Distribution and Structural Analysis of Polycyclic Aromatic

Article pubs.acs.org/EF

Distribution and Structural Analysis of Polycyclic Aromatic Hydrocarbons Abundant in Coal Tar Pitch Xiaohua Fan,† Youqing Fei,*,†,‡ Lei Chen,† and Wei Li† †

College of Materials Science and Engineering, Hunan University, Changsha, Hunan 410082, China State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha, Hunan 410082, China

‡

ABSTRACT: High-resolution matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) combined with other analytical techniques and aromatic structure analysis were employed to characterize and identify large polycyclic aromatic hydrocarbons (PAHs) in coal tar pitch and its hexane-soluble fraction. Nonsubstituted aromatics in high abundance up to 850 Da can be extracted successfully with defined molecular formula from MALDI spectra, and mapped systematically into a 50/24 matrix, indicating a structural growth pattern by cooperative ortho-condensation and baycondensation. The matrix provided a quantitative summary, with their quantity and chemical structures defined essentially by MS intensity and molecular formula of PAHs isomers, respectively. It also revealed a trend that the PAHs grown alternatingly at ortho and bay positions exhibit much higher concentrations than those by ortho- or bay-condensations alone, even though their relative abundance by MS intensity may drop as aromatic rings grow with condensation polymerizations. Such compositional insights may be valuable for better utilization of coal tar pitch as well as understanding the overall reactions that lead to the formation of such a complex mixture including very small and very large polycyclic aromatic compounds.

1. INTRODUCTION Coal tar pitch (CTP) is a valuable aromatic hydrocarbon resource and has been produced in large scale for centuries, as a byproduct of metallurgic cokes since the first industrial revolution. In China, it is estimated that over 15 million tons of coal tar (containing ∼50 wt % CTP) are produced annually in recent years,1 with a significant portion consumed simply as a fuel. Both poor utilization and mishandling may result in serious environmental and health issues,2 because some polycyclic aromatic hydrocarbons (PAHs) in CTP are carcinogenic and mutagenic.3−5 After proper processing, however, the CTP is useable for binders6,7 and pitch cokes,8 and with more advanced processing, it can be a valuable precursor of advanced carbon materials, including carbon fibers,9−12 needle cokes,13 and carbon−carbon composites.14 High yield and quality of the resultant carbon, stable and sufficient source, and low price are the among important benefits. Chemically, CTP is an extremely complex mixture that consists of several thousands to millions of different species, mostly PAHs. The chemical components have significant influences on the processing behavior of pitches and the properties of the final products.15−17 A better understanding of CTP components at molecular level can be a key to solving many issues mentioned above and beyond. Extensive work on the characterization of pitches has been carried out using various analytical methods,18−20 and an extensive review was recently published for polydisperse hydrocarbon mixtures, including CTP.21 For characterization at the molecular level, earlier chromatographic and spectrometric techniques, for example, gas chromatography−mass spectrometry (GC-MS),22,23 high-performance liquid chromatography (HPLC),24 field ionization mass spectrometry (FIMS), and field desorption mass spectrometry (FD-MS),25,26 © 2017 American Chemical Society

had been limited to relatively low molecular mass compounds. With advanced analytical techniques developed, especially matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), high-resolution molecular ions can be obtained at high molecular mass.27−29 Great efforts have been made to validate high molecular ion signals by second approaches, including size exclusion chromatography (SEC) and synchronous ultraviolet-fluorescence spectroscopy (UV-F),30−33 and assisted sometimes with proper separation techniques, including dense-gas extraction (DGE) and thinlayer chromatography (TLC).34−36 Large amounts of molecular mass (MM) data have also been treated collectively to evaluate analytical techniques and to extract some characteristic features of samples.37,38 For instance, Herod and co-workers have fractionated CTP and studied the detectable MM range, average structural parameters, and structural relationships of major PAHs in certain fractions by MALDI-MS and SEC.23,39−42 Müllen et al. compared the petroleum pitch and CTP, in terms of MM distributions, carbon number and ion mobility, and other molecular structural characteristics by highresolution MALDI-MS and ion mobility mass spectrometry.43 Their compositional analysis statistically indicated that PAHs in CTP are grown by linear and nonlinear extension of benzene rings. Other efforts have also been made to examine individual MS peaks to obtain specific molecular information. Herod and colleagues attempted to identify certain individual MS peaks from laser desorption-MS (LD-MS) assisted with GC-MS, as PAHs and their isomers, for a coal tar and soluble fractions of CTPs.40,41 Müllen et al. examined the MM increments of 24 Received: November 22, 2016 Revised: March 9, 2017 Published: April 3, 2017 4694

DOI: 10.1021/acs.energyfuels.6b03113 Energy Fuels 2017, 31, 4694−4704

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Energy & Fuels and 26 Da, or the “24/26 rule”, on predominant MALDI peaks for CTP and petroleum pitch.43 They found that the 24/26 rule was valid for the CTP, but not petroleum pitch, although the MM increment of 26 Da (C2H2) was not verifiable with CTP based on aromatic structure or chemistry. On the other hand, the work by Thies et al. at Clemson University has been directed to separating different oligomers from petroleum pitch and identifying series of individual MALDI peaks in monomer and dimer fractions, in terms of methyl substitutions, base aromatic cores, and their condensations for the oligomers, via extensive separation, preparation, and confirmation processes, including DGE, preparative-scale gel permeation chromatography (prep-scale GPC), high-performance liquid chromatography with photodiode array detection (HPLC/PDA), and MALDI-post source decay (PSD).32,36,44 Nonetheless, an overall picture on major constituents has not been obtained yet for CTP, in terms of molecular structure as well as their quantity in a systematic fashion. The present study was undertaken to investigate the MM distribution and structural features of abundant PAH species in CTP. GC-MS and high-resolution MALDI-MS were employed first to investigate molecular structures of certain predominant PAHs with known molecular mass in the lighter hexane-soluble fraction, and to establish relationships between the molecular structures and MM distribution patterns of these aromatic species identified. Upon the confirmation of overall molecular structural characteristics by FT-IR and NMR, the MALDI spectra of the parent CTP sample were subjected to more extensive analysis based on aromatic structural characteristics. The MM distribution patterns of abundant PAH components were carefully examined, and a 50/24 matrix diagram was developed to reconstruct PAHs structural growth patterns by benzene ring-addition via ortho-condensation and bay-condensation. Combined with relative intensity of MALDI-MS, an overview for major PAHs in CTP was eventually obtained quantitatively and structurally.

