## 3 Results
### Characterization of Cellulose by 1D and 2D NMR Spectroscopy
#### 3.3.1 General Information Obtained by 1D Solid-State CPMAS \({}^{13}\)C NMR Spectroscopy  
As already expected from the decreasing H/C\({}_{\text{atm}}\) with higher temperature (Table 1), a considerable shift of the chemical composition towards higher aromaticity (160 to 110 ppm) at the expense of \(O\)-alkyl C (110 to 45 ppm) is evidenced from the solid-state CPMAS NMR spectrum of the cellulose (Figure 3a), pyrolyzed at 350 \({}^{\circ}\)C for 1 h.  
The spectrum is coherent with previous spectra obtained from other pyrolyzed cellulose [39] and lignocellulosic residues [23]. Further intensity occurs between 215 and 200 ppm and is assigned to aldehyde/ketone C and in the carboxyl C region at 174 ppm. Note that such signals were not expressed in the spectrum obtained from cellulose charred at 350 \({}^{\circ}\)C under oxic conditions [29] (Figure 3b), confirming that during pyrolysis and oxic combustion different processes occur. Comparably, the considerable intensity in the chemical shift region assigned to \(O\)-alkyl C (90-45 ppm) and alkyl C (45-0 ppm) in the spectrum of the pyrolyzed cellulose is missing in that obtained from the cellulose charred in the presence of oxygen. The resonances in the aromatic C region (160 to 95 ppm) of 1D NMR spectra of bio- or pyrochars are often interpreted to be caused by carbons located in a polycondensed aromatic network [40; 41]. Note that if such aromatic systems are too large, magnetization transfer to their core carbons may be inefficient and the latter may not be detected by the CP technique. However, analyzing the CP dynamics of the present char suggested full CP of all Cs at t\({}_{\text{c}}\) = 1000 \(\upmu\)s (Figure S1; Table S1). Thus, we can assume that almost all if not all aromatic C occurs in small ring systems. Note that the chemical shift of the main signal at 127 ppm could also be explained with protonated phenyl C units [36; 42]. The smaller shoulder, peaking at 153 ppm is in the chemical shift region assigned to _O_-aryl C (160 to 140 ppm). For lignocellulosic feedstocks such as wood and its biochars, this region is often assigned to phenols in lignin residues (Figure 4, structure 1) or to surface aromatic C substituted by a hydroxyl groups or involved in ether bonds (Figure 4, structure 2). In all cases, their protonated adjunct Cs resonance around 110-120 ppm. Of course, for the cellulose studied here, the first explanation isn't appropriate since lignin was not present in the feedstock, but the second could still be valid. Alternatively, C close to oxygen in furan-like intermediates may equally contribute to signals between 140 and 160 ppm (Figure 4, structure 3-11). The neighboring protonated C would give a signal around 100 to 114 ppm. Such compounds were also evidenced during thermal degradation of wood [23]. These furans may be part of hydroxymethylfurfural (HMF) (Figure 4, structure 6), which were suggested to be formed during hydrothermal carbonization of glucose [43], but also as a degradation product of cellulose during pyrolysis [44]. [PERSON] et al. [43] proposed that HMF condensates to polyfuran chains which finally form a condensed aromatic network (Figure 4, structure 7, 8, 12). Figure 3: Solid-state 1D NMR spectra of charred cellulose produced after pyrolysis of 350 \({}^{\circ}\)C (**a**) and incomplete combustion in the presence of oxygen for 4 and 8 min (**b**) [29]. The asterisks indicate spinning side bands, which result from the modulation of the magnetic field at the spinning frequency and contain intensity of the parent signal (in this case the aryl C). 3.2 Solid-State 2D \({}^{1}\)H-\({}^{13}\)C HETCOR NMR Spectra of the Cellulose Derived Pyrochar  
**Alcohols, Furans, Alkene C and Protonated Aromatic C in Cellulose Biochar Produced at 350 \({}^{\circ}\)C:** In order to obtain more insights into the nature of the aromatic network of the pyrochar produced at 350 \({}^{\circ}\)C, the respective solid-state 2D \({}^{1}\)H-\({}^{13}\)C HETCOR NMR spectra are analyzed in more detail (Figure 5).