70-P2 mixture
28.9 23.9 31.6 17.4 13.5 19.7 19.7 21.1 13.6 21.0 19.4 21.6
16.54 ± 1.24
420
6.7 4.1 7.8 3.6 5.6 3.3 4.4 4.7 5.3 8.3 10.4 8.6
64.7 ± 2.6
SS70-P1 mixture
35.3 38.4 32.1 6.9 9.6 4.5 15.4 12.1 17.8 51.3 48.8 52.5
49.1 ± 1.5
10.98 ± 0.59
feedstock
798
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Figure 1. Optical texture of the solid products: (a) MP; (b) MP-P1; (c) MP-P2; (d) MP-P3.
3.2.3. Scanning Electron Microscopy Analysis and Nitrogen Adsorption Measurement. Scanning electron microscopy (SEM) analysis was used to investigate the surface morphology of the specimens, and SEM images are presented in Figure 2. The SEM photographs showed that the surfaces of MP-P1 and MP-P2 were relatively flat and cavities were hardly seen, but there was formation of pores in the surface of the MP sample, and MP-P3 sample had smaller but more intensive pores. Combined with comparison between the specific surface areas and pore volumes listed in Table 5, it can been concluded that the P3 addition could generate more pore structures which easily form when a large amount of volatiles is generated during the carbonizing reactions and releases quickly,11 accompanied by fast increasing viscosity of the system. 3.3. Mechanism of Co-carbonization Modification. 3.3.1. Relationship between Carbonizing Kinetics and Mesophase Development. It has been reported34,35 that, during the carbonization process, the constituent molecules of feedstock undergo a set of consecutive reactions, in which the molecules of the intermediate generated by condensation reactions change progressively the insolubility. So in this work, overall kinetics parameters34 of carbonization were determined by measuring the content of toluene insoluble (TI) formed at different temperatures as a function of soaking time (shown in Figure 3). Author: It is clear that the formation of TI during the carbonization of each feedstock follows an apparent first-order kinetic behavior over a wide range of conversion. The deviations to first-order kinetics for high TI conversion are also seen in Figure 3a,d. This experimental finding is analogous ́ to those of Eser and Jenkins,32 Ebrahimi et al.,34 and RodriguezReinosoa et al.35 These authors only used experimental data in the linear zone, eliminating the rest of the data. In this work a similar criterion has been applied.
pitch and also accelerates the co-carbonized reactions by cracking of long side chains. 3.2.2. XRD Analysis of Resultant Solid Products. The microcrystalline parameters of each sample are calculated and presented in Table 4. Among all of the XRD patterns, there was Table 4. Microcrystalline Parameters of the Resultant Solid Products microcrystalline parameters sample
d002/Å
Lc/nm
n
MP MP-P1 MP-P2 MP-P3
3.4394 3.4771 3.4728 3.4389
2.54 3.51 3.19 2.40
8.39 11.09 10.19 7.96
an obvious peak at 2θ ≈ 25.6−26.0°, corresponding to (002) reflection of carbon structures due to the stacking of aromatic layers.33 The interlayer spacings d002 of MP-P1 and MP-P2 were larger than that of MP, and the d002 of MP-P3 was minimum. The growth of interlayer spacing can be attributed in the increasing proportion of the side-chain substituents in constituent molecules of MP-P1 and MP-P2 samples, leading to intensified space steric effect. Similarly, d002 slight reduction in MP-P3 is due to a lack of side chain substituents. The stacking height Lc of MP-P1 and MP-P2 rose to 3.51 and 3.19 nm, respectively, while the Lc of MP-P3 slightly reduced to 2.40 nm. Ultimately, the addition of P1 and P2 promoted the stacking layers number (n) of the specimens but adding P3 decreased it. The variations of Lc and n show that the fluidity of the reaction system was promoted by addition of P1 and P2 and the quick increase in viscosity of the system during cocarbonization of SS70-P3 mixtures restricts the mobility and orientation of mesogens. 799
DOI: 10.1021/acs.energyfuels.5b01920 Energy Fuels 2016, 30, 796−804
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Figure 2. SEM images of the solid specimens: (a) MP; (b) MP-P1; (c) MP-P2; (d) MP-P3.
