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A COMPARATIVE STUDY OF THE PROPERTIES OF THE COAL EXTRACTIVE AND COMMERCIAL PITCHES Peter Nikolaevich Kuznetsov, Evgeniy Sergeevich Kamenskiy, and Ludmila Kuznetsova Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00158 • Publication Date (Web): 04 Apr 2017 Downloaded from http://pubs.acs.org on April 10, 2017
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A COMPARATIVE STUDY OF THE PROPERTIES OF THE COAL EXTRACTIVE AND COMMERCIAL PITCHES Peter N. Kuznetsov*, Evgeny S. Kamenskiy, Ludmila I. Kuznetsova Institute of Chemistry and Chemical Technology SB RAS, Federal Research Center «Krasnoyarsk Science Center SB RAS», 50-24 Akademgorodok, Krasnoyarsk, 660036 Russia
Received:
2017
Revised:
2017
Published:
Abstract Three extractive pitches were produced by using thermal dissolution of the medium-ranked coals at 380 °C in the anthracene oil. The empirical properties such as the chemical composition, solubility in quinoline and toluene, softening point, and content of carcinogenic polycyclic hydrocarbons, and also molecular and structural parameters of the extractive pitches were characterized in comparison with three commercially available pitches, including typical coal-tar pitch, petroleum-derived pitch and blended pitch derived from the mixture of coal-tar with petroleum feedstock. The molecular and structural properties of pitches were studied using FTIR, 1H and 13C NMR and XRD techniques. It was shown that, the average molecule of the extractive pitches was composed of predominantly aromatic rarely substituted pericondensed nuclei, like that in coal-tar pitch. The spatial structures of both the extractive and reference pitches consisted of predominantly disordered carbon matter with small amount of rather ordered nanosized “graphite-like” stacks. In terms of the technical specifications, the extractive pitches irrespectively of coal used met the requirements for the pitch binder and resembled commercial blended petro-coal-tar pitch. A remarkable merit of the extractive pitches compared to coal-tar pitch was low carcinogenicity (two-three times as low).
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1. Introduction The coking of bituminous and subbituminous coals is a large-scale process for the production of coke for the steel industry. This process yields also a little coal-tar (3-5%) as a by-product, from which a host of chemicals necessary for the production of advanced carbon materials and valuable organic substances can be separated.1,2 The main industrial application of coal-tar is as a feedstock for the production of coal-tar pitches having favorable combination of high coking ability and low viscosity in the molten state. These properties are responsible for the high level of physical and mechanical properties of the advanced carbon materials produced: anode composites, electrodes, pitch cokes, graphite and graphite-based construction materials, diamond, fullerenes, mesocarbon microbeads, carbon fibers and foams, activated carbons, molecular sieves, electrical-coal articles, tap-hole mix, refractory materials, roofing materials, electrical products, various carbon materials for nuclear and missile equipment. Actually, coal-tar is currently the main source of pitch for the production of these materials, the most amount (more than 75%) being traditionally consumed as a binder and an impregnating agent for the manufacture of carbon anodes and graphite electrodes. The need for coal-tar pitch and the requirements to its quality increase continuously in different branches of industry, in the increasingly developing aluminum industry, in particular, where it plays a crucial role as a binder for carbon anodes. The availability of coal-tar depends inevitably on the production of coke for the metallurgical industry. However, coke manufacturing has decreased significantly last years in most countries and is projected to decline further because of decreasing demand due to improvements in metallurgical technology: use of pulverized coal instead of coke, steady shift from traditional blast furnace to electric one, and other technological innovations. These differently directed trends in demand and in production of coaltar are faced with the urgent problem of search for the coal-tar pitch substitutes. Another important target is to mitigate environmental danger of pitches.2 The petroleum-derived pitch substitutes have an important advantage over the coal-tar pitches due to low carcinogenic emission.3,4 However, they show usually poor coking and binding properties and often enhanced sulfur content, and currently have limited use as a binder in the production of high quality anodes. The combination of petroleum-derived feedstocks and coal-tars allows the quality of the compounded pitches ACS Paragon Plus Environment
