(731h) Structural Characterization and Molecular Representation of Refined Unburned Carbon Recovered from Fly Ash

In order to improve and efficient utilization of fly ash derived from coal-based power plants, a structural study of refined unburned carbon (RUC) recovered from fly ash was carried out with multiple analytical tools, including X-ray diffraction (XRD), Raman spectroscopy, solid-state ¹³C nuclear magnetic resonance (¹³C-NMR) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Laser desorption ionization time-of-flight mass spectroscopy (LD-TOF-MS), and high-resolution transmission electron microscopy (HRTEM). These analytical data was used to construct a molecular representation of GUC by using a built-in Amorphous Builder module in Materials Studio.

In the present study, GUC was recovered by a two-step process including refining of fly ash by demineralization with HF/HCl/HF process and then graphitization and crystallization by heat treatment up to 2500 ^oC.

The crystalline dimensions such as interlayer spacing (d₀₀₂), stacking height (L_a), and average lateral size (Lc) of GUC was calculated from XRD spectrum and found to be 0.338, 19.60, and 13.15 nm respectively. Moreover, the increase in peak intensity of D and G band was clearly observed during the heat treatment for carbon graphitization. The aromaticity (f_a) of RUC was found in the range of 0.45-0.51 by the multiple techniques. The relative intensity ratio of D and G band of GUC was 0.47.

The –OH, aliphatic and aromatic C–H, C=O, C-C, and C-O functional groups was clearly observed in FTIR and XPS spectrums. The quantitative analysis of carbon and oxygen functionalities such as –OH, aliphatic and aromatic C–H, C=O, C-C, C-O groups was found to be 12.1, 38.7, 10.4, 28.5, and 10.3% respectively from C1s and O1s XPS profile. The molar content of different carbon types such as aliphatic, aromatic, and carbonyl carbons of GUC was 37.5, 57.1, and 5.1% calculated from ¹³C-NMR spectrum. Four aromatic rings per clusters with 3-4 substitute side chains on each aromatic cluster was calculated from ¹³C-NMR spectrum. The aromatic layers was clearly observed in the HRTEM image in the form of fringes with a consistent crystalline dimension from XRD.

250 aromatic fringes was selected from the HRTEM image, and the heteroatoms and aliphatic side chains obtained from different analytical tools was used to construct 250 individual RUC fragments. A random cross-linking of individual fragments with different combinations of aromatic, aliphatic, and oxygen linkages till observing a consistent distribution of molecular weight observed in LD-TOF MS. A final molecular model was presented with 52 unique individual molecules with a molecular formula C₇₇₅₄H₄₅₇₆O₁₀₉N₄₈S₃₂. The structural and molecular properties obtained from analytical tools are consistent with the constructed model. This is the first molecular representation of unburned carbon recovered from fly ash ever proposed, which may lead to further understand the unburned carbon from fly ash and improve their utilization.

References:

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