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Senneca, O.; Apicella, B.; Russo, C. Methods for Chemico-Physical Characterization of Refineries Solid Wastes. Encyclopedia. Available online: (accessed on 18 June 2024).
Senneca O, Apicella B, Russo C. Methods for Chemico-Physical Characterization of Refineries Solid Wastes. Encyclopedia. Available at: Accessed June 18, 2024.
Senneca, Osvalda, Barbara Apicella, Carmela Russo. "Methods for Chemico-Physical Characterization of Refineries Solid Wastes" Encyclopedia, (accessed June 18, 2024).
Senneca, O., Apicella, B., & Russo, C. (2022, June 08). Methods for Chemico-Physical Characterization of Refineries Solid Wastes. In Encyclopedia.
Senneca, Osvalda, et al. "Methods for Chemico-Physical Characterization of Refineries Solid Wastes." Encyclopedia. Web. 08 June, 2022.
Methods for Chemico-Physical Characterization of Refineries Solid Wastes

In the delayed coker the residues of the vacuum distillation units are cracked into lighter hydrocarbons, producing coker gas oil, but also a solid carbonaceous byproduct which is called “pet-coke”. This is a highly viscous, black, sticky material, it remains liquid as long as it is transported hot, but solidifies otherwise. Its insoluble fraction in n-heptane is called asphaltene, which thus represents the heaviest fraction of crude oil.  The detailed and complete analysis of heavy petroleum fractions requires the use of many different analytical tools because of their chemical complexity and low volatility. However, the molecular weight (MW), the aromaticity and, in general, the chemical functionalities, are the key properties to determine for guiding their processing technology development. In particular, size exclusion chromatography (SEC) and mass spectrometry (MS) are the most eligible techniques for MW determination, while Raman and Fourier Transform Infrared (FTIR) spectroscopy can provide information on aromaticity and chemical functionalities, respectively.