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to be significantly improved.5,6 A blend with 50 % petroleum-derived fraction has been found to be as good as the reference coal-tar pitch, the emissions of benz(a)pyrene decreasing by 50-60 %.6 One should note, however, that coal-tar and petroleum-derived pitch feedstocks both are the by-products and their availability inevitably depends on the coke-making and petroleum processing technologies. The key feature that makes coal-tar pitch attractive to carbon industry is a polycondensed aromatic nature of its constituents.1,2,7 Coal in its native form is known to contain polycondensed aromatic nucleus and this is a reason why it would be prudent to think again about coal as a source of pitches. In this context, coal dissolution aimed to extract polycondensed aromatics naturally occurring in the native coal matter seems to be a promising alternative for the coal coking process. The most studies carried out on thermal coal dissolution were aimed traditionally to the production of liquids as a feedstock for different fuels.8-10 Typical example is the SRC-I technololgy for coal processing into pitch-like product aimed for energy generation.9,10 The extensive studies on the dissolution of coals into polyaromatic products were carried out by Derbyshire and Whitehurst in the early 1980s.11 In the solvents such as pyrene, the coals with carbon contents in the range 82-88 wt. % were found to give up to 70-80 % of high-boiling products at relatively low temperatures of 370-400 °C and in the absence of hydrogen gas. Last years, thermal dissolution of coals to obtain pitch-like products was carried out by several authors.12-17 A review on the production of pitches from the products of alternative coal processes was reported recently.15 In the paper12, a bituminous fat coal was thermally dissolved and the properties of the products were characterized. The dissolved products obtained were shown to have better caking properties than that of fat coal itself. The authors14 reported that the application of hydrotreated polyaromatic solvent with hydrogen donating activity allows the extract yield of 73 % to be obtained from the low-rank coals at 400 °C. According to the data16, pitch materials can be produced even from the anthracite dissolution in a hydrogendonor solvent. Garcia et al17 studied a possibility of extraction of pitch-like matter from the coals by using supercritical fluids. The carbonization of the extractive pitch-like product was shown to give an anisotropic semicoke (73-78 %) with a fine mosaic texture. ACS Paragon Plus Environment
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In USA, the targeted studies to obtain reliable data to enable a technical evaluation of the process for the production of high quality pitch by a low severity process of solvent extraction of bituminous coals were conducted at the University of Kentucky and West Virginia State University.18 The principal objective was to produce pitches that can be used as binders and as a source in the manufacture of high grade coke for anode production, fundamental to the aluminum industry, and also as a feedstock for the synthesis of low cost carbon fiber. The bench-scale tests have demonstrated that coal dissolution in the anthracene oil at 400 °C and 60 minutes of residence time reached up to 85 %. The technology developed at West Virginia State University and patented by Quantex Research Corp. includes coal dissolution at 400-425 °С and a pressure of 1.4 MPa. Japanese researchers have elaborated the effective process for the production of the ashless Hypercoal by using mild dissolution of subbituminous and bituminous coals at temperatures of 360-380 °C in the mixture of bicyclic aromatic hydrocarbons as a solvent.19-23 Medium-ranked coals were found to be the most suitable for conversion into pitch-like products.19 Hypercoal showed good ductile and coking properties with rather high softening point of 240 to 270 °C. It can be used as an active additive to coking charge to get needle coke and other valuable carbon materials besides the fuels and chemicals.22 The results of the thermal dissolution of bituminous and subbituminous coals to produce the extractive pitches have been reported recently in our papers.24-26 In the medium of anthracene coal-tar fraction, the yields of pitch containing product at temperatures of 350-380 °C were up to 80 %. The extraction of pitchlike products occurred very selectively, only a little distillates and gases were obtained. This paper aimed to characterize the properties of the extractive pitches in comparison with the typical commercial reference pitches. Conventional routine methods which describe the requirements for pitch binder, such as proximate and ultimate analysis, the solubility in quinoline and toluene, softening point, and content of carcinogenic polycyclic hydrocarbons were used. However, these empirical parameters are only indicative of the properties of pitches because they represent a complex material containing hundreds of compounds with different functionalities and molecular structure, and a broad molecular weight distribution. For this reason, more detailed characterization was made by using FTIR, NMR and XRD techniques to ACS Paragon Plus Environment
