**4. Conclusions**

Formosolv treatments of Paulownia, under optimum conditions, produced a cellulose-enriched pulp (80%) and a solubilization of the 78.5% of the initial lignin in the solid. The physicochemical and spectroscopic characteristics of the formosolv lignin and its comparison with the MWL showed that the lignin underwent depolymerization phenomena but also recondensation, leading to a molecular weight distribution with high polydispersity. Delignification proceeded mainly by the breaking of β-O-4′ linkages from arylglycerol β-aryl ether units. Other bonds were also affected but resisted better the solvolytic treatment.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4395/10/8/1205/ s1, Figure S1. Main substructures identified in the MWL and PFL of Paulownia. (**A**) Aryglycerol β-aryl ethers; (**A'**) γ-OH acylated aryglycerol β-aryl ethers; (**B**) resinols; (**C**) phenylcoumarans; (**D**) spirodienones; (**E**) dibenzodioxocins; (**I**) cinnamyl alcohol end-groups; (**J**) cinnamaldehyde end-groups; (**G**) guaiacyl units; (**S**) syringyl units; (**S'**), oxidized syringyl units bearing a carbonyl group at Cα; Figure S2. 1H NMR spectra of MWL and formosolv Paulownia lignin; Figure S3. TGA (dashed) and DTG (bold) curves of MWL and PFL; and

Table S1. Assignments of carbon chemical shifts (δ, ppm) in <sup>13</sup>C NMR spectrum of MWL and formosolv lignin; Table S2. Assignments of <sup>13</sup>C-1H correlation signals in the HSQC NMR spectrum of the obtained lignin fractions.

**Author Contributions:** Conceptualization, E.D. and A.d.V.; methodology, A.d.V.; software, E.D. and A.d.V.; validation, A.d.V. and G.G.; formal analysis, E.D., A.d.V. and G.G.; investigation, E.D.; resources, A.d.V. and G.G.; data curation, E.D., P.G.d.R. and A.d.V.; writing—original draft preparation, E.D. and A.d.V.; writing—review and editing, P.G.d.R., G.G., A.R. and P.G.; visualization, E.D., P.G.d.R., A.d.V., G.G., A.R. and P.G.; supervision, A.d.V., G.G., A.R., P.G.; project administration, A.d.V., G.G.; funding acquisition, G.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by MINECO (Spain) in the framework of the projects "Development of processes for the integral use of fast-growing biomass by means of the production of bioethanol and chemical products" with reference CTQ2012-30855 and "Multistage processes for the integral benefit of macroalgal and vegetal biomass" with reference CTM2015-68503-R, by Consellería de Cultura, Educación e Ordenación Universitaria (Xunta de Galicia) through the contract ED431C 2017/62-GRC to Competitive Reference Group BV1, and by the CITACA Strategic Partnership ED431E 2018/07, programs partially funded by European Regional Development Fund (FEDER).

**Acknowledgments:** Pablo G. del Río would like to express his gratitude to the Ministry of Science, Innovation and Universities of Spain for his FPU research grant (FPU16/04077). Authors are grateful to Alberto Núñez and Jorge Otero from the Research Support Services of the Univeridade da Coruña for conducting and advising on the interpretation of NMR, FTIR, and TGA techniques. Finally, the authors thank Jalel Labidi and his team, from the Universidad del País Vasco, for the technical support in the analysis of SEC samples.

**Conflicts of Interest:** The authors declare no conflict of interest.