TMS) was chosen as the solvent. All the NMR spectra analyses as well as phase and baseline correction were performed via the MestReNova program. In order to reduce the uncertainty caused by manual adjustment, at least three repeat analyses were made, and the average values were used as the final results. Fourier transform infrared spectral (FT-IR) analysis was carried out on a Nicolet 6700 FT-IR spectrometer using regular KBr disc method. The spectrum was recorded from 4000 to 650 cm−1 at a resolution of 4 cm−1.

2. EXPERIMENTAL SECTION

molecular ion signals clearly recorded from around 200 to over 550 Da and strong peaks observed between 250 and 450 Da. The overall outline of spectrum roughly looked like a Gaussian distribution and was comprised of a series of peak clusters separated apparently from each other. Each cluster essentially contained a dozen of individual peaks, usually including a predominant peak with maximum intensity. These peak clusters can be divided into a sequence of main clusters and minor clusters, which alternately distributed with a spacing of around 12−14 Da between their predominant peaks. The predominant peaks of main clusters were observed at 252, 276, 302, 326, 350, 376, 400, 426, and 450 Da, apparently separated by increment of 24 or 26 Da. The increments of 24/26 have been observed with aromatic hydrocarbons formed in different systems, including electric discharging, thermal pyrolysis, and combustion flames.46,47 Below 200 Da, there was essentially no peak observed, which was consistent with literature.29 This was probably due to the volatilization of light molecules in the vacuum of the MALDI instrument. Although MALDI mass spectrometry has been proven to be a powerful technique for pitch samples, MALDI-MS alone normally cannot provide reliable structure information. For pitch-like mixtures, up to now no single technique could produce satisfactory results independently regarding the molecular structure for the entire sample.48 Thus, it is necessary to validate based on other analytical methods in order to correctly interpret the signals

3. RESULTS AND DISCUSSION 3.1. Characterizations of HS Fraction. Although hexane is a relatively weak organic solvent with no polarity, the light HS fraction contains smaller and representative PAHs of the parent CTP. As the lightest and major component of CTP samples, the HS fraction may play an important role in providing molecular characteristics of PAHs for the light fraction and whole CTP sample as well. 3.1.1. MALDI-MS Analysis of HS Fraction. Figure 1 shows the high-resolution MALDI spectrum of HS fraction, with

Figure 1. MALDI-TOF mass spectrum of the HS fraction.

A soft CTP made from a large domestic commercial plant was used in this study, with the elemental composition C = 92.55 wt %, H = 4.75 wt %, N = 0.99 wt %, S = 1.00 wt % and softening point 58.3 °C. The quinoline insoluble (QI) fraction was removed by high temperature centrifugation. Hexane-soluble fraction (HS) was obtained by solvent extraction and accounted for about 56 wt % of the parent CTP on QIfree basis. A Bruker Autoflex III TOF/TOF mass spectrometer (Bruker Daltonics, Leipzig, Germany), equipped with a 200-Hz smartbeam laser, was employed to determine the MM distribution of the parent CTP and its HS fraction. The MALDI spectra were recorded in the reflection mode. The solvent-free sample preparation method was used, with 7,7,8,8-tetracyanoquino-dimethane as matrix.29,45 Gas chromatography−mass spectrometry (GC-MS) analysis of the HS fraction was carried out on a Thermo Finnigan Trace 2000 PolarisQ MSn ion trap, equipped with a DB-5 quartz capillary column (30 m × 0.25 mm i.d., 0.25 μm film thickness). The carrier gas was He at the flow rate of 1 mL min−1. The HS fraction and solvent dichloromethane were mixed at a ratio 0.8 mg mL−1, and about 1 μL of solution was injected with the injector at 280 °C and a split ratio of 10:1. The temperature was programmed to hold 150 °C for 3 min, then heat to the final temperature 300 °C at 5 °C min−1 rate, with the final temperature maintained for 15 min. The transfer line into the ion trap was kept at 230 °C. The mass spectrometer scans from m/z 40 to 650 and electron ionization (EI) at 70 eV was used. The nuclear magnetic resonance spectra (NMR) were performed on a Bruker AM-400 spectrometer. Chloroform-d (contains 0.03 vol % 4695


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Figure 2. GC-MS chromatogram of the HS fraction. The peak numbers corresponding to the compounds are identical in Table 1 and Figure 3, to indicate aromatic structure relationships.

molecular structural relationships could be built on three base units that were shared extensively as MMs increased. Such a relationship may look like a “family tree”, and a similar approach was taken earlier by Herod et al.41 It is interesting to notice that the molecular structure developments of these abundant PAHs were highly analogous. The larger molecules appeared to be formed by adding benzene rings successively in certain positions of smaller aromatic units, with their MMs increased by a multiple of 24 or 50 Da. The molecular structures of predominant PAHs may contain a five-membered ring or purely six-membered ring. They could be grown from acenaphthene (m/z = 154) and phenanthrene or anthracene (m/z = 178) by increasing a benzene ring consecutively. But the way of buildup was not random, the mass increments of 24 and 50 Da actually were the results of bay-condensation and ortho-condensation, respectively, as will be discussed in details later (see Figure 10). Briefly, the structural changes were caused by incorporating CC (24 Da) to an available three-sided bay (arm-chair) position,51,52 and CCHCHC (50 Da) to one side (CC bond consisting of two secondary carbons) on base aromatic rings,53 respectively. These two reactions can also be the examples of nonlinear and linear extension of benzene rings.43 Although limited to relatively small MMs, GC-MS results allowed us to identify some strong peaks from the main clusters and minor clusters in the MALDI spectra, essentially as even-carbon-numbered PAHs (bare PAHs) and odd-carbonnumbered aromatics (methylated PAHs), respectively. The mass increment of 14 Da between predominant peaks (i.e., from the main and minor clusters) is essentially due to methylation,47 and is not a focus of this study. 3.2. Characterization of CTP Sample. 3.2.1. NMR Analysis of CTP Sample. Both 1H NMR and 13C NMR spectra of the CTP sample, as shown in Figure 4, displayed strong signals in aromatic regions, but weak in aliphatic ones, confirming high aromaticity and a low degree of substitution. For the 1H NMR spectrum (Figure 4a), the protons with NMR chemical shifts between 0.5 and 4.5 ppm were attributed to the aliphatic hydrogens (Hal) that attached to the carbons of side

