The molecular structure of Inner Mongolia lignite utilizing XRD, solid state 13C NMR, HRTEM and XPS techniques

The chemical-structural properties of Inner Mongolia lignite were investigated by utilizing X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectrum (XPS) and solid-state 13C nuclear magnetic resonance spectrum (NMR). The aromaticity of the lignite coal determined by XRD was 0.7 and compared well with the 13C NMR result of 0.64. The structural parameters from XRD result indicated that lignite coal was poorly organized. HRTEM aromatic fringe image analysis revealed that Inner Mongolia lignite coal contained 31.0% (<5.9 Å), 37.5% (5.9-9.9 Å) and 31.5% (10-24.9 Å) fringes. The oxygen functional groups, detected from XPS fitting analysis were consisted of 38.06% OH, 16.96% CO, 26.93% CO and 18.05% COOR. Similarly, ketone (200-240 ppm), carboxyl (175-200 ppm), methoxyl (45-60 ppm) and O-substituted alkyl (60-100 ppm) were found with the relative contents of 0.078, 0.066 and 0.13 NMR results. The molecular model of the lignite was constructed on the basis of HRTEM (aromatic fraction) and XPS (oxygen functional groups). This molecule formula is C166H130O49. Oxygen in the molecule mainly exists in the form of phenolic/alcoholic hydroxyl oxygen, ether oxygen, carbonyl oxygen and ester oxygen. In addition, there is a good agreement in the peak ranges and compositions through comparing the simulated infrared spectrum of the lignite coal molecular model and the measured infrared spectrum. This molecular model will be used to investigate the lignite flotation mechanism and beneficial for improving its flotation efficiency.