Fractional Extraction And Thermal Dissolution Of Xiaolongtan And Shengli Lignites

To effectively isolate and concentrate the organic species (OSs) of Shengli (SL) andXiaolongtan (XLT) lignites, direct thermal dissolution (TD) of lignites, and fractionalextraction (FE) of lignites and sequential TD of FE residues were conducted. Then the extracts,thermal soluble fractions (SFs) and residue were characterized with many modern instruments.Basing on the dissolution profile of OSs, the TD mechanism in different conditions andcomposition and structural characteristics of the two lignites was discussed.Direct TD experiments shows that SFsyield increases with temperature in the same solvent,and SFsyields in various solvents are different. The OSsin lignites have high yield in alcoholsand acetone, followed by EA, and their yields in benzene are the lowest. Extracts yields of thetwo lignites are obvious lower than the SFsyields, while FE have obvious swelling effect onthe macromolecular network structure of lignites.Carbon disulfide (CDS) and benzene show good dissolution effect on low polar componentsin lignites, such as alkanes, aromatic hydrocarbons, steroids and terpenoids, etc. Ethyl acetate(EA) can react with the covalent bonds in lignites through transesterification and alcoholysis,therefore the SFsin EA are rich in alkanes and esters. During TD, the condensation reaction ofacetone catalyzed by mineral matters in lignites influence the SFsyields and their compostion.However, many nitrogen-containing organic compounds (NCOCs) are isolated due to theforming of N-H…O hydrogen bonds between acetone and NCOCs. Alcohols can promote thedissolution of OSsfrom lignites through alcoholysis, alkylation and ester exchange reactionsduring TD. Additionally, ethers and esters are the main compounds in the tetrahydrofuran (THF)extracts.The whole FE and sequential TD process can be divided into three stages. The first stageincluding FE and TD below240oC is physical extraction process, and the main components inthe soluble OSsare the free compounds wraped in the macromolecular network structure oflignites, and the second stage (240-270oC) is supercritical fluid extraction process. However,the SFsin the third stage (above300oC) originate from pyrolysis, supercritical fluid extractionand the reactions of solvent with OSsin lignite.The molecular weight (MW) of the extracts and SFsfrom two lignites distributes in80-600.Cadalene and n-alkanes continuously from C13to C29are detected in both lignites, while theaverage MW of n-alkanes from XLT is larger than those in SL, and the odd-carbon numberpredominance in XLT is more obvious. Moreover, the RC of biomarkers in XLT is higher thanin SL, such as straight-chain alkanes acid methyl esters, cadalene and tocopherol, etc. It ispossibly suggest that the biological origin of both lignites are connected with higher plant, while the maturity of XLT is lower than SL. The main compounds in SFsabove270oC fromboth lignites are alkyl benzene, alkyl naphthalene, phenols, methoxy aromatic compounds and2-isopropyl-1-methoxy-4-methyl-benzene, which indicate that the macromolecular structureand bridged bonds of these lignites are similar. Most of NCOCs and sulfur-containing organiccompounds in the two lignites are nitrogen-containing heterocyclic and benzothiophenecompounds.These results indicate that FE and TD using conventional solvent in mild conditions canisolate and enrich the OSs of lignite effectively, then characterize the composition and structurecharacteristics of soluble OSsat molecular level using modern analysis instruments. Thesefinding are beneficial for identifying the macromolecular sturctral characteristics of lignite andpromoting the clean and effective utilization of lignite resources.