Investigation on chemical and structural properties of coal- and petroleum-derived pitches and implications on physico-chemical properties (solubility, softening and coking)

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Fuel

journal homepage:www.elsevier.com/locate/fuel

Full Length Article

Investigation on chemical and structural properties of coal- and

petroleum-derived pitches and implications on physico-chemical properties (solubility,

softening and coking)

C. Russo

, A. Ciajolo

, F. Stanzione

, A. Tregrossi

, M.M. Oliano

, A. Carpentieri

, B. Apicella

a_{Istituto di Ricerche sulla Combustione, IRC-CNR, P.le Tecchio 80, 80125 Napoli, Italy}

b_{Dipartimento di Scienze Chimiche, Università di Napoli“Federico II”, Complesso Monte Sant’Angelo 21, 80126 Napoli, Italy}

G R A P H I C A L A B S T R A C T A R T I C L E I N F O Keywords: Coal-tar pitch Petroleum pitch Spectroscopy

Polycyclic aromatic hydrocarbons Mass spectrometry

SEM

A B S T R A C T

The structural properties and composition of pitches are particularly important for the optimization of pitch processing to high-value products and practical application. Because of their complexity, pitch analysis requires both chemical and spectroscopic tools, as those applied in the present work to coal and petroleum-derived pitch samples. In particular, a large variety of methods, including chromatography, elemental analysis, thermo-gravimetry, mass spectrometry, UV–Visible and FTIR spectroscopy, was applied for evidencing the different characteristics of commercial solid pitch derived from petroleum and coal. The interrelation among some pitch properties was investigated by analyzing petroleum and coal-derived pitches having different softening points and volatility. It was observed that properties as the softening point and the coking yield are mainly related to the pitch volatility rather than on the pitch source (coal or petroleum), whereas some other features as the solubility and the oxidation reactivity of coke produced appeared correlated with the source characteristics.

1. Introduction

Pitches derived from different sources (coal, petroleum and

synthesis from aromatics) are carbon materials relevant for the manu-facture of many important industrial products, e.g. electrodes, carbon fibers, etc. [1]. Among pitch properties, the graphitization (or

https://doi.org/10.1016/j.fuel.2019.02.040

Received 9 December 2018; Received in revised form 25 January 2019; Accepted 7 February 2019

⁎_{Corresponding author.}

E-mail address:apicella@irc.cnr.it(B. Apicella).

Available online 23 February 2019

0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

graphenization) propensity under thermal treatment is currently very interesting from the application point of view since the increasing im-portance of graphene-like materials. Synthetic pitches having very high carbon yields and graphitizability, have been obtained by means of catalytic polymerization of polycyclic aromatic hydrocarbons (PAH) (e.g. naphthalene pitch by naphthalene polycondensation [2]). They can be used for a very broad range of applications, but are not eco-nomically convenient since their production cost is rather high.

Coal tar pitch (CTP) is a carbon material produced in large scale as by-product of metallurgic cokes since the first industrial revolution. Both poor utilization and mishandling may result in serious environ-mental and health issues, since PAH massively present in CTP are car-cinogenic and mutagenic [3]. More recently, petroleum pitches (PP) have been considered a suitable alternative to coal tar pitches pre-senting a main different chemical feature that is a significant aliphatic substitution degree of large polycyclic aromatic hydrocarbons (LPAH) (> C24) constituting the main pitch components[4]. As a consequence of the different composition, the quality of carbon products obtained by CTP and PP thermal transformations can be very different. The com-positional and structural knowledge of parent pitches as well as of in-termediates andfinal products derived from pitch thermal treatment is thus required especially to investigate and foresee their graphenization (or amorphization) tendencies.

Because of a huge number of components, pitch complexity is generally described with an“average structure” obtained mainly from elemental analysis, and average molecular mass [5,6]. However, the bulk properties analysis does not provide the chemico-physical detail necessary to foresee the behaviour and suitability for a specific appli-cation of pitches of different origin and chemico-physical character-istics [7, 8 and references therein]. The present wok is framed in this context as regarding the relationships between structural and compo-sitional features of pitches and some physico-chemical properties as solubility, softening and coking that have been investigated by char-acterizing different pitches as petroleum- and coal tar-derived pitches with different softening points and volatility. Detailed information is given on similarities and differences in terms of mass distribution, chemical functionalities and structure by crossing information derived from chemical and spectroscopic techniques.

2. Experimental

2.1. Materials

The pitches object of this work, kindly provided by RÜTGERS Basic Aromatics GmbH (Castrop-Rauxel, Germany), are listed below. Coal tar- and petroleum-derived pitch samples having different softening points and volatility were suitably selected for the study.

1. Electrode binder (EB): coal tar pitch (Electrode Binder BX 95KS, CAS NO.: 65996-93-2) with softening point of 110–115 °C (DIN 51920/ ASTM D 3104).

2. Carbores (Carb): specially modified coal tar pitch (CARBORES® P, CAS NO.: 121575-60-8) with softening point of 235 °C (DIN 51920/ ASTM D 3104).