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compare the molecular composition and spatial microstructure of the carbon matters of the extractive and reference pitches. 2. Experimental Section Coals and solvent used. The extractive pitch samples were prepared from three different-ranked coals taken from Kaa-Khem and Chadan deposits (Tyva Republic, Russia) and "Chertinskaya-Coke” mine (Kuznetsk Basin). Table 1 lists the proximate and ultimate analysis data for the coals and solvent used. The anthracene oil (AO) which is known to contain both the active hydrogen donors and the compounds with the solvating properties was used as the solvent. Its boiling range was between 250 to 360 °C. The coal samples were ground to a fraction of less than 1.0 mm and dried in a vacuum oven at 80 °C. The process of coal dissolution was carried out following previously optimized procedure using an experimental unit equipped with 2 dm3 autoclave with mechanical stirrer.24 Shown Fig.1 is the schematic diagram of the experimental unit used. The autoclave was charged with 900 g of coal/solvent slurry with the proportion of 1:2 by weight, purged carefully with nitrogen, then it was brought to the operating temperature (4-5 °C per min heating rate) to commence the digestion stage. The reaction was carried out at the temperature of 380 °C for 60 min residence time at 1.5-2.0 MPa autogenous pressure. At reaction completion, the autoclave was allowed to cool to 250 °C and the gases and vapor products vented through the collection vessel to depressurize the autoclave. The molten digest was then drained through a valve at the bottom of the autoclave into a heated cylinder-settler where pitch containing extract was separated from the ash and coal residue at 250 °C for 3 hours. After cooling to ambient temperature, the solid product was recovered from the settler, and its bottom part (consisted of mainly ash and coal residue) was separated to produce the ashless pitch containing extract. The pitch samples, labeled as EP-1, EP-2 and EP-3, derived from G, GF and F coals, respectively, were prepared by vacuum distillation of pitch containing extracts at 360 °C (corresponding to normal conditions). The reference pitch samples of different origins including typical coal-tar pitch used in aluminium industry as a binder, labeled as CTP, and two representative pitches prepared on a large scale unit from a petroleum ACS Paragon Plus Environment
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feedstock, labeled as PP, and from a petroleum-coal tar blend, labeled as PCTP. Details about the preparation technology of the reference samples are business confidential. Coal and pitch characterization. The technical properties of pitch samples were characterized using conventional routine methods such as proximate and ultimate analysis, fractionation with quinoline and toluene according to ASTM D-2317-66 and ASTM D-2318-66 standards, softening point, and content of carcinogenic polycyclic hydrocarbons. The contents of C, H, N, S in coal, pitch samples and in AO solvent were determined by using a Flash EATM 1112 analyzer. The concentration of the benz(a)pyrene (BaP) and other carcinogenic polycyclic aromatic hydrocarbons (PAHs) in the toluene soluble pitch fractions was analyzed using Shimadzu LC20 high-performance liquid chromatograph. The softening point of pitches was determined by using “ring and ball” method. NMR studies. The NMR spectra were recorded for the toluene soluble pitch fractions. The solution was filtered of through a double paper filter and dried in a rotary evaporator and then under vacuum. Complete toluene removal from the pitch samples was controlled by NMR spectra, drying was repeated if necessary. Dried extract was then dissolved in chloroform-d and the NMR spectra were recorded by using a Bruker Avance III Fourier spectrometer with proton resonance frequency of 600 MHz. 1H NMR spectra were recorded with 8 signal accumulation for 3.6 s and 5 s relaxation delay. 13C NMR spectra were obtained using inverse-gated technique. Each spectrum was recorded by accumulation of 4500 signals and 15 c relaxation delay between scans. Tetramethylsilane (TMS) was used as the chemical shift reference. The assignments of the chemical shifts of each NMR absorption was made according to the literature data.27-29 FTIR studies. The FTIR spectra were recorded on a Vector 22 spectrometer (Bruker, Germany) with a resolution of 4 cm-1. The samples were prepared in the form of KBr discs containing 1 mg of thoroughly crushed pitch. Selected spectral regions were studied by curve-fitting analyses to separate overlapping bands by using a commercially available data processing program (OriginPro, OriginLab Corporation) assuming Gaussian shape of bands. The positions and numbers of the bands were initially established based on the references30-34 and then fit to the experimental envelope by a minimum-least-squares iterative procedure. ACS Paragon Plus Environment
