Analysis of Carbon Tetrachloride-Extractable Species from Daxing Bituminous Coal

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.