Article pubs.acs.org/EF
Structural Characterization of Large Polycyclic Aromatic Hydrocarbons. Part 1: The Case of Coal Tar Pitch and NaphthaleneDerived Pitch Valentina Gargiulo,*,† Barbara Apicella,† Michela Alfè,† Carmela Russo,† Fernando Stanzione,† Antonio Tregrossi,† Angela Amoresano,‡ Marcos Millan,§ and Anna Ciajolo† †
Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche (CNR), Piazzale Vincenzo Tecchio 80, 80125 Napoli, Italy ‡ Dipartimento di Scienze Chimiche, Università di Napoli “Federico II”, Complesso Monte Sant’Angelo 21, 80126 Napoli, Italy § Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom ABSTRACT: The different thermal behaviors and solubilities of large and structurally different polycyclic aromatic hydrocarbon (PAH) mixtures featuring coal tar pitch (CP) and naphthalene synthetic pitch (NP) samples could be read in light of their different molecular weight (MW) distribution and spectroscopic features. The number-average MW obtained by mass spectrometry for CP (417 Da) and NP (691 Da) resulted to be lower in comparison to the values evaluated by size-exclusion chromatography (SEC) (796 and 824 Da for CP and NP, respectively) because of the different response of the detector of mass spectrometry to low- and high-MW components. Hence, SEC showed to be more suitable for the analysis of PAH mixtures overlapping and covering a higher mass range in comparison to mass spectrometry. Insights into structural PAH features were given by means of spectroscopic analysis [infrared (IR), ultraviolet−visible (UV−vis), and fluorescence], allowing for the discrimination between different families of PAHs as ortho-fused PAHs and rylenes interspersed with aliphatic (mainly naphthenic) groups, mainly featuring CP and NP, respectively. Besides showing the different aromaticity and aliphatic/aromatic hydrogen distribution, the improvement of Fourier transform infrared (FTIR) and UV−vis absorption analysis put also in evidence the contribution of carbon-rich particle impurities and PAH aggregates in CP and NP, respectively.
1. INTRODUCTION The properties of large polycyclic aromatic hydrocarbons (PAHs), defined as that class of PAHs with aromatic cores having C ≥ 24,1 are important from the fundamental and technological point of view. Large PAHs are the constituting units and/or important formation intermediates of carbon materials as soot, interstellar dust, mesophases, carbon black, and graphenic materials, among others. Beyond the limit of 24 carbons, the huge number of isomers and the related difficulties in easily obtaining individual large size PAHs, as well as their high insolubility, volatilization temperature, and thermal degradability, limit the study of their properties and structures. Important readily available resources of large PAH mixtures are fossil-fuel-derived and synthesized pitches, which can also be used in place of purposely synthesized individual PAHs as realistic models of the aromatic structural units constituting carbon materials. PAH-containing samples, such as pitches, also present a peculiar behavior depending upon their chemical structure and molecular weight (MW), i.e., the formation of a discotic nematic liquid crystal phase, referred to as a carbonaceous mesophase. This mesophase, nucleating from the isotropic liquid after the pitch heat treatment, is typically involved in the production of carbon materials, such as carbon fibers.2 It was noticed that the mesophasic behavior is not observed for any single pitch constituent,3 although carbonization of a single PAH and liquid-crystal development at a high pressure have also been observed,4 but it occurs to different extents depending upon the PAHs that constitute the pitches. © XXXX American Chemical Society
Most of the pitch characterization work has been early devoted to determining the features and properties of coal tar pitch (CP).5 However, the attention is currently more focused on catalytically synthesized6 and petroleum pitch samples,7 mainly because of the high environmental impact of CP production plants. Regardless of the source, a better understanding of pitches at the molecular level is necessary for the processing technology development. Because of the chemical complexity and low volatility of pitches, different analytical tools have been used for their detailed and complete analysis, such as Fourier transform infrared (FTIR) spectroscopy,8 thermogravimetry (TG), differential thermal analysis (DTA), and liquid and gas chromatography.9,10 Optical microscopy, high-temperature ultracentrifugation,11 and nuclear magnetic resonance (NMR) analysis12 have been applied to measure features of pitch samples as the mesophase character. Solvent fractionation has also been employed to sharpen and simplify the composition to be analyzed and is commonly used for the evaluation of pitch quality for their industrial use.13 Solvent fractionation has also been used to eliminate the volatile fraction and gather the insoluble fraction that is more prone to form mesophase carbon materials, even though it was early established that solubility and mesophase formation are unconnected phenomena.14 Received: June 15, 2015 Revised: August 7, 2015