observed in MALDI-MS. For relatively low MMs, GC-MS is an excellent choice for this purpose. 3.1.2. GC-MS Analysis of HS Fraction. The GC-MS chromatogram of HS fraction is showed in Figure 2, with significant peaks numbered and corresponding compounds identified in Table 1. As expected, the abundant species identified were nonsubstituted aromatics, including certain isomers. The dominant species were phenanthrene (#16), anthracene (#17), fluoranthene (#28), pyrene (#29), triphenylene (#42), chrysene (#43), and perylene (#53) along with their benzo and methyl derivatives, showing MMs up to 302 Da. The MMs of these PAHs were also observed in the MALDI spectra (Figure 1), but only accounted for a small part. In addition to a few small aromatics with MM less than 200 Da, some heterocyclic and hydrogenated PAHs were also detected at much lower concentration. No aliphatic compound was detected in the HS fraction. Except for the hydrogenated PAHs that may be formed in a hydrogen-rich reducing environment, these results were consistent with those reported widely in the literature.22,23,49,50 Therefore, the compounds identified in HS fraction can be classified principally into four types: (1) the primary constituent of substituent-free PAHs, (2) some methylsubstituted PAHs, (3) small amount of partially hydrogenated PAHs, and (4) small amounts of heterocyclic aromatics. The upper limit of MM (302 Da) by GC-MS was obviously much lower than that of MALDI. This is principally due to the low volatility of majority unidentified compounds with high MMs to be carried by mobile gas through chromatographic column (e.g., dibenzopyrene at best in this study). Clearly, the GC-MS method exhibited its limitations to separate and identify larger PAH species. Nonetheless, GC-MS is one of the most versatile and reliable techniques for analyzing the light PAHs in pitch materials. More importantly, the small numbers of relatively small PAHs identified allow us to investigate the molecular formula and structure of some PAHs in CTP, and to compare the results obtained from MALDI. Figure 3 summarizes the molecular structures of abundant aromatic compounds identified in the HS fraction, and 4696


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Energy & Fuels Table 1. Aromatic Components Identified in HS Fraction by GC-MS peak no.

m/z

potential PAH compound

formula

relative intensitya

peak no.

m/z

potential PAH compound

1 2 3 4 5 6 7 8 9 10 11 12 13 14

142 142 154 158 154 168 166 168 168 180 180 180 180 182

C11H10 C11H10 C12H10 C12H14 C12H10 C12H8O C13H10 C13H12 C13H12 C14H12 C14H12 C14H12 C14H12 C14H14

V V V V W V W V V V V V V W

34 35 36

216 216 218

37 38

230 234

39 40

226 232

15 16 17 18

184 178 178 206

C12H8S C14H10 C14H10 C16H14

V S W V

41 42 43 44 45 46 47 48

228 228 228 242 242 242 240 256

19 20 21 22 23 24 25 26 27 28 29 30 31 32

192 192 204 206 206 206 204 206 204 202 202 218 216 203

C15H12 C15H12 C16H12 C16H14 C16H14 C16H14 C16H12 C16H14 C16H12 C16H10 C16H10 C16H10O C17H12 C15H9N

W W W V V V V V V S S V W V

49 50 51 52 53 54 55 56 57 58 59 60 61 62 63

252 252 252 252 252 252 266 266 276 276 276 278 278 276 302

2-methyl-fluoranthene 7H-benzo[de]anthracene 5,6-dihydro-4H-benz[de] anthracene 5,6-dihydrochrysene benzo[b]naphtha[2,1-d] thiophene isomers benzo[ghi]fluoranthene isomers 1,2,3,4tetrahydrobenzanthracene isomers benzo[a]anthracene triphenylene chrysene 9-methyl-benz[a]anthracene 6-methyl-chrysene 5-methyl-benzo[c]phenanthrene 9H-cyclopenta[a]pyrene 7,8,9,10-tetrahydrobenzo[a] pyrene benzo[e]fluoranthene benzo[k]fluoranthene benzo[e]pyrene benzo[a]pyrene perylene benzo[b]acephenanthrylene 3-methyl-benzaceanthrylene 10-methylbenzo[a]pyrene indeno[1,2,3-cd]pyrene benzo[ghi]perylene dibenzo[def.mno]chrysene dibenz[a]anthracene benzo[b]chrysene indeno[1,2,3-cd]fluoranthene 1,2,4,5-dibenzopyrene

33

216

2-methylnaphthalene 1-methylnaphthalene biphenyl 1-phenyl-cyclohexene acenaphthene dibenzofuran fluorene 2,4a-dihydrofluorene 2-methyl-1,1-biphenyl 9,10-bihydro-anthracene 9,10-bihydro-phenanthrene 1-methyl-9H-fluorene 2-methyl-9H-fluorene 1,2,3,4-tetrahydro-phenanthrene isomers dibenzothiophene isomers phenanthrene anthracene 1,2,3,10btetrahydrofluoranthene 2-methyl-phenanthrene 2-methyl-anthracene 4,5-dihydro-acephenanthrylene 3,6-dimethyl-phenanthrene 1,7-dimethyl-phenanthrene 9,10-dimethylanthracene 2-phenyl-naphthalene 2,7-dimethyl-phenanthrene 1,9-dihydropyrene fluoranthene pyrene benzo[b]naphtho[2,3-d]furan 11H-benzo[a]fluorene naphtho[2,1,8-def ]quinoline isomers 1-methyl-pyrene