petroleum pitch asphalt bitumen petroleum coke combustion analysis pyrolysis

1. Terminology for Refinery Residues

Before starting with the description of the diagnostics, it is necessary to give some clarifications as regards the definition of asphaltenes, petroleum pitches and pet-coke.
The source of the crude oil affects in a variable way the content and characteristics of asphaltenes, which constitute its most refractory (or the heaviest) fraction [1]. Asphaltenes provides very low economic value and causes adverse effects to the oil industry.
They are known to be complex mixtures of species having aromatic moieties substituted with heteroatoms (e.g., N-, O-, S-) or linked with aliphatic bridges [2].
Even if carbon and hydrogen are the most abundant elements (>90 wt.%) [3], there are some metallic elements in asphaltenes, usually present in the form of metalloporphyrins [4], which are distributed in the range of 0–4000 ppm (nickel and vanadium are the most abundant). Asphaltenes are operationally defined as compounds which are insoluble in aliphatic hydrocarbons (n-pentane or n-heptane), and soluble in aromatics (toluene and benzene) [5]. Asphaltenes are dark-brown-to-black friable solids without a definite melting point, and usually foam and swell upon heating, leaving a carbonaceous residue [2].
Apart from elemental composition, all asphaltenes chemical properties had been the subject of a debate; for example, their MW had been estimated distributing in a range from hundreds to millions of mass units [6], leading to speculation about self-aggregation [7].
Asphaltenes are critical to all aspects of petroleum use, hindering its production, transportation and refining. For all these reasons, they have been defined as the “cholesterol of petroleum” [8]. Such hindrances on production and processing have made asphaltenes one of the most focused materials in petroleum research. Asphaltenes are also valuable products from oil refineries to be used as paving materials on roads, shingles for roofs, and waterproof coatings. Thus, the ultimate goal is to either separate asphaltenes from the petroleum liquids before entering the refining processes or “upgrade” it to a less refractory (or lighter) fraction. For both cases, it is mandatory to understand the fundamental chemistry of asphaltenes.
A general consensus has been reached on asphaltenes molecular structure by using different diagnostic techniques, converging on aromatic systems having on average 4–20 fused rings. By contrast, the debate on asphaltene MW has not been resolved. On the basis of different hypotheses on MW, two different molecular structures involved in asphaltenes self-aggregation have been proposed: the “archipelago” or alternatively the “island” structure [7][9][10].
Petroleum pitches are carbonaceous materials derived from organic precursors by relatively low temperature processes (for example distillation at <700 K). They contain a large range of molecular types and masses. In particular, petroleum pitches are residues from heat treatment and distillation of heavy petroleum fractions. They are generally considered consisting of oligomers of alkylated polycyclic aromatic hydrocarbons (PAHs), with the overall MW ranging from approximately m/z 200 to 2000 [11][12][13].
After heating, pitches generally melt giving an isotropic fluid. As heating is continued above 660 K, alignment of lamellar molecules occurs, leading to the formation of nematic discotic liquid crystals (mesophase) [14]. Indeed, pitch falls into the category of glass-forming materials, which have no defined melting point, but pass through a temperature region, “the glass transition region”, before forming a viscous liquid. This temperature is strictly dependent on the process and the conditions used for producing petroleum pitches and it is a key factor in determining almost all of the physical properties of a pitch. In particular, longer times and higher temperatures result in more aromatic pitches with higher anisotropic contents and, in general, higher “glass transition temperatures”.
Pitches derived from petroleum are used as precursors in the manufacture of important industrial products, as electrodes and carbon fibers and the matrix phase of carbon–carbon composites [15][16]. The accurate description of the structural composition of pitches is thus fundamental in the optimization of pitch processing to high-value products.
The presence of a very large number of compounds makes difficult pitch characterization. Their separation into fractions is therefore required before characterization.
Pet-coke is a carbonization product of high boiling carbon fractions obtained in petroleum processing. When the carbonization temperature is below 900 K, the coke is named “green coke”. The heat treatment of “green coke” to about 1600 K forms the “calcinated coke”, which has a mass fraction of hydrogen less than 0.1 wt.%. From petroleum pitches in delayed coker two commercial grades cokes are produced: “regular coke” (or sponge coke, anisotropic) for use in anode manufacture (filler for electrodes); “needle coke” for electrode production (oriented, with high graphitizability) [14]. Thus, the nature of the parent material being carbonized and the carbonization conditions affect the properties of coke.

2. Solubility as a Characterization Technique

In order to sharpen and simplify the composition analysis of petroleum residues and also for the evaluation of their quality for industrial use [17], solvent fractionation was often employed [17][18]. Solvent solubility is also used as an operational criterion for separating and defining the different petroleum heavy fractions, as illustrated in the following. An important advantage with respect to other characterization techniques is that solvent fractionation is amenable to be applied at larger scales.
Therefore, solvent fractionation can be considered as a method of characterization for petroleum bottom fractions and, in the case of pitches, it has been effectively used as a method of modifying the chemical structure prior to and during the processing of fibers [14]. The elimination of volatile pitch fraction for gathering the insoluble fraction, which is more prone to form mesophase carbon materials, has also been pursued by solvent fractionation. However, it was early established that solubility and mesophase formation are unconnected phenomena [19] and mesophase properties are related to the nature of the precursor and to the preparation procedure [20].
Unfortunately, petroleum industries have tended to use different solvents and nomenclature, but considering the different approaches, in the following, the most widely applied extraction/solubilisation scheme for heavy petroleum fractions will be resumed.
The scheme is reported in Figure 1. It uses quinoline (standard ASTM D2318 or ASTM D7280 test methods) and toluene (standard ASTM D4312 or ASTM D4072 test methods), even if pyridine is also employed.
Figure 1. Extraction/solubilisation scheme.
Recently, quinoline was substituted by N-methyl pyrrolydone (NMP), which is cheaper and less toxic and separates similar percentage of insolubles [13]. Successive extractions with n-heptane, petroleum ether or n-hexane have been used to separate the lighter pitch components, in order to allow the characterization of the higher MW aromatic species, concentrated in the toluene-insoluble fraction [11][14]. The relative proportions of quinoline-insoluble and toluene-insoluble fractions determine the pitches softening and coking properties, which in turn are important for their applicability in various manufacturing processes [17].
As observable in Figure 1, asphaltenes are the compounds insoluble in aliphatic hydrocarbons such as n-pentane or n-heptane, and soluble in aromatics such as toluene and benzene [5]. Their solubility has up to now been used for defining them operationally, as already described before. By contrast, coke is generally insoluble in the most used organic solvents.

