**1. Introduction**

The current economic system, based on a linear model, has led to humankind's overreliance on non-renewable fossil resources causing its depletion, besides entailing harmful consequences for the environment, society, economy and health [1]. This global reality is provoking an unsustainable

situation with consequences still incalculable. In an attempt to mitigate this situation, in recent decades, an unceasing search for alternative strategies has been encouraged to find new and suitable production systems founded in the use of renewable resources as raw material within a biorefinery context. This would suppose a transition from the traditional linear economic model to a circular economy, more efficient and greener, moving the current trend towards a global and sustainable bioeconomy [2].

In this scenario, lignocellulosic materials are promising candidates as feedstock to obtain biofuels, building blocks, bio-chemicals, food additives, adhesives, or cosmetics, among others [3,4]. Lignocellulosic materials (LCM) present a three-dimensional and recalcitrant structure mainly composed of cellulose (homopolymer made up of glucose units), hemicelluloses (heteropolymer made up of different sugars) and lignin (aromatic polymer). The integral use of LCM involves a selective separation of its components through fractionation treatments following the biorefinery concept [5]. Therefore, the selection of the adequate fractionation process is key to achieve an efficient utilization of all fractions.

Organic solvent pretreatment (also known as organosolv) is an emerging alternative to conventional pulping processes, since it allows fractionating the LCM into cellulose, lignin, and hemicellulose, with a high purity of all fractions opening the possibility of its integral exploitation [6]. The organosolv fractionation process provides an efficient and clean way of transforming lignocellulose into valuable products, facilitating a subsequent recovery of all the fractions obtained [7]. This kind of treatment uses a mixture of an aqueous organic solvent and water, with or without the addition of a mineral acid, which dissolves most part of the lignin and hemicellulose from the raw material [8]. Moreover, these procedures present interesting advantages such as: easy solvent recovery and recyclability, free sulfur, low-cost investment and environmentally friendly [9].

Short-chain organic acids have been considered good solvents for lignin in the delignification of LCM [10]. Particularly, formic acid has attracted considerable attention as a delignification agent, due to its ability to achieve a selective fractionation of the biomass showing high efficiency, both non-wood, hardwood and softwood biomass [9]. The mixture of LCM and concentrated aqueous solutions of formic acid at boiling temperature, with the addition of small quantities of hydrochloric acid employed as a catalyst, is known as formosolv. In this work, the formosolv treatments were performed at atmospheric pressure in all cases, that is, depending on the composition of the cooking liquor, in the range 105–109 ◦C. Nevertheless, the temperature and pressure can be raised in order to reduce the time of reaction [9]. This process yields a cellulose rich pulp, an aqueous fraction rich in hemicellulosic sugars and a lignin fraction [11]. Among the LCM, Paulownia species are rapid-growth trees with a high biomass production rate per year (50 t/ha/yr) and a low demand of water, which make it very suitable for intercropping systems as it protects the crops from adverse climatic conditions, benefiting the harvest yields. In addition to its uses as wood to build from plywood for musical instruments, other applications suggested for Paulownia wood include its exploitation as a source for pulp due to its fast development and uniform growth [12].

Several works have reported the use of Paulownia species in a biorefinery framework. For example, Domínguez et al., (2017) [13] subjected *P. tomentosa* to hydrothermal pretreatment to solubilize the hemicellulosic fraction yielding a solid fraction that was evaluated to obtain bio-ethanol. Gong and Bujanovic (2014) [14] purposed a sequence based on hot water extraction to solubilize most of the hemicelluloses followed by the delignification in acetone/water in the presence of oxygen for the production of cellulose and lignin from *P. tomentosa* and *P. elongata*. However, to the best of our knowledge, scarce research has been performed to fractionate Paulownia by formosolv pulping to recover cellulose and lignin.

The resulting cellulose of the fractionation process can be used for the production of pulp, derivatives, nanofibrillated cellulose or fermentable glucose (after the cellulose hydrolysis) depending on its physicochemical properties [14].

Lignin is a polyphenolic amorphous material originated from the random oxidative coupling of three main *p*-hydroxycinnamyl alcohol monomers (*p*-coumaryl, coniferyl, and sinapyl alcohols), which are representative of the *p*-hydroxyphenyl (H-units), guaiacyl (G-units) and syringyl (S-units) phenylpropanoid units, respectively [15,16]. Due to its polyphenolic chemical structure, it can be employed in the manufacture of adhesives, epoxy, phenolic resins, and polyolefins, as well as in a variety of novel applications.

The objective of this work was the systematic study of the operational variables of formosolv cooking (concentrations of formic and hydrochloric acids and reaction time) to obtain the highest yield of delignification of Paulownia wood. In addition, a secondary target was to identify the main changes caused during the delignification process on the Paulownia lignin, by means of different analytical methods such as FTIR, NMR, high-performance size-exclusion chromatography (HPSEC) and TGA, comparing with pristine lignin used as reference.