C17H12

W

a

formula

relative intensitya

C17H12 C17H12 C17H14

M M W

C18H14 C16H10S

W W

C18H10 C18H16

M W

C18H12 C18H12 C18H12 C19H14 C19H14 C19H14 C19H12 C20H16

S S M W W W W S

C20H12 C20H12 C20H12 C20H12 C20H12 C20H12 C21H14 C21H14 C22H12 C22H12 C22H12 C22H14 C22H14 C22H12 C24H14

S S M S S M W W W S W W S V M

V = very weak; W = weak; S = strong; M = medium.

related to bay-condensed (broadly as a peri-condensation) internal aromatic carbons and protonated aromatic carbons (Car1,3).55 The ratio of Car1,2 and Car1,3 was about 1:4, indicating significant contents of bay-condensed and protonated aromatic carbons. Although the quantitative analysis of 13 C NMR had certain limitations due to signal-noises ratio, the overall trend observed was consistent with the previous GC-MS results, and other NMR results reported in the literature.54−56 3.2.2. FT-IR Analysis of CTP Sample. The FT-IR spectrum of the CTP sample (Figure 5) exhibited two strong absorption bands at 1434 and 1621 cm−1, which were due to the stretching vibrations of CC on aromatic ring. The strong band at 3040 cm−1 was assigned to aromatic CH stretching. Two relatively weak bands at 2919 and 2854 cm−1 were caused by stretching of aliphatic CH, providing a clear evidence for alkyl substitutes in CTP. Furthermore, three distinct bands observed at 700−900 cm−1 were assigned to the out-of-plane bending modes of aromatic CH, which were related to the number of adjacent hydrogens on various aromatic rings, again indicating the presence of alkyl substitution. The existence of three bands indicated the alkyl substitution of PAH aromatic backbone more likely occur at the β position,36,57 which was also confirmed earlier by GC-MS. Therefore, the FT-IR results were

chains, and the aromatic hydrogens (Har) linked directly to aromatic carbons were in the range 6.5−9.5 ppm. The ratio of Hal and Har was approximately 1:7.2, also reflecting high aromaticity. Moreover, the aliphatic signals between 4.5 and 2.0 ppm were related to protons (Hα) on the methyl group connected directly to aromatic rings. The chemical shifts in the ranges 2.0−1.0 and 1.0−0.5 ppm were weak, belonging to methylene (Hβ) and propyl or longer aliphatic protons (Hγ), respectively. Clearly, the major compounds in CTP were PAHs, and a small number of aliphatic substituents were short and mostly methyl, as reported in the literature.20,52,54 The carbon types in the CTP can also be examined by 13C NMR in Figure 4b, the major peaks concentrated in the aromatic region (100−140 ppm), and few peaks were observed in the aliphatic region (0−70 ppm). The predominance of the aromatic signals over the aliphatic ones was even stronger than that of 1H NMR, and nearly 93% of the carbon was aromatic, indicating high aromaticity and very limited alkyl-containing compounds in the CTP, and being consistent with high C/H as well. The aromatic region between 160 and 129.5 ppm was assigned to the ortho-condensed (formerly, cata-condensed) aromatic carbons, and aromatic carbons with heteroatoms or aromatic substituents (Car1,2); the rest 129.5−108 ppm was 4697


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Figure 3. Molecular structures of the major PAHs identified by GC-MS in the HS fraction and preliminary structural relationships observed for PAHs growth. Two important reactions to add a benzene ring to specific positions of aromatic base: ortho-condensation (ΔMw = 50) and baycondensation (ΔMw = 24). PAHs are numbered the same as in Figure 2 and Table 1.

parent CTP and HS fraction may indicate their compositional similarity, in terms of molecular structure of PAHs. While preliminary molecular structures and structural relationships of limited number of PAHs in the HS fraction had been explored earlier, significant efforts were directed to look carefully into the MALDI spectra of the parent CTP from different angles. Figure 7 illustrates the magnified mass spectrum in the MM range of 200−500 Da, and it was not difficult to recognize some interesting patterns embedded. A repeated sequence of the mass differences (24−24−26 Da) starting from 178 Da was obvious between the predominant peaks within the main clusters, e.g., 178, 202, 228, 252, 276, 302, 326, 350, 376 Da. The MMs of these predominant peaks could be classified and linked to form two routes, based on the order of MM values added initially by 24 or 50 Da. For example, Route I is indicated by dotted arrows, and the solid arrows show Route II in Figure 7, with some basic concepts indicated earlier in Figure 3. The MM distributions of both routes gradually increased from 178 Da (marked with a hollow star, with its lower intensity already explained in section 3.1.1) to higher MMs by alternate increments of 24 and 50 Da. The only difference between two routes was that Route I increased MM first by 24 Da rather than 50 Da for Route II, but both Routes shared the same MMs at a fixed interval of 74 Da (marked by solid stars). To display sufficient details, Figure 7 exhibits only a small section with MMs up to 500 Da, while similar patterns were recognized up to about 850 Da. In order to investigate other detailed distribution characteristics from a different angle, a further magnified section with a

not only supported by the GC-MS and NMR results, but also consistent with the literature.33,58,59 All these results provided an overall picture that the main components of the CTP sample were essentially bare PAHs plus small number of methylated aromatics. 3.2.3. MALDI-MS Analysis of CTP Sample. Figure 6 shows the MALDI spectrum of whole CTP sample. A wide range of MM distribution roughly from 200 to over 1600 Da was observed with particularly strong signals distributing in 200− 600 Da range. The MM distribution range of strong peaks (200−600 Da) was similar to that (200−550 Da) of its HS fraction, probably reflecting some important roles of the HS fraction in the parent CTP sample. Although much broader than that of the HS fraction earlier, the mass spectrum of the parent CTP sample also encompassed a series of main peak clusters and minor peak clusters. Both clusters had significant differences in intensity, and one after another alternately distributed to form a full MALDI spectrum up to 850 Da, where the boundaries between the main and minor clusters became increasingly hard to recognize. Such a situation may be caused by a growing number of isotopes with MM increase, additional series of aromatic compounds, and some coupling reactions occurring randomly between two aromatic hydrocarbons. Regardless of the main or the minor, each peak cluster consisted of approximately a dozen of peaks, and usually contained a predominant peak with low-intensity peers around. Similar to the HS fraction, the mass difference between predominant peaks in the adjacent main clusters was 24 or 26 Da. The similarity in signal distribution pattern between the 4698


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Figure 6. MALDI mass spectra of CTP sample: (a) full mass spectrum and (b) a magnified region between 900 and 2000 Da.