mixtures did at 410 °C (k, 7.48 × 10−5 and 8.20 × 10−5 s−1, respectively) but OTI values of MP-P1 and MP-P2 samples are also larger, which demonstrates that the relative rate of carbonization is not the only factor to determine the mesophase development of resultant solid products. So the structural features of the mesogen precursors from carbonization of each feedstock were sought by using solvent extraction, FTIR, and 1H NMR analysis to explain this phenomenon. 3.3.2. Structural Analysis of Mesogen Precursors Formed in the Early Stage of Carbonization. When petroleum pitch is carbonized, TI was always regarded as the precursor of mesogens. So the difference in molecular structures of TI formed at the initial stage of carbonization was investigated to further explore the modified mechanism. 3.3.2.1. Solvent Extraction and Elementary Analysis. Under the soaking time of 2 h and the pressure of 8 MPa, solid samples obtained carbonization from SS70 at 410 °C, SS70−P1 mixture at 420 °C, SS70−P2 mixture at 420 °C, and SS70−P3 mixture at 410 °C were labeled as S1, S2, S3, and S4, respectively. Among the solvent fractions of these intermediates, the predominant fraction was toluene solubles (TS) for all four samples (seen in Table 7). Additionally, S1 and S4 have not only a toluene insoluble−pyridine soluble fraction (TI-PS, 39.6 and 38.8 wt %, respectively) but also a pyridine insoluble fraction (PI, 10.3 and 13.5 wt %, respectively), whereas S2 and S3 contain only a TI-PS fraction (45.7 and 57.8 wt %, respectively). These observations imply that the TI fractions (including TI-PS and PI fractions) from S1 and S4 are heavier than those from the S2 and S3. Besides, it should be pointed out that all feedstocks well-dissolved in toluene and no TIs were generated when additives P1, P2, and P3 were separately carbonized alone for 2 h at 420, 420, and 410 °C respectively. Thus, this manifests that these toluene insolubles are mainly generated from reactions of highly reactive species in SS70 pitch. The atom C/H ratios of TI fractions from S1, S2, S3, and
Table 5. Specific Surface Areas and Pore Volumes of the Carbonized Products sample
specific surface area/(m2·g‑1)
pore volume/(×10‑3 cm−3·g−1)
MP MP-P1 MP-P2 MP-P3
7.89 0.91 1.18 15.62
5.91 2.44 6.02 13.45
The calculated rate constants together with the associated activation energies and preexponential factors are summarized in Table 6. Comparing with the rate constant of direct carbonization of SS70 pitch, it can be observed that, at the same temperature, the rate constants of co-carbonization of SS70−P1 and SS70−P2 mixtures showed decline in contrast to growth of that in co-carbonization of SS70-P3 mixtures. So it can be safely concluded that adding P1and P2 into SS70 pitch lowers the rate of carbonization while the addition of P3 can accelerate it. It is well-conceivable that a high carbonized rate causing a rapid increase in the viscosity of reaction medium can reduce the mobility of the constituent molecules and then suppress the mesophase development. A cross-examination of Table 3 (or Figure 1) and Table 6 reveals that the modified effects of additives P1 and P2 in enhancing mesophase development are seen to be commensurate with its reduced rates of carbonization at the reaction temperature of 410 or 420 °C. The activated energy was frequently used to compare the relative rates of carbonization. However, in this study, the least activated energy was for co-carbonization of SS70-P1 mixtures, followed by co-carbonization of SS70-P2 mixtures, direct carbonization of SS70 pitch, and co-carbonization of SS70-P3 mixtures. Therefore, it should be noted that consideration of the activation energies alone without taking into account the preexponential factors can be misleading.27 It is noteworthy that SS70−P1 and SS70−P2 mixtures carbonized at 420 °C (k, 8.40 × 10−5 and 9.87 × 10−5 s−1, respectively) more rapidly than SS70 pitch and SS70−P3 800
DOI: 10.1021/acs.energyfuels.5b01920 Energy Fuels 2016, 30, 796−804
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Figure 3. First-order plots for toluene insoluble formation from carbonization of the feedstock: (a) SS70 pitch; (b) SS70-P1 mixtures; (c) SS70-P2 mixtures; (c) SS70-P3 mixtures.
