*3.3. FTIR of Paulownia Wood and Residual Solid*

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FTIR spectroscopy is a fast and simple technique that allows extracting valuable information about the existence of functional groups and their changes during chemical treatments. Figure 2 shows the fingerprint region of the FTIR spectra of Paulowia wood (P), and the solid residue after the formosolv treatment (PFP). The assignment of the bands was made based on data published elsewhere [9,28]. −

**Figure 2.** FTIR spectra of Paulowia wood (P) and Paulownia formosolv pulp (PFP).

Several typical bands of lignin can be clearly seen in the spectra, which involve aromatic skeleton vibrations, which disappeared or were greatly diminished in the residual solid of the formosolv treatment due to extensive delignification. For example: 1594, 1504, 1420 cm−<sup>1</sup> , and the broad

band around 1230 cm−<sup>1</sup> that contains the contributions of vibration modes from G and S units. In addition, the band located around 1650 cm−<sup>1</sup> reflects some loss of C=O conjugate aryl ketones in lignin. The absorption band in the Paulownia wood spectrum at 1730 cm−<sup>1</sup> , attributable to the presence of acetyl groups of the hemicellulose, disappeared after organosolv treatment, indicating the solubilization of the majority of the hemicellulose. In addition, a new absorption band appeared in the spectrum of formosolv pulp, at lower wavelength (1710 cm−<sup>1</sup> ) that is assigned to the incorporation of formate groups to the propyl chain of lignin [11]. Finally, the bands located at 1325 and 1265 cm−<sup>1</sup> , originated by the stretching of C–O bonds in S and G-units, respectively, modified their relative intensity in an evident way. Although it is not possible to confirm the exclusive contribution of G-units in the band at 1265 cm−<sup>1</sup> , its great decrease in intensity seems to indicate an important loss of guaiacyl structures [30].