Figure 4. NMR spectra of CTP sample: (a) 1H NMR and (b) NMR.

13

C Figure 7. Magnified MALDI spectrum of CTP sample in the range of 170−500 Da with predominant MM signals marked. Two different routes shown by Route I (MM increase repeated by 24/50) and Route II (MM increase repeated by 50/24), with the increment of 26 Da apparently resulting from crossed differences between two routes (e.g., 302 − 276 = 26).

Figure 5. FT-IR spectrum of CTP sample.

MM range from 350 to 550 Da is displayed in Figure 8. An important observation was that there were a number of peaks with their signal intensity excessively protruding from that of their adjacent peers, as marked by orange and blue colors for two groups of series with the increments of 24 and 50 Da, respectively. Such protruded peaks in MALDI spectra have received little attention so far in the literature. Because both carbon and hydrogen atoms have their own isotopes, PAHs with multiple H and C atoms can normally produce isotopederived consecutive peaks with gradually reduced intensity, for

Figure 8. Section of magnified CTP spectrum indicating the protruding peaks along with predominant peaks (blue and orange series indicating the MM increments of 50 and 24 Da, respectively), with the inset showing some effects related to isotopes.

example, a mass range of 524−531 Da shown by the inset in Figure 8. Thus, the protruding signals in the main clusters 4699


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Figure 9. Matrix designed for quantitative analysis of PAHs, with each cell containing intensity, molecular mass, and formula. From cell to cell, molecular mass varied by 24 and 50 Da, vertically and horizontally, respectively. Green region: Cells on the diagonal line having MM increment of 74 Da for specific predominant PAHs to share MMs with Routes I and II; green cells above and below the diagonal indicate the growth of aromatic ring via Route II (50/24) and Route I (24/50), respectively. Blue region: formed from many blue series or rows, with each series made from a sequence of protruded MMs with 50 Da increment and started from a green cell on Route II. Orange region: formed from many orange series or columns, with each series made from a sequence of protruded MMs with 24 Da increment and started from a green cell on Route I.

probably were not simply peaks of the isotope(s)-containing molecules, but likely different compounds, and the intensity of protruding signals in the spectra actually was the result of a superposition. It was interesting to notice that these protruding peaks in either orange or blue group could be divided into many series, with each series having a fixed MM increment of 24 or 50 Da. For example, the orange series of 448−472−496 was extended from the predominant peak of 424 with the increment of 24 Da; while the blue series of 426−476 was originated from the predominant peak of 376, growing by an increment of 50 Da. Additionally, the intensity of these protruded peaks in the same series clearly displayed a gradually decreasing trend with the increase of their MMs. Although the

MM range from 350 to 550 Da is displayed in Figure 8, such a pattern can extend to about 850 Da in the MALDI spectra. Similar trends can also be observed from the data reported for a CTP with MMs between 450 and 500 Da.43 3.2.4. Mapping of Predominant and Protruded PAHs. To understand MM distribution characteristics of abundant PAHs, a 50/24 matrix diagram was conceived and shown in Figure 9. Similar to the periodic table for the organization of different elements, this matrix is used to organize varied types of PAHs, based on the data from MALDI spectra and structural insights from GC-MS data and structural analysis. This summary diagram was constructed from the top-left cell at 178 Da, by adding 50 Da from the left to right in each row, and 24 Da from 4700


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Figure 10. Examples of two routes for PAHs to grow in a full cycle, as two benzene rings added alternatingly by bay/ortho- (24/50) or ortho/baycondensations (50/24) (total MM increase kept at 74 Da). Green sides indicate the bay position, and blue line shows an ortho position on a PAH base.

line contained PAHs that were identified clearly by GC-MS in the HS fraction, but showed relative lower concentrations. These lighter PAHs in CTP may be affected by coal tar distillation and evacuation as discussed earlier, rather than originally formed during the coal coking process, i.e., thermodynamics and/or kinetics. Interestingly, the protruding molecules in the blue and orange series observed in the MALDI spectra exactly occupied the upper and the lower sides of the green region, respectively, as shown by orange and blue cells in Figure 9. Each specific column or row was formed by filling an orange or blue series of protruded MMs, originated from to a predominant PAH in the green region. For example, the orange 448−472−496 series in the fourth column started from the predominant molecule 424 Da in the green cell. Certainly, as observed in MALDI spectra, the PAH molecules along the same blue rows or the same orange columns had gradually reduced their relative intensity (illustrated by brighter gradient colors) with the increase of MMs. Thus, PAHs with up to 70 carbon atoms and MMs around 850 Da have been recognized and analyzed in the diagram with sufficient accuracy. The rest of the cells, gray and colorless, were the areas of unrecognized peaks and unextractable data, respectively. Because of mapping all extractable data both from predominant and protruded peaks into the 50/24 matrix, three color-coded regions with welldefined shapes were displayed, validating the MALDI results

the top to bottom in each column. As the starting point of the diagram, 178 Da (A1 in Figure 9) was the MM of anthracene and phenanthrene, two isomers of the smallest PAH identified in the HS fraction and two base units for molecular structural relationships depicted in Figure 3. This diagram was markedly different from the “24/26 rule”,46 because the increment of 26 Da could not be explained for CTP by aromatic chemistry.43 For the 50/24 matrix, however, any single step move (e.g., from cell to cell) along each row or column had a recognized meaning in aromatic chemistry by ortho-condensation or baycondensation. The key differences between the 24/26 rule and the 50/24 matrix method can be better understood by examining how MALDI data from different types of PAH are organized by the matrix. By filling the cells of diagram with the PAHs data of predominant peaks from the main clusters, a green region was produced (in Figure 9) where these predominant PAH species distributed exactly along a diagonal band (of three lines). Moreover, the distribution of PAH peaks belonged to Route I (as dotted line in Figure 7 and Figure 9) and Route II (solid lines in Figure 7 and Figure 9) displayed stepped paths as the MM increased, occupying the upper and the lower lines, respectively. The cells on the central/diagonal line were occupied by shared MMs from both Routes. The MMs values in green region along three diagonal lines can be calculated by formulas shown in Figure 9. The top-left area inside the yellow 4701