3. Mass Spectrometry (MS)

A mass spectrometer generates ions from either organic or inorganic compounds by many different methods and separates them on the basis of their mass-to-charge ratio (m/z) for the qualitative and quantitative detection by their respective m/z and abundance [21]. Advances in MS and, in particular, in the ionization and injection techniques of liquid and solid samples [22][23], made this analytical technique a promising tool for the analysis of a complex matrix of heavy aromatic tarry species.
Laser desorption-mass spectrometry (LD-MS) also with matrix, i.e., Matrix-Assisted Laser Desorption/Ionisation Mass Spectrometry (MALDI-MS), Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) and atmospheric pressure mass spectrometry (APPI-MS), have been successfully applied to heavy carbonaceous materials. Laser Desorption/Ionization (LDI) techniques were first developed in the early seventies [24] but only in the late eighties [25], with the introduction of MALDI, they became an established method for the MS of macromolecular compounds. The role of matrix is to absorb UV laser radiation and to give the energy to the analyte, often not absorbing in UV region, for ionizing it in a softer way. However, many polycondensed systems, especially with aromatic moieties and, therefore, strong UV absorption, have the so-called “to be self-matrix” property [26][27], which means the capability of the sample (or a part of it) to act as matrix by itself, without the necessity of an external matrix addition.
FT-ICR MS, introduced in 1974, is a high resolution mass analyzer based on the cyclotron frequency of the ions in a fixed magnetic field. The excitation field, after ions excitation, is removed, and the ions are rotating at their cyclotron frequency in phase (as a “packet” of ions). These ions induce a charge on a pair of electrodes as the packets of ions pass close to them. A Fourier transform is employed for extracting the resulting signal from the data, giving a mass spectrum. FT-ICR MS is the only mass-analysis method capable of resolving the chemical constituents of petroleum [28].
APPI-MS is a relatively new soft ionization technique [29]. With APPI-MS system, a liquid solution of a sample is directly introduced into the mass analyzer. A discharge Kr lamp generates photons in a narrow range of ionization energies. If the ionization energy (IE) of a molecule is lower than the photon energy, a direct ionization occurs, whereas if the molecule has a higher IE values, it is necessary the addition in the analyte solution of a dopant, i.e., a molecule with a high IE, which can promote the ionization.
With its continuous technological development, MS has represented the tool at the basis of an intense debate on the chemical properties and molecular composition of asphaltenes [6][9].
MS applied to asphaltenes has three main concerns: fragmentation, gas-phase aggregation, and the inability to volatilize the heaviest asphaltene fraction [30]. These concerns explain the reason why the true MW range of asphaltenes is still an object of high-pitched debate [6][10]. The controversy has not been resolved by studies employing MS with a soft ionization source like LDI. Indeed, different groups have reported mean MW spanning two orders of magnitude for similar LDI setups and asphaltene samples [31][32][33][34]. However, the variation of some experimental parameters, such as laser pulse energy [35] and sample concentration, can heavily affect the measured mass spectrum. In general, heavier measured masses are observed if a higher pulse energy or concentration are employed, even if a too high energy can even cause fragmentation and it is necessary to choose a good compromise [31]. This result has been explained in different ways. The supporters of the only presence of low-mass range in asphaltenes hypothesize that, with higher pulse energy or concentration, aggregation of asphaltenes is promoted in the plasma plume produced in LDI and therefore the large masses are only artifacts [7][36][37].
On the opposite, the supporters of high masses range [9][31][34] asserted that relatively low laser pulse energy is insufficient to volatilize or ionize the heaviest components of asphaltenes.