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XRD analysis. The XRD patterns for the powdered pitch samples packed into aluminium holder were recorded by using a PANalytical X’Pert PRO diffractometer with CuKα radiation and step-scanning (2Θ=0.2°, 25 s/step) between 2Θ from 5 and 55° under strictly the same conditions. The XRD peaks were deconvoluted using a data-processing program assuming Gaussian shape of bands. The parameters of the stacking structure of the pitch carbon matter were estimated from the (002) and (10) reflections which were corrected with absorption, polarization, and atomic scattering factors according to recommendations described in.35-38 The average distance between the polyaromatic layers, d002, in the stacking structure was determined from the (002) peak position using Bragg’s equation. The average thickness (Lc) of the stacks was evaluated from the width at the half maximum of the (002) peak using Scherrer’s equation, and the number of aromatic layers in the stacks by means of the equation N = (L c d 002 ) + 1 . The average diameter of layers (La) was calculated from the width at the half maximum of the (10) peak. 3. Results and Discussion 3.1. Chemical composition of pitches. The elemental analysis data for the extractive and reference pitch samples are shown in Table 2. The extractive pitches had higher content of hydrogen and oxygen and less carbon as compared to coal-tar pitch. Petroleum pitch differed with a highest content of hydrogen and sulfur as compared to other pitches. In general, the data in Table 2 show thus that chemical composition of the extractive pitches resembled that of PCTP sample except for oxygen and sulfur contents. 3.2. Technical specifications. Shown in Table 3 are the technical specifications of the pitch samples. The extractive pitches contained an enhanced portion of the toluene-insoluble α-faction and significantly less quinoline-insoluble α1-faction compared to coal-tar pitch. Petroleum and petro/coal tar pitches differed with less α-fraction, i.e. with better solubility in toluene. The softening points of the extractive pitches varied 105 °C to 115 °C depending on the coal, higher than those for the reference pitches. According to deashing procedure used, the ash contents in the extractive pitches were 0.5% to 1.0%, more than that in the reference pitch samples. Thus, the deashing technique needs to be improved by additional hot filtration through a fine glass membrane like that described in.18,23 ACS Paragon Plus Environment
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An important pitch quality parameter is the content of carcinogens. The data in Table 3 show that benz(a)pyrene ( BaP) concentration and benz(a)pyrene equivalent (BaPE, which takes into account other carcinogenic PAHs) in the toluene soluble fractions of the extractive pitches were close to those in the PCTP sample and much less (twice or more) than in the reference CTP sample. PP petroleum-derived pitch differed with much less carcinogenicity. Thus, with the exception of the enhanced ash content, technical specifications of the extractive pitches were approaching for coal-tar and petro/coal tar pitches. An important merit of the EP pitches was low carcinogenicity as compared with CTP. 3.3. Molecular composition of pitches. FTIR spectra data. Displayed in Figure 2 are the FTIR spectra for the extractive and reference pitch samples. All the spectra show the same typical absorbance bands in the 3600-500 cm-1 region, differing in intensity. The summary of the peak/band assignments based on the references29-34 are displayed in the Table 4. A large number of parameters associated with the chemical characteristics of pitches can be derived from the intensities of FTIR spectra.32,33,39 All pitches showed significant absorbances centered at 3050 cm-1 which indicated high aromaticity of pitch molecules. The results of curve-fitting analysis of the overlapped bands in the spectral region between 3100 cm-1 and 2800 cm-1 showed that the best fits were obtained with a reduced number of bands typical for pitches and bituminous coals: one (or two) band in the aromatic C-H region and four main bands in the aliphatic C-H region related to the stretching vibration of CH3 and CH2 groups (Fig.2). According to 31-33, the ratio of the integrated absorbance area in the aromatic region (A3100-3000cm-1) to that in the aliphatic region (A3000-2800 cm-1) was used as a relative hydrogen aromaticity index (Iar) for pitches: I ar = (A 3100-3000cm -1 A 3000-2800cm -1 )
(1).
From the data for Iar displayed in Table 5, it follows that the CTP pitch sample was characterized with strong aromaticity as compared to EP and PP pitches. In the spectral region between 900 cm-1 and 700 cm-1, several out-of-plane C-H deformation bands are observed. The spectrum for CTP pitch showed the most absorbance centered at 750 cm-1 indicating sparsely
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substituted aromatic rings. The aromatic rings in EP pitches had little more substituents, and PP pitch differed with higher extent of aromatic ring substitution as compared to other pitches. As for the structure of the saturated aliphatic groups, the spectra show that absorbance profiles depended upon pitch origin. The spectrum for CTP pitch between 3000 cm-1 and 2800 cm-1 showed much less proportion between the absorbance intensity centered at 2958 cm-1 (asymmetric stretching vibration of CH3 groups) and that at 2920 cm-1 (asymmetric stretching vibration of CH2 groups) as compared to other samples. According to29, the extinction coefficient for the asymmetric vibration of methylene group was about half that for the asymmetric vibration of methyl group. Taking this ratio of the extinction coefficients into account, the ratios between the numbers of CH3 groups to those of CH2 groups were calculated by using the Eq.2: CH 3 CH 2 = 0.5(A 2958cm -1 A 2920cm-1 )
(2).