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Figure 11. Illustration of structural growth patterns by known PAHs up to 400 Da, via different benzene ring-addition routes, by alternating orthocondensation and bay-condensation.

column in the orange region and row in the blue region, in addition to the green diagonal band). A nearly opposite trend in the top-left area was largely related to the CTP production process where vacuum distillation (may include the vacuum in MS instrument) was carried out to remove lighter fractions (e.g., up to anthracene oils). The partition effects between gas and liquid phases may partially explain a Gaussian shape of MM distribution in the HS fraction, whereas soluble−insoluble partitions in organic solvent extraction should be responsible for the other part. Also, the highest intensity along a single row or column may occur slightly away from the diagonal line as MM increased beyond 400 Da. For example, E4, F5, G5, H5, and I6 of the green and orange cells for E, F, G, H, and I rows in Figure 9, and D5, E6, E7, E8, and F9 of green and blue cells for the fifth to ninth columns. This was probably an indication that for a given degree of bay- or ortho-condensation, the cooperating ortho- or bay-condensations need to occur to generate a specific PAH species with high populations. The MMs of abundant PAHs included in the 50/24 matrix were extended to a little over 850 Da, although lots of signals with much lower intensity were observed almost continuously with the MMs above 1600 Da, and even earlier on the background of MALDI spectra. While some discrimination of larger MM species by MALDI ionization process may be a factor, other minor reaction mechanisms cannot be excluded from possible routes for higher MM PAHs at relatively low population, besides ortho- and bay-condensations. For example, the coupling reactions between a PAH and any available reactant (including another PAH) may randomly result in a large number of hydrocarbons with a wide range of MMs.

itself. In addition to the MM value, two other key parameters, i.e., molecular formula and relative intensity, were also included to display a full range of abundant PAH species in the CTP sample quantitatively and comprehensively, since the intensity of molecular signals within a reasonable range essentially reflects their relative contents in the sample.60 3.2.5. Distribution of Abundant PAHs in CTP. Starting from green cells on the diagonal of 50/24 matrix, a decreasing trend in the MM intensity had been observed extensively with protruded PAHs in each blue (50 Da) series or orange (24 Da) series, caused by ortho-condensation or bay-condensation alone. The former had tendency to extend more cells (i.e., longer blue cells) in the row than the latter (i.e., shorter orange cells) in the column in Figure 9. This is likely due to the fact that for a given PAH more choices were normally available at its side (of two secondary carbons) than the three-sided armchair or bay positions where two different condensation reactions have to occur (illustrated later in Figure 10). While this was an important point to build the matrix for aromatic structure analysis, but the 50/24 matrix actually can reveal more insights about PAHs abundantly present in CTP. Except a few cells at the top-left area, in the direction parallel to the diagonal, the green cells showed a decreasing intensity as the MM increased, and the cells on the central diagonal line had the highest intensity up to certain MMs, row-wise as well as column-wise. These nearly monotonic trends strongly indicated the importance of the cooperative ortho- and bay-condensations that significantly influenced the PAH formation of CTP. Furthermore, the formation of PAHs was apparently controlled by kinetics, rather than thermodynamics, because decreasing population trends were clearly and monotonically observed as the MM increased, for every series of PAH species (along the 4702


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Energy & Fuels 3.2.6. Structural Analysis and Illustration of Abundant PAHs in CTP. Based on these results, especially a number of clear trends observed from the 50/24 matrix, the cooperative ortho- and bay-condensations were found to be responsible for the formation of predominant PAHs in CTP, on a macro scale. It is worthwhile to investigate a few specific examples regarding some effects of cooperative ortho- and bay-condensations on the structural growth from three small PAHs, phenanthrene, anthracene, and fluoranthene (Figure 10). Depending on the structures of base PAHs, specifically whether the base has a bay position, e.g., phenanthrene, there can be no difference in the final product if a full cycle of cooperative ortho/bay reactions is completed, regardless via Route I (bay/ortho or 24/50) or Route II (ortho/bay or 50/ 24), even though their intermediates can be different, shown by two examples in the top of Figure 10. For a PAH without any bay position, e.g., anthracene (with three rings fused in a straight line), the Route I is not allowed, because no bay position is available for a more ring to be formed initially by adding -CC- (24 Da). The same rule is applicable to a fivemembered ring-containing PAH, fluoranthene (at the bottom of Figure 10). On the other hand, the same PAH product may be produced from the same route (II), but from different reactant bases, i.e., anthracene and phenanthrene, as shown in the middle of Figure 10. These examples clearly indicate possible selectivity of cooperative condensations, relationships between MM increments and molecular structure. All these factors can result in a large number of possibilities/ combinations of intermediates and final products, especially as the MMs of PAHs increase. To further illustrate the growth patterns of some abundant PAHs in CTP, several species identified with slightly larger MMs of 276 and 302 Da were selected as the base reactants to demonstrate possible aromatic structures of the PAHs that have a known CAS number with MM up to 400 Da, via Route I and Route II, in Figure 11. Since MMs in the 50/24 matrix were recognized up to around 850 Da, the largest PAHs extended by such a growth pattern likely have aromatic structures with their size a little more than twice of the largest PAHs shown in Figure 11. On the other hand, the number of resultant PAHs with high MMs can increase exponentially as cooperative ortho/bay-condensations progress further, making the identifications of numerous individual isomers with specific molecular structure even more challenging. Nonetheless, to our knowledge, this probably is the first time that a molecular structurebased tool, 50/24 matrix, was developed as an effective way to validate the high-MM data of CTP, enabling us to examine the distribution of abundant PAHs in CTP systematically and quantitatively.