Mullins and coworkers [38] developed a method for overcoming the experimental condition dependence of LDI ionization. The method is named two-step laser mass spectrometry (L2MS). This is a two-color technique in which the laser desorption and laser ionization steps are spatially and temporally separated. In this way, laser desorption forms a plume of neutral molecules, bypassing the problem of aggregation in plume. The obtained L2MS data support the hypothesis that LDI asphaltenes mass spectra peaking at high MW masses result from aggregation in the LDI plasma plume.
By using FT-ICRMS, asphaltenes structure were hypothesized to be not an extension of maltene (n-heptane solubles, see Figure 1) compositional space to a higher and higher carbon number but an extension to higher degrees of aromaticity. These results suggest that asphaltenes may be composed primarily of small molecules (less than ~2 kDa) with highly condensed aromatic ring systems [39]. In order to investigate deeply the effects of ionization source and mass analyzer, some authors [35][40][41] compared Laser Desorption/Ionization-Time of Flight mass spectrometry (LDI-TOFMS) with APPI-MS for asphaltenes analysis. Both the MS spectra of asphaltenes, obtained with the two techniques, present similar MW range with a broad structure extending in the range m/z 300–1100 (the peaks around m/z 200, in the case of APPI-MS spectrum, are due to toluene) with peaks periodicities of 1 (peaks at every nominal mass), 2 and 14 units, indicating the presence of homologues with different unsaturation degrees [27].
The very good agreement between LDI-MS and APPI-MS, even if using APPI ionization, the fragmentation should be reduced, suggests that the masses detected are representative of the true MW of the sample. The mass range found, between m/z 300–1100 with a maximum at m/z 500–600, is in very good agreement with previous works [27][42][43], whereas the eventual presence of higher masses (>100,000 u), as found by other authors [44][45], cannot be investigated by APPI-MS because the mass limit of the ion trap MS is about 3000–4000 u for single-ionized analytes. Adding mathematical tools for mass spectra interpretation, such as Fourier transform (FFT) and double bond equivalent (DBE) number analysis, also with the support of other techniques such as X-ray diffraction (XRD) and Size exclusion chromatography, the occurrence of aryl-linked core structures [46] was hypothesized to feature asphaltenes, along with island and archipelago structures [35].
As regards petroleum pitches, they are often difficult to be analyzed via MS for a number of reasons, which include low solubility, low volatility, broad MW range, and high molecular mass components. To overcome these challenges and the effect that they have on traditional ionization approaches, some authors [47][48] applied solid state MALDI-MS and found that petroleum pitches have a polymeric nature. Indeed, monomers, dimers and trimers could be identified. Moreover, MALDI results indicated that monomer pitch fraction was dominated by methylated derivatives of 4–5 rings PAH, whereas the dimer pitch fraction had the most prevalent species consistent with condensation reactions of the most common monomer species with an accompanying loss of 4–6 hydrogens.
Other authors [10] by using tandem MS/MS option of the APPI-MS apparatus inferred the occurrence of functional groups on the aromatic components of pitches [21]. In particular, for the petroleum pitch investigated, both the first (MS2) and the second (MS3) fragmentation occur with the loss of fragments at m/z = 15 (methyl-) and the MS2 also with the loss of m/z = 14 and 29 fragments (ethyl-), allowing the identification of the parent peak as an alkyl-substituted PAH. In order to deepen the analysis of pitches on the whole sample, LDI-TOFMS was also applied, since the APPI mass spectral analysis was limited to the pitch fraction which is soluble in toluene. Consistently with the APPI spectra, the LDI-TOFMS spectra, show a peak continuum without specific regular sequences peaked around m/z 250 and 500, respectively, suggesting the presence of monomeric and dimeric species as already found in special petroleum pitches by MALDI [47][48].
Zhang et al. used MALDI−TOF MS to characterize and compare petroleum and coal tar pitch using a solvent-free sample preparation [49]. In agreement with the other cited studies, they hypothesized for the petroleum pitch sample the presence of a wider array of large PAHs with short aliphatic side chains.