3. PP118: petroleum pitch (ZL 118 CAS NO 68187-58-6) with soft-ening points of 113 °C (DIN 51920/ ASTM D 3104).

4. PP250: petroleum pitch (ZL 250 M, CAS NO.: 68187-58-6) with softening point of 252 °C (DIN 51920/ ASTM D 3104).

2.2. Analytical methods

2.2.1. Elemental analysis

The elemental analysis was performed on a CHN 2000 LECO ele-mental Analyzer.

2.2.2. Scanning electron microscopy (SEM)

SEM analysis was performed using a FEI INSPECT S.

2.2.3. Thermogravimetric (TG) analysis

Thermal behavior (volatility and oxidation reactivity) was studied by TG analysis performed on a Perkin-Elmer Pyris 1 TG analyzer. The pitch samples were heated from 50 to 750 °C at a rate of 10 °C min−1in both inert atmosphere (N2, 40 mL min−1) and oxidative environment (air, 30 mL min−1).

2.2.4. Mass spectrometric techniques

2.2.4.1. Gas chromatography– mass spectrometry (GC–MS). The GC–MS instrument used for qualitative and quantitative determination of PAH was an Agilent HP6890/HP5975. The gas chromatograph was equipped with a DB-5 ms capillary column (60 m × 0.25 mm i.d., 0.25-mmfilm thickness). Helium was used as carrier gas with a constant flow of 1.0 mL min−1. The samples were dissolved in dichloromethane (DCM) or acetone for GC–MS analysis.

2.2.4.2. Atmospheric pressure photoionization mass spectrometry (APPI-MS). The APPI mass spectra were obtained on an Agilent 1100 Series MSD Trap (Agilent Technologies, Palo Alto, CA, USA)., The toluene soluble fractions of pitches were directly injected into the APPI source by using a syringe pump (5μl/min). The spectrometer is equipped with the MSn_{functionality so that ions coming from ion source are separated} by mass-to-charge ratio (MS1), after then ions of a mass-to-charge ratio chosen by the operator (parent peak) can be selected and fragment ions are created by collisinduced dissociation, photodissociation, ion-molecule reaction, or other processes. The resulting ions are then separated and detected in a second stage of mass spectrometry (MS2). The procedure can be repeated n times (MSn_{), up to the complete} disappearance of the parent peak. More details on the mass spectrometric system are reported in[9].

2.2.4.3. Laser desorption ionization time offlight mass spectrometry (LDI-TOFMS). Laser Desorption Ionization-Time-of-Flight Mass Spectrometry spectra were recorded on positive reflectron mode on a AB SCIEX TOF/TOF™ 5800 System. The target was prepared by depositing on the metallic sample plate a volume variable from 1 and 10μl of a solution of the sample dispersed in DCM. Matrices were not added as all of the investigated samples are able to absorb the laser beam (λ = 337 nm) acting as a self-matrix.[8,10–13].

2.2.5. Mass spectrometric tools

2.2.5.1. Fast Fourier transform (FFT) analysis. FFT method has been used to compute the discrete Fourier transform (DFT) of a repetitive signal like the intensity of mass spectrum peaks. The result of a FFT is simply equal to that of a DFT performed on the same input. Many FFT software packages can give several output results, such as the magnitude, power, phase and amplitude of the transformed data. Among these results, the mean square amplitude power, represents the number of ions contributing to the mass frequency (f = 1/m). In order to have a clearer physics meaning, the independent variable, the frequency f, may be converted to the mass period (m = 1/f). A commercial graphic software (Origin) has been used offering a higher flexibility in the parameters choice for data analysis. More details on the method as applied to mass spectra are reported in[14].

2.2.5.2. Double bond equivalence (DBE) calculation. DBE, also called degree of unsaturation, is calculated from the structure of the chemicals considering that each π bond or ring will generate one DBE. If the compound contains the elements C, H, O, and N, the DBE for the general formula CxHyNzOnis calculated as follows[15]:

= − + +

DBE x y/2 z/2 1

atom and one C atom, respectively, the mass that bestfits the theore-tical molecular weight of chemical formula CxHycan be calculated for each mass peak. Because of the very low contribution of inorganic elements including Nitrogen (Table 1), the carbon number and DBE number of petroleum and coal tar -derived pitches investigated in this work were determined for each mass peak considering the simplified formula for CxHy hydrocarbons: DBE = x− (y/2) + 1 by an home-made software. Iso-abundance graphs were drawn in the Origin soft-ware by plotting the DBE number and the relative intensity against the carbon number.

2.2.6. UV–Visible absorption spectroscopy

UV–Visible spectra of pitch samples suspended in N-methylpyrroli-done (NMP), were measured on HP 8453 Diode Array spectro-photometer using a 1-cm path-length quartz cuvette. The interference of the solvents on the UV absorption limited the acquisition of the spectra down to 260 nm.

2.2.7. FTIR spectroscopy

FTIR analysis of pitches 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. The quantitative analysis of hydrogen bonded to aromatic and aliphatic carbon, hereafter named aromatic and aliphatic hydrogen, respectively, was performed[16].