The MALDI spectra of the parent CTP exhibited 50/24 patterns for abundant PAHs more extensively and clearly up to 850 Da, with abundant PAHs mapped nearly perfectly into a 50/24 matrix to obtain their molecular formula. The 50/24 matrix, developed as a molecular structure-based tool, has been shown not only to validate MALDI-MS data effectively, but also to examine the distribution of abundant PAHs in CTP systematically and quantitatively. The compositional results from 50/24 matrix analysis on MALDI data can provide valuable insights not only for better utilization of coal tar pitch but also for better understanding the overall reactions that lead to the formation of complicated PAH systems by cooperative ortho- and bay-condensations.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +86 0731 88821930. ORCID

Xiaohua Fan: 0000-0002-8923-9792 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is principally sponsored by the Principal Scientist Programs of the 985 Project, other support by the State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body at Hunan University (No. 734215002) is also acknowledged.

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REFERENCES

(1) Du, M. M. Guangzhou Huagong 2011, 39 (20), 29−30 (in Chinese with English abstract). (2) Trosset, R. P.; Warshawsky, D.; Menefee, C. L.; Bingham, E. Investigation of selected potential environmental contaminants: asphalt and coal tar pitch; Cincinnati University: Cincinnati, OH, 1978. (3) Burstyn, I.; Boffetta, P.; Heederik, D.; Partanen, T.; Kromhout, H.; Svane, O.; Langard, S.; Frentzel-Beyme, R.; Kauppinen, T.; Stucker, I.; Shaham, J.; Ahrens, W.; Cenee, S.; Ferro, G.; Heikkila, P.; Hooiveld, M.; Johansen, C.; Randem, B. G.; Schill, W. Am. J. Epidemiol. 2003, 158 (5), 468−478. (4) Friesen, M. C.; Demers, P. A.; Spinelli, J. J.; Eisen, E. A.; Lorenzi, M. F.; Le, N. D. Am. J. Epidemiol. 2010, 172 (7), 790−799. (5) Friesen, M. C.; Benke, G.; Del Monaco, A.; Dennekamp, M.; Fritschi, L.; de Klerk, N.; Hoving, J. L.; MacFarlane, E.; Sim, M. R. Cancer Causes & Control 2009, 20 (6), 905−916. (6) Kubica, K. Karbo-Energochem.-Ekol. 1997, 42 (4), 141−145. (7) Snyder, D. R.; Wombles, R. H.; Golubic, T. A. U.S. Patent 20050081752A1, 2005. (8) Yamashita, M.; Shibata, K. Japan Patent 60152591A, 1985. (9) Alcañiz-Monge, J.; Cazorla-Amorós, D.; Linares-Solano, A.; Oya, A.; Sakamoto, A.; Hosm, K. Carbon 1997, 35 (8), 1079−1087. (10) Mora, E.; Blanco, C.; Prada, V.; Santamaría, R.; Granda, M.; Menéndez, R. Carbon 2002, 40 (14), 2719−2725. (11) Montes-Morán, M. A.; Crespo, J. L.; Young, R. J.; García, R.; Moinelo, S. R. Fuel Process. Technol. 2002, 77−78, 207−212. (12) Ji, Y. B.; Li, T. H.; Lin, Q. L.; Fang, C. G.; Wang, X. X. Key Eng. Mater. 2007, 334−335, 165−168. (13) Mochida, I.; Fei, Y.; Korai, Y.; Oishi, T. Fuel 1990, 69 (6), 672− 677. (14) Matzinos, P. D.; Patrick, J. W.; Walker, A. Carbon 1996, 34 (5), 639−644. (15) Mochida, I.; Fei, Y. Q.; Korai, Y.; Fujimoto, K.; Yamashita, R. Carbon 1989, 27 (3), 375−380. (16) Eser, S.; Wang, G. Prepr. Symp. - Am. Chem. Soc., Div. Fuel Chem. 2004, 49 (1), 15−17.

4. SUMMARY AND CONCLUSIONS The present work has been performed in attempt to more quantitatively and comprehensively characterize the nonsubstituted, large aromatics, i.e., PAHs, abundant in coal tar pitch, using high-resolution MALDI-MS combined with other instrumentation and structural analysis. Beginning with the lighter HS fraction, in addition to a few methylated and hydrogenated PAHs, a small number of PAHs with high abundance were separated and identified by GC-MS and compared with the analytic results of MALDI-MS. A preliminary PAH “family tree” was constructed and linked to ortho- and bay-condensations. 4703