It follows from the last column in Table 5 that the aliphatic groups in the molecules of the pitches were represented mainly by the methylene groups, they being by far the most abundant in the CTP pitch sample. This is consistent with lower intensity ratio between the bands at 1378 cm-1 (CH3 bending vibration) and at near 1450 cm-1 (CH2 bending vibration) for CTP sample as compared for EP and PP samples. The same conclusion based on the FTIR evidences for coal tar pitches was reported in.40 An additional qualification of the aliphatic structure of the pitches was derived from the analysis of the profiles and the positions of the slightly resolved bands near 1450 cm-1 (insertion in Figure 2). One can see that, at least, two kinds of CH2 groups were present in the saturated structures of the pitches. In the spectrum for the CTP sample, the band at 1455 cm-1 had lower intensity as compared to that at 1440 cm-1. However, the inverse proportion is observed for the EP pitches. According to41, the band centered at 1455 cm-1 can be due to CH2 in the alkyl substituents in the aromatic rings, and that at 1440 cm-1 can be assigned to CH2 in the hydroaromatic rings or/and, perhaps, in a isolated CH2 groups (e.g. methylene bridge between aromatic rings). If so, it may indicate a preponderance of CH2 in the alkyl groups in the EP pitches, and in the
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hydroaromatic cycles (or/and may be in methylene bridges between the aromatic rings) in the CTP pitch. The mode of CH2 group distribution in the PP pitch was less specific. The spectra between 1750 and 1650 cm-1 indicated different C=O groups in the PP and EP pitches, however, in small concentrations. In all spectra, weak absorbance was observed also at 1925 cm-1 due to the presence of compounds with cumulated C=C=C bonds.
3.4. 1H NMR data. The 1H NMR spectra of the toluene soluble fractions of different pitches were qualitatively similar, they all showed predominant resonances in the aromatic region (9.5-6.3 ppm) and very poor resonances in the aliphatic one (below 4.5 ppm). However, the relative signal intensities varied from one spectrum to another so that integration of the spectral regions related to specific hydrogen atoms was made. The assignement of the chemical shifts of each NMR signals was made according to the references.2729
Displayed in Figure 3 is the 1H NMR spectrum for the EP-3 extractive pitch as an example. The integrated data on the distribution of hydrogen between different structural positions in the extractive and in the reference pitches are shown in Table 6. One can see that, in the pitches studied, except for PP sample, the aromatic hydrogen accounted for 57.3 to 77.1%, and the hydrogen in the alkyl substituents in α- and βpositions to the aromatic rings did 20.9% to 36.5 %. Very small portion of hydrogen occurred in γ-position (0.7-5.1%), in olefin species (0.4-3.4%) and in oxygen functionalities (0.4-1.2%). The hydrogen distribution for PP pitch was less specific. The results on hydrogen distribution in the CTP and PP reference samples were in good agreement with the literature data reported in.28,42-44 For example, the proportions of the aromatic hydrogens in several petroleum-derived pitches have been found45 to be 46-58%, close to that in the reference PP sample (46.2%, Table 6). Based on the 1H NMR data in conjunction with the elemental analysis data, the Brown-Ladner structural parameters such as the aromaticity (fa), degree of aromatic ring condensation (Har/Car) and substitution (σ), and the number of carbon atoms (n) in the alkyl substituents in the average aromatic molecule were calculated according to the formulas45: ACS Paragon Plus Environment
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fa =
C H − H α x − H βγ y CH
H ar Car =
σ=
(3)
Hα x + H ar + O H C H − Hα x − H βγ y
(4)
Hα x + O H H α x + H ar + O H
n=
(5)
H α + H βγ Hα
(6)
where, Hα is the portion of hydrogen in alpha-position to aromatic ring determined by 1H-NMR; Hβγ - the portion of hydrogen in beta and more remote position to aromatic rings; C/H and O/H are the atomic ratios from chemical analysis; and x and y are the constants both equal to 2.0. It follows from Table 7 that the structural parameters of the EP samples were almost similar, i.e. they hardly depended on the rank of coal which is in line with the data of Rahman14 who compared pitch-like products extracted from the different coals. The reference CTP sample had almost same aromaticity as EP samples, however differed with higher degree of aromatic ring condensation ( H ar C ar = 0.48 ) as compared to both the extractive (0.63-0.67) and petroleum-based reference samples (0.52-0.59). The aromatic molecules of all the pitches had typically low extent of aromatic ring substitution (σ=0.12-0.22) and short alkyl substituents, n, between methyl and ethyl radicals, depending on the pitch.