Table 6. Rate Constants (k), Activation Energies (Ea), and Preexponential Factors (A) of Each Carbonization Process k × 104/s−1 feedstock
400 °C
410 °C
420 °C
430 °C
440 °C
Ea/(kJ·mol−1)
SS70 SS70-P1 SS70-P2 SS70-P3
0.393 0.375 0.382 0.423
0.748 0.557 0.620 0.820
1.392 0.840 0.987 1.575
2.482 1.222 1.535 3.012
4.333 1.793 2.392 5.702
239 156 183 260
weight/% TS
TI-PS
PI
S1 S2 S3 S4
50.1 54.3 42.2 47.7
39.6 45.7 57.8 38.8
10.3 0 0 13.5
1.40 4.80 6.12 6.32
× × × ×
1014 107 109 1015
distinct and stretching peaks of aliphatic groups attached to aromatic compounds at 2850−2960 cm−1 are not very strong any more. The CO stretch vibration peaks (near 1700 cm−1) seen in the FTIR spectroscopy of SS70 pitch disappear. So an assumption can be made that these TI fractions are derived from condensation of reactive species of SS70 pitch, initiated by cracking of a long aliphatic side chain and/or oxygen functionalities. The FTIR indices Iar and Ios of TI fractions from S1 (0.59, 0.455) and S4 (0.63, 0.469) are larger than those from S2 (0.43, 0.376) and S3 (0.50, 0.363). It reveals that the aromatic molecules in TI fractions from S1 and S4 contain less aliphatic side chains but larger size of aromatic rings. 3.3.2.3. 1H NMR Analysis of TI-PS Fraction. The constituent hydrogen distributions of TI-PS fractions from S1, S2, S3, and S4, named as TI-PS-S1, TI-PS-S2, TI-PS-S3, and TI-PS-S4, are presented in Table 8. The comparison of aromatic degree fA and condensation index HAU/CA between TI-PS fractions also implies additives P1 and P2 can effectively suppress the
Table 7. Distributions of Solvent Fractions in Samples Obtained by Carbonization at 2 h and 4 MPa sample
A/s−1
S4 were 1.62, 1.23, 1.31, and 1.68, respectively. It also shows that adding P1 or P2 can restrict the excessive cross-linking reactions between highly reactive species in SS70 pitch. 3.3.2.2. FTIR Analysis of Toluene Insolubles. The FTIR spectra of SS70 pitch and toluene insoluble from S1, S2, S3, and S4 were shown in Figure 4. Comparing with the FTIR spectra of SS70 pitch, it can be found that the stretching vibration peak of aromatic hydrogen (3048.2 cm−1) becomes 801
DOI: 10.1021/acs.energyfuels.5b01920 Energy Fuels 2016, 30, 796−804
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in the subsequent carbonization process. Due to a large amount of long aliphatic side-chain substituents in P3, the addition of P3 contributes to the self-condensation of reactive species of SS70 pitch and causes adverse effect on the mesophase development. These findings demonstrate that the additives induce a different modified effect on the structure of molecules formed in the early stage of carbonization, which also plays an important role in mesophase formation and development. 3.3.3. Influences of Solvent Fractions on Mesophase Development of SS70 Pitch. The aforementioned experimental evidence indicates that the components of SS70 pitch can be broadly divided into two groups that differ in their carbonizing reactivity. So in this work, the SS70 pitch was separated into maltene (69.54 wt %) and asphaltene (30.46 wt %) fractions to investigate the influence of solvent fractions on the optical textures of solid products obtained from carbonization of the parent pitch. 3.3.3.1. Properties of Maltene and Asphaltene Fractions. The elemental composition together with 1H NMR analysis of the solvent fractions was given in Table 9. The carbon content between the solvent fractions is rather similar. However, as expected, the maltene fraction contains more hydrogen content (10.81 wt %) than the asphaltene fraction (8.59 wt %), so the atom C/H ratio of the maltene fraction is less. And the heteroatoms (S, N, and O), tend to concentrate in the asphaltene fraction, especially oxygen atoms. Certainly, the presence of reactive groups attached to aromatic nuclei, e.g., phenolic, carboxylic, and the presence of heteroatoms, all lead to rapid reaction rate and decreasing size of optical textures.30 Moreover, the calculated fA of the maltene fraction (0.31) was smaller than that of the asphaltene fraction (0.48) while the condensation index HAU/CA of maltenes was 0.65, larger than the 0.51 of asphaltenes. These results prove that the asphaltenic fraction contains substantial high-molecular-weight polycyclic aromatics and heterocycles, which contribute to highly reactive carbonization, while the maltene fraction is featured with the relatively small aromatic molecules attached with long aliphatic side chains. 3.3.3.2. Mesophase Development in Carbonization of Solvent Fractions. Figure 5 showed the optical textures of solid samples obtained from direct carbonization of solvent fractions at the pressure of 8 MPa. When solvent fractions were carbonized at 410 °C for 6 h, the asphaltene fraction was carbonized too fast to only produce mosaic structures, whereas only mesophase spheres appeared from the maltenic fraction due to its low reactivity. But their parent pitch exhibits a relatively developed mesophase at the same condition. So it can be deduced that during the carbonization of SS70 pitch, the asphaltene fraction can promote the reactivity of maltene fraction; at the same time, the maltene fraction can maintain the fluidity of system and disperse asphaltenes as well as free radicals to avoid excessive cross-linking reactions between asphaltenic molecules. When the carbonizing temperature was raised to 420 °C, the maltene fraction generated a well-
Figure 4. FTIR spectra of (a) SS70 pitch and toluene insolubles from (b) S1, (c) S2, (d) S3, and (e) S4.