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Energy & Fuels (17) Liu Y, E. S. Relationships between Molecular Composition of FCC Decant Oils and Mesophase Development. 22nd Biennial Conference on Carbon; University of California: San Diego, 1995. (18) Martin, Y.; Garcia, R.; Sole, R. A.; Moinelo, S. R. Chromatographia 1998, 47 (7-8), 373−382. (19) Guillen, M. D.; Iglesias, M. J.; Dominguez, A.; Blanco, C. G. Fuel 1995, 74 (11), 1595−1598. (20) Kershaw, J. R. Polycyclic Aromatic Compounds 1993, 3 (3), 185− 197. (21) Herod, A. A.; Bartle, K. D.; Morgan, T. J.; Kandiyoti, R. Chem. Rev. (Washington, DC, U. S.) 2012, 112 (7), 3892−3923. (22) Afanasov, I. M.; Kepman, A. V.; Morozov, V. A.; Seleznev, A. N.; Avdeev, V. V. J. Anal. Chem. 2009, 64 (4), 361−365. (23) Karaca, F.; Morgan, T. J.; George, A.; Bull, I. D.; Herod, A. A.; Millan, M.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2009, 23 (13), 2087−2098. (24) Fetzer, J. C.; Kershaw, J. R. Fuel 1995, 74 (10), 1533−1536. (25) Drake, J. A. G.; Jones, D. W. Fuel 1983, 62 (7), 835−839. (26) Fujioka, Y. Tetsu to Hagane 1999, 85 (2), 189−194. (27) Herod, A. A.; Zhang, S. F.; Carter, D. M.; Domin, M.; Cocksedge, M. J.; Parker, J. E.; Johnson, C. A. F.; John, P.; Smith, G. P.; et al. Rapid Commun. Mass Spectrom. 1996, 10 (2), 171−177. (28) Lazaro, M. J.; Herod, A. A.; Domin, M.; Zhuo, Y.; Islas, C. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1999, 13 (14), 1401− 1412. (29) Edwards, W. F.; Jin, L.; Thies, M. C. Carbon 2003, 41 (14), 2761−2768. (30) Lázaro, M.-J.; Herod, A. A.; Cocksedge, M.; Domin, M.; Kandiyoti, R. Fuel 1997, 76 (13), 1225−1233. (31) Herod, A. A.; Lazaro, M. J.; Domin, M.; Islas, C. A.; Kandiyoti, R. Fuel 2000, 79 (3−4), 323−337. (32) Burgess, W. A.; Pittman, J. J.; Marcus, R. K.; Thies, M. C. Energy Fuels 2010, 24 (8), 4301−4311. (33) Gargiulo, V.; Apicella, B.; Alfe, M.; Russo, C.; Stanzione, F.; Tregrossi, A.; Amoresano, A.; Millan, M.; Ciajolo, A. Energy Fuels 2015, 29 (9), 5714−5722. (34) Herod, A. A.; Kandiyoti, R. J. Chromatogr. A 1995, 708 (1), 143−160. (35) Islas, C. A.; Suelves, I.; Carter, J. F.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2000, 14 (19), 1766−1782. (36) Cristadoro, A.; Kulkarni, S. U.; Burgess, W. A.; Cervo, E. G.; Räder, H. J.; Müllen, K.; Bruce, D. A.; Thies, M. C. Carbon 2009, 47 (10), 2358−2370. (37) Herod, A. A. Rapid Commun. Mass Spectrom. 2010, 24 (17), 2507−2519. (38) Herod, A. A.; Bartle, K. D.; Kandiyoti, R. Energy Fuels 2007, 21 (4), 2176−2203. (39) Islas, C. A.; Suelves, I.; Millan, M.; Apicella, B.; Lazaro, M.-J.; Herod, A. A.; Kandiyoti, R. J. Sep. Sci. 2003, 26 (15−16), 1422−1428. (40) Millan, M.; Morgan, T. J.; Behrouzi, M.; Karaca, F.; Galmes, C.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2005, 19 (13), 1867−1873. (41) Parker, J. E.; Johnson, C. A. F.; John, P.; Smith, G. P.; Herod, A. A.; Stokes, B. J.; Kandiyoti, R. Fuel 1993, 72 (10), 1381−1391. (42) George, A.; Morgan, T. J.; Alvarez, P.; Millan, M.; Herod, A. A.; Kandiyoti, R. Fuel 2010, 89 (10), 2953−2970. (43) Zhang, W.; Andersson, J. T.; Raeder, H. J.; Muellen, K. Carbon 2015, 95, 672−680. (44) Burgess, W. A.; Thies, M. C. Carbon 2011, 49 (2), 636−651. (45) Przybilla, L.; Brand, J.-D.; Yoshimura, K.; Räder, H. J.; Müllen, K. Anal. Chem. 2000, 72 (19), 4591−4597. (46) Beck, M. T.; Keki, S.; Szabo, P. T.; Zsuga, M. Tetrahedron 1999, 55 (6), 1799−1806. (47) Alfe, M.; Apicella, B.; Tregrossi, A.; Ciajolo, A. Carbon 2008, 46 (15), 2059−2066. (48) Morgan, T. J.; George, A.; Alvarez, P.; Millan, M.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2008, 22 (5), 3275−3292. (49) Fernandez, A. M.; Barriocanal, C.; Diez, M. A.; Alvarez, R. Fuel 2012, 101, 45−52.

(50) Gargiulo, V.; Apicella, B.; Stanzione, F.; Tregrossi, A.; Millan, M.; Ciajolo, A.; Russo, C. Energy Fuels 2016, 30 (4), 2574−2583. (51) Rieger, R.; Müllen, K. J. Phys. Org. Chem. 2010, 23 (4), 315− 325. (52) Guillén, M. a. D.; Díaz, C.; Blanco, C. G. Fuel Process. Technol. 1998, 58 (1), 1−15. (53) Fetzer, J. C. Stud. Surf. Sci. Catal. 1996, 100, 263−271. (54) Kershaw, J. R.; Black, K. J. T. Energy Fuels 1993, 7 (3), 420− 425. (55) Díaz, C.; Blanco, C. G. Energy Fuels 2003, 17 (4), 907−913. (56) Andresen, J. M.; Dennison, P. R.; Maroto-Valer, M. M.; Snape, C. E.; Garcia, R.; Moinelo, S. R. Prepr. Pap. - Am. Chem. Soc., Div. Fuel Chem. 1994, 39 (3), 777−781. (57) Silverstein, R. M.; Webster, F. X.; Kiemie, D. Spectrometric Identification of Organic Compounds, 7th ed.; Wiley: Hoboken, NJ, 2002. (58) Kershaw, J. R. Polycyclic Aromat. Compd. 1993, 3 (3), 185−197. (59) Alcaniz-Monge, J.; Cazorla-Amoros, D.; Linares-Solano, A. Fuel 2001, 80 (1), 41−48. (60) Kulkarni, S. U.; Raeder, H. J.; Thies, M. C. Rapid Commun. Mass Spectrom. 2011, 25 (19), 2799−2808.

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Distribution and Structural Analysis of Polycyclic Aromatic

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