## 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
Note that the spectrum obtained with t\({}_{\rm c}\) = 50 \(\upmu\)s (Figure 5a) shows only cross peaks if the \({}^{13}\)C is in close proximity \({}^{1}\)H. In the most cases this is the case if the two nuclei are directly bound. Figure 4: Chemical structures (1–18) that are potentially present in the pyrochar produced from cellulose pyrolyzed at 350 \({}^{\circ}\)C for 1 h. Their \({}^{13}\)C chemical shifts are given in blue and their \({}^{1}\)H chemical shifts are indicated in orange. Potential cross polarization of unprotonated carbons by neighboring protons of adjunct carbons are indicated by orange arrows. The chemical shift assignments were obtained from literature values [23; 36; 45; 46; 47; 48] and [[https://www.chemicalbook.com](https://www.chemicalbook.com)]([https://www.chemicalbook.com](https://www.chemicalbook.com)) (accessed on 20 January 2021). Cross peaks are visible in the chemical shift region of \(O\)-alkyl C/H (\(\delta\)C/\(\delta\)H 77 to 64 ppm/3 to 5 ppm). The \({}^{13}\)C signals around 75 ppm were previously interpreted as deriving from residual or \"final\" carbohydrates. The latter were suggested to be intermediates, formed during heating up to 325 \({}^{\circ}\)C and converting into aromatic structures at higher temperatures [39]. However, the HETCOR spectrum in Figure 5a reveals no signal assignable to anomeric C (\(\delta\)C/\(\delta\)H 95 to 110 ppm/3.5 to 4.5 ppm) of this intermediate or of any other carbohydrate. This suggests that if such intermediates were formed in our experiment they did not survive heating up to 350 \({}^{\circ}\)C. The missing of intensity in this region indicates further that in contrast to common interpretation, the signal in the chemical shift region of \(O\)-alkyl C in spectra of biochars may derive from alcohols or ether rather than from residual  
Figure 5: Solid-state 2D \({}^{1}\)H-\({}^{13}\)C HETCOR NMR spectra from cellulose charred at 350 \({}^{\circ}\)C for 1 h obtained with \(\mathrm{t_{c}}=\) 50 \(\upmu\)s (**a**), 200 \(\upmu\)s (**b**) and 1000 \(\upmu\)s (**c**). Lines indicate the chemical shifts of selected the cross peaks. \({}^{13}\)C and \({}^{1}\)H chemical shifts of cross peaks are given in blue and orange, respectively. labile carbohydrate residues. Thus, we may conclude that already at 350 \({}^{\circ}\)C almost all carbohydrate units have been transformed either into aromatic moieties and alkyl residues or were degraded and released as volatile compounds. Typical chemical shifts for aliphatic alcohols would occur around \(\delta\)C/\(\delta\)H 90 to 50 ppm/3.5 to 5.5 ppm. Interpretation of \({}^{13}\)C NMR spectra of soil organic matter, compost, or biochar often neglects that in addition to aromatic C, the region between 160 and 110 ppm covers resonances of olefin C in alkenes. In the present sample, they may have been formed during heat-induced partial degradation of cellulose units. Their \({}^{1}\)Hs resonates between 6.5 and 4.5 ppm and can explain the cross peaks in the respective area in the 2D spectrum in Figure 5a. Indications for pyrochar constituents other than phenyl units are revealed by the \({}^{13}\)C signal appearing between 120 and 110 ppm at 8C/\(\delta\)H 111 pm/6.3 ppm which is most likely caused by protonated C3 in furans (Figure 4, structures 4-6) [23]. For benzofurans, C6 can contribute to 8C/\(\delta\)H 111 ppm /7.4 ppm (Figure 4, C6 in structure 3). In the following, we will have a closer look for indications that may support the aromatization during cellulose pyrolysis via the HMF pathway suggested for glucose [43]. HMF (Figure 4, structure 6) gives \({}^{13}\)C signals at 178 ppm (C6), 152 ppm (C1), 124 ppm (C2), 110 ppm (C3), 162 ppm (C4), 57 ppm (C5) ([[https://www.chemicalbook.com/SpectrumEN_67-47-0_13](https://www.chemicalbook.com/SpectrumEN_67-47-0_13) CNMR.htm]([https://www.chemicalbook.com/SpectrumEN_67-47-0_13](https://www.chemicalbook.com/SpectrumEN_67-47-0_13) CNMR.htm), accessed on 10 January 2021).