A
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colored (dark brown) and would, therefore, absorb the laser beam acting as a self-matrix.19−21 Each target was prepared by depositing a volume variable from 1 and 5 μL of a solution/suspension of the sample in NMP on a metallic plate. The samples, after deposition, were heated in an oven at 60 °C to achieve a complete evaporation of the solvent. A typical analysis consists of 100−150 co-added spectra collected on the whole spot surface (1−2 mm in diameter) to overcome the nonhomogeneity of the sample on the target plate. More details are given in a previous paper.22 Spectra in positive reflectron mode were recorded on a Voyager DE STR Pro instrument (Applied Biosystems, Framingham, MA). Acceleration and reflector voltage setups were as follows: target voltage of 20 kV, first grid at 96% of the target voltage, and delayed extraction in the 150−600 ns range. The times of flight were externally calibrated (two-point calibration algorithm) for each sample plate and different sample preparations. Calibration and further data processing used the computer software provided by the manufacturer. The mass resolution, in the operative conditions employed for the LDI−TOFMS in reflectron configuration, is around 3500, calculated as the ratio of the mass of interest, m, to the difference in mass, Δm, defined by the width of a peak at 50% of the peak height: m/Δm.23 2.3. IR Spectral Analysis. Quantitative FTIR analysis of aliphatic and aromatic hydrogen15 was performed on solid sample dispersions prepared by mixing and grinding the pitch samples (0.25−0.5 wt %) with KBr. The KBr dispersions were compressed at 10 tons for 10 min into thin disks having a mean thickness value of 0.035 cm. FTIR spectra in the 3400−600 cm−1 range were acquired in the transmittance mode using a Nicolet iS10 spectrophotometer. 2.4. TG Analysis. Thermal behavior (volatility and reactivity) was studied by TG analysis performed on a PerkinElmer Pyris 1 TG analyzer. The pitch samples were heated from 50 to 750 °C at a rate of 10 °C min−1 in both an inert atmosphere (N2, 40 mL min−1) and oxidative environment (air, 30 mL min−1). 2.5. UV−vis Absorption Spectroscopy. UV−vis spectra of the parent pitches, dissolved/suspended in NMP, were measured on a HP 8453 diode array spectrophotometer using 1 cm path length quartz cuvettes. The interference of the solvent (NMP) on the UV absorption limited the acquisition of the spectra to 260 nm. 2.6. Fluorescence Spectroscopy. Fluorescence spectra were acquired on a PerkinElmer LS-50 spectrofluorimeter using a xenon discharge lamp as the excitation light source and a gated photomultiplier with modified S5 response for operation to about 650 nm as the detection device. Monochromators were of the Monk-Gillieson type and covered the following ranges: excitation of 200−800 nm and emission of 200−900 nm. The wavelength accuracy was ±1.0 nm, and the wavelength reproducibility was ±0.5 nm. Instrumental parameters were controlled by the Fluorescence Data Manager PerkinElmer software. The fluorescence measurements were performed on samples very diluted in NMP to avoid concentration quenching and/or other phenomena that could distort the spectral shape affecting the fluorescence intensity and quantum efficiency evaluation. Sample concentration ranges from 0.5 to 2 mg L−1, on the basis of the sample. Synchronous fluorescence emission spectra were measured by applying a simultaneous scanning of the excitation and emission wavelengths, keeping constant the difference between the wavelengths (Δλ = 10 nm).
This work is focused on the detailed comparative analysis of a CP and a synthetic naphthalene pitch (NP), which have a similar hydrogen/carbon (H/C) atomic ratio (around 0.5) but different structural characteristics, corresponding to different physicochemical properties as the softening point, and coking yield. Thermogravimetry has been used to evaluate the thermal behavior (volatility, coking yield, etc.) of pitch samples, whereas the MW distribution has been measured by both size-exclusion chromatography (SEC) and laser desorption ionization time-offlight mass spectrometry (LDI−TOFMS). The hydrogen content and the aliphatic/aromatic hydrogen distribution have been measured by a quantitative infrared (IR) spectroscopic analysis15 and used to infer specific compositional and structural features as the aliphatic functionalities and aromatic structures. The characteristics of aromatic moieties have also been inferred by ultraviolet−visible (UV−vis) absorption and emission (fluorescence) spectroscopy. On the whole, the improvement of these diagnostics, notably of spectroscopic tools, as applied to differentiate large PAH systems featuring pitch, is also useful for ferreting out the possible different nanosized sp2 aromatic domains, which characterize ordered and disordered carbon materials. Additional results on the pitches analyzed in the present work are presented in a companion paper regarding the analysis of their solvent-separated fractions.