Table 8. Distribution of Hydrogen in the TI-PS Fractions and Structural Parameters Calculated by B−L Method weight/% sample
Har
Hα
HN
Hβ
Hγ
fA
HAU/ CA
σ
TI-PS-S1 TI-PS-S2 TI-PS-S3 TI-PS-S3
55.5 41.9 45.7 62.4
28.4 35.7 33.7 26.9
5.2 8.4 7.1 4.1
7.7 9.6 9.8 4.7
3.2 4.4 3.7 1.9
0.86 0.77 0.80 0.88
0.52 0.63 0.60 0.52
0.20 0.30 0.27 0.18
condensation of reactive species during the early stage of carbonization. And the difference in the parameter σ, represented as the substitution degree of aromatic ring surrounding hydrogens, clearly suggests that more aliphatic side-chain substituents are retained in the molecule of toluene insolubles obtained from co-carbonization of SS70−P1 and SS70−P2 mixtures. Those results are consistent with the characterization by using FTIR. Based on the aforementioned characterized results, it is deduced that the toluene insolubles from the S1 sample are formed mainly by self-condensation between reactive species in SS70 pitch, which are regarded as condensed aromatic molecules highly substituted with substituents and/or functional groups containing heteroatom; and then these selfcondensation compounds, due to large molecular size and weight, are prone to aggregation from the reaction medium into isotropic or mosaic optical structures and interference with carbonization of less reactive species to form the welldeveloped mesophase structures. During the co-carbonization of SS70−P1 and SS70−P2 mixtures, free radicals from reactive species of SS70 pitch can be stabilized by H-transfer reactions, thereby facilitating the formation of molecules with less condensation and aromaticity degree. These molecules can diffuse well in the medium to form well-developed mesophase Table 9. Elementary Analysis of the Solvent Fractions weight/%
weight/%
material
C
H
S
N
O
C/H
Har
Hα
HN
Hβ
Hγ
fA
HAU/CA
maltenes asphaltenes
86.30 86.26
10.81 8.59
0.68 0.64
0.83 1.40
1.38 3.11
0.67 0.84
7.4 12.1
12.2 16.4
8.2 10.9
51.5 40.7
20.7 19.9
0.31 0.48
0.65 0.51
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DOI: 10.1021/acs.energyfuels.5b01920 Energy Fuels 2016, 30, 796−804
Article
Energy & Fuels
Figure 5. Optical texture of solid samples from the carbonization of SS70 pitch and its fractions: (a−c) carbonizing at 410 °C; (d−e) carbonizing at 420 °C.
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developed mesophase in comparison with the poorly developed mesophase from SS70 pitch and the asphaltenes. That is to say, the excellent carbonization performance of the maltenes is deteriorated by too rapid reactions of asphaltenes during the direct carbonization of SS70 pitch. Therefore, among the cocarbonization of SS70−additives mixtures, additives P1 and P2 extend the “dominant partner effect” on the development of mesophase by providing the necessary physical fluidity of the system such as the maltenes, and possibly some chemical stability for the asphaltenes via the dilution effect and Htransfer reactions,29 thus recovering the performance of maltenes and also improving that of asphaltenes.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b01920. Experimental designs and corresponding results of carbonization of SS70 pitch and SS70-additives mixtures (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel./Fax: +86-0532-86984629. E-mail:
[email protected]. com.
4. CONCLUSION
Notes
The authors declare no competing financial interest.
Comparing with direct carbonization of SS70 pitch, the reduced rates of co-carbonization with additives P1 or P2 at the same experiment temperatures are attributed to naphthenic or aromatic hydrogen transfer reactions and dilution effect. And decreasing aromaticity degree as well as relative size of aromatic molecules of toluene insoluble fraction formed at an early stage of the carbonization processes promotes solubility in the matrix and then contributes to a well-developed mesophase of the solid products. Owing to high heteroatoms content and molecule weight, the asphaltene fraction acts as a rapidly carbonizing species and interferes with the excellent carbonization performance of the maltene fraction. During the cocarbonization, additives P1 and P2 extend the dominant partner effect on the development of mesophase by providing the necessary physical fluidity of the system and possibly some chemical stability for the asphaltenes via the dilution effect and H-transfer reactions.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant 21176259) and the Fundamental Research Funds for the Central Universities (Grant 15CX05009A).
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