*3.7. <sup>31</sup>P NMR Spectroscopy of Lignins*

Figure 5 shows the <sup>31</sup>P NMR spectra of the phosphitylated MWL and PFL, and Table 4 shows the calculated distribution of the different OH groups. According to the bibliography [25–27], the signals can be classified according to the different resonances of each type of the phosphitylated hydroxyl groups present in the molecule (Figure 5). Thus, the signals corresponding to aliphatic OH (150.6–145.2 ppm) of the PFL decreased very significantly (61%) after the organosolv treatment. Several mechanisms of esterification (formylation) and acid-catalyzed dehydration can explain this decrease [6,7,41]. The signals produced by G and S-type units were much broader in the PFL due to recondensation, as was also reflected in the HSQC spectra. In fact, the area of signals increased almost four times after the formosolv treatment due to the presences of 5-substituted structures which include syringyl, and β-5′ , 4-O-5′ and 5-5′ linked units. The signals due to G-units also showed condensation processes since the spectrum reflected similar qualitatively similar characteristics to those of the S-units. At 135.8 ppm a signal, absent in MWL, appeared due to the formic acid carboxyl group [42]. The spectrum area corresponding to the carboxyl groups (136.3–134.21 ppm) revealed more intense signals in PFL. Table 4 reflects the content of the different hydroxyl, in mmol OH/mg lignin, depending on the content of the internal standard (cholesterol) and according to the methods already published [27].

β ′ ′ ′

**Figure 5.** <sup>31</sup>P NMR spectra of MWL and PFL.