2. EXPERIMENTAL SECTION The CP sample analyzed in this work was obtained from the hightemperature coking of a British coal and has been previously used as a standard in the development of analytical techniques at the Imperial College group.16 It has a H/C atomic ratio of 0.54. AR mesophase pitch, kindly provided by Mitsubishi Gas Chemical Company, is a 100% mesophase pitch synthesized by polymerization of naphthalene.6 AR NP has a H/C atomic ratio equal to 0.59. Solvent extraction with heptane of CP and NP was carried out to obtain their heptane-soluble (HS) and heptane-insoluble (HI) fractions. The HI was further extracted with toluene, giving the toluene-soluble (TS) and toluene-insoluble (TI) fractions. The compositions of CP and NP in terms of solvent-separated fraction percentages are reported in Table 1.
Table 1. Weight Percentages of Solvent-Separated Fractions heptane insolubles (HI)
CP NP
heptane solubles (HS) (wt %)
toluene solubles (TS) (wt %)
toluene insolubles (TI) (wt %)
14.4 0.7
44.4 30.1
41.2 69.2
2.1. SEC. The SEC analysis of pitch samples was carried out by elution with N-methyl pyrrolidone (NMP) on a PLgel polystyrenepolydivinylbenzene individual pore column (Polymer Laboratories, Ltd., U.K.; particle size of 5 μm diameter and a pore dimension of 50 nm). This column is able to separate polystyrene (PS) standards in the molecular mass range from 100 to 50 000 Da. The relation between retention times and molecular mass of PS has shown to be held also for PAH standards [toluene, dicoronylene, naphthalene, dimethylanthracene, pyrene, and a standard PAH mixture (Supelco EPA 525 PAH mix A)].9,16−18 The injection volume was 250 μL, and the analyses were performed at a constant temperature of 80 °C with a flow rate of 0.5 mL min−1. The online detection of species eluted from the SEC column used a HP1050 UV−vis diode array detector measuring the absorbance signal at fixed absorption wavelengths (350 and 500 nm). 2.2. LDI−TOFMS. LDI−TOFMS without using matrices was applied in this work because all of the investigated samples were
3. RESULTS AND DISCUSSION 3.1. Thermal Behavior. The TG analysis of pitch samples allows for following their thermal behavior involving devolatilization and pyrolysis reactions, with the latter ones resulting in the formation of a carbonaceous residue in the form of semicoke. Figure 1 reports the CP and NP TG profiles measured in an inert and oxidative environment (left panel) and the corresponding derivative TG (DTG) curves (right panel). B
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Figure 1. TG profiles of CP and NP in an inert (N2) and oxidative environment (air) (left panel) and corresponding derivative TG (DTG) curves (right panel).
Figure 2. LDI−TOFMS (reflectron configuration) spectra of CP and NP.
different nature of the pitch precursor affects the structure and, thereby, the oxidation reactivity of coke formed in an inert environment. The reactivity toward oxygen during heat treatment of both pitch samples leads to the increase of the coke yield, as shown by the TG profiles measured in an oxidative environment, also reported in Figure 1. A general reduction of the weight loss in comparison to the inert environment, up to the temperature where burnoff occurs (around 600 °C), is noticeable. Specifically, the CP weight loss diminishes from 50 wt % in inert conditions to about 30 wt % in air, whereas the weight loss of NP measured in inert conditions is wholly cleared in an oxidative environment, and even a net weight increase can be observed at the beginning of the heat treatment (low temperature). This can be attributed to the easy oxygen attack on the many alkyl (mainly naphthenic) groups featuring NP, to form ketone functions.27 Upon further heating, the oxygenated cross-links between pitch molecules favor aromatic condensation and stabilization until the complete NP burnoff. Despite the different thermal behaviors in inert conditions and different reactivities toward oxidative attack, it is noticeable that the burnoff of both the CP and NP residues (left after heating to 500 °C) occurs at a very similar temperature (Tmax ox ∼ 600 °C). This could suggest that the oxygen intervention during CP and NP heating does not only increase the coking yields but also leads to structurally similar types of coke. 3.2. Molecular Mass Distribution by LDI−TOFMS and SEC. A continuous mass sequence from m/z 150 to about 1000−2000, approximately centered at m/z 300−400, characterizes the LDI−TOFMS profiles of CP and NP reported in Figure 2. This mass spectral shape is similar to that obtained in previous works by field desorption and laser desorption−mass spectrometry.6,21,28,29 The maximum of the NP molecular mass