3. Results and discussion

An overall view of the main differences in CTP and PP quality is given in Tables 1 and 2reporting the ultimate analysis and solvent solubility data, respectively. The H/C atomic ratio between 0.5 and 0.6 of Carb and EB is typical of unsubstituted PAH mainly featuring coal tar pitches [7,8], whereas higher H/C values (0.7–0.8) characterize pet-roleum pitches (Table 1) as a consequence of a more relevant presence of alkyl-substituents groups[4]. A low content of heteroatoms (below 2–3 wt%) can be generally observed for CTP and PP samples; specifi-cally, in comparison to petroleum pitches (PP250 and PP118), only a slightly higher nitrogen and inorganic content for coal tar pitches (Carb and EB) can be noticed.

3.1. Solubility and extract analysis

Beside ultimate analytical data, the most common used test for pitch

characterization/classification is the extraction/solubilisation in qui-noline and toluene that are important parameters qualifying pitch ap-plicability in various manufacturing processes[17].

As generally occurs for coal tar pitches, quinoline insolubles (ASTM D2318or ASTMD7280) and toluene insolubles (ASTMD4312or ASTM D4072), reported inTable 2, result to be higher for the Carb sample having the higher softening point (Table 1) and higher coking tendency as represented by the residue measured by TG at 750 °C in N2(Table 1). Differently from CTP samples, the petroleum pitches (PP118 and PP250) solubilities are quite similar each other, both in quinoline and toluene, in spite of their different softening points and coking properties (Table 1). It is also remarkable that the PP solubility is much higher in comparison to CTP samples (Table 2). Beside the different chemical structure of CTP and PP, to the lower solubility of CTP also contribute fine coke particles produced by thermal cracking of the volatiles formed during coke[18]. Pitch solubilities in other solvents as DCM, NMP and acetone have been also tested (Table 2). Acetone has been tested as alternative less toxic solvent in respect to DCM that is the solvent commonly used for the analysis of chromatographable two- to seven-numbered ring PAH. Interestingly enough, NMP-insolubles percentage is similar or even lower than quinoline-insolubles percentage, con-firming that NMP is a suitable alternative to quinoline as cheaper and less toxic solvent[19].

DCM and toluene solubles percentages are comparable (Table 2) and the use of these solvents allows the analysis of molecular compo-nents by means of GC–MS and APPI-MS techniques, respectively, as reported in the followings.

Overall, the extraction/solubilization tests above reported have shown that, differently from CTP, the solubility of petroleum-derived pitches is very high in most of the common pitch solvents in-dependently on their softening points and volatility.

GC–MS analysis resulted to be effective for partly analyzing only EB and PP118 pitch samples having lower softening points in respect to Carb and PP250 (Table 1). The different PAH quality and quantity of EB and PP118 can be observed inTable 3, reporting the percentages of individual two- to seven-ring PAH as measured by GC–MS analysis of DCM extracts. DCM is generally considered the most effective solvent for PAH analysis of carbonaceous products, however we have also tested the effectiveness of acetone as possible solvent alternative to the toxic DCM (Table 2). The PAH content measured in EB and PP118 by GC–MS analysis of acetone solubles, along with a bar diagram com-paring the relative abundance of each detected species in acetone and in DCM, is reported in“Supplementary Material” (Table 1S and Fig. 1S). It is remarkable that similar amounts of two- to seven ring PAH (up to 300u) could be evaluated in DCM and acetone extracts of EB and PP118 pitch samples in spite of the lower pitch solubility in acetone (Table 2). It derives that acetone is a suitable alternative for performing PAH analysis of high-volatile pitches as the lower solubility power of acetone regards the heavier components not analyzable by gas chromatography. Actually, it is generally remarkable that heavy components make up most of EB and PP118 as the content of total PAH identified by GC–MS (GC-PAH), also reported inTable 3, is rather low, namely 24.55 wt% and 9.72 wt% for EB and PP118, respectively. As regards the EB and PP118 composition, it is noticeable that alkyl-substituted PAH, scarcely present in EB (< 8 wt% of total GC-PAH), are the predominant PAH

Table 1

Elemental composition, softening point and residual coke of pitches after volatilization in TG apparatus in air up to 450 °C, and in N2up to 450 °C and 750 °C.

H/C atomic ratio

C. wt% H. wt% N. wt% Others. wt%* _{Softening point}

(°C) % TG Residue at 450 °C in Air % TG Residue at 450 °C in N2 % TG Residue at 750 °C in N2 EB 0.58 92.7 4.4 0.9 2 110–115 67.8 43.9 33.4 Carb 0.52 93.1 4.1 0.7 2.1 235 99.0 79.4 62.2 PP118 0.77 92.2 5.9 0.2 1.7 113 69.1 49.0 36.5 PP250 0.72 93.7 5.6 0.1 0.6 252 90.5 66.3 47.8 * by difference. Table 2

Percentages of solvent-insoluble fractions of pitches.

