Composition Characterization And Catalytic Hydrocracking Of Organic Matters In Lignites

Lignites are rich in organic oxygen and show great potential as chemical feedstocks for producing value-added oxygen-containing species(OCSs). Detailed studying the compositions of organic matters in lignites at the molecular level is crucial to the efficient utilization of lignites. On this basis, the key to value-added utilization of lignites is developing conversion reactions which can break some covalent bonds in lignites under mild conditions and selectively produce small-molecular organic compounds. In this study, composition characterization of organic matters in lignites was performed using a three-step degradation, including sequential ultrasonic extraction, sequential thermal dissolution, and ruthenium ion-catalyzed oxidation(RICO), combined with various advanced analytical techniques. In addition, supercritical methanolysis of ultrasonic extraction residues(UERs) from lignites with Na OH and catalytic hydrocracking of lignite over magnetic solid base were studied in order to develop promising approaches for obtaining OCSs from lignites.FTIRS, XPS, solid-state 13 C NMRS, and TGA were used to directly characterize oxygen functional groups, surface element forms, carbon skeleton structures, and covalent bond types in Xianfeng lignite(XL), Xiaolongtan lignite(XLT), and Shengli lignite(SL). Most of the organic matters(ca. 90 wt%) in lignites were converted into soluble organic species via the three-step degradation followed by chemical composition characterization. Small-molecular compounds rich in aliphatic moieties which are free or bound in lignites by weak non-covalent bonds were mainly released during ultrasonic extraction. Hydrogen bonds and ?-π interactions along with some weak covalent bonds in lignites could be broken by sequential thermal dissolution at 320 oC, leading to significant increase in yields of soluble species. Phenols and nitrogen-containing species(NCSs) which can easily form hydrogen bonds and ?-π interactions were mainly released during thermal dissolution. Series of biomarkers, including long-chain n-alkanes,n-alkenes(1-alkenes, 2-alkenes, and 3-alkenes), n-alkan-2-ones, and alkylbenzenes(n-alkylbenzenes, n-alkyltoluenes, and n-alkyl-p-xylenes) were detected with GC/MS in the soluble portions from ultrasonic extraction and thermal dissolution. The n-alkanes could be derived from higher plant waxes or decarboxylation of n-alkanoic acids during coalification. The long-chain alkylbenzenes could result from n-alkanoic acids undergoing reduction, cyclization, α-methyl rearrangement, and aromatization. The n-alkan-2-ones were supposed to be formed through microbially mediated β-oxidation of the corresponding n-alkanes or result from β-oxidation of n-alkanoic acids, followed by decarboxylation.RICO of thermal dissolution residues(TERs) suggests that the insoluble macromolecular network in lignites primarily consists of condensed aromatics with rings ≥3. The aromatic units are mainly connected each other in form of biaryls, along with a small amount of methylene bridges with carbon number <8. Methyl is the dominant alkyl side chain on aromatic rings with low contents of alkyl groups with carbon number >2. Longer alkyl side chain and methylene bridges primarily exist in the TER from XL, which is consistent with the 13 C NMR analysis that XL contains longer methylene than XLT and SL. Nitrobenzenecarboxylic acids produced from RICO could result from the degradation of macromolecular nitroaromatics in lignites.The total yield of soluble portions from supercritical methanolysis of UERs significantly increases with increasing Na OH content, with the highest yield at Na OH/UER of 1 g/g. The soluble portions obtain under optimal conditions were separated into 4 fractions(E1-E4) and analyzed with GC/MS. The results show that phenols are the major GC/MS-detectable species in E1 and E2 with relative contents more than 50%, whereas almost all the alkanoic acids, alkanedioic acids, benzoic acids, and phenyl-substituted alkanoic acids were enriched in E4. The ESI FTICRMS analysis shows that the O1-O6 class species are the predominant compounds in E1-E4, with 0-14 double bond equivalent values and 9-34 carbon numbers. They could be assigned to acidic species, such as aliphatic acids, 1-4 ring arenols, and 1-4 ring aromatic acids. Supercritical methanolysis followed by subsequent separation techniques could be a feasible approach for producing value-added OCSs from lignites. ESI FTICRMS analysis also reveals sulfur-containing species(SCSs) and NCSs in E1-E4, detection of which is difficult with GC/MS. S atom in SCSs is mainly present in thiol and thiophene ring and N atom in NCSs mainly exists in pyrrole ring and amino, providing important information on S and N forms occurring in lignites.The key to obtaining value-added chemicals, especially OCSs, is preparing appropriate catalysts which can selectively break covalent bonds(especially-C-O- bonds) in lignites. In this investigation, a supported magnetic solid base catalyst with strong magnetism and basicity was prepared through chemical precipitation, sol-gel, and impregnation methods, and was used for hydrocracking of XLT. The results show that the total yield of soluble organic species from XLT hydrocracking at 300 oC increases from 59 wt% under non-catalytic conditions to 78 wt% under catalytic conditions. Among the soluble organic species, the yields of phenols and arenes remarkably increase. Alkylphenols with C1-C8 in alkyl group(s) dominate in the phenols and alkylbenzenes are the major arenes. Benzyloxybenzene and dibenzyl ether were subjected to catalytic hydrocracking over magnetic solid base to discuss reaction mechanism and they were completely converted into hydrocracking products at 250 oC and 200 oC, respectively. Hydrocracking products of coal-related model compounds in cyclohexane and methanol are different because methanol participated in reaction. Magnetic solid base catalyzes heterolysis of H2 to a mobile H- which can attack the C atom with higher electropositivity in the model compound, leading to cleavage of bridge bond and forming hydrocracking products. The catalyst can also facilitate heterolysis of CH3 OH to CH3O- which can also break the bridge bond like H-. Therefore, phenols and arenes from catalytic hydrocracking of XLT could result from the cleavage of –C–O– bridge bonds connecting aromatic rings in lignite catalyzed by magnetic solid base.