*3.5. <sup>13</sup>C NMR Spectroscopy*

<sup>13</sup>C NMR spectra of MWL and PFL (Figure 3) provide interesting information about several functional features of MWL and the changes suffered after the organosolv treatment. The assignment of signals was made based on previous published works [32–35].

The spectrum of MWL showed the characteristic peaks assigned to methoxy groups (55.9 ppm), carbonyl carbon in acetates (169.6 ppm), carbons 3 and 5 (C3,5) in etherified syringyl (S) units (152.2 ppm), C<sup>3</sup> in etherified guaiacyl (G) units (149.1 ppm), C<sup>4</sup> in etherified G units and C3,5 in non-etherified S units, and C<sup>4</sup> in S units (138.2 ppm). The aromatic carbons 2, 5 and 6 of the G structures resonated at 111.1, 114.7 and 119.1 ppm, respectively, while the C2,6 in the S units produced a signal around 104 ppm.

Between 70 and 90 ppm, the signals of C<sup>β</sup> in β-O-4′ , and C<sup>α</sup> in β-5 and β-β ′ were overlapped with those belonging to the carbons of the residual carbohydrates, complicating an accurate assignment, but probably the sharpest signal in this area (71.9 ppm) could be due to C<sup>α</sup> of β-O-4′ linkages. Other identifiable peaks were: C<sup>γ</sup> in units with oxidized C<sup>α</sup> (65.6 ppm), C<sup>γ</sup> in substructures β-O-4′ (62.9 ppm), methylene carbons in the α,β positions of the side chain (29.1 ppm), the methyl carbon in acetates (21.1 ppm), and the primary carbon in methyl groups of the propyl chain (15.3 ppm). The absence of typical *p*-hydroxyphenyl group signals (167, 161, 158 ppm) agrees with those found in the literature [36,37] and reflects that the Paulownia hardwood lignin is GS-type [35].

β ′

γ

**Figure 3.** <sup>13</sup>C NMR spectra of milled wood lignin (MWL) and Paulownia formosolv lignin (PFL).

<sup>β</sup> β ′ <sup>α</sup> β β β′ <sup>α</sup> β ′ <sup>γ</sup> <sup>α</sup> <sup>γ</sup> β ′ α β After the formosolv treatment, the lignin recovered from the liquor (PFL) showed important changes compared to MWL. The acetate groups were almost completely eliminated from the structure (absence of signals at 169.6 and 21.1 ppm). At the same time, two intense signals (163.4 and 162.4 ppm), not appearing in the MWL spectrum, reflected the introduction of formate groups into the molecule [38]. The effects of delignification are clearly reflected in <sup>13</sup>C NMR spectrum of PFL. The depolymerization of lignin through the disruption of β-O-4′ bonds and to some extent, β-5′ and β-β ′ , was reflected by the decrease in the intensity of many of the typical signals of these substructures in the 90–52 ppm range: specifically, 71.9, 65.6, and 62.9, whose assignment was mentioned above. Additionally, the solubilization of a large proportion of the remaining carbohydrates in the MWL causes this area of the spectrum to have less signals than MWL. The distribution of aliphatic carbon signals is modified, appearing more and as different types of signals: more methylenic carbons and less methyl groups.

In the region of aromatic carbons (166–102 ppm), the changes produced by delignification were clearly visible. The signals of carbons 2, 5 and 6 in the G-units (111.1, 114.7 and 119.1 ppm) were broader and with a larger area, which is compatible with a partial recondensation of lignin. In addition, the signals C3,5 in the S-units (152.2 ppm) and C<sup>3</sup> in the G-units (149.1 ppm) increased their intensity. However, this increase was more important in the signal due to guaiacyl groups, indicating a greater tendency of these groups to undergo recondensation processes. The integration of the clusters due to the signals of aryl bonds [33] that is C–O (161–141 ppm), C–C (141–124 ppm) and C–H (124–100 ppm) indicated different percentage between the values of MWL and PFL of −2%, 31% and −21%, respectively. These values can be explained by the formation of a higher proportion of C–C links and a loss of C–H, which provide an additional symptom of the existence of lignin condensation reactions.

Table S1 (Supplementary Material) collects the assignments of carbon chemical shifts (δ, ppm) in the <sup>13</sup>C NMR spectrum of MWL and formosolv lignin.
