Dissociation Of Non-covalent Bonds And Depolymerization Of Covalent Bonds In Shaerhu Subbituminous Coal And Mengdong Lignite In Mild Conditions

The high oxygen content limits the industrial application of low rank coals.However,it is beneficial to obtaining value-added oxygen-containing species.A comprehensive investigation on the compositions of organic matters at the molecular level is crucial to the efficient utilization of low rank coals.Due to the complex compositions of coals,it is impossible to obtain integrated compositional information of coals by a simple method.Therefore,the coal samples in the present study were subjected to sequential ultrasonic extraction,sequential thermal dissolution(TD)at different temperatures,and ruthenium ion-catalyzed oxidation(RICO)to dissociate the non-covalent bonds and depolymerize the covalent bonds in organic matters of coals,obtaining the structural information on soluble potions and carbon skeleton structures,which provides theoretical basis and technological support for the efficient utilization of low rank coals.SEM,element analyzer,XRPES,solid-state 13 C NMR,and TGA were used to directly characterize the organic matters in raw coals,extraction residues,and TD residues.It could be concluded that hydrocarbons are enriched in extracts,and oxygen-containing organic compounds are mainly dissolved out during TD.After extraction and TD,the particle size is uniform and the surface is smooth.The number of substituent groups attached to aromatic rings is nearly unchanged,while the chain length is much shorter compared with raw coals,which might be related to the dissolution and cleavage of long-chain arenes.Bridged linkages are mainly connected with aromatic carbons rather than aliphatic carbons in TD residues.ML and SSBC were sequentially extracted under ultrasonication with petroleum ether,carbon disulfide,methanol,acetone,and isometric carbon disulfide/acetone mixed solvent to afford extracts(E1–E5)and extraction residues(ERML and ERSSBC).The extracts consist of soluble micromolecules which are free or bound in coal matrixs by weak non-covalent bonds,and the dissociation of the weak non-covalent bonds leads to the release of soluble micromolecules bound in coal matrixs.The extraction yields of ML and SSBC are 4.8% and 4.1% with the highest yield in E3.E1 and E2 are lower in yield and higher in GC/MS-detectable species number,in which alkanes,steroids,and arenes are predominant.The biomarkers,alkanes and steroids,provided important information on the main origin of organic matters in coals.Nitrogen-containing and oxygen-containing organic compounds mainly appeared in E3,and arenes with larger aromatic rings are mainly enriched in E4 and E5.The nitrogen exists in various forms,and the sulfur mainly appears in thiophene rings.According to analysis with electrospray ionization Fourier transform ion cyclotron resonance mass spectrometer,ultrasonication extraction can destroy hydrogen bonds and p-p interactions to dissociate compounds with higher molecular weight,as well as dissolve compounds with smaller molecular weight.O2-O4 class species are dominant in Ox class species with double bond equivalent value = 5-17 and carbon number = 14-42.The hydrogen bonds between heteroatoms in NxOx class species are complicated,which might form “dimer structure”,resulting in a larger double bond equivalent value(7-31)and carbon number(13-65).The existing forms of oxygen are furan rings,R-O-R',RCOOR',and RCOR'(R and R' denote alkyl or aryl),and the nitrogen mainly exists in benzoquinoline,acridine,pyridine,or indole rings and amino groups.ERML and ERSSBC were subjected to sequential TD at 100,150,200,250,and 300 oC with cyclohexane,methanol,and ethanol to afford soluble potions(SP1-SP3).Raising temperature and choosing nucleophilic solvent can improve the yield of TD.According to the results,cyclohexane can dissociate or depolymerize the most organic matters in ERSSBC,while the highest yield of ERML achieved in SP3.The total TD yields of ERML and ERSSBC are 37.7% and 33.1%,respectively.The main soluble portions from TD are arenes,arenols,and esters,which could result from the dissociation of intermolecular interactions between soluble portion and ER or the depolymerization of weak covalent bonds in ER through different reactions.Methylarenes could be generated from the cleavage of Ca–C? bonds in longer-chain alkylarenes.The breakage of ether bonds leads to the formation of arenols,and esters could originate from esterification and transesterification.Noteworthily,most of esters are methyl esters and aromatic esters and produced at lower TD temperatures in SP2,while in SP3 ethyl esters and aliphatic esters are dominant in esters which are obtained at higher TD temperatures.The different distribution of esters in SP2 and SP3 may be related to the difference in steric hindrance and nucleophilicity of methanol and ethanol and dissociation energy of different oxygen-containing bridged linkages,resulting in different pathways.According to the breakage of oxygen-containing bridged bonds in benzyloxybenzene and dibenzyl ether,it could be concluded that thermal effect leads to the cleavage of oxygen-containing bridged bonds during TD with cyclohexane,and both thermal effect and nucleophilicity of methanol will facilitate the breakage of oxygen-containing bridged bonds during TD in methanol,resulting in the difference in product distributions.During TD with cyclohexane,dibenzyl ether was converted into toluene and benzaldehyde.When using alcohol solvents,the oxygen atom in alcohols can attack the aliphatic carbon connected to oxygen atoms in oxygen-containing bridged bonds with higher electropositivity,leading to the cleavage of oxygen-containing bridged bonds and formation of arenols and esters.Raw coals,extraction residues,and TD residues of ML and SSBC were subjected to RICO.According to the RICO results,arenes with alkyl side chains can be dissociated and depolymerized into extracts and SP,especially after TD,the length of side chains is significantly reduced.-(CH2)2-is the most abundant methylene bridged linkages connecting aromatic rings.After extraction and TD,the length of methylene bridged linkages is also significantly reduced,resulting in an evident increase in yield of biaryls.Compared with extraction,TD can dissolve a(11)w-diarylalkanes efficiently.Besides condensed arenes,biaryls are rich in coal structures.