4. Size Exclusion Chromatography (SEC)

Size exclusion chromatography (SEC) is surely the most used method for determining the MW of a polymer. SEC separates polymers on the basis of molecular hydrodynamic volume or size. For performing SEC analysis, polymers are dissolved in an appropriate solvent and injected into a column packed with porous particles. The SEC mobile phase is generally the same solvent used to dissolve the polymer. As the polymer elutes through the column, molecules that are too large to penetrate the pores of the packing elute in the interstitial or void volume of the column. Smaller polymer molecules can penetrate into the pores and access greater pore volume and, therefore, elute at a later time. Thus, high MW material elutes first from a SEC column, followed by low MW components. Since SEC is a relative and not an absolute MW technique, columns must be calibrated with polymer standards of known MW or an on line light scattering detector must be used [50]. Other detectors based on a single wavelength absorption, or Diode array UV-visible absorption or fluorescence, can be also employed, according to the properties of the sample.
For calibrating the system, it is necessary to use a calibrant structurally quite similar to the analyte, in order to present the same relation between molecular size and molecular mass. For pitches and asphaltene components with masses greater than 500 u, this presents difficulties because the molecular structures are not generally known.
SEC, in contrast to MS, is not limited by volatility considerations, but it is limited by sample solubility in the chosen solvent. Most of the SEC work uses NMP as solvent and eluent for petroleum-derived liquids, like pitches and asphaltenes [51]. Asphaltenes present SEC chromatograms, using NMP as eluent, with bimodal distributions [35][52][53], with an early eluting peak, in correspondence of column porosity exclusion, and a later eluting peak in the range resolved by the column. On the basis of calibration performed by using polystyrene and PAHs, the excluded peak normally corresponds to apparent masses in excess of 20,000 u, whereas the low MW peak corresponds to masses of about 1000 u. This bimodal distribution is a standard feature of SEC using NMP and raises two important questions: (1) if the early peak is formed by very large molecules or by aggregates of small polar molecules (2) the significance of the valley between the two peaks [51]. Aggregation in asphaltenes is another hot topic in research. Some authors used the SEC in NMP system with various approaches to determine whether aggregation has a significant influence on mass estimates. In all these studies, reviewed in [51], no evidence could be found for aggregation. Other authors, instead, by using mainly time-resolved fluorescence depolarization [7], Nuclear Magnetic Resonance [54] and MS [33] confirmed the low MW of asphaltenes and the occurrence of aggregation, claiming that NMP not only aggregates asphaltenes, but also flocculates a significant fraction of asphaltene.
As regards pitches, most of the literature on SEC regards coal pitches [18][51][55][56] and reports MW distributions with a dominant peak at around 1000 Da and another smaller peak around 30,000 Da. Similar bimodal MW distributions, even if with the lower MW peak slightly shifted toward higher MW range, were found for petroleum pitches [14].

