| Coal tar is a by-product of metallurgical coke production.Due to the large scale of steel industry,the annual output of coal tar in China has reached 15 million tons in recent years.Heavy coal tar pitch accounts for more than half of it,and is a unique and valuable polycyclic aromatic hydrocarbons(PAHs)resource.Compared with developed countries,there are still significant gaps in the development and application of high added value of coal tar pitch,including coal-based carbon fiber and needle coke.In addition,the promotion of nano-carbon materials represented by graphene in a global scale,results in some large graphene-like and controllable PAHs have been successfully synthesized in recent years.Such graphene-like molecular materials attract widespread attentions,but high difficulty and cost of synthesis as well as limited structures are all issues.In fact,since the first industrial revolution,the extremely diverse and complex structures of large PAHs in large-scale coal tar pitch have not been revealed.In this study,high-resolution matrix-assisted laser desorption/ionization time-of-flight mass spectrometry(MALDI-TOF MS)has been demonstrated to successfully recognize PAHs molecular peaks abundant in coal tar pitch,in terms of basic molecular structure by aromatic building blocks,based upon extensive analyses of PAHs growth patterns,in combination with other modern analysis techniques.The relationships between basic aromatic structure(i.e.,ortho condensation and bay condensation)and molecular weight(Mw)of PAHs were elucidated from three to more than twenty aromatic rings.In further investigations of the research program,in-depth and systematical investigations were carried out for several different pitches,their solvent-extracted fractions and thermally polymerized products.Thus,the main contents and results of this thesis are divided into four related parts:(1)The establishment of a new method for interpreting PAHs molecular peaks in the MALDI spectra of coal tar pitch: Certain small aromatics in light fraction were firstly identified by Gas Chromatography-Mass Spectrometer(GC-MS),and the growth patterns of detected PAHs were carefully studied.Subsequently,based on the systematic analysis of distribution characteristics with the strong molecular peaks of the whole pitch,the PAHs peaks of high abundance were recognized with specific aromatic characteristics by the growth patterns of PAHs.As a key break-though,the 50/24 Da Mw increase rule were found to be valid not only for small PAHs,but also for much higher Mw PAHs peaks in MALDI spectra,resulting from th e ortho-condensed and bay-condensed growth of aromatic structure.Therefore,PAHs with Mw up to 850 Da in the pitch sample were successfully interpreted and further analyzed by 50/24 matrix.(2)A comparative study on the chemical composition and structure of various coal tar pitches: In combination with FT-IR 、 NMR 、 GC-MS 、 TG and rheological analysis,MALDI-MS was used to analyze the molecular characteristics and Mw distribution of large PAHs in three more coal tar pitches and to compare the molecular composition and structure in relations with the properties of different pitch samples.The study has found that the PAH peaks in the MALDI spectra of other three pitches exhibited the same 50/24 Da distribution characteristics as the previous one,and the structural parameters from 4 to 23 ring PAHs that are abundant in three pitches were obtained.These results not only confirmed the applicability of 50/24 matrix methodology in analyzing large PAHs in different pitch samples,but also provided a new method for comparing small differences between different pitch samples at molecular level.On the other hand,the averaged analysis results from FT-IR and NMR revealed that the chemical compositions of d ifferent coal tar pitches were qualitatively similar.However,the identification of small molecules by GC-MS and the interpretation of higher Mw PAHs peaks in MALDI spectra allowed detailed comparisons of different pitches in compositions at molecular lev el.(3)Structural characteristics of PAH molecules in different extraction fractions of coal tar pitch: After coal tar pitch was sequentially separated by both soxhlet extraction and anti-solvent precipitation,the separation effects were evaluated by MALDI MS at molecular level.Large PAHs enriched in different fractions were examined,and their topological structures were analyzed in detail.The results showed that a series of solvent extraction could separate the polydispersed coal tar pitch into narrow fractions with specific Mw distribution;and the aromatic ring size and Mw of PAHs abundant in the separated fractions tended to have higher values as the solubility reduced,with certain overlap between two adjacent fractions.It was found that different growth routes of PAHs result in diverse structures in topology.The PAHs grown extensively by bay-condensations had tight cluster structures with mostly zigzag edges,while ortho-condensations tended to form long and narrow PAH molecules,with their periphery containing more bay positions.The large PAHs with same number of aromatic rings can have very different Mw,carbon numbers,DBE,structural configurations and edge structures due to the different type of aromatic rings formed.(4)A molecular-level examination of thermal polymerization of coal tar pitch leading to the formation of mesophase: The isotropic phase and mesophase in the thermal-polymerized product of coal tar pitch were separated by high-temperature centrifugation.Polarized light microscope,XRD,FT-IR and MALDI MS were used to analyze the separated phases to follow the changes of molecular composition,microcrystalline structure and optical texture caused by mesophase formation.The results indicated that high-temperature centrifugation could effectively separate isotropic pitch and mesophase.Isotropic pitch and mesophase showed similar Mw distribution(720~3700Da),while the main distribution range was higher for mesophase.Large aromatic molecules produced in thermally polymerized p itch were the basis to form the mesophase.Moreover,the Mw distribution of polymerized products did not conform to the 50/24 growth pattern,significantly differing from the parent pitch.The prolongation of thermal polymerizations can increase the high M w components in the mesophase,leading to more ordered macromolecular accumulation as well as better orientation in optical texture. |