3.5. 13C NMR data. While 1H NMR spectra reflect the composition of the periphery of the polycondensed aromatic molecules,
13
C NMR spectra provide also accurate information related to the internal carbon
framework, a factor that most strongly affects pitch properties. The 13C NMR spectra showed very strong resonances in the region between 100 and 160 ppm (centered at near 127 ppm) associated with different kinds of aromatic carbons.27-29,44 Very weak resonances (near to signal-to-noise ratio) were observed below 65 ppm region which indicated little content of aliphatic carbon groups. In Figure 4,
13
C NMR spectrum for EP-3 extractive pitch is displayed as an example. Good
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separation between the aromatic and aliphatic bands confirmed directly that the extractive pitch samples composed of predominantly aromatic molecules. Shown in Table 8 are the normalized integrated data on the distribution of carbon atoms between the chemical groups in the molecules of the EP pitches. One can see that the apparent carbon-aromaticity factors Fa = C ar C total were 0.91-0.94, close to that published in the literature for coal-tar pitches.42,44 Carbon atoms bonded to oxygen and aliphatic carbons accounted only for 3-6 % and 3-4 %, respectively. The NMR values for the proportions between the CH3 and CH2 groups in Table 8 were consistent with the FTIR estimates in Table 5. An important characteristics of the aromatic nucleis of the pitches could be derived from the
13
C NMR
spectra by evaluating the proportion between the pericondensed carbons (i.e., carbon atoms belonging to three aromatic rings) to the catacondensed carbons (i.e., carbons belonging to two aromatic rings): the higher this proportion, the more highly condensed rings. For this purpose, the aromatic spectral region was subdivided into two subregions. According to44, the region from 108 to 129.5 ppm was assigned to the resonances from the quaternary pericondensed aromatic carbon atoms and also from tertiary aromatic carbons (protonated). The signals in 160-129.5 ppm region were due to quaternary catacondensed aromatic carbons and also to quaternary aromatic carbons which were not pericondensed but bonded to aliphatic chains. One can see from Table 8 that the pericondensed carbons+tertiary protonated aromatic carbons in EP samples predominated (57-55%) over the catacondensed carbons+aromatic carbons bonded to aliphatic chains (34-36%). The accurate proportion between solely pericondensed and catacondensed carbon atoms is difficult to estimate because the signals from the pericondensed carbons overlapped with those from the tertiary carbons. Nonetheless, since there were a little aliphatic carbons and hydrogens, the error involved in deducing this proportion seems to be relatively small. Therefore, one can assume that the ratio between the intensities of the resonances in the 129.5-108 ppm region to that in the 160-129.5 ppm region reflects the proportion mostly between the pericondenced and catacondensed carbons. If so, then the data in Table 8 may ACS Paragon Plus Environment
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imply that aromatic structures in the extractive pitches consisted of predominantly pericondensed polynuclear sheets rather than catacondensed chains, which is in line with the literature data for coal-tar pitches.42,44 The petroleum-based pitches had approximately equal proportion between the contents of pericondensed (3847%) and catacondensed carbons (36-47%), i.e. the structures of their aromatic clusters were less specific.44 Finally, it should be noted that polycondensed, and more significantly, pericondensed type of pitch structure was a reason why the proportions of aromatic carbon atoms estimated from the 13C NMR spectra (91-94%, Table 8) were larger than those of the aromatic hydrogen atoms estimated from the 1H NMR spectra (76.577.1 %, Table 6).
3.6. Spatial structure. The XRD patterns of all the pitches showed a typical broad and asymmetrical reflection in the 2Θ region from 8° to 34° due to a specific intermolecular ordering of carbon matter and a weak reflection centered at 2Θ of about 44° due to intramolecular ordering. The result of curve-fitting analysis showed that the asymmetrical reflection attributed to intermolecular ordering was best simulated by a superposition of three Gaussians. Shown in Figure 5 are the examples of the deconvolution of the (002) peaks for the extractive and reference pitches. Similar three-component structure of organic matter was revealed by the authors38 in low-rank coals. According to35-38, the Gaussians in Figure 5 were assigned to three carbon structures represented by rather ordered “graphite-like” component (at 2Θ of 24.7-25.0°) and by two poorly ordered γ-components (at 2Θ of 18.2-19.0° and 10.0-11.0°). “Graphite-like” component is considered to consist of the flat polyaromatic molecules (graphenes) stacked in parallel. Other aromatic and aliphatic molecules located at the periphery of the polyaromatic layers and not included in the stacking structure represented less-ordered γ1-fractions (at 2Θ of 19.0-19.5°) and least ordered γ2-fraction (at 2Θ of 10.0-10.5°). One can see from Fig 5 that dominant proportion of carbon in all the pitches occurred in the disordered γ1-matter (61% to 68%) and much less proportion did for the least ordered γ2-matter(9% to 15%). The most ordered “graphite-like” matter accounted for 19% to 27% in different pitches. The stacked graphene layers and the structural groups of the
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γ-components represented thus rather ordered and disordered carbon matter, and their combination formed pitch microtexture. Shown in Figure 6 is a diagram where the structural parameters of the “graphite-like” carbon matter of different pitches are compared. One can see that the graphene layers in all the pitch samples had nearly same diameters (22.0-24.8 Å). As for the arrangements of the graphenes, the stacks in the PP sample had more thickness (18.1 Å) and numbers of layers (6.1) as compared to other pitch samples, the inter-layer spacing being 3.57 to 3.62 Å in all the pitches. The XRD data showed thus that the spatial structure of the carbon matter of the extractive pitches resembled that of PCTP sample.
