Study of the structure of semi-coke using EPR and NMR spectroscopy methods

. The work carried out comprehensive studies of a number of industrial semi-cokes using EPR and NMR methods. Apply of EPR spectroscopy, it was shown that in all the studied semi-cokes there are two types of radical structures, the first of which are low-molecular aromatic radicals.The second type is conjugated polyaromatic structures, in which the unpaired electron is delocalized. Line shape analyzing in EPR spectra, it was found that there are differences in the presented line of isotropic cokes. In this work was shown that EPR is a highly sensitive method that makes it possible to monitor changes in the molecular structure during the production of semi-cokes. The results of the EPR studies obtained in this work are in good agreement with the results of NMR spectroscopy. The results obtained show that changing the composition of the feedstock and technological operations to obtain high-quality anisotropic cokes is impossible without assessing the molecular structure of the intermediate coking products.


Introduction
Sustainable development of heavy industry, in particular metallurgy, requires the creation of electrodes with specified electrical properties.One of the materials for their formation is anisotropic (so-called needle) coke, the economic availability, chemical purity and high electrical conductivity of which make it the most profitable and in demand [1].The main criteria for assessing the quality of coke for its applicability in electrode materials are based on three factors: composition and amount of impurities, crystal structure (presence of longrange order and anisotropy), and porosity [2].
Based on the known criteria for assessing the quality of coke, we can definitely say that this is a complex and multicomponent task.For solution of it requires the use of many physico-chemical research methods.It should be noted that the quality of coke is largely determined by the choice of feedstock, its chemical purity and the conditions of its processing and preparation for production processes [3][4][5].
The most informative indicator of coke quality for its use in electrode materials is the degree of anisotropy [6], which makes it possible to classify cokes by assigning them scores from 0 to 7. From isotropic (so-called shot cokes) to completely anisotropic, for example, needle cokes.The crystal structure of cokes determines their physical and mechanical characteristics, as well as their areas of application [3][4][5].
The main influence on the crystalline structure of coke is exerted by the chemical and molecular composition of the feedstock [2,[7][8][9].In the work [7], it was demonstrated that there is a significant difference in the structures of cokes obtained from raw materials in which there is a predominance of a certain type of organic structures such as paraffin, naphthenic or aromatic.Thus, when producing coke from oils in which there is a predominance of paraffin structures, which determine the growth of carboide formations in one direction during the coking process, which as a consequence leads to the formation of anisotropic needle coke.A lower degree of anisotropy is observed in cokes obtained from resins.Such cokes have a spongy structure and a low degree of anisotropy; fibrous formations are also recorded in their structure.The least structured coke is formed when asphaltenes are used as feedstock, which leads to the formation of an isotropic structure that arises due to the growth of dense colloidal formations uniformly in all directions.
Formation of ordered graphite-like structures during the coking process through successive elimination of aliphatic structures from conjugated aromatic systems [10][11][12].Breaks of C-C bonds as a result of high-temperature processes occur continuously according to a homolytic mechanism with interruptions at atoms around unpaired electrons (paramagnetic centers (PMC)).With increasing temperature, the number of aliphatic fragments in the total organic mass leads to an increase in the number of conjugated aromatic rings until the formation of a graphite-like structure for the countable recombination of aromatic radicals [13].It is also known [14][15] that during the pyrolysis of organic matter, inclusions of matrix radical molecules are removed, which ultimately reduces the distance between polycyclic aromatic radicals with high molecular weight and leads to the formation of bonds between high-molecular-weight radical structures due to dipole-dipole interaction.The study of the formation of radical structures in coke and semi-coke is extremely important for refining and refining technological processes for the production of anisotropic cokes.
The PMCs formed as a result of pyrolysis can be localized on carbon atoms surrounded by protons (aliphatic radicals or low molecular weight aromatic radicals), and can also be delocalized on a conjugated aromatic system (high molecular weight conjugated aromatic radicals).Assessing the nature of radical structures in the EPR spectroscopy method is possible by analyzing the shape and linewidth of the signal observed in the spectrum [16][17].It is known [18][19][20] that the largest contribution to the line width of the EPR spectra of carbon materials is made by the spin-spin interaction between PMCs localized on carbon atoms located in the aromatic ring or on carbon atoms in aliphatic radicals.While the greatest contribution to the decrease in linewidth comes from the exchange interactions of PMCs delocalized on the conjugated polycyclic aromatic system.
It is worth noting that recording EPR spectra of cokes with a significant degree of gritization is impossible due to their electrical conductivity (the presence of electron gas), which makes the task of searching for a resonant frequency when setting up an EPR spectrometer impossible.
To optimize production processes in order to obtain anisotropic cokes and to improve technological coking processes, in addition to studying the structural and shape-size characteristics [21], it is also necessary to study the structural-molecular and radical compositions at various stages of coking.The optimal objects for this are semi-cokes, the study of which is possible using EPR and NMR spectroscopy methods due to their relatively low electrical conductivity.