% Toluene Insoluble % Quinoline Insolubles % DCM Insolubles % Acetone Insolubles % NMP Insolubles EB 25 6 20.5 33.2 8.3 Carb 45 15 45.3 64.3 10.9 PP118 13 0.7 12.4 38 < 1 PP250 12 < 1 12.2 33.7 < 1

identified in PP118 (about 60 wt% of total GC-PAH).

PAH evaluation by GC–MS allows calculating an important pitch quality parameter which is the carcinogenicity, indicated by the benz (a)pyrene-equivalent (BaPE) concentration [20,21]. It was calculated by applying Eq.(1) [21]

=

BaPeq SPAHconcTEFPAH (1)

where BaPeqis the carcinogenic risk relative to BaP of the PAH mixture, PAHconcis the concentration of individual PAH (only the unsubstituted PAH) and TEF is the toxic equivalent factor of individual PAH. A list of the employed TEF for BaPeq calculation is reported in the Supplementary material (Table 3S).

BaPE is 12.7% for EB and 1.5% for PP118 (Table 3), confirming the much higher carcinogenicity of coal tar pitches with respect to petro-leum pitches.

3.2. Molecular weight distribution

Some compositional similarities and differences between coal tar and petroleum pitches have been evidenced in previous work mainly focused on APPI-MS spectral analysis of low volatile CTP and PP[22]. Fig. 1reports the APPI-MS spectra of the analyzed pitches.

The tandem MS/MS option of the APPI-MS apparatus has been also used for the MSn functionality analysis to infer the occurrence of functional groups on the aromatic components of pitches[23]. In par-ticular, as observable inFig. 2which shows, as an example, the MS2_and MS3_{spectra of an isolated peak (at m/z = 456) from the spectrum of} PP118 (Fig. 1), both thefirst (MS2) and the second (MS3) fragmentation occur with the loss of fragments at m/z = 15 (Fig. 2, appearance of peaks at m/z = 441 and 426, respectively) and the MS2_{also with the} loss of m/z = 14 and 29 fragments (Fig. 2, appearance of peaks at m/ z = 442 and 427), allowing the identification of the parent peak as an alkyl-substituted PAH. Similar features are presented for the isolation and fragmentation of other peaks along all the investigated range (up to

1000 u) and the same occurs when tandem MS/MS is applied to PP250 sample. Therefore, the application of MS/MS to APPI spectra of pitches revealed that the PP are mainly constituted by methyl- (first loss of m/ z = 15), dimethyl- (second loss of m/z = 15) and ethyl- substituted PAH (loss of m/z = 14 and 29), in all the investigated range. This finding is consistent with the higher abundance of alkyl-PAH measured by GC–MS in PP118 (Table 3).

As the APPI mass spectral analysis could be performed just on to-luene extracts, LDI-TOFMS technique has been used to analyze the whole pitch directly deposited on a target. The resolution of the LDI-TOFMS system here employed (m/Δm = 10,000) is higher with respect to that of APPI-MS (m/Δm = 2000). Moreover, laser power was low enough to prevent photofragmentation occurrence, as evaluated in previous works[24,25]guarantying the reliability of TOFMS LDI-TOFMS spectra of coal tar and petroleum pitch samples are shown in Fig. 3.

Consistently with the APPI spectra reported inFig. 1, LDI-TOFMS spectra of EB and Carb present the maximum around m/z 300 and 400, respectively, and are dominated by two sequences of higher and lower intensities with a spacing of m/z 24 attributed to even- and odd-numbered PAH[11,26]. It is noticeable that, in comparison to APPI-MS, LDI-TOFMS spectra of both CTP and PP samples are extended toward slightly higher masses, (see insets of thefigures), whereas lighter spe-cies, up to 200 u, are detected to a lower extent since they are partially lost into the high-vacuum pumping system. LDI-TOFMS and APPI-MS spectra of PP118 and PP250 show a peak continuum without specific regular sequences peaked around m/z 250 and 500, respectively. In-terestingly, the high-MW distribution is centered around a maximum (m/z = 500) which is almost twice the low-MW distribution maximum (m/z = 250), which could be somehow traced to monomeric and di-meric species found in special petroleum-pitches[4,27].

In agreement with the LDI-TOFMS analysis, the MW distributions evaluated by size exclusion chromatography (SEC) analysis [28–33]

Table 3

Unsubstituted PAH and alkyl-PAH content in EB and PP118, along with their BaPeq, as evaluated by GC–MS of DCM extracts.