4. Conclusion Three extractive pitches were produced by using thermal dissolution of the medium-ranked coals at 380 °C for one hour in the anthracene oil. The empirical, molecular and structural pitch properties were characterized in detail in comparison with the commercially available reference pitches including typical coal-tar pitch, petroleum-derived pitch and blended pitch derived from the mixture of coal tar with petroleum feedstock. It was shown that, in terms of the empirical properties, the extractive pitches irrespectively of coal used met the requirements for the pitch binder and resembled commercial blended petro-coal-tar pitch. A remarkable merit of the extractive pitches compared to widely used commercial coal-tar pitch was low carcinogenicity (two-three times as low). The molecular structure of the extractive pitches was represented mostly by the sparsely substituted pericondensed aromatic nuclei like those in coal-tar pitch. The aliphatic fragments consisted of short alkyl groups, methylene groups predominantly, the latter being by far the most abundant in the coal-tar pitch. The differences in the distribution of the methylene groups between the structural positions in the molecules of different pitches were revealed by the FTIR and NMR spectra: in the extractive pitches, they were represented to a larger extent by the substituents in the aromatic rings, but methylene groups in hydroaromatic cycles (or/and may be in methylene bridges between the aromatic rings) prevailed in the CTP pitch. The mode of methylene group distribution in the PP pitch was less specific. ACS Paragon Plus Environment
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The spatial structures of both the extractive and reference pitches were simulated by three kinds of structural components with different ordering. The most ordered “graphite-like” component accounting for 19% to 27% was represented by the nanosized stacks of five-six flat polyaromatic molecules with the interlayer spacing of 3.57 to 3.62 Å. In terms of spatial structure, the carbon matter of the extractive pitches closely resembled that of the blended pitch derived from the mixture of coal-tar and petroleum feedstock.
Author information Corresponding Author * E-mail address:
[email protected];
[email protected] Phone: +73912494849
Notes The authors declare no competing financial interest.
Acknowledgements The authors are grateful to Dr. S. Kocytsina (Siberian Federal University) for vacuum distillation of the pitch containing extracts and to Dr. E. Marakushina (RUSAL Engineering and Technology Centre, Russia) for supplying the reference pitch samples and for determination of carcinogenicity. The authors thank also Krasnoyarsk Regional Analytical Center for recording the FTIR and NMR spectra.
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List of Tables: Table 1. The characterization coals and solvent used Coal, solvent
Designation
Kaa-Khem gaseous G
Аd ,
Vdaf,
Elemental composition, wt.% on daf
%
%
С
Н
N
S
Оdif
10.4
45.2
78.0
6.2
1.2
0.3
14.3
5.2
35.8
84.0
5.4
1.1
0.6
8.9
2,7
0,5
5,0
coal Chadan
gaseous- GF
fat coal Chertinsk fat coal
F
25,9
37,5
86,0
5,8
Anthracene oil
AO
-
-
89.3
5.5
5.2*
*Total content of N+S+O
Table 2. Elemental composition of the pitches of different origins
Pitch sample
Elemental composition, wt.% Pitch feedstock
С
Н
N
S
O
С/Н,
O/H,
at.
at.
