Hydrochar structural determination from artifact-free Raman analysis

Abstract Hydrochar was studied using Raman spectroscopy to understand thermal damage potentially introduced during analysis. Analysis of glucose hydrochar at power densities less than 5 GW m−2 indicated a furan-rich material, while analysis at power densities greater than 50 GW m−2 indicated an arene-rich structure. Thermal tests using conventional heating revealed furan-to-arene transition in the temperature range between 300 and 500 °C, suggesting that Raman analysis at power densities greater than 5 GW m−2 converts furans to arenes. A heat transfer model predicted temperatures consistent with the transition identified experimentally and indicated pelletizing the hydrochar as an approach for acquisition of artifact-free Raman spectra without sacrificing signal-to-noise. Subsequent experimental analysis of hydrochar-KBr pellets confirmed the potential effectiveness of the approach. The artifact-free Raman spectrum of glucose hydrochar was re-analyzed and found to be much more consistent with structures inferred from solid state nuclear magnetic resonance (NMR) than previously thought. Hydrochar is a thermally sensitive material and should be studied with great care when subjected to temperatures greater than 300 °C or (equivalently) laser powers greater than 5 GW m−2. The approaches described here should be used for future work in order to get a proper description of char structure.