CP shows in an inert environment a weight loss of about 50 wt %, which early begins at a temperature next to the typical softening point of CPs (around 100 °C) and ends around 300− 400 °C. This weight reduction can be mainly attributed to the volatilization of low-MW PAHs, typically contained in CP.24,25 This result was confirmed by comparison to the TG profile of a two- to seven-ring standard PAH mixture, which was shown to completely vaporize in this temperature range (data not shown). It is noteworthy that the TG weight loss (50 wt %) of CP is comparable to the sum of the percentages of lighter species constituting soluble fractions (HS and TS) (58.8 wt %) (Table 1). On the other hand, the slowing weight loss rate at higher temperatures (400 °C) and the flatness of the TG profile after 400 °C are ascribable to the heavier species contained in the TI fraction mainly contributing to the final coke yield of about 50 wt %. The TG profile of NP exhibits a relatively low weight loss (about 25 wt %) occurring in a much higher temperature range (400−500 °C), which is beyond the typical softening point range of NP (275−295 °C). Similar to CP, the NP weight loss almost corresponds to the soluble components represented by the TS fraction (30 wt %) (Table 1). However, in this case, the NP weight loss is mainly due to the release of gaseous species in the form of hydrogen and C1−C3 hydrocarbons deriving from decomposition/condensation reactions that precede and prevail on NP component vaporization.26 This, in turn, causes the activation of radical sites on the residual carbon material and the consequent occurrence of condensation/polymerization reactions, thereby leading to the high coke yield (about 80 wt %)26,27 observed (Figure 1). It was noticed that the maximum oxidation temperature of the NP residue left after heating in an inert environment was slightly higher (620 °C) than that of the CP residue (570 °C), suggesting that the C
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typical of polydispersed coal-derived materials.10,35 When the detection wavelength is changed, from 350 to 500 nm, the contribution of species with low MW (200−400 Da) is reduced, confirming their attribution to small PAH, typically contained in CP,24,25 which are known to exhibit a negligible visible absorption with respect to larger size aromatic species featured by a remarkable absorbance in the visible region.36 In comparison to CP, the SEC profile of NP (right panel of Figure 3) exhibits a more regular and continuous distribution extending in a slightly sharper MW range (200−3000 Da) and peaked around 600−700 Da, regardless the detection absorption wavelength. This suggests a higher MW homogeneity of NP components that is consistent with the peculiarity of the NP synthesis6 involving a polycondensation route based on a unique aromatic precursor (naphthalene).6,26 The wider MW range and the higher MW maximum exhibited by the SEC profiles in comparison to LDI−TOFMS spectra (Figure 2) are remarkable. The difference in the MW distribution of pitch samples is well-evidenced by comparing the number-average molecular weight (Mn) evaluated on their LDI−TOFMS and SEC profiles. The Mn has been evaluated in the 150−4000 Da range assuming the intensity of the mass spectrometric and SEC signals proportional to concentration through the formula
distribution appears clearly shifted at higher MW (m/z 400) with respect to CP (m/z 300). The complexity of mass spectra does not allow for distinguishing either the regular even and odd carbon number sequences of mass peaks typical of combustion-formed PAH products nor the sequence of oligomeric systems where aromatic units are connected by, e.g., C−C single bonds (oligo-aryl type), −CH 2 groups typically identified in petroleum pitches.30 Fast Fourier transform (FFT) analysis, recently developed for mass spectral analysis of complex aromatic mixtures, such as asphaltenes and pitches,31 allowed us just to individuate periodic mass gaps around 12 and 14, indicating the presence of a huge number of PAHs increasing by a carbon atom or CH and CH2 groups. SEC analysis, coupled with suitable detectors measuring the refractive index, the light scattering, or the UV−vis absorption properties, is an alternative method for MW analysis generally used to separate polymers into diverse MW species eluting at increasing elution times as their MW decreases. Actually, SEC analysis has the advantage of overlapping and covering a higher MW range with respect to gas chromatography−mass spectrometry (GC−MS) and LDI−TOFMS, which are blind to the heavier PAH species not easily volatizable and/or ionizable. However, different from the mass spectrometric analysis, the MW determination by SEC requires the calibration with standard species structurally similar to the mixture components to be analyzed. The UV−vis detector is generally preferred for the SEC analysis of polydispersed hydrocarbon/ carbon mixtures as those found in heavy products coming from combustion or coal, biomass, and petroleum treatment.32−34 By conversion of the SEC elution times into MWs according to the calibration curve built with PS and PAH standards,18 the SEC profiles of pitch samples (measured at fixed absorbance wavelengths) can be considered representative of MW distribution profiles. The area-normalized SEC profiles of CP and NP measured in the UV (350 nm) and visible (500 nm) regions are reported in Figure 3.