EB PP118

Unsubstituted PAH % Alkyl PAH % Unsubstituted PAH % Alkyl PAH %

Acenaphthene 0.02 Naphthalene. 1.2-propenyl == Acenaphthene 0.06 Naphthalene. 1.2-propenyl 0.03

Fluorene == 1.1-Biphenyl. 2-methyl == Fluorene 0.01 1.1-Biphenyl. 2-methyl 0.01

Phenanthrene 0.18 Phenanthrene. 1-methyl == Phenanthrene 0.17 Phenanthrene. 1-methyl 0.03

Anthracene 0.12 Phenantrene. 2-methyl == Anthracene 0.07 Phenantrene. 2-methyl 0.35

Cyclopenta[def] phenanthrene 0.10 Phenantrene. 9-ethyl == Cyclopenta[def] phenanthrene

0.06 Phenantrene. 9-ethyl 0.02 Benzo(a)fluorene 0.35 Phenantrene. 4.5-dimethyl == Benzo(a)fluorene 0.24 Phenantrene. 4.5-dimethyl 0.06 Fluoranthene 1.43 Phenantrene. 2.5-dimethyl == Fluoranthene 0.20 Phenantrene. 2.5-dimethyl 0.22

Pyrene 1.05 Naphthalene. 1-phenyl 0.30 Pyrene 0.46 Naphthalene. 1-phenyl 0.06

Benzo[ghi]fluoranthene 0.15 Pyrene. 1-methyl 0.21 Benzo[ghi]fluoranthene 0.03 Pyrene. 1-methyl 1.23 Cyclopenta[cd]pyrene == Pyrene. 1.3-dimethyl 0.09 Cyclopenta[cd]pyrene 0.03 Pyrene. 1.3-dimethyl 0.93 Benzo[a]anthracene 2.42 1.1-Biphenyl. 2-phenylmethyl == Benzo[a]anthracene 0.49 1.1-Biphenyl. 2-phenylmethyl 0.13

Chrysene 1.52 Chrysene. 5-methyl == Chrysene 0.36 Chrysene. 5-methyl 0.19

Benzo[b]fluoranthene 3.61 Benzo[a]anthracene. 1-methyl 0.07 Benzo[b]fluoranthene 0.44 Benzo[a]anthracene. 1-methyl 0.27 Benzo[k]fluoranthene 1.28 Chrysene. 4-methyl == Benzo[k]fluoranthene 0.09 Chrysene. 4-methyl 0.10

Benzo[e]pyrene 1.24 Chrysene. 6-methyl == Benzo[e]pyrene 0.36 Chrysene. 6-methyl 0.22

Benzo[a]pyrene 3.12 Benzo[a]anthracene. 8.12-dimethyl == Benzo[a]pyrene 0.55 Benzo[a]anthracene. 8.12-dimethyl 0.33 Perylene 0.49 Benzo[a]anthracene. 4.12-dimethyl == Perylene 0.08 Benzo[a]anthracene. 4.12-dimethyl 0.18 Indeno[1.2.3-cd]pyrene 2.88 Benzo[a]anthracene. 7-ethyl. 12-methyl == Indeno[1.2.3-cd]pyrene 0.12 Benzo[a]anthracene. 7-ethyl. 12-methyl 0.04 Dibenz[a.h]anthracene 0.43 Perylene. 3-methyl == Dibenz[a.h]anthracene 0.07 Perylene. 3-methyl 0.12 Benzo[ghi]perylene 1.40 Benzo[a]pyrene. 10-methyl == Benzo[ghi]perylene 0.27 Benzo[a]pyrene. 10-methyl 0.17 Dibenzo[a.i]pyrene 0.64 Benzo[j]aceanthrylene. 3-methyl 0.71 Dibenzo[a.i]pyrene 0.05 Benzo[j]aceanthrylene. 3-methyl 0.42 Coronene 0.26 Benzo[a]cyclopropane [cd]pentalene.

1-methyl

== Coronene 0.03 Benzo[a]cyclopropane [cd]pentalene. 1-methyl

0.11

Benzo[a]pyrene. 7.10-dimethyl == Benzo[a]pyrene. 7.10-dimethyl 0.09

Benzo[ghi]perylene. 4-methyl 0.32 Benzo[ghi]perylene. 4-methyl 0.18

Triphenylene. 2-methyl 0.15 Triphenylene. 2-methyl ==

Total unsubstituted PAH 22.69 Tot Alkyl PAH 1.86 Total unsubstituted PAH 4.23 Tot Alkyl PAH 5.49

Total PAH 24.6 Total PAH 9.71

show the MW maximum of petroleum pitches shifted toward higher MW with respect to the coal tar pitches (seeSupplementary material, Fig. 2S).

Fast Fourier Transform (FFT) analysis, set up in previous work[14], has been applied to the LDI-TOFMS spectra for extracting mass peri-odicities that can indicate preferential growth routes and/or fragmen-tation patterns related to the chemical composition of pitches. The FFT method applied to the LDI-TOFMS spectra gives at a glance the mole-cular difference between coal tar and petroleum pitches as shown in Fig. 4reporting the mass difference for each pitch sample. Specifically, the 12 and 24 m/z peaks typical of unsubstituted PAH, feature coal tar pitch samples. The peak at m/z 24 presents a standard deviation (evaluated as half of FWHM of the peaks) of ± 2, including also the gap at m/z 26, which indicates the same PAH sequence but with more hydrogenated PAH [13]. The sum of the two gaps at m/z 24 and 26 gives the increment of m/z 50, which was found also forflame-formed PAH and attributed to molecular growth through the addition of C4H2 [34]. Typical of alkyl-substituted PAH are the FFT of petroleum pitches showing the m/z 1 and 2 peaks dominating on the other unique low-intensity peak at m/z 14 (Fig. 4). It is worth to underline that very

similar results were obtained by applying the FFT method to APPI-MS spectra confirming that there is no bias imparted by the LD ionization at the laser power employed in the present work.

