<sup>1</sup>H NMR Spectroscopy of MWL and Formosolv Lignin

Figure S1 shows the <sup>1</sup>H NMR spectra of MWL and formosolv Paulownia lignin. The <sup>1</sup>H NMR spectra of MWL and PFL showed that the proton signals in non-oxygenated carbons (methyl and methylene groups, δ = 0.8–1.2 ppm) were fewer, and less intense in the organosolv residue, which indicates structural modifications in the side chain of the phenylpropane units. The methyl acetate protons resonated in the range of 1.8–2.1 ppm and disappeared almost completely after the treatment, due to the hydrolytic effect of the acidic medium in the black liquor. Peaks at 2.50 and 3.33 ppm were due to residual proton signals in DMSO-d<sup>6</sup> and water, respectively [31].

Between 4 and 5.5 ppm, a large number of signals originated from several protons of the propyl side chain of lignin. In this region of the spectrum, resonances due to the different protons of the hemicelluloses components also appear, but their contribution should be very low. The areas of these signals are smaller in the organosolv residue, which indicates the changes in the functionalities of the side chain. Specifically, the signal at 4.44 ppm (part per million), which originated from the γ protons in β-O-4′ aryl ether units, decreased drastically in the spectrum of PFL due to delignification.

The range between 6.3 and 7.5 ppm showed the different signals of the aromatic protons, and between 8 and 9 ppm (part per million), those of the phenolic hydroxyl protons. The sharp peak at 8.1 ppm originated from carboxylic functionalities (formic acid) incorporated into a lignin molecule.
