High-Level Computer Molecular Modeling for Low-Rank Coal Containing Metal Complexes and Iron-Catalyzed Steam Gasification

Low-rank coal is a complex mixture; consequently, it is necessary to develop simpler molecular representations for computational modeling. Our modeling objective has been to develop molecules suitable for semi-empirical (SE) computations of low-rank coal containing transition-metal complexes. These molecular models contain oxygen functional groups that are macro-ligands, forming coordination complexes with specific three-dimentional (3D) orientations; consequently, we develop models that encapsulate the properties of low-rank coals and can form metal complexes. The large computer resources required for SE calculations of these molecules limited their size; of the models examined, those containing numerous short links between phenyl groups caused excessive strain and were unsuitable to model transition-metal complexes. Computations (SE) of our models provided data on (i) hydrogen bonds of coal containing water, (ii) formation of aqua−inorganic species and transition-metal complexes, (iii) pyrolysis chemistry involving transformations of metal hydroxide/oxides, (iv) routes for H₂ and CO formation, and (v) mechanism of iron-catalyzed steam gasification. Our char models, on the basis of transformations of the coal model, were consistent with low-temperature pyrolysis; these were disordered structures with some phenyl groups spaced between 0.35 and 0.4 nm. Smaller models of char and chars containing transition-metal clusters were optimized with SE and density functional theory (DFT) computations; these models were useful in modeling the mechanism of catalytic steam gasification. Our modeling of the mechanisms of iron-catalyzed steam gasification was consistent with experimental data