The DBE number[15,35,36]and the planar limit, defined by max-imum DBE values at a given carbon number[37], give important in-formation on the molecular composition as applied on complex and crowded mass spectra.

In the case of the pitches studied in the present work, the percentage of heteroatoms is lower than 3% for coal pitches and lower than 2% for petroleum pitches (Table 1). Therefore, the DBE could be calculated with the simplified formula (DBE = x − (y/2) + 1, see Experimental Section), neglecting the contribution of heteroatoms and considering the general hydrocarbon formula CxHy. The resolution of the instru-ment is about 10,000 that is relatively high, allowing the DBE calcu-lation up to 60 C (MW < 1000 Da). Moreover, the DBE analysis results reported inFig. 5are in good agreement with those obtained by Müllen et al. on a ultra high-resolution instrument[35]for Carb and PP250, as shown in detail in the following, thereby demonstrating the reliability of DBE in the case of pitch samples, even when evaluated on relatively lower resolution mass spectra. Based on the assigned elemental

Fig. 1. APPI-MS spectra of EB, Carb, PP118 and PP250 pitches.

composition, the carbon number and DBE number have been de-termined for each mass peak, obtaining the iso-abundance plots of the DBE number for Carb, EB, PP118 and PP250 along with the planar limit line. The isoabundance plots of all pitch samples are reported inFig. 5. Each dot in the iso-abundance plot represents one kind of

hydrocarbons with the same chemical formulas while the abscissa is the carbon number and the ordinate is the DBE number of that species. The relative content of species is obtained from the mass peak intensity ratio to the sum intensity of all mass peaks and is represented by the colour of the dot with the colorimetric scale also reported in thefigure. DBE

Fig. 3. LDI-TOFMS spectra of all the pitches.

has been evaluated in the carbon atom number range between 12 and 60 where the higher mass peak intensity ratio was found.

The increasing value of DBE number with the carbon number is demonstrative of the increasing number of double bonds and/or aro-matic rings with the molecular mass increase. Indeed, for example benzene (1 ring) has a DBE of 4, naphthalene (2 rings) has a DBE = 7 whereas coronene (7 rings) has a DBE = 18. PP118 and EB show higher DBE intensities at DBE≥ 12, whereas DBE of Carb and PP250 are al-most completely deprived of the lower values in the range of DBE≤ 18, as confirmed by the complete absence of GC-PAH (DBE ≤ 18).

It is remarkable that the variety of LPAH observed in coal pitches is smaller with respect to the petroleum pitches (much narrower iso-abundance plot distributions), as already reported for the Carb and PP250 cases[35]. In particular, DBE profile for EB (Fig. 5) presents a very narrow sequence of red points (peaks with the higher intensity) which correspond to the even PAH sequence of the well-known stabi-lomers grid[38]in the range above m/z 400 as also found for flame-formed tars[11,26].

The planar limit line, defined by Cho et al.[37]as the line generated from the iso-abundance plots by connecting maximum DBE numbers at given carbon numbers, is useful for following the growth of the con-densed molecules during carbonization. The slope of planar limit line, i.e. the DBE/carbon number ratios, is 1 for the growth with addition of linear benzene rings 0.75 for the progressive addition of nonlinear benzene rings and 0.25 for the growth of aromatic cores for successive incorporation of saturated cyclic rings [35]. The slopes obtained for Carb and EB, namely 0.779 and 0.798, respectively, indicate that LPAHs in coal tar pitches are mainly formed by linear and nonlinear extension through benzene rings addition. The slopes for PP250 and

PP118 are 0.706 and 0.717, respectively, suggesting that their growth occurs through the formation of saturated cyclic rings or the addition of saturated alkyl chains, beside aromatic rings addition [35]. It is re-markable that the slopes evaluated for Carb and PP250 are in agree-ment with those evaluated on the same samples in other work (0.7779 and 0.7088 for Carb and PP250 in[35]) in spite of the different mass spectrometer resolution. Summing up, the analysis of LDI-TOFMS spectra by FFT and DBE methods puts well in evidence the occurrence of LPAH with highly condensed aromatic cores and either no or short aliphatic side chains, and LPAHs with aliphatic sides as the major species in the coal tar pitch and petroleum pitch samples, respectively.

3.3. Aromaticity

The different aromatic (aliphatic) character of coal tar and petro-leum pitches indicated by the mass spectral analysis, has been further investigated by spectroscopic analysis. The height-normalized UV–Visible spectra of the pitches are reported in Fig. 6. Some fine structure, typical of relatively small PAH as those detected by GC–MS, can be observed just in the EB spectrum The mass specific absorption coefficients (ε) in the UV and visible portion of spectra are parameters related to the content and quality of aromatic species[39]. The com-parison of the mass specific absorption coefficients in the UV and visible of pitches, reported in the inset ofFig. 6, gives indications on the dif-ferent sizes and structures of aromatic moieties featuring petroleum and coal tar pitches. Specifically, the higher mass absorption coefficients (ε) of CTP (EB and Carb), both in the UV (300 nm) and in the visible (500 nm) range indicate their higher aromatic carbon content in respect to PP118 and PP250 samples.