Mn =
∑i Ii MWi ∑i Ii
where Ii is the signal intensity and MWi represents the MW for each point i of the SEC profile and mass spectrum. The Mn values obtained by LDI−TOFMS for CP (417 ± 24 Da) and NP (691 ± 41 Da) are significantly different and lower in comparison to the Mn values evaluated from the SEC profiles (measured at 350 nm) that are 796 ± 62 and 824 ± 49 Da for CP and NP, respectively. The smaller Mn values measured by LDI−TOFMS can be due to the different detector responses to low- and high-MW components. In fact, an intrinsic limitation of the LDI−TOFMS technique applied to blends of low- and high-MW species is the predominance of the signals as a result of the low-MW species obscuring the high-MW species detection.37,38 Actually, the discrepancy between Mn values derived from SEC and LDI−TOFMS analyses is particularly evident just for CP featured by the co-presence of lighter and heavier species, as evidenced by the TG and SEC analyses reported above. To this regard, the MW analysis of solventseparated species of pitches can overcome the shortcomings of LDI−TOFMS and SEC analyses for a more reliable MW evaluation of pitches. It is worth remarking that the Mn has been evaluated by SEC using the detector absorption wavelength of 350 nm because, at this wavelength, it can be assumed an almost equal response factor for all species included in the MW range featuring pitches. However, in Figure 3 it can be clearly seen that the MW evaluation for the CP case shifts toward higher values using an absorption detector wavelength focused in the visible region (500 nm). In particular, the Mn of CP increases from 796 ± 62 to 1212 ± 122 Da, passing from 350 to 500 nm as the absorption wavelength. As mentioned before, this behavior demonstrates that different absorption properties, and hence, different structures correspond to PAHs having different MWs. On the other hand, the use of a visible centered detection wavelength, e.g., 500 nm, can be considered as a useful way for putting in evidence the MW distribution of
Figure 3. MW distribution of CP and NP measured by SEC at 350 and 500 nm as detection wavelengths.
Both pitches exhibited a dominant peak below 20 000 Da and another smaller peak around 30 000 Da (not shown) next to the exclusion limit of the column (around 50 000 Da18). Because of its low contribution and uncertain origin, this peak has not been considered for the characterization and comparison of pitch properties. The SEC profile of CP covers the 200−4000 Da range (left panel of Figure 3) and exhibits a broad and skewed shape D
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A distinct peak at 1600 cm−1 emerges among the bands in the 1700−1000 cm−1 region for both CP and NP. It is due to the CC ring stretch mode arising from irregular (asymmetric) aromatic units41,42 as PAH containing ortho-substituted end rings or substituted with aliphatic (linear or cyclic) chains. Consistent with the predominance of aliphatic C−H stretch peaks, the peaks at 1450 and 1370 cm−1, assigned to the bending of C−H aliphatic bonds,43 are more clearly distinguishable in the NP spectrum. Conversely, a broad massif can be clearly observed for CP in the 1300−1100 cm−1 region. This is a very complex region where the vibrations of carbon−carbon bonds strongly overlap, as typically exhibited by complex carbon defective networks featuring solid carbon particles.15,44,45 Eventually, the peaks in the 900−700 cm−1 region are remarkable, due to the “out-of-plane” (OPLA) bending modes of aromatic C−H bonds.46 OPLA modes are sensitive to the ring substitution and give rise to different components according to the number of adjacent hydrogen atoms on a ring, named solo, duo, trio, and quatro.44 On the basis of the FTIR spectra of standard PAH and alkyl-substituted PAH, the solo, duo, trio, and quatro modes can be assigned to specific wavenumber regions.15,44 Isolated hydrogens (solo) are located in the 890−870 cm−1 range, whereas two adjacent hydrogens (duo) are located between 