A molecular model for Illinois No. 6 Argonne Premium coal: Moving toward capturing the continuum structure

A large-scale molecular model for Illinois No. 6 Argonne Premium coal is generated based on an automated construction approach in an effort to move toward capturing the continuum structure. The model contains 50,789 atoms within 728 molecules and is the largest, most complex coal representation constructed to-date. The aromatic ring size distribution was based on multiple high-resolution transmission electron microscope (HRTEM) lattice fringe micrographs and was duplicated with automated construction protocols (Fringe3D) in molecular modeling space. Additional structural data was obtained from the abundant literature assessing this Argonne Premium coal. Organic oxygen, nitrogen, and sulfur functionalities were incorporated primarily into the polyaromatic structures according to X-ray photoelectron spectroscopy and X-ray adsorption near-edge structure spectroscopy data. Aliphatic carbons were in the form of cross-links (bridges and loops) and pendant alkyl groups based on the combination of laser desorption ionization mass spectrometry (LDIMS), ruthenium ion catalyzed oxidation, elemental analysis, and NMR data in good agreement with the literature. Bound and bulk water was also included. Construction of the coal molecules was performed by use of Perl scripts developed in Materials Studio to eliminate personal bias and improve the accuracy and the scale of the structure generated. The large-scale model captured a broad and continuous molecular weight distribution in accordance with LDIMS data here ranging from 100 to 2850 Da, enabling inclusion of structural diversity to capture a portion of the continuum structure. A theoretical pyridine extraction yield, determined by a group contribution approach, was in agreement with the experimental value. The extract and residue representations were generated from the large-scale Illinois coal model and showed consistency with NMR, elemental analysis and LDIMS trends. The distribution of heteroatomic classes and double bond equivalents was also well-defined experimentally based on electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. These data further constrain the molecular weight of extractable material and was consistent with limited pyridine extractability and model heteroatom classes. Future work will be well served by staying within the limits established by the approach and increasing the structural diversity (sampling frequency through increased scale) to better capture the complex nature of coal structural diversity, i.e., the continuum.