Pyrolytic behavior of coal-related model compounds connected with C–C bridged linkages by in-situ pyrolysis vacuum ultraviolet photoionization mass spectrometry

• Published in: Fuel (Guildford) 2019-04, Vol.241, p.533-541
• Main Authors: Zhou, Yang, Li, Lu, Jin, Lijun, Zhou, Jian, Shi, Zhiwei, Li, Yang, Hu, Haoquan
• Format: Article
• Language: English
• Subjects: Aromatic compounds; Bonding strength; Chemical bonds; Cleavage; Coal; Coal-related model compounds; Conjugation; Covalence; Covalent bonds; C–C bridged bond; Dimers; Environmental effects; Free radicals; Homology; Hydrogen bonding; Hydrogen bonds; Hydroxyl groups; Intermediates; Internet; Linkages; Mass spectrometry; Mass spectroscopy; Monomers; Organic chemistry; Photoionization; Polypropylene; Polystyrene; Polystyrene resins; Polyvinylpyrrolidone; Py-VUV-PIMS; Pyrolysis; Recombination; Scientific imaging; Spectroscopy; Temperature; Vacuum
• ISSN: 0016-2361; 1873-7153
• DOI: 10.1016/j.fuel.2018.12.046

Proposed reaction pathways of five coal-related model compounds’ pyrolysis process consist of three stages: Firstly is the chain initiation, the primary decomposition process of polymer molecules produces large amounts of alkylated radicals accompanied by a small amount of residual matter through C–C bond homolysis in the thermal field. Then, the collision between free radicals and other molecules will take place during the chain propagation stage, resulting into the formation of new radicals and molecules. Lastly, annihilation of free radicals by combination with each other terminates the chain reaction. And most of the polymer samples degrade into small molecules as the released compounds, and very small amounts of heavy compounds remained as residue. [Display omitted] •Application of an in-situ Py-VUV-PIMS to analyze five polymeric coal-related model compounds.•Comprehensive analysis of the pyrolysis reaction based on experiments and theoretical calculation.•Correlations of cleavage temperature of C–C bridged bonds with corresponding chemical structure.•A proposed radical dominant mechanism for cleavage of Cal–Cal bridged bonds in selected coal-related model compounds. Coal pyrolysis is generally considered to initiate by cleavage of weak covalent bonds bridged between aromatic rings. The C–C covalent bonds are predominant bridged linkages among the framework of coal, which have a critical influence on coal pyrolysis reaction but lack of direct online observation. Five representative coal-related model compounds containing C–C bridged bonds, polypropylene (PP), polyvinylpyrrolidone (PVP), poly-4-vinylpyridine (P4VP), polystyrene (PS) and poly-4-vinylphenol, were selected to pyrolyze in an in-situ pyrolysis vacuum ultraviolet photoionization mass spectrometry (Py-VUV-PIMS). The soft ionized mass spectral detection can provide product distribution and evolved information of original products and intermediates, which are the key evidence to understand the initiation, recombination and reaction routes during pyrolysis process. Online analysis of pyrolysis reaction was found to be priority with cracking of C–C bond in the five polymer samples, and homologous free radicals and original products, which mainly include monomers, dimers and their alkyl and alkenyl radicals, were detected. The results reveal the cleavage temperature of C–C covalent bond is deeply related to its chemical environment, and obvious conjugation effect and steric effect for the bridged