Moreover, the increasing size of the aromatic moieties as going from PP118 to PP250, EB and finally to Carb is demonstrated by the in-creasing visible absorption coefficient, reported in the inset ofFig. 6.

The aliphatic/aromatic character of the pitch samples, qualitatively analyzed by mass spectrometry and UV–Visible spectra, has been evaluated by semiquantitative FTIR analysis previously developed on carbonaceous materials[16]. FTIR spectra of pitches reported inFig. 7 show three main absorption regions related to the different C–H bonds (aliphatic and aromatic) and the carbon network [5,40–43]. The in-tensity of the aromatic C–H stretching peak (around 3050 cm−1_{) is} lower compared to that of the aliphatic C–H stretching signals (peaks around 2925 cm−1) in the PP118 and PP250 spectra. The opposite can be observed in the Carb and EB spectra. The 900–700 cm−1 wave-number region (also reported in the inset of Fig. 7) is the other sig-nificant FTIR region where peaks related to out-of-plane (OPLA) bending modes of aromatic C–H bonds named solo (890 and 870 cm−1), duo (850 and 810 cm−1), and trio/quatro (790 and 720 cm−1) can be noticed. The trio/quatro hydrogen peaks are the most intense for all the samples, however their intensity relative to the solo and duo peaks is much higher for EB and Carb, indicating the pre-dominance of unsubstituted PAH in comparison to PP118 and PP250

featured by a larger degree of aromatic substitution with aliphatic groups. This observation is in line with the mass spectral analysis above reported.

The semiquantitative FTIR method set up in previous work [7,16,44]has been applied to determine the hydrogen linked to aro-matic (Har) and to aliphatic (Hal) carbon reported inTable 4. It can be seen that the higher hydrogen content of petroleum pitches is due to the higher concentration of hydrogen linked to aliphatic carbon. The re-liability of the method is testified by the agreement between the total H/C ratio derived from FTIR analysis, also shown inTable 4, and that measured by elemental analysis (Table 1).

Table 4also reports the percentage of protonated aromatic carbon (hydrogen-bonded aromatic carbon), sp3_{-bonded aliphatic carbon (CH,} CH2and CH3), and sp2-bonded aromatic carbon (calculated subtracting the total hydrogen and aliphatic carbon contributions from the total pitch mass). For sp2_{carbon compounds a decrease of the protonated} aromatic hydrogen implies an increase of the size of the aromatic moieties. For PP118 and PP250 the lower amount of sp2-bonded carbon combined with the lower amount of protonated aromatic carbon in-dicates that hydrogen was replaced by aliphatic functionalities at the periphery of the aromatic clusters. The lower amount of protonated aromatic carbon and the higher sp2_{-bonded carbon for Carb (Table 4)} confirm the larger size of the aromatic moieties and the larger aromatic character as inferred by UV absorptivity (Fig. 6). The higher coking yield in comparison to PP250 (Table 1), and the lower reactivity to oxidation of the coke formed could be attributed just to the strong aromatic character of Carb evidenced by spectroscopic analysis.

3.4. Thermal behaviour

Based on the detailed characterization of pitches, the thermal be-havior in terms of volatilization, reactivity and coking yield can be interpreted. The TG and TG derivative (DTG) curves of all pitches, measured under airflow, are reported inFig. 8, grouping together EB with PP118 (left panel ofFig. 8), and Carb with PP250 (right panel of Fig. 8) as presenting similar TG profiles. Indeed, in comparison to TG in inert environment analyzed in previous work[22]the reduction of the weight loss and/or the weight gain from the beginning of TG heating cause the enhancement of carbonaceous residue formation at high temperatures (around 400–500 °C), before the complete burnoff oc-currence (around 600 °C). The comparison between the carbonaceous residues left in air and in inert environment at 450 °C, also reported in Table 1, demonstrates the enhancing effect of oxidative environment on the carbonaceous residue formation as previously found for natural and synthetic pitches[22,45–47].

In the case of EB and PP118, the early volatilization phase, occur-ring up to around 300 °C (weight loss around 30 wt%), is rather similar to that measured in inert environment up to 300 °C demonstrating that the lighter components volatilize without undergoing oxygen uptake. Only after thefirst volatilization phase, (above 300 °C), there is oxygen intervention which enhances the residue formation from EB and PP118. Summing up the more volatile and softer EB and PP118 pitch samples present both in inert and oxidative environment a lower coking ten-dency in respect to Carb and PP250.

