Characterization Of Intermediates And Radicals In Coal Pyrolysis And Investigation On Reaction Mechanism

The main purpose of this dissertation is to investigate the mechanism of coal pyro lysis at the molecule level. The coal based model compounds which represent the individual structural units are the main research objects. Molecular-beam mass spectrometry technique coupled with vacuum ultraviolet single-photon ionization is employed to identify the reactants, radicals, and products. Semi-quantitative analysis and theoretical calculation at mPW2PLYP level are used to support the experimental observation. The major contents and results in this work are summarized as follows:(1) Phenyl ethers are selected as the model compounds for studying the pyrolysis behavior of ether bond in coal. Anisole is the most simple phenyl ether, its pyrolysis process has experienced the following reaction:bond homolysis of PhO-CH3 to form phenoxy radical followed by a multistep ring-reduction/CO elimination to generate cyclope ntadienyl radical. With temperature increasing, cyclopentadienyl radical abstracts acetylene to form propargyl radical. In our experiment, benzene isn’t derived from propargyl-propargyl radical recombination, it generated from the reaction of methylcyclopentadiene abstracts two hydrogen atoms to generate benzene via a bicyclic intermediate and ring expansion. Phenol is the main product in the pyrolysis of phenyl ethers. It generates from addition of hydrogen atom and phenoxy radical at higher temperatures. However, the formation of phenol at lower temperatures in pyrolysis of phenyl ethyl ether has another reasonable explanation. Theoretical calculation indicates that the non-radical reaction is dominant at low temperature and the β-H is a key factor for the non-radical reaction.(2) Methyl substituent effect is studied through the thermal decomposition of three methyl anisole isomers. The initial pyrolytic step for each isomer is methoxy bond homolysis to eliminate methyl radical. The pyrolysis processes of o-CH3-C6H4-OCH3 and p-CH3-C6H4-OCH3 are analogous. The o-and p-methyl phenoxy radicals abstract a hydrogen atom and subsequently rearrange to 6-methylen-2,4-cyclohexadien-l-one and 4-methylene-2, 5-cyclohexadiene-1-one, respectively. Both of the two compounds have the reasonable conjugate structure, which make them stable. The m-methyl phenoxy radical can’t directly form related the stable products. However, our theoretical calculations indicate that the w-methyl phenoxy radical via isomerization pathways to generate 6-methylen-2, 4-cyclohexadien-1-one or 4-methylene-2,5-cyclohexadiene-1-one.(3) The pyrolysis of model compounds which containing bridge structures serve as the model bridge structure in low-rank coal have been performed. The length of C-C bridge bond plays an influential role in cracking of corresponding bonds in the diarylalkanes pyrolysis process. The C-H bond scission is dominant in pyrolysis of diphenylmethane at low temperatures, while the C-C bond scission competes with it when the temperature increases. The symmetrical homo lysis of bibenzyl is the dominant reaction at low temperature, and will compete with the unsymmertrical cleavage reaction with temperature increasing. Theoretical calculations on the formation mechanism of fluorene during pyrolysis of diphenylmethane suggest that the diphenylmethyl radical goes through the H transfer process and subsequently suffers a ring closure to form fluorene. Moreover, the pyrolysis mechanism of model compounds which containing heteroatoms in bridge bond can be obtained from experimental results.(4) In-situ vacuum ultraviolet photoionization and electron impact mass spectrometry is applied to detect thermal decomposition fragments of two lignites. Experimental results show that H2, H2O, CO, and CO2 are dominant inorganic gas products and their characteristic temperatures are in accord with the temperatures of related chemical bonds cleavage. Mononuclear aromatic compounds are dominant organic pyrolytic products, and many olefin species are also identified. The differences between sample A and B on macro molecular structures make the diversity of pyrolytic products. In addition, the peak of H2S and CH3SH are both clearly observed in mass spectra of sample B, which could come from thioether bonds decomposition.(5) A novel catalytic reactor coupled with vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry (VUV-SPI-MS) is used for understanding the mechanism of catalytic conversion of methane. Mass spectrum of radical (methyl) and intermediates (C3, C4, and C7) are obtained, which provide strong experimental evidence for catalytic mechanism speculation. Intensity of mass signals of major products is detected as a function of methane flow rate at 1223 K. The results show that increasing the CH4 flow rate in the VUV-SPI-MS reactor favors the formation of C2H4, whereas lower flow rates (corresponding to longer residence times) promote cyclization of intermediates leading to aromatics.