Ultrasonic/microwave-assisted Extraction Of Liuhuanggou Bituminous Coal And Sequential Thermal Dissolution Of Extraction Residues

Liuhuanggou coalfield of Xinjiang Uygur Autonomous region is more promising for large-scale exploitation and utilization owing to lower transportation costs.Understanding thestructure and composition of organic matter in Liuhuanggou bituminous coal?LBC?is crucially important for its efficient,clean,and value-added utilization.In recent years,microwave-assisted extraction?MAE?and ultrasonic/microwave-assisted extraction?UMAE?are flourishing methods for extracting biomass and environmental samples and ultrasonic-assisted extraction?UAE?is widely used in coal extraction.But applications to MAE and UMAE for coal,especially comparison between MAE,UMAE,and UAE in bituminous coal extraction were seldom reported.In this paper,MAE,UAE,and UMAE were separately conducted on LBC with four solvent to explore the difference between extracts 1-4?E₁-E₄?from MAE(E_M1-E_M4),UAE(E_U1-E_U4)and UMAE(E_UM1-E_UM4)and obtained extract residues?ERs?from UAE?ER_U?,MAE?ER_M?and UMAE(ER_UM).Then the ER_UM and LBC were subjected to sequential thermal dissolution?TD?at 300 ^oC with cyclohexane,toluene,and ethanol to afford soluble portions 1-3?SP₁-SP₃?and dissolution residues from LBC(SP_L1-SP_L3,R_L)and ER_UM(SP_ER1-SP_ER3,R_ER).Multiple detections were involved to investigate the products of extraction and TD,including GC/MS,ESI/time-of-flight mass spectrometer?ESI/TOF-MS?,FTIR and NMR,et al.The carbon types in LBC mainly consist of aliphatic?22.2%?and aromatic?73.7%?carbons.Each aromatic cluster of LBC contains two or three aromatic rings on average,and each ring have 2-3 substituents.The aromatic and carbonyl carbon of ER_UMM and ER_M are fewer than those of ER_U,indicating that the microwave irradiation?MWI?contributes to the dissolution of aromatic and carbonyl-containing species.Besides,different extraction methods make little difference in the function groups and carbon types among LBC and ERs.The oxygen-containing structures were enriched in dissolution residues since many aliphatic moieties were transferred to soluble portions.Most of surful in LBC is inorganic and it remained in residues after TD.The carbon number of n-alkanes?NAs?in extracts and SPs are similar without obvious carbon preference,indicating significant contributions of bacteria,microbial wax,and/or higher plant wax to the organic matter in LBC.The highest total extract yield of 10.6 wt%was obtained by UMAE.The UMAE and MAE can obtain more aliphatics,aromatics and oxygen-containing species than UAE,while MAE is the most effective for extracting highly condensed arenes from coal.The molecular masses of species in E_UM3-E_UM4 are smaller than that in E_U3-E_U4 and E_M3-E_M4 and the yields of E_UM3-E_UM4are much higher than that of E_U3-E_U4 and E_M3-E_M4,suggesting that associated species could be dissociated by UMAE.MAE and UMAE are idealtools to obtain value-added chemicals for their high selectivity of extracting heteroatom-containing species?HACSs?.In TD section,10.4 and 4.9 wt%of organic matter was released from LBC and ER_UM by TD,respectively.More aliphatic species were dissolved from LBC,while more species containing C=O and C-O-C were riched in SP_ER1-SP_ER2.Based on GC/MS analysis,varities of alkenes were detected in SP₁-SP₂,which could be attributed to the hydrocracking of C?—C?bond or alkane pyrolysis under thermal effect.According to Orbitrap-MS analysis,large amounts of HACSs were condensed in SP₃ with molecular masses of m/z 150-175.The N₁ and N₁O₁ species dominate in SP₃ while less sulfur-containing species were identified.N₁ species contain 1-3 rings on average.The carbon number?CN?of N₁O_x-N₄O_x?x=1-4?is 10-30 with double bond equivalent?DBE?of 5-12.Both the quantities and categories of N₁O_x-N₄O_x?x=1-3?species in SP_L3 is higher than that in SP_ER3,proving that N₁O_x-N₄O_x?x=1-3?species were embed in coal skeleton and being condensed in E_UM1-E_UM4M4 by UMAE.