Carbonaceous materials derived from the thermal carbonization of carbohydrates and similar molecules in aqueous solutions under autothermal pressure have been shown to have potential applications as catalysts, energy storage materials, and water purification adsorbents. In the literature, char materials synthesized under liquid water conditions are termed “hydrothermal chars”; relative to pyrolysis-derived biochars, hydrothermal chars less highly aromatic and more highly oxygenated.
Despite on-going effort, improved characterization methods are required to develop rigorous structure-property relationships to guide design of hydrothermal chars for specific applications. Many techniques appear in the literature, ranging from elemental analysis (primarily combustion), various functional group identification (vibrational spectroscopy techniques and NMR), and extended structure characterization (primarily C-XANES). To date, infrared and Raman spectroscopy have been used strictly for functional group identification. However, the specific locations and intensities of vibrational absorption bands can provide a much richer understanding. Fully mining the vibrational data sets is limited by the complexity of the spectra, and this is the challenge that we are attempting to address.
In this work, we have used density functional theory, the available literature of model compounds and carbonaceous solids, and our own vibrational spectroscopy and microscopy for hydrothermal chars as complementary tools for understanding complex vibrational spectra. Specifically, we have modeled the Raman and infrared spectra of polyaromatic hydrocarbons and functionalized aromatic compounds. We can then view the individual vibrational modes of these model compounds to get a better understanding of how complex molecular structure is represented in Raman and IR spectra. Specific emphasis has been placed on understanding the molecular significance of the G and D bands, graphite and defect assignments based on spectroscopy of low-defect graphitic materials, for the poorly ordered and defect-rich hydrothermal char. In simple polyaromatics the wavenumber position of these bands was found to decrease with increase in the number of aromatic rings. The relative intensities of the characteristic peaks in the D and G band bands were found to fluctuate based on the number of rings, and the symmetry of the molecule. That is, whether the molecule consisted of rings arranged in the shape of simple polygons such as pyrene, or merely as a straight chain of rings as in tetracene. For example, molecules consisting of polygonal structures have decreasing wavenumber location of their D band, an A1G ring breathing mode, with increasing molecular size. Similar trends have been discovered for more complex aromatic compounds such as arylfurans and aromatic carboxylic acids. In addition, defects in the aromatic rings have been investigated including methyl and aldehyde side chains. The primary vibrational mode associated with the methyl group appears at 1465 +/- 10 cm-1, and is largely independent of the number of rings in the aromatic structure. In terms of the influence of the methyl group on the characteristic aromatic vibrations, the effect is prominent for condensed aromatic structures with 1, 2, and 3 rings and then decreases for larger aromatic structures. The DFT simulation results were then used to examine experimental data for glucose and xylose hydrothermal chars.