Pyrolysis Behaviors Of Coals And Model Compounds By Time Of Flight Mass Spectrometry

During coal pyrolysis, a large number of complex chemical reactions occur, and the distribution and formation of pyrolysis products are closely related with the molecular structure of coals. Therefore, it is important for the understanding of the weak bond structure of coal in term of the molecular level by the investigation of the distribution and formation of pyrolysis products, especially primary pyrolysis products, and the relationship between pyrolysis products and the molecular structure of coal. Due to complexity of coal structure, it is difficult to investigate the pyrolysis mechanism by using the real coal. To understand the molecular structure of coal, it is advisable to compare the pyrolysis behaviors of coals with suitable model compounds with well-defined structure. In the paper, diphenyl ether, poly(2,6-dimethyl-1,4-phenylene oxide) and several coals were selected to investigate the pyrolysis behaviors by using time of flight mass spectrometry.Firstly, time of flight mass spectrometry (TOF-MS) with a resolution of1400combined with vacuum ultraviolet (VUV) photoionization was applied to study the pyrolysis behaviors of diphenyl ether at300-1200°C, and the possible routes of the products were proposed. The results showed that, during the pyrolysis process of diphenyl ether, cyclopentadienyl was formed due to the decarboxylation of phenoxy at700°C. Although benzene radicals are produced by the direct dissociation of diphenyl ether at700°C, there is no benzene formed because of the lack of hydrogen radicals till to the temperature above1000°C. With the production of hydrogen radical, the benzene radicals can combine with hydrogen radicals to form benzene. Another pathway for the formation of benzene is by the dehydrogenation of C6H8, which comes from the combination of cyclopentadienyl and methyl radicals.To further improve the resolution of TOF-MS, a high resolution TOF-MS with resolution of4000(cumulative1s) was constructed. And a new ion source was designed to study the pyrolysis behaviors of poly(2,6-dimethyl-1,4-phenylene oxide) and several coals including Xilinguole coal, Zhundong coal, Shenhua Coal, C67coal. The results showed that the new designed TOF-MS can record intermediates and products during pyrolysis of samples quickly and comprehensively.To explain the relationship between the pyrolysis behaviors of coals and molecular structure of coals, the formation of products released during pyrolysis of poly (2,6-dimethyl-1, 4-phenylene oxide) were investigated. Two ionization methods (EI and PI) were used to investigate the formation rules of small molecule compounds (such as H2O, CO, CO2, CH4and H2) and macromolecular compounds (alkyl benzene, naphthalene, phenanthrene, phenols, alkenes) during pyrolysis of coals. The results showed that the distribution of hydrocarbons and phenolic compounds are greatly affected by H/C and O/C. The formation rules of small molecule compounds, hydrocarbons and phenols reveal a lot of information on weak bond structure of coals:aliphatic carboxylic acids presented in the side chain of coals begin to decarboxylate from200°C; when the temperature was raised to300°C, ether bonds in the side chain and methylene bridge bonds begin to break up, accompanied by the release of phenols, hydrocarbons and other compounds. In the stage, aromatic carboxylic acids and aromatic carboxylic acid anhydrides begin to decarboxylate. When the temperature was raised above450°C, the dissociating rate of methyl and methylene which directly linked to the aromatic rings accelerates. Long chain hydrocarbons are more likely to disintegrate to short chain hydrocarbons at higher temperature. With the further increase of temperature, oxygen-containing heterocyclic rings begin to fracture accompanied by the formation of CO, and the condensation rate of macromolecular structure of coals accelerates, accompanied by the generation of H2.