Comprehensive evaluation of polycyclic aromatic units in three liquefaction-derived heavy fractions from Zhunnan lignite

Abstract

The structural characteristics of polycyclic aromatic units in three liquefaction-derived heavy fractions from Zhunnan lignite, including thermal dissolution (TD) in cyclohexane (HF C ), ethanolysis (HF E ), and two-step sequential TD in cyclohexane-ethanol (HF CE ), were investigated in depth. Combining multiple destructive tools, the intrinsic correlation among dissociated/degraded fragments and organic macromolecular networks, involving the types of alkyl side chain, methylene bridged bond, heteroatom, and aromatic nucleus, was also constructed. The results indicate that HF C with a smaller average molecular weight may be rich in -OH, >C-O-, and >C O units, while HF CE has higher aromaticity. In addition, abundant arenes and arenols were detected in the volatiles released from HF C flash pyrolysis, but macro/ultramolecular groups dominated in HF E and HF CE . Obviously, the differences in thermochemical reactions further confirmed that the component distribution of volatiles is mainly related to the direct gasification of small/medium molecules and the cleavage of weak/moderate strength covalent bonds. The analyses with quadrupole exactive orbitrap mass spectrometer and ruthenium ion-catalyzed oxidation show that HF C also contains abundant oxynitrides, while HF CE may be dominated by cata -condensed aromatics and polyaryl units, followed by few peri -condensed aromatics, long-chain saturated rings, aromatic rings with longer aliphatic side chains, and polymethylene linkage units. Comprehensive evaluation of these difficult-to-detect parts is beneficial to the resource and diversified utilization of such special soluble portions with potential environmental hazards. • HFCE has the highest aromaticity, while HFC is rich in O-containing FGs. • HFC also contains abundant >C-O- moieties and oxynitrides. • HFCE may be dominated by cata-condensed aromatics and polyaryl units. • The inner link among degraded fragments and organic structures was revealed.