5. Raman Spectroscopy

Raman spectroscopy is a particularly appropriated technique for carbon material characterization. An accurate multi-wavelength Raman scattering study of carbon materials is able to give information on the structure of disordered carbons in terms of sp2 bonds configuration (aromatic rings or chains) of the number of aromatic rings fused in a graphitic cluster [57][58].
A graphite crystal has one Raman mode, centered at 1580 cm−1 and labelled G band [59]. This band is associated with the in plane bond stretching of sp2 carbon atoms. Thus, this mode is not necessarily activated by the presence of aromatic rings and is always located in the range 1500–1630 cm−1 in aromatic and olefinic molecules. The Raman spectrum of micro-crystalline graphite shows an additional peak at 1350 cm−1, the so-called D peak, that arises due to the disappearance of in plane long range symmetry. These two peaks, D and G, with different widths, positions, and relative intensity dominate the Raman spectrum of whichever carbonaceous material [60].
The D peak is correlated to the breathing mode of sp2 aromatic carbon; therefore, its activation strictly requires the presence of aromatic rings. Tuinstra and Koenig [59] showed that the ratio between the D mode intensity, I(D), and the G mode intensity, I(G), increases as the size of graphite crystals, La, decreases. Starting from a purely aromatic network of infinite size as graphite crystal, as the disorder is generated, the D peak is activated and I(D)/I(G) will start to increase as the degree of disorder rises. However, with the growth of defect numbers and the shortening of aromatic layer size at about 2–3 nm, the Tuinstra and Koening relationship can no longer be applied. Indeed, for very disordered carbons, aromatic carbon content decreases and aromatic layers become smaller and more tortuous in favour of a rise of odd-member rings and chains structure. Thus, in amorphous carbons with La smaller than 2 nm the rise of the D peak indicates ordering and its strength is proportional to the probability of finding sixfold rings. The non monomodal relation between I(D)/I(G) ratio and the size of aromatic cluster is often neglected in literature leading to uncorrected interpretation of the results.
A detailed analysis of Raman spectra requires deconvolution procedures. The simplest fit consists of two functions to deconvolute the two main Raman features, D and G peaks. More complex deconvolution approaches can be adopted according with the spectrum complexity. Indeed, in particular conditions minor contributions can arise on the sides of the G and D peaks and a multi-peak fit is necessary [61][62]. In spite of the great efforts devoted to the interpretation of the different modes that populate the Raman spectra of carbonaceous materials, there is presently no consensus concerning the attribution and the physical activation mechanism of several of them [63][64][65]. Thus, a fitting cannot be adopted a priori, especially for disordered carbons as asphaltenes and pitch, and it has to be adapted to the spectral features of the sample. The choice of the fitting procedure, including the number of components and the type of function to use, cannot be arbitrary and the goodness of a deconvolution has to be estimated also on its ability to build spectral parameters having physical meaning and able to be related to the samples properties.
Raman spectroscopy so far has been largely employed to characterize carbonaceous material nanostructure. However, Raman spectroscopy can provide useful information also on molecular structures featuring complex mixtures of organic compounds. The exploitation of this technique for this class of compounds is particularly recommended also in consideration of the fact that it can provide also information on the hydrogen content. Indeed, in the Raman spectra of hydrogenated compounds, a relevant background due to photoluminescence phenomena superimposed upon the Raman scattering signal is present, in very hydrogenated samples this photoluminescence is so strong that the spectrum appears featureless. As an example in Figure 2, the Raman spectrum of a petroleum pitch (see PP118 in [13]) is reported. The hydrogen content of PP118 is so high (H/C 0.77) that Raman peaks cannot be detected. An experimental strategy to perform Raman spectroscopy of so fluorescent carbon is to anneal the sample in an inert atmosphere in order to volatilize the lighter part, being usually the more hydrogenated. Figure 2 reports the Raman spectrum of the same petroleum pitch after annealing in N2 in thermogravimetric apparatus to remove the most fluorescent part. This operation makes it possible to measure the spectrum and to deconvolve it to derive parameters useful to the structural characterization of the most aromatic part of the sample as the I(D)/I(G) ratio.
Figure 2. Raman spectra of a petroleum pitch (PP raw) and of the same petroleum pitch after annealing at 450 °C (PP N2 450 °C) in nitrogen.
Empirical equations have been proposed to relate the H/C atomic ratio to the ratio between the intensity of the photoluminescence signal, measured by deriving the background slope fitted with a straight line, and the intensity of the G peak [66][67]. In particular, for the petroleum pitch annealed at 450 °C in N2 reported in Figure 2, a I(D)/I(G) ratio of 0.46 was evaluated corresponding to an aromatic size of 0.85 nm, typical of aromatic molecules with about eight aromatic rings. However, the Raman-derived H/C ratio is estimated to be 0.73 or 0.55 according with the formulas proposed in [66][67], respectively. This high hydrogen content suggested the presence of some aliphatic carbon.
Asphaltenes were analysed by Raman spectroscopy confirming the size of 4–20 fused aromatic rings on which other different diagnostic techniques have converged [61][68]. Raman spectroscopy has been also widely used to characterized pitch and pitch-derived materials such as carbon fibers and foams to evaluate their disorder degree. Raman spectroscopy has also been demonstrated to be a powerful technique for coke microstructural properties analysis [69][70] and for following their structural modification after proper treatments to improve their properties for application purpose [71].

