Structural Characterization of Hydrothermal Carbon Spheres by Advanced Solid-State MAS 13 C NMR Investigations

The local structure of carbon spheres obtained via the hydrothermal carbonization process is characterized by using a combination of advanced solid-state 13C NMR techniques. Glucose was chosen as the starting product because it offers the possibility of 13C isotopic enrichment and is regarded as a model compound for more complex polysaccharides and biomass, as reported in recent studies. A number of 13C solid-state MAS NMR techniques (single pulse, cross polarization (CP), inversion recovery cross polarization (IRCP), INEPT, 13C-13C proton-driven magnetization exchange and 13C-13C double quantum-single quantum (DQ-SQ) correlation experiments) were combined to retrieve information about binding motifs and C-C closest neighbor relations. We found that the core of the carbonaceous scaffold is composed of furan rings crosslinked by domains containing short keto-aliphatic chains instead of otherwise expected graphenetype sheets, as mainly reported either for hydrothermal carbon spheres or for biomass-related carbons obtained by low-temperature (< 350°C) pyrolisis treatment.