EP-1
Coal G
88.8
5.0
1.5
0.3
4.4
1.48
0.06
EP-2
Coal GF
89.1
5.1
1.5
0.4
3.9
1.46
0.05
EP-3
Coal F
90.5
5.1
1.9
0.4
2.1
1.48
0.03
СTР
Coal tar
92.7
4.4
1.2
0.5
1.2
1.75
0.02
PP
Petroleum
91.7
5.7
0.8
1.3
0.5
1.34
0.01
91.1
5.0
1.5
1.0
1.4
1.52
0.02
PCTP
Petroleum fraction coal tar blend
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Table 3. Technical specifications of the pitches of different origins BaPc,
BaPEd,
mg/g
mg/g
2.3
5.4
15.7
38.6
7,8
5,0
14,6
111
40.4
3,1
4,6
13,6
0.2
88
35.1
10.5
11.5
30.0
PP
0.06
89
28.6
0.4
2.4
4.6
PCTP
0.14
103
24.0
4.4
5.4
16.3
Pitch sample
αa, %
Ash,
Softening
wt %
point, °C
EP-1
0.6
105
37.5
EP-2
0.5
115
EP-3
1.0
СTР
a
α - toluene-insoluble fraction;
benz(a)pyrene content;
d
b
α1b, %
α1- quinoline-insoluble fraction;
c
BaP –
BaPE – benz(a)pyrene equivalent, calculated in
accordance with6.
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Table 4. The summary of peak/band assignments in FTIR spectra of the pitches Mode of vibration
Spectral region, cm
Peak centered
-1
at cm-1
N-H and OH stretching vibration
3200-3600
3425
C-H stretching vibration in aromatic rings
3000-3100
3050
C-H stretching vibration in aliphatic groups
3000-2800
asymmetric stretch in CH3
2958
asymmetric stretch in CH2
2920
symmetric stretch in CH3
2878
symmetric stretch in CH2
2856
C=C=C cumulated bonds
1925
C=O stretching in acids, eithers, carbonyls
1610-1750
С=С stretching in aromatic rings
1610-1547
C-H deformation vibration in aliphatics
1490-1360
1925
1600
CH2
1450
CH3
1378
Deformation vibration in O-H and C-O groups C-H out-of plane bending in aromatic rings
1030-1300 700-900
isolated hydrogen in the aromatic ring
880
two or three adjacent aromatic hydrogens
810-850
four adjacent aromatic hydrogens
750
Table 5. FTIR data on semiquantification of pitch aromaticity and CH3/CH2 ratios for different pitch samples Pitch
Hydrogen aromaticity
CH3/CH2b
index, Iara
sample EP-2
0.37
0.50
EP-3
0.39
0.62
PP
0.40
0.58
CTP
0.63
0.16
a
Iar, ratio of the absorbance at 3100-3000 cm-1 to that at
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Table 6. NMR data for proton distribution in the specific structural groups in the pitches Integration interval, ppm. 10,5-9,0
Proton proportion, %
Chemical form
EP-2 EP-3 aliphatic protons bonded to C atom adjacent to O 1.1
CTP PCTP
PP
1.1
0.8
0.4
1.2
atom, Ho 9,0-6,7
aromatic protons, Har
77,1
76,5
64.9
57.3
46.2
6,7-4,5
aliphatic protons in olefins, Hol
0,5
0,4
1.2
2.5
3.4
4,5-2,0
aliphatic protons in CH3, CH2 and CH groups in 15,6
14,1
13.6
21.3
32.6
6,8
14.4
15.2
15.9
1,2
5.1
3.3
0.6
α-position to an aromatic ring, Hα 2,0-1,0
aliphatic protons in CH3, CH2 groups in β- 7,0 position to an aromatic ring or in CH groups in βor further, Hβ
1,0-0,0
aliphatic protons in γ-position or further to an 1,0 aromatic ring, Hγ
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Table 7. Structural characteristics of the average molecules of different pitches Pitch
far
Har/Car
σ
n
EP-2
0,92
0.67
0,14
1,5
EP-3
0,93
0.63
0,12
1,6
СTР
0,91
0.48
0,12
2,4
PCTP
0,87
0.52
0,18
1,9
PP
0.84
0.59
0,22
1,6
sample
Table 8. 13С NMR integrated data on the distribution of carbon atoms in different structural positions in the molecules of the extractive pitches Chemical shift, δ, ppm
Chemical group
Carbon proportion, % EP-2
EP-3
0-24
Aliphatic CH3
1
1
25-45
Aliphatic CH2 and CH
2
3
55-70
Aliphatic C-O
4
2
Total aliphatic carbons
7
6
108-129.5
pericondensed Car + protonated Car
57
58
129.5-160
catacondensed Car + Car bonded to
34
36
Total aromatic carbons
91
94
C=O
2
