FTIR Analysis of the Correlation Between the Pyrolysis Characteristics and Molecular Structure of Ultrasonic Extraction Derived From Mid-Temperature Pitch

The regulation of molecular structure and thermal conversion behavior of coal tar pitch is the key to preparing high-quality coal tar pitch-based carbon materials. In order to further clarify the relationship between pyrolysis behavior and molecular structure of coal tar pitch, 8 kinds of extracts derived from Medium temperature pitch (AGMP) were extracted by ultrasonic extraction with 8 kinds of organic solvents at room temperature in this paper. The PeakFit v4. 12 software was used to perform peak fitting the infrared spectra of the extracts in four regions of 700 similar to 900, 1 000 similar to 1 800, 2 800 similar to 3 000 and 3 000 similar to 3 100 cm(-1) to achieve the fine structure information about various functional groups. In addition, six molecular structure parameters (I-1- I-6) were introduced to characterize the relationship between molecular structure and pyrolysis activation energy of the extracts. FTIR spectra analysis shows that the 8 extracts are complex compounds composed of condensed aromatic ring structures mainly comprised of aliphatic hydrocarbon side chains containing oxygen and nitrogen and other heteroatoms. Moreover, due to the difference in the extractant' s structure, the extract's molecular structure parameters are also slightly different. The extract obtained from the linear structure extractant has a higher content of chain hydrocarbons (I-5) , and the extract obtained from the ring structure extractant has more aromatic rings substituted structures (I-6). Thermogravimetric analysis (TGA) was used to study the thermal weight loss behavior of 8 extracts at different heating rates (3, 6, 10, 15 K . min(-1)). Under the condition of equal conversion rate without considering the reaction mechanism, the pyrolytic activation energy (E-a) were calculated and analyzed using the Flynn-Wall-Ozawa method and Kissingr-Akahira-Sunose method. The results indicated that the eight extracts' pyrolytic activation energies ranged from 78 to 116 kJ . mol(-1), which are closely related to the structure and content of functional groups. The structural parameters of the extracts obtained by IR quantitative are correlated with the pyrolysis activation energy. The fitting results of the one-element linear equation E-a = f (I-i) between the structural parameters of the extracts and the pyrolysis activation energy found that the aromaticity index (I-3) and the degree of branching (I-5) are the two main indicators that determine the pyrolysis activation energy of the extracts. The fitting results of positive and negative correlation between the pyrolysis activation energy with every single index (I-i) indicate the difficulty of the structure being destroyed by pyrolysis. Considering the interaction of various infrared structural parameters, the fitting relationship model between the E-a of AGMP extracts and the infrared spectral structure indexes was shown as followed : E-a = - 4 294. 53I(1) + 73 812. 16I(2) +207 673. 32I(3) - 20 324. 20I(4) - 168. 56I(5) + 857. 86I(6). The results, which combined analysis of FTIR spectra, reveal more details about thermal characterization and kinetic characterization and they can be expected to lead to a well-understood coal tar pitch's pyrolysis.

Automated summary

The regulation of molecular structure and thermal conversion behavior of coal tar pitch is essential for preparing high-quality carbon materials. This study investigated the relationship between pyrolysis behavior and molecular structure of coal tar pitch by extracting 8 kinds of extracts using different organic solvents. The infrared spectra analysis revealed that the extracts are complex compounds composed of condensed aromatic ring structures with aliphatic hydrocarbon side chains containing oxygen and nitrogen. The thermal weight loss behavior of the extracts was studied using thermogravimetric analysis, and the pyrolytic activation energies were calculated. The results showed that the pyrolytic activation energies are closely related to the structure and content of functional groups. The aromaticity index and degree of branching were found to be the main indicators determining the pyrolysis activation energy. The fitting relationship model between the pyrolytic activation energy and the infrared spectral structure indexes was established, providing a better understanding of coal tar pitch's pyrolysis.