Construction and Evaluation of a Medium-Rank Coal Molecular Model Using a Hybrid Experimental–Simulation–Theoretical Method

To aid the in-depth understanding of the coal structure and its mechanism of reactivity and adsorption in medium-rank coal, a plausible molecular structural model for TunLan no.2 coal in China was constructed by combining corrected solid-state 13C-nuclear magnetic resonance spectroscopy (13C NMR), Fourier transform infrared spectrometer (FTIR), X-ray photoelectron spectroscopy (XPS) and ultimate analysis using a hybrid experimental–simulation–theoretical method. The aforementioned methods yield a molecular model formula of C158H110O8N2S with a weight of 2196.63. The aromaticity of the constructed model matches well with that determined by corrected 13C NMR (0.81) and FTIR (0.80). To further validate the molecular model, we computed its quantum chemical properties. The cleavage sequences of various model bonds agree reasonably well with the study results obtained by thermogravimetric mass spectrometry (TG/MS). By adding periodic boundary conditions, the average density of the simulated model (1.32 g/cm3) is compared with the true relative density (1.34–1.39 g/cm3) probed by experiments; the calculated value is slightly smaller because the model construction does not consider the presence of minerals or small molecules. Despite some unavoidable defects, the comparison between the simulated and experimental–theoretical results validates the molecular model and lays a solid foundation for an in-depth study of medium-rank coal structure and its reactivity and the coalbed-methane adsorption mechanisms.