Analyses Of Oxygen-and Nitrogen-containing Compounds In Lignites And Hydrogen Bonding Interactions Between Their Model Compounds And Water Molecules

Hydrogen bonding interactions between organic oxygen/organic nitrogen in lignite and water molecules result in that lignite is rich in the "molecular water",and the molecular waters is difficult to remove which limit the efficient utilization of lignite.In this work,the occurrence forms of organic oxygen and nitrogen in two lignites,i.e.Huozhou?HZ?lignite from Shanxi province and Xinghe?XH?lignite from Inner Mongolia Autonomous Region,and their hydrogen bonding interactions with water were investigated using the method of experiment combined with theoretical calculation.Firstly,the lignites were fractionally extracted with petroleum ether,carbon disulfide,methanol,acetone and isometric disulfide/acetone mixed solvents to dissolve the soluble small molecular compounds in the organic species of the lignites affording extracts,then the extraction residues were thermal dissolved in toluene,methanol,acetone and isometric toluene/methanol mixed solvents?ITMS?at 320 ? to dissolve the organic macromolecule clusters and the components combined with weak oxygen bridged bonds in the organic species of the lignites affording to soluble portions?SPs?.The contents and occurrence forms of organic oxygen and nitrogen in raw coals,extraction residues and thermal dissolution residues were characterized with elemental analyzer,Fourier transform infrared spectroscopy?FT-IR?,X-ray photoelectron spectroscopy?XPS?and thermogravimetry?TGA?,and the composition and structural characteristics of oxygen-and nitrogen-containing organic compounds in extracts and SPs were identified with FTIR and gas chromatography/mass spectrometry detector?GC/MS?.Base on the results from the experiments,a series of related organic oxygen and nitrogen model compounds in the lignites were selected or designed,then the hydrogen bonding interactions between model compounds and water molecules were investigated through quantum chemistry calculation in combination with density functional theory?DFT?and atom in molecules theory?AIM?to reveal the interaction mechanism between water molecules and organic oxygen/organic nitrogen in the lignites,and the existence form of the moisture in the lignites.The total extract yields of HZ and XH in fractional extraction are 7.03% and 7.86%,respectively,and the extract yields of HZ and XH in methanol are as high as 3.53% and 4.17%,which possibly is ascribed to that the hydrogen bonds formed between methanol and small molecules soluble in lignite weakened or destroyed the original hydrogen bonds between the small molecules soluble compounds and the organic macromolecular skeleton of lignite.There exist obvious synergic effects between toluene and methanol during thermal dissolution,therefore the SPs yields of the extraction residues from HZ and XH in ITMS are as high as 45.76% and 40.14%,respectively.GC/MS analyses show that the oxygen-containing organic compounds?OCOCs?in the extracts are dominated with ketones,ethers,alcohols and phenols,while the OCOCs in the SPs are mainly composed of phenols,ketones,esters and alcohols.The nitrogen-containing organic compounds?NCOCs?in the extracts are dominated with amines,pyridines,nitriles and nitro compounds,while the NCOCs in the SPs are mainly composed of amines,pyrazoles,imidazoles and pyridines.The intensities of the stretching vibration peaks from association hydroxyl,C=O and C-O-C groups in the FTIR spectra of extraction residues and thermal dissolution residues are dramatic weaker than those of raw coals.XPS analyses show that there are three forms of organic oxygen in the two lignites,including carbonyl?C=O?,carbon oxygen single bond?C-O?and carboxyl?COO-?,and the organic nitrogen mainly exist in the forms of pyridine nitrogen?N-6?,amine nitrogen?N-NH₃?,pyrrole nitrogen?N-5?,quaternary nitrogen?N-Q?and pyridine oxide?N-6-O?.Compared to raw coals,extraction residues and thermal dissolution residues contain lower of C-O content,and higher N-5 and N-6 contents,and N-6-O was undetected in the thermal dissolution residues.Additionally,the lowest oxygen contents in the thermal dissolution residues of HZ and XH are 10.10 % and 11.30 %,respectively,which are obvious lower than those of raw coals indicating that organic oxygen compounds in lignite can be easily dissolved out during thermal dissolution.Nevertheless,the nitrogen contents of thermal dissolution residues are similar to those of raw coals suggesting that the organic nitrogen compounds in lignite are mainly constituted with condensed nitrogen heterocycles and difficultly are dissolved during thermal dissolution.Phenol was selected as the model compound of phenols in lignite to calculate its hydrogen bonding interactions with water molecules at B3LYP/6-311++G** level.The results indicate that phenol can form a variety of structures of phenol-?H₂O?n?n=1-3?complexs,and the most stable phenol-water complex is the complex with a water chain configuration which formed from a O-H···O hydrogen bond and terminated with a O···H-O hydrogen bond on the phenolic-hydroxyl of phenol.The hydrogen bonding interactions between 24 oxygen-containing functional groups/14 nitrogen-containing functional groups with water molecule were calculated at the M06-2X/6-311+G** level.The results indicate that the order of combining ability for the oxygen-containing functional groups are carboxyl > ester > phenolic hydroxyl > carbonyl alcohol > alkyl > ether > benzene ? electron cloud,and that for the different nitrogen-containing functional groups are amide > pyridine > pyridine > quinolone > aniline > carbazole > pyrrole > benzonitrile.Additionally,the synergic effects among the different functional groups enhanced their hydrogen bonding interactions with water molecules.The hydrogen bonding interactions between the lignite basic structure unit model of organic oxygen?UMO?/the lignite basic structure unit model of organic nitrogen?UMN?and water molecules were calculated at the M06-2X/6-31+G* level.The results show that these structure unit models can form a variety of UMO-?H₂O?n?n=1-16?and UMN-?H₂O?n?n=1-12?complexes with water molecules.The water molecules formed on the UMO and UMN are mainly dominated by water-cage clusters and water-film clusters respectively,and four ring configuration is the dominated existence form among water clusters structure.