Structural analysis of hydrothermal char and its models by density functional theory simulation of vibrational spectroscopy

Density Functional Theory (DFT) and experimental measurements were used to develop a systematic method for interpreting the Raman spectra of hydrothermal char (hydrochar). Average band locations, relative intensities, and their trends relative to structural features were determined for the G, D, and Kekulé bands. When combined with several other less prominent vibrational modes, including vibrations associated with aromatic, ether, alkyl, and carbonyl bonds, the calculated average locations reproduced all major features of hydrochar Raman spectra. Two model structures were found that could reproduce the main features of the hydrochar Raman spectrum and its elemental analysis: 1) a structure consisting of arene domains comprised of 6-8 rings connected via aliphatic chains or 2) a furan/arene structure consisting primarily of single furans and 2 or 3ring arenes. NMR confirmed that the furan/arene ratio of glucose hydrochar is approximately 1:1, consistent with the furan/arene structure supported by Raman spectra. This work establishes an interpretation method for hydrochar Raman spectra and reconciles the main features of hydrochar structures determined using Raman spectroscopy with those based on other methods.

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