850 and 810 cm−1. Peaks due to three adjacent hydrogens (trio) are located in the 790−750 cm−1 region and partly overlapped with four adjacent hydrogen (quatro) peaks detected in the 750−720 cm−1 region. It can be noticed that the trio/quatro hydrogen peaks are the most intense for CP in comparison to the solo peak, as typically found in CP.8 This indicates the predominance of PAH with ortho-fused end rings.47 Unlike CP, the FTIR spectrum of NP (upper part of Figure 4) exhibits similar intensities of the different aromatic hydrogen signals in the OPLA region, suggesting the higher substitution degree of PAH components. Overall, the different relative contributions of aromatic and aliphatic C−H stretchings highlight the different compositions and, in particular, the quite different aromaticities/aliphaticities of CP and NP. However, the band intensity ratio of aromatic and aliphatic C−H stretching signals, around 3000 cm−1, is not so reliable as the aromaticity parameter24,39 because it does not take into account their different absorption strengths and the presence of substituted (not hydrogenated) carbon. Actually, it has been found that aliphatic hydrogen has a much higher response (2 or 3 times) in comparison to aromatic hydrogen.15 Hence, the evaluation of aromaticity from the ratio of aromatic and aliphatic C−H stretching signals can be misleading, and the quantitative analysis of aliphatic and aromatic hydrogen is instead required. The quantitative analysis of aliphatic and aromatic hydrogen and carbon is generally carried out by sophisticated and time-consuming 1H and 13C NMR spectroscopy techniques.48,49 A newly developed FTIR method, described in detail in a previous work,15 has been applied here for quantitatively determining the aliphatic and aromatic hydrogen contents for CP and NP. This quantitative FTIR method, briefly described in the following paragraph, has allowed for determining the weight percentages of aromatic and aliphatic hydrogen (in the form of methyl CH3, methylene CH2, and methyne CH groups) and, thereby, the evaluation of aliphatic and aromatic carbon. These data are summarized in Table 2. The quantification method of aromatic and aliphatic hydrogen by FTIR analysis of the C−H stretching region
the heavier pitch fraction, avoiding the pre-separation of the whole pitch samples. In summary, the determination of an average MW appears to be misleading when the PAH mixture covers a too wide range from light PAH to higher MW species, as is the case of CP. SEC analysis showed to be more reliable for the MW analysis of pitches overlapping and covering a higher MW range with respect to the mass spectrometric analysis. Even though the MW determination is important for pitch characterization, it does not provide insights into the different structures and functionalities of the pitch components, also involved in their solubility and coking yield, which can be instead furnished by the spectroscopic analysis reported below. 3.3. Spectroscopic Analysis. FTIR and UV−vis absorption and emission (fluorescence) spectroscopies have been applied to pitch samples to give insights into their functionalities and, in particular, their aromatic nature. 3.3.1. FTIR Spectroscopy. FTIR spectra of pitches reported in the literature show three main absorption regions, which give information on different C−H bonds (aliphatic and aromatic) and the carbon network.8,24,39,40 The high-frequency bands, around 3000 cm−1, are due to the stretching vibrations of aliphatic and aromatic C−H bonds, while in the mediumfrequency range, the absorption of the carbon skeleton results in a broad combination of peaks between 1700 and 1000 cm−1, where carbon−carbon stretching and aliphatic C−H bending modes strongly overlap each other. A third group of bands characteristic of the aromatic C−H out-of-plane bending modes occurs between 1000 and 600 cm−1. The FTIR spectra of CP and NP pitches, reported in Figure 4, exhibit many bands
Figure 4. FTIR absorption spectra of CP and NP.
