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

Abstract

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, inversion recovery cross-polarization, INEPT, 13C−13C proton-driven magnetization exchange, and 13C−13C double-quantum−single-quantum 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 cross-linked by domains containing short keto-aliphatic chains instead of otherwise expected graphene-type sheets, as mainly reported either for hydrothermal carbon spheres or for biomass-related carbons obtained by low-temperature (<350 °C) pyrolysis treatment.

Deactivation of sulfonated hydrothermal carbons in the presence of alcohols: Evidences for sulfonic esters formation

“…The spectra of the as-prepared samples ( Figure 3) are in good agreement with the data reported in the literature [40,41]. Three distinct zones can be observed in the spectrum.…”

“…The high intensity of the aliphatic band demonstrates de presence of incompletely carbonized material, in agreement with the rather high oxygen and hydrogen contents. The aromatic groups in the hydrothermal carbon are mainly furans, as demonstrated by the band at 140-160 ppm, either with directly bound furan rings (bifuryl moieties) or with aliphatic or carbonyl spacers [40,41]. Simple sulfonation should not significantly modify this spectrum but it is not the case as shown in Figure 3.…”

Morphological and structural differences between glucose, cellulose and lignocellulosic biomass derived hydrothermal carbons

“…Then, an analysis of the HTC yield and elemental composition of the hydrothermal carbon obtained from these three different precursors will be presented. Finally, we will discuss the results of 13 C solid state NMR experiments, which have already proved to be extremely helpful to determine the final chemical structure of our HTC materials [13] [14].…”

“…Glucose especially, being one of the cheapest and most abundant carbohydrates, has been extensively studied. A detailed investigation of its transformation mechanism into HTC carbon has been carried out by using advanced 13 C solid-state NMR experiments [14]. This characterisation technique has proved to be a very powerful tool for the analysis of carbon materials [15], revealing important hints regarding the final structure of the HTC materials.…”

Chemical, structural and energy properties of hydrochars from microwave-assisted hydrothermal carbonization of glucose

“…S1 (Supplementary information) provided information about its chemical composition, and also confirmed the results from the FT-IR. The peaks between 14 and 60 ppm are due to the presence of aliphatic carbons [8,35], while those between 100 and 160 ppm usually referred to as the aromatic region are all due to C=C bond, but between 140 and 160 ppm are specifically due to the oxygen bound O-C=C (O-aryl) [35]. The peaks between 170 and 200 ppm are due to the presence of carboxylic acid, aldehydes or ketones moieties [36].…”