Experimental part
The EPR spectra of the semi-cokes samples were recorded on a Bruker EMX micro 6/1 EPR spectrometer (Germany), operating in the X-band, at room temperature in an air atmosphere.The frequency of microwave radiation when recording spectra was ≈ 9.8 GHz, the power of microwave radiation varied from 1.9 mW to 200 mW; modulation signal frequency -100 kHz with an amplitude of 1 G.The concentration of paramagnetic centers was calculated by comparison with a standard sample (Mn 2+ ions doped with MgO).Simulation of EPR spectra and line shape analysis were carried out in the EasySpin program [22] using the Simultispin graphical interface [23].
High-resolution 13 C NMR spectra in solids were recorded on a Bruker Avance III 300 W NMR spectrometer (Germany) at a frequency of 75 MHz using the standard crosspolarization and magic angle spinning (CPMAS) technique.To obtain quantitative data, spectra were simulated using the Dmfit program.

Results and discussion
The low degree of graphitization of semi-cokes samples made it possible to study them using EPR and NMR spectroscopy.All EPR spectra (for example, Fig. 1) of the samples under study contain two lines with the same g-factor value: a narrow one with a width of 2-2.5 G against the background of a wide one (5-6 G).Next, studies were carried out on the influence of the supplied microwave radiation power on the ESR signal of the samples.As a result, it was revealed that only in sample semi-coke No. 1, with increasing microwave power, a central narrow band appears in the EPR spectrum (Fig. 2), which was absent at low powers.
The fact that the central narrow component is not observed in the spectrum of sample No. 1 is due to the overlap of the wide and narrow components of the spectrum due to the fact that the sample has the smallest width of the "broad" component of all the presented samples.PMC concentrations and g-factors are shown in Table .1, from which it can be seen that the samples differ from each other insignificantly in these parameters.To exclude vanadium complexes from the EPR signal, which can be transmitted as a narrow line in the EPR spectrum [24], EPR spectra with vanadyl ion were simulated.A comparison of the experimental and simulated spectra showed a discrepancy between the lines (Fig. 3).Thus, the appearance of a central narrow line in the EPR spectrum can be associated with the structural features of the radicals of organic masses of semi-coke.
It is known from works [18][19][20] that the main contribution to the line broadening of EPR spectra comes from spin-spin interactions of unpaired electrons localized on simple (non-conjugated) aromatic rings.A decrease in the linewidths of EPR spectra in carbon materials is associated with the exchange interaction of unpaired electrons delocalized on a conjugated high-molecular aromatic system.Thus, the narrow component of the EPR spectrum of the studied images may be associated with the presence of conjugated polyaromatic radicals.The broad component is associated with the presence of low molecular weight aromatic radicals.
Analysis of the line shape of the EPR spectra of the samples (Table 2) showed that sample No. 1 has the smallest linewidth of the central narrow component, which indicates an increased role of exchange interactions of unpaired delocalized electrons on the conjugated system of polyaromatic radicals that form packets.The increase in this effect is associated, according to data from [28], with an increase in the amount of PMC in such structures and a decrease in the distance between the layers of polyarenes in the stacks.According to NMR spectroscopy, the samples are mainly aromatic structures with a small number of relatively short aliphatic fragments (Table 3).In sample No. 1, the smallest number of -CH2-groups and the largest amount of carbon in the aromatic rings (Ar-C) are observed.Thus, it can be assumed that shorter terminal aliphatic chains contribute to the convergence of polyaromatic structures and the formation of packets.

Conclusion
Using EPR and NMR spectroscopy methods, it was shown that semi-cokes have some differences in the molecular structure.Thus, EPR and NMR spectroscopy methods are informative methods for studying the processes of evolution of the carbon frame at the semicoking stage.

Table 1 .
PMC concentrations in semi-cokes samples and g-factor values.

Table 2 .
Results of deconvolution of EPR spectra of samples.