Analyzing Coals Using Fourier Transform Infrared Spectroscopy

Scientists from the National University of Mongolia in Ulaanbaatar, Mongolia created new means of analyzing solvent-based extraction of bituminous coals. Their findings were published in the Journal of Spectroscopy (1).

hot red coals among black ash, wallpapers for mobile devices, abstract | Image Credit: © Dmitriy Popov - stock.adobe.com

The climate conditions of long winters with extremely low temperatures (up to −40° C) have demanded an environmentally friendly and clean way of using coal for energy, especially when considering current economic circumstances and the lifestyles of traditional dwelling inhabitants. Solvent extraction is widely used in the study of coal’s compositional or structural characteristics with traditional solvents, such as benzene, toluene, methanol, chloroform, dichloromethane, pyridine, and N-methyl pyrrolidone. When solubilizing coal, given the disassociation and rearrangement of functional groups, there is an increased number of polycyclic aromatic hydrocarbons (PAHs) or hydrogen bonding formation in the coal structure.

In this study, the scientists highlighted what types of coal influence traditional and innovative environmentally friendly solvents-based extraction. Bituminous types of coal ionic liquid (IL)-based extraction provides more promising results compared to sub-bituminous coal, including many types of hydrogen bonds to be revealed, a shortage of aliphatic chain, a breakage of oxygen-containing structures, and removal of minerals or oxygen-containing groups. Fourier transform infrared spectroscopy (FT-IR) is the method used in the current study, typically being used to reveal renovations and rearrangements in the functional groups of coal structures. For obtaining qualitative and quantitative data on the distribution and contents of the functional groups, Beer–Lambert theory-based FT-IR was used.

Using FT-IR spectroscopy, solubility was investigated for two types of Mongolian coals, bituminous Tavantolgoi and sub-bituminous Alagtogoo coals. Bituminous coal is a type of coal that holds a high heating (Btu) value, while sub-bituminous coal has low-to-moderate heating values (2). Two types of solvents, pyridine and IL ([Bmim]Cl−), were used for the structural characterization and a solubility degree in the coals. According to the scientists, “The main characteristic point of Mongolian coals is their great content of trace elements or various minerals inclusion.” Some heavy elements were prominent in the coal mass containment, including silicon, aluminum, sulfur, and selenium. Moreover, oxygen content in coal was shown to play a key role in solvent-based extraction production.

The coal samples were analyzed using a scanning electron microscope (SEM) equipped with backscattered and secondary electron detectors with energy dispersive X-ray spectrometry (EDS). The SEM-EDS system provides detailed imaging information about the morphology and surface texture of individual particles, in addition to the elemental composition of samples. The elemental composition of the coal samples was determined using a characteristic X-ray spectrum up to 20 kV on the coals’ surface.

Bituminous and sub-bituminous types of coals [Bmim]Cl−-based extraction both provide promising results, such as various types of extra hydrogen bonds occurring to confirm good solubility, and excellent removal of minerals or oxygen-containing groups in the low-frequency region. Low oxygen content in the fundamental structure of Tavantolgoi coal is an important part of this extraction process. Overall, traditional solvents have the advantage of being easily processible, with room temperature procedures producing virtually similar results to IL treatment in the low-frequency region. Quantitatively, one can confirm various results with this approach, such as extract aromatization and aliphatic chain length shortages.

References

(1) Sambuu, M.; Ganbold, N.; Ganbold, E-O. Effects of Pyridine and Ionic Liquid ([Bmim]Cl) Extraction on the Structure of Mongolian Bituminous Coals. J. Spectrosc. 2025, 2025 (1), 9958603. DOI: 10.1155/jspe/9958603

(2) What Are the Types of Coal? USGS 2024. https://www.usgs.gov/faqs/what-are-types-coal (accessed 2025-3-4)