Hydrochar structural determination from artifact-free Raman analysis

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 furanrich 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 tempera ture range between 300 and 500 degrees 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 degrees 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. (C) 2020 Elsevier Ltd. All rights reserved.