The Interactions Between Lignite Model Compound And Water Molecules&Pyrolysis Properties Of Lignite: A Theoretical Study

Lignite is an abundant fossil resource and will play an important role in meetingthe world’s energy needs in the future. It has been found that lignite has low calorificvalues due primarily to high oxygen and moisture contents. Naturally lignite has acomplicated macromolecular network structure consisting of different size organicstructural units linked by covalent and noncovalent bonds. The molecular structuresand physical-chemical properties are still less known at present. Quantum chemicalcalculations as an important tool at the molecular level can provide useful insight intocharacterizations of the structure and properties of coal. To investigate the detailedwater adsorption properties and pyrolysis behavior of lignite, we perform a series ofdensity functional calculations on lignite-related model compounds. The main resultsare summarized as follows:(1) The interaction energy exists differences between water molecule and variousfunctional groups in lignite, the sequence is carboxyl> phenolic hydroxyl>carbonyl>benzene. The total hydrogen-bonded energy significantly enhances with the increaseof the number of water molecules because hydrogen bond interacts cooperatively. Thewater molecules would prefer to form small clusters in the surface of lignite. Thefeatures of water clusters are clearly different from pure water clusters, e.g. the waterclusters (n≥4) tend to form quadrilateral rings structure.(2) Theoretical investigation of effects of confined environment on the properties ofwater molecules, based on single-walled carbon nanotubes (SWCNT) as simplemodel compound. The results indicate that the geometries of confined watermolecules changed significantly due to the confinement effect of SWCNT comparedto water molecules in vacuum, for example, the (H2O)6cluster can form chain-likeconfiguration via hydrogen bond. The weak interactions mainly include H···πhydrogen bond interaction and O···π orbital interaction between confined watermolecules and SWCNT, resulting in the decrease of the O-H electron density ρ. Itgives rise to the red shift of the majority of the O-H stretching modes inside SWCNTcompared to vacuum vibrational frequencies. Furthermore, another phenomenon isthat a slight charge transfers from confiend water molecules to SWCNT.(3) The possible pyrolysis channel of the different kinds of oxygen functional groupshave been studied theoretically. The present results indicate that the way ofdehydration and reaction activation energy exist differences for oxygen functional groups, even in the same cross-linking reaction, there are more than one reactionchannel. The four-membered ring or six-membered ring structures are typicallytransition state structures for all reaction paths. Meanwhile, the activation energy canbe reduced for the dehydration reaction of oxygen functional groups involving inwater molecules as the proton transfer. Besides, addition of metal ions would be favorof this kind of reaction.(4) The cleavage properties of the different types of bridge bond in lignite modelcompounds have been explored by theoretical calculation, and exploring substituenteffect on bond dissociation enthalpies (BDEs). For73lignite model compounds, theaverage BDEs are51.0kcal/mol,66.1kcal/mol and66.2kcal/mol for the α-O-4,β-O-4and α-β types of model compounds, respectively. The C-O BDEs will decreasewhen the adjacent arene rings are substituted by electron donating groups (-OH,-OCH3and-CH3), while the results are opposite for the electron withdrawing groupssuch as carboxyl group. Substituent effects on C-C BDEs are consistent with C-OBDEs.