Abstract
Soxhlet extraction (SE), ultrasonic-assisted extraction (UAE), and microwave-assisted extraction (MAE) were carried out on Daxing coal with carbon tetrachloride. The extracted components were analyzed by GC-MS while the residues of the coal after extractions were analyzed by FT-IR spectroscopy. The obtained IR spectra indicated that the functional groups were barely changed in strength for the coal before and after extractions concluding that the macromolecular structures of coal were not destroyed in the extraction processes. XRD diagrams showed the peak around θ = 47 was totally disappeared by all the three extractions, indicating that the graphite-like structural substances in the coal were totally destroyed in the extraction processes. GC/MS analysis showed that: (1) The SE method extracted the least number of substances from the coal; on the other hand, the extracted compounds are largely chlorinated which can be explained by the free radical mechanism. (2) MAE extracted 75 organic compounds of which 53 are oxygen-containing substances. A small portion of non-alkanes (1.19%) was found, which is in contrast to the other two extraction methods. Moreover, a few biomarker compounds were also identified including hexaoxane, 2-methylcholest-3-ene, 6,9,12-tripropylheptadecane, and 17α-21β-28,30-bisnorhopane. (3) The three extraction methods gave totally different extraction patterns for the same coal, highlighting that the extraction method can dominate the outcome of the extracted products. The mechanisms behind these extraction processes are discussed. This study provides a base for the future choice of the extraction methods in terms of outcomes of the extraction products.
Highlights
Coal is an extremely complex and heterogeneous material
The results show that more small molecular substances are dissolved in the extraction process, which emphasizes the macromolecular structure of the coal
The Daxing coal was extracted by CCl4 using three different extraction methods
Summary
Coal is an extremely complex and heterogeneous material. It is a challenging task to characterize the structure of coal due to the complex and heterogeneous nature. Understanding the origin, chemical structure, and carbon skeleton are important for coal exploitation and further utilization [1]. Comprehensive utilization of coal is of critical importance for the development of the economy. It has been demonstrated that coal extraction is a good method for achieving the above goals since many aromatic compounds are obtainable from the extraction process.
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