occurring in the three main spectral regions and differing from each other by their intensity. The main difference is observed in the 3150−2800 cm−1 region related to the distribution of aromatic and aliphatic C−H bonds. The much higher intensity of the aromatic C−H stretching peak (around 3050 cm−1) than the group of aliphatic C−H stretching signals (peaked at 2925 cm−1) in CP and, in contrast, the predominance of aliphatic C− H signals over the aromatic C−H peak observed for NP are an evident indication of the higher CP aromaticity (Figure 4). With regard to the CH2 and CH3 aliphatic group distribution, the prevalence of the peak at 2925 cm−1, as a result of asymmetric stretching of CH2 groups, is detected, regardless of the pitch. E
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that it is mainly constituted of highly substituted aromatic systems; hence, the very high CH2/CH3 ratio (about 8) can hardly be attributed to long alkyl chain substituents. Moreover, PAH with long alkyl chains should reflect in a significant solubilization of NP that is not observed (Table 1).6,26 As mentioned before, the significant presence of CH2 groups bridging aromatic rings and/or included in naphthenic groups has to be considered for NP, consistently with previous work.6,26 From the quantitative FTIR analysis of aromatic and aliphatic hydrogen, the total hydrogen percentage, the protonated aromatic carbon, and the total aliphatic (sp3-bonded) carbon percentage (calculated by summing the CH, CH2, and CH3 percentage and assuming negligible the quaternary aliphatic carbon) have been obtained (Table 2). The sum of total hydrogen and aliphatic carbon percentages accounts for about 9 and 20 wt % of CP and NP, respectively. The remaining mass is due to sp2-bonded aromatic carbon and includes protonated and unprotonated aromatic carbon. The total sp2-bonded carbon calculated in this way shows the higher aromatic carbon percentage for CP (approximately 90 wt %) in comparison to NP (80 wt %). The size and basic structure of aromatic moieties have been inferred by UV−vis absorption and fluorescence emission analyses. 3.3.2. UV−vis Absorption and Fluorescence Spectroscopies. The comparison of UV−vis absorption spectra and fluorescence spectra of CP and NP reported in Figure 5 gives information on the different sizes and structures of aromatic moieties. The most intense feature of CP and NP absorption spectra compared in Figure 5a is the UV absorption band typical of aromatic systems. The fine structure of the CP spectrum, along with the high absorption coefficient values in the UV region steeply decreasing while going toward the visible, until approximately 400 nm, are typical features of a series of relatively small ortho-fused end-ring PAHs,1,36 belonging to either the PAH series called K-region PAHs or phenacenes.51 The absorption coefficient of NP appears to be much lower in the UV and smoothly decreasing toward the visible while going through a broad maximum around 400−500 nm. Generally, the low UV absorption and these longer wavelength transitions feature PAHs with higher numbers of fused aromatic rings and/or with the higher ratio of isolated double-bond carbon/sextet.52 Moreover, the NP absorption spectral shape is more typical of pericondensed PAH1 belonging to the PAH series named rylenes that is the group of PAHs built up by peri-
Table 2. FTIR Parameters of CP and NP aromatic hydrogen, Har (wt %) aliphatic hydrogen, Hal (wt %) hydrogen total (wt %) Har/(Har + Hal) H/C (atomic) methyl carbon, CH3 (wt %) methylene carbon, CH2 (wt %) methyne carbon, CH (wt %) CH2/CH3 trio/quatro abundancea protonated aromatic carbon (wt %) sp3-bonded carbonb (wt %) sp2-bonded carbonc (wt %)
CP
NP
3.16 0.86 4.01 0.79 0.50 0.92 2.92 1.66 3.18 0.79 37.91 5.50 90.48
1.87 2.42 4.29 0.44 0.54 1.38 10.63 3.65 7.70 0.67 22.38 15.66 80.06
a
Trio/quatro abundance is the relative abundance with respect to the total aromatic hydrogen. bsp3-bonded carbon is the sum of aliphatic (CH3 + CH2 + CH) carbon percentages. csp2-bonded carbon is the total carbon subtracted from aliphatic carbon.
(3100−2800 cm−1) is based on the careful selection of standard molecules for the calculation of the absorption strength of each vibrational mode obtained by deconvolution of IR peaks, provided their assignment to specific C−H bonds.15 To this regard, just for NP, the deconvolution procedure showed the occurrence of a peak around 2830 cm−1 as a result of the symmetric stretching of two adjacent CH2 groups featuring species as acenaphthene (2 CH2 groups), 9,10-dihydrophenanthrene, or 1,2,3,4-tetrahydronaphthalene (4 CH2 groups). This bears out the significant presence of aliphatic carbon in the form of naphthenic groups.6,26 It is noteworthy that the total hydrogen content of CP and NP, obtained by summing the aromatic and aliphatic hydrogen percentages, leads to H/C values around 0.5 (Table 2) that are consistent with those measured by elemental analysis. The relative abundance of the aromatic hydrogen, expressed as the Har/(Har + Hal) ratio, also reported in Table 2, shows the quite different hydrogen distribution in CP and NP. In particular, the high Har/(Har + Hal) of CP (0.79) (Table 2) and the low CH2/CH3 ratio (around 3) confirms that CP is mainly constituted of PAHs with a low degree of ring substitution with very short alkyl chains. The substitution mainly occurs on the edge of PAH systems in the form of methyl, ethyl, or at most propyl groups, as indicated by NMR studies.50 The low Har/(Hal + Har) ratio of NP (0.44) bears up
