Molecular Compositional Features And Mild Conversion Of Organic Matter In Zhaotong Lignite

Lignites are abundant coal resources in China,accounting for ca.13%,but they are low-quality fuel because of their high moisture content,low calorific value,and easy spontaneous combustion.On the other hand,many chemicals,especially oxygen-containing chemicals,could be obtained from lignites due to their high oxygen content and volatile matter.Therefore,it is necessary to develop non-fuel use technology for realizing value-added utilization of lignites.The key issue is to understand molecular compositional features and mild conversion mechanisms.In this thesis,a three-step depolymerization method,including ethanolysis,NaOH/ethanolysis,and oxidation with NaClO,combined with ruthenium ion-catalyzed oxidation(RICO),pyrolysis,and modern analytical instruments were used to investigate molecular compositional features of organic matter in ZL and possibility of mild conversion.In addition,ethanol-soluble portion(ESP)was subjected to column chromatographic separation and lignite-related hydrogen bond interaction and alkanolysis mechanism were calculated using density functional theory(DFT).Carbon skeleton structure and elemental composition on the surface of ZL were examined with solid-state 13 C NMR and XPS.The results show that carbon skeleton of ZL mainly consists of aliphatic(52.3%)and aromatic(42.2%)carbons and each aromatic cluster contains 2 rings on average.Oxygen and nitrogen atoms on the surface of ZL mainly exist in C–O,COO,C=O,pyrrole,quaternary nitrogen,and amino moieties.Organic matter in ZL was completely converted and soluble portions with high yields of 64.9%,28.3%,and 15.0% were obtained by the three-step depolymerization method.Noncovalent bonds and weak covalent bonds were destroyed via ethanolysis and relatively strong covalent bonds were cleaved via NaOH/ethanolysis,while C–C bridged bonds and aromatic rings were destructed via oxidation with NaClO.In total,812 organic compounds were detected in ESP with GC×GC/TOF-MS,and they can be classified into alkanes,arenes,ethers,alcohols,ketones,aldehydes,esters,carboxylic acids,nitrogen-containing species,sulfur-containing species,and others,among which many species were not reported previously and the most abundant compounds are alcohols and esters.Thousands of heteroatom-containing species were identified with ESI FTICRMS,including acidic oxygen compounds,neutral nitrogen compounds,and basic nitrogen compounds.Their possible structures were proposed based on double bond equivalent and carbon number.The number of species in soluble portions obtained from NaOH/ethanolysis is much less than that in ESP.Products obtained from oxidation with NaClO are dominated by benzenecarboxylic acids and no chloro-substituted benzenecarboxylic acids were detected,suggesting that residue from NaOH/ethanolysis(ER')is suitable for preparing benzenecarboxylic acids.According to the distributions of carboxylic acids from RICO of ZL,ethanolyzed residue(ER),and ER',bridged bonds,side chains on aromatic rings,and condensed aromatic rings can be inferred.ZL,ER,and ER' are rich in –CH2CH2– and –CH2CH2CH2– bridged linkages connecting aromatic rings.Comparing with ZL,the length of alkylene bridges in ER and ER' is relatively short,which could be ascribed that long alkylene bridges were cleaved via ethanolysis.According to the distributions of benzenecarboxylic acids,ZL has relatively high condensation degree than ER and ER',which is in agreement with the results of solid-state 13 C NMR.At the same temperature,mass losses decrease in the order: ZL > ER > ER'.Significant differences in the distributions of the volatile species from pyrolysis of ZL,ER,and ER' at 500 oC were observed.Phenols,nitrogen-containing compounds,and alkanes are the most abundant group components from pyrolysis of ZL,ER,and ER',respectively.ESP was sequentially eluted with petroleum ether,ethyl acetate/petroleum ether mixed solvents with increased ethyl acetate to petroleum ether ratio,ethyl acetate,methanol/ethyl acetate mixed solvents with increased methanol to ethyl acetate ratio,and methanol.A series of alkanes,arenes,ethyl alkanoates,phenols,phenylmethanols,and nitrogen-containing compounds were enriched to large extents.Phenol…phenol(PP),phenol…benzene(PB),phenol…oxydibenzene(PO),phenol…quinoline(PQ),benzoic acid…benzoic acid(BB),and cyclic system(CS)were selected as lignite-related model compounds to investigate lignite-related hydrogen bond interaction using DFT.The results exhibit that the strength of hydrogen bonds in different complexes increases in the order: PB < PO < PP < PQ < CS < BB.Alkanolysis simulation of lignite-related model compounds(LRMCs)demonstrates that alkanolysis includes nucleophilic attack,hydrogen transfer,and bond cleavage.Under the same conditions,the reactivities of LRMCs toward alkanolysis decrease in the order: benzyloxybenzene > anisole > phenethoxybenzene > oxydibenzene.For the same LRMC,the activation energy of ethanolysis is much lower than that of methanolysis,i.e.,ethanolysis proceeds much easier than methanolysis.