It can be concluded that the CTP and PP thermal behaviour, in particular the coking tendency, appears (inversely) related to the pitch components volatility, regardless the pitch source (coal or petroleum). However, it can be noticed that thefinal burnoff of residual coke occurs at slightly higher temperatures for coal-tar derived pitches (Tmax ox∼ 650 °C) in comparison to those derived from petroleum (Tmax ox∼ 630 °C) (Fig. 8). The difference in the maximum oxidation tem-perature of the carbonaceous residue should suggest that the reactivity of the residue formed in oxidative environment from the coal-derived (more aromatic) pitches is lower than that shown by the residue formed from the petroleum (more aliphatic) pitches. Hence, it can be deduced that, although the yield of carbonaceous residue formed both in

EB Carb PP118 PP250

ε

ε

Fig. 6. Height-normalized UV–visible spectra of pitches dissolved in NMP. In the inset: mass absorption coefficients (ε) in the UV (300 nm) and visible (500 nm).

Fig. 7. Mass absorption coefficient of PP118, PP250, EB (electrode binder) and Carb in the mid-IR region. In the inset a magnification of the spectra arbitrary shifted in the OPLA region is reported.

oxidative (below the combustion temperature of the residue) and inert environment is mainly dependent on pitch volatility, CTP and PP un-dergo different oxygen uptake routes because of their different struc-tural properties so far described, leading to residues and/or cokes of diverse oxidative reactivity.

The residues from CTP and PP present also different morphology, as observable from SEM images reported inFig. 9: the Carb residue pre-sents a more compact structure with respect to PP250 residue, in agreement with its lower reactivity. Work in progress regards a deeper characterization of the pitches residues, which is complicated by their scarce or no solubilization in solvents.

4. Conclusions

The compositional complexity of pitches requires the use of an array of chemical and spectroscopic techniques that is mandatory for eluci-dating pitch properties and interrelating physicochemical parameters as H/C, softening point, solvent solubility and thermal behaviour. Many chemical and spectroscopic methods were applied for evidencing the different characteristics of commercial solid pitch derived from

petroleum and coal. The interrelation among some pitch properties have been investigated by analyzing petroleum and coal-derived pit-ches having different softening points and volatility.

The extraction/solubilization tests showed that the solubility of petroleum-derived pitches is very high independently on their softening points and volatility. Among the tested solvents acetone showed to be a suitable alternative, less toxic than the other solvents, for PAH analysis by gas chromatography. Mass spectrometry and spectroscopic analysis of coal tar and petroleum pitches provided useful information on their different composition, in turn important to interpret their thermal be-havior in oxidative environment and to address their possible applica-tion.

Mass spectrometry and spectroscopic analysis indicated the relative abundance of light components for PP118 consisting with the lower softening point and higher volatility in comparison to PP250. However, in spite of the different volatility, both PP presented a rather high and similar solubility in typical solvents as toluene, quinoline, etc. as related to similar chemical features like the aliphatic substitution degree of aromatic components. Indeed, the“average” petroleum pitch molecule is characterized by a larger presence of alkyl substituents on aromatic

Table 4

Percentage of hydrogen, protonated aromatic carbon, sp3_{-bonded aliphatic carbon, and sp}2_{-bonded aromatic carbon, along with H/C ratio derived from FTIR}

analysis. Aromatic hydrogen, Har, wt% Aliphatic hydrogen, Hal, wt% Total hydrogen, wt% H/C (FTIR) CH3wt% CH2wt% CH wt% Protonated aromatic carbon, wt% sp3_-bonded carbon, wt% sp2_-bonded carbon, wt % PP118 2.64 3.47 6.11 0.78 3.59 12.42 6.03 31.73 22.04 71.85 PP250 2.79 3.18 5.97 0.76 3.46 11.02 5.69 33.51 20.17 73.87 EB 3.60 0.97 4.57 0.57 0.85 3.75 1.61 43.19 6.22 89.21 Carb 3.35 1.08 4.43 0.56 0.97 3.95 2.16 40.16 7.07 88.50

Fig. 8. Thermogravimetric profiles and relative DTG in oxidative environment of EB and PP118 (left), and Carb and PP250 (right).

components, and a higher MW with respect to the coal pitch molecules. In the case of coal-derived pitches, the lighter MW distribution, corresponding to a higher content of GC-PAH and smaller size of aro-matic moieties, justifies the lower softening point and the higher vo-latility and reactivity of EB in comparison to Carb. As regards these physical-chemical properties, EB and PP118 from one side, and Carb and PP250 from the other one, present similar softening points and volatilities, even though they are structurally different in terms of aromatic/aliphatic content, MW distribution, and composition.

The thermal behaviour of pitch samples, specifically the coking propensity, analyzed by TG, correlates more with their volatility rather than with the chemical functionalities and structure of pitch indicating a lack of correlation with the source (petroleum or coal). Nevertheless, some relationship of the properties of the products obtained by pitch heating with the pitch composition and structure was suggested by the observation of a different reactivity of the coke formed in oxidative environment.

Acknowledgements

The authors are very grateful to Mr. Jens Stiegert (RÜTGERS Basic Aromatics GmbH, Castrop-Rauxel, Germany) for kindly supplying the solid pitch samples.

Luciano Cortese (IRC-CNR) is acknowledged for SEM images.

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps:// doi.org/10.1016/j.fuel.2019.02.040.

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