Figure 5. UV−vis absorption spectra (left panel) and fluorescence emission spectra (right panel, with excitation wavelength fixed at 350 nm) of CP and NP. F
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Energy & Fuels annellation of naphthylenes,51 which are probable products of the NP synthesis process. The high fluorescence efficiency of smaller PAHs can be used to distinguish their contribution from that of large PAHs that are highly absorbing in the visible region but not fluorescing.53 The fluorescence emission spectra of both CP and NP, reported in Figure 5b (excitation wavelength = 350 nm), are characterized by a broad emission peak in the 300−700 nm wavelength range that is the region where two- to seven-ring PAHs fluoresce.54 The CP fluorescence spectrum peaks at about 440 nm and shows a more structured shape in comparison to the NP fluorescence emission spectrum peaked at a higher emission wavelength (around 490 nm). Consistent with the higher UV absorption of CP, the upshifted maximum fluorescence emission for CP demonstrates the large abundance of unsubstituted PAHs of smaller size with respect to NP. To better resolve the PAH composition of CP and NP, the simultaneous scanning of excitation and emission wavelengths with a fixed wavelength difference, Δλ, known as synchronous fluorescence,55,56 has been applied. Indeed, synchronous fluorescence is very selective to classes of PAHs with different ring numbers, which generally appear as well-discriminated peaks providing fingerprints of specific aromatic moieties in complex PAH mixtures.1,54 The discrimination in different peaks located in the 300−500 nm range mainly as a result of three- to six-ring PAHs can be noticed in the synchronous fluorescence spectrum of CP compared to that of NP in Figure 6.
Figure 7. Absorption coefficients of CP and NP absorption edges as a function of the photon energy.
aromatic systems, the optical band gap analysis is meaningless for evaluating the size of aromatic components57−59 and, at most, could be indicative of the largest size of aromatic clusters.57 However, it is remarkable that, below the optical gap value named E04 (the value of the photon energy at which the absorption coefficient α is equal to 104 cm−1),59 the strong decrease of the absorption edge in the visible region, normally featuring spectra of PAHs of small and large size,51 is not observed. It can be suggested that interactions (π stacking) between PAH molecules should be responsible for the absorption tail into the visible region. The occurrence of π stacking is indeed favored between PAHs having a large aromatic core,58 such as those present in NP (detected by fluorescence), and can also be the reason for its high anisotropic character and insolubility (Table 1). In contrast, the aggregation is less favored in the CP case because of the higher abundance of small PAHs demonstrated by the higher UV absorption, the fluorescence upshifted with respect to that of NP, and the higher CP solubility (Table 1). On the basis of the FTIR analysis showing for CP the contribution of complex carbon defective networks featuring solid carbon particles, it can be inferred that insoluble solid carbon impurities (soot and coke) brought in the CP during the production process could be mainly responsible for the absorption tail in the CP case. Ongoing work devoted to the deep characterization of the smaller PAH fraction of pitches separated by extraction from larger PAHs and/or solid carbon particles will be reported in the second part of this series focused on the analysis of solventseparated fractions of CP and NP.
Figure 6. Synchronous fluorescence spectra (Δλ = 10 nm) of CP and NP.
In contrast, the absence of individual peaks in the NP synchronous spectrum can be observed. The unique large synchronous fluorescence signal, extending much above 400 nm, indicates the broad and continuous distribution of larger aromatic fluorophores in NP. Despite the larger contribution of small PAHs, evidenced by fluorescence analysis, it is remarkable that CP exhibits a significant visible absorption tail that is of intensity similar to that of NP. The visible absorption tails are better evidenced in Figure 7, reporting on a logarithm scale the absorption coefficients (cm2 g−1) as a function of the photon energy. Figure 7 clearly shows the extension from UV to the near-IR range of the absorption spectra for both CP and NP, owing to different aromatic moiety sizes, each one with its local band gap that extends to lower energy values as larger as the aromatic size.57 Because of the absorption overlaying of different
4. FINAL REMARKS The comparative structural study of large PAH systems featuring CP and NP samples has been carried out by a multi-array analytical approach involving TG, chromatographic, mass spectrometric, and spectroscopic analyses. The higher solubility (in heptane and toluene) and the lower coking yield of CP in comparison to NP correspond to some differences in MW distributions, measured by LDI−TOFMS and SEC analyses, and more striking structural differences G
DOI: 10.1021/acs.energyfuels.5b01327 Energy Fuels XXXX, XXX, XXX−XXX
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