Structural Characterization Of Small Molecular Components In Coals Based On GC/Q-TOF MS

Coals are rich in aromatic rings, alkyl groups and heteroatom containing compounds, and the compositions of coals are very complex. The key step to improve the efficiency of clean utilization for coals is understanding the compositions and molecular structures of coals. Due to the complexity in structure and composition of coals, gas chromatography/mass spectrometry(GC/MS) has been proven a powerful tool for species identification in coals. However, GC/MS cannot detect all of the components in the thermal dissolution products of coals because of its resolution and selectivity. As an enhanced GC/MS, gas chromatography/quadrupole time-of-flight mass spectrometry(GC/Q-TOF MS) has been paid great attention in the analysis of complicated matrix due to higher resolution. GC/Q-TOF MS has been proven a powerful tool for the identification of unknown compounds via a second ionization of MS/MS, which is suitable for the structural eluciation of organic species in coals.GC/Q-TOF MS was applied to analyze coal related model compounds(MCs), investigated selective regularities of the characteristic ion under different collision induced dissociation(CID) and the GC/Q-TOF MS fragmentation pathway of MCs. Three coal samples were thermally dissoluted with cyclohexane and methanol, respectively. Thermal soluble protions(TSPs) were analyzed using GC/Q-TOF MS to reveal the similarities and differences, which provide the basis for the reasonable and efficient utilization of coal and the research of coal structure.It is concluded that the different types of compounds have different characteristic ion and suitable CID in the second ionization of MS/MS. So it have different mechanism of cracking. The precursor ions of some aromatic compounds without branched chain such as fluorene, acenaphthylene and etc. are hard to crack even with a high CID voltage. In the CID process, the precursor and product ions of some aromatic compounds with branched chains first lose methyl radical, and then the compound is opened to lose an acetylene molecule and form a stable ion. The branched chain containing heteroatoms is easy to break compared with heteroatom-containing compounds. There are chemical bond breaking(such as C-C, C-N, C-S, and C-Br), hydrogen transfer, rearrangement reactions(such as indene), and elimination of neutral fragments(such as CO and C2H2) during the CID process.TSPs were characterized by GC/MS and GC/Q-TOF MS. The amount of compounds identified by GC/Q-TOF MS is larger than GC/MS, due to the high sensitivity and high resolution of GC/Q-TOF MS. Therefore, the GC/Q-TOF MS was preferred to characterize all the TSPs. The compounds which have much relative content and could not identify by GC/Q-TOF MS were analyzed using a second ionization of MS/MS to identify its structure, such as the characterization of 1,1’-(4,4’-methylenebis(4, 1- phenylene)) diethanone. The results indicated that the TSPs have higher yield in methanol compared to cyclohexane for the three coals. The thermal dissolution process is supercritical extraction under the condition of 300 oC. Methanol can penetrate into macromolecular structure of coal to damage chemical bonds. A series of chemical reactions occurred in coal, such as alcoholysis, alkylation and ester exchange reaction, resulting in covalent bond cleavage. So the relative content of oxygen compounds(phenols, esters, ethers, ketones, and aldehydes) in TSPs with methanol is higher than that with cyclohexane. As a non-polar solvent, cyclohexane is more conducive to extraction of alkanes and aromatic compounds.