6. FTIR Spectroscopy

FTIR spectroscopy detects the vibrational motions characteristic of chemical functional groups in a specimen. Almost independently from the neighboring molecular structure, a chemical functional group absorbs infrared radiation in a specific wavenumber range, being the energy of light absorbed distinctive only of the chemical bond, whatever the sample. The interaction with the infrared radiation allows chemical bonds to stretch, contract, twist and bend in plane or out of plane. The infrared spectra of carbonaceous materials are characterized by three main absorption regions. In Figure 3 the infrared spectra of a petroleum pitch and an asphaltene are reported, showing in grey the most significant spectral regions for studying them. The bands peaked around 3000 cm−1 due to the stretching of the C–H bonds, being hydrogen bonded both to aromatic or aliphatic carbon. In the medium-frequency range, between 1600 and 1000 cm−1, mainly the absorption of the carbon skeleton occurs, resulting in a broad combination of peaks. A third group of bands is located between 600 and 1000 cm−1 and they are due to the aromatic C-H out of plane (OPLA) bending modes and to the long chain methyl C–H rock.
Figure 3. Infrared spectra of a petroleum pitch and an asphaltene sample.
Important information on the structure and composition of organic carbons can be derived by a deep analysis of their C–H vibrational modes in the high frequency region, where methyl, methylene and methine stretches arise together with aromatic stretch, allowing to discriminate the presence of these different bonds [72]. The OPLA peaks occur at decreasing wavenumbers as the number of adjacent hydrogens located at the edge of polyaromatic systems increases [73]. A deep analysis of these peaks can give important information on the degree of substitution and condensation of aromatic systems.
FTIR spectroscopy is one among the most used techniques to identify the constituent compounds of petroleum heavy fractions. It has been used to evaluate the aromatic and aliphatic hydrogen content of asphaltenes [74] and also to evaluate the asphaltene content in petroleum crude oil [75][76]. It was also extensively used to determine the composition of pitches [77] and of their relative fractions or to follow changes in the pitch composition when, for instance, they were heated, treated with solvents [13][18][56][78][79] or oxidated [80]. FTIR spectroscopy has also provided useful insights in the characterization of compositional properties of complex materials such as coke, furnishing information on the distribution of the aliphatic functionalities [81]. Being the intensity of the signal proportional to the concentration of the absorber, i.e., the functional group, infrared spectroscopy can be used for some quantitative analyses.
Elemental analysis can directly measure the total hydrogen content in a specimen. However, more details on its structure can be derived quantifying the amount of aromatic and aliphatic hydrogen.
In this regard, each C-H stretching peak is characterized by different absorption strength. Ristein et al. [82] deconvoluted the C-H bond stretching region into its different absorption peaks and multiplied each of them by the relative absorption strength, derived from standard molecules. However, it was found that the absorption strength and position of methyl group depended even in simple molecules on the hybridization state of adjacent carbon atoms [82][83] not allowing the quantification of the hydrogen content.
Later on, Russo et al. [84] performed a systematic study of functional groups absorption features of several standard molecules, in terms of peak shape, position, width, intensity, and electronic configuration of their surrounding atoms. This allowed a method to develop for discriminating and quantifying the different kinds of aliphatic and aromatic hydrogen and also for evaluating aliphatic and aromatic carbon in complex carbonaceous materials as soot, pitch and asphaltene [13][18][56][85]. A detailed quantification and speciation of hydrogen content of the asphaltene and petroleum pitch, whose infrared spectra are showed in Figure 3, is reported elsewhere (see PP250 in [13][84] for asphaltene). It has been calculated that aliphatic hydrogen is 3.18 wt.% and 6.27 wt.% and aromatic hydrogen is 2.79 wt.% and 0.79 wt.% in the petroleum pitch and in the asphaltene sample, respectively.
It can be noticed that, in spite of the predominance in the spectra of aliphatic hydrogen (peaked at 2925 cm−1), aromatic hydrogen content contribute is not negligible. This is due to the much higher calibration factor (low response) of aromatic hydrogen in comparison to the different kind of aliphatic hydrogens.


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