*3.1. Formosolv Process: Delignification of Paulownia Wood*

The formic acid delignification was carried out with the objective to obtain: (i) a high glucan content in the solid phase, potentially useful in later enzymatic hydrolysis stages, (ii) a high delignification degree and (iii) a high solubilization of hemicellulosic sugars, leading to a solid phase mainly composed of glucan.

Paulownia wood was treated with different concentrations of formic and hydrochloric acid. In order to obtain a quantitative description of the effects of these variables on the fractionation process, the experiments were carried out according to a second order face-centered factorial design. The structure of the factorial design, analysis of the pulps and liquors are shown in Table 2, and regression parameters for the variables' solid yield, viscosity and glucan in the pretreated pulp are displayed in Table 3.


**Table 3.** Regression parameters for the dependent variables: solid yield, viscosity and glucan.

<sup>a</sup> Coefficients significant at 99% confidence level based on the Student's t-test; <sup>b</sup> coefficients significant at 95% confidence level based on the Student's t-test; <sup>c</sup> coefficients significant at 90% confidence level based on the Student's t-test.

Formic acid treatment produced an important solubilization of Paulownia wood, ranging the pulp yield between 45.2 (in experiment 1, carried out at the highest values of the three independent variables, time = 60 min, FA: 95%, HA: 0.1%) and 58.3% (in experiment 14, time = 45 min, FA: 80%, HA: 0.075%). According to the data of Table 2, the yield decreased when the independent variables increased, with more influence of time and FA concentration, according to the data of Table 3.

The highest delignification (78.5% of lignin removal) was achieved under the conditions of experiment 11 (see Table 2), corresponding to a kappa number value of 27.7 (10.6% Klason lignin). The most significant variables on the results of the KN were the duration of the treatment and the concentration of formic acid, but none of them were significant at the 95% confidence level.

Regarding the hemicellulosic sugars recovery in liquors, significant differences were found between the operational conditions, achieving a concentration of hemicellulosic sugars varying in the range of 14.3–18.3 g of sugar/100 g of Paulownia. The operational range employed allowed to reach very similar percentages of dissolution among the experiments (about 17% of hemicelluloses were solubilized from Paulownia wood during the pretreatment) (Table 2). In the optimum conditions to remove the maximum lignin content (experiment 11), the distribution of sugars in black liquor was largely dominated by xylooligosaccharides (66.9% of total dissolved carbohydrates, 2.2 g/L), followed by glucooligosaccharides (14.3% of total dissolved carbohydrates, 0.47 g/L) and arabinooligosaccharides (7.5% of total dissolved carbohydrates, 0.24 g/L). The presence of monosaccharides was reduced up to 11.0% by mass, while their products of decomposition, furfural and hydroxymethylfurfural, only accounted for 0.3% of the mixture of dissolved carbohydrates.

The high glucan content is directly related to the cellulose purity. In our case, the values were very close to 70% in the best experimental combinations (experiments 1, 3, 11). The regression equation of this variable indicates that long treatment times and high FA concentrations promote a high percentage of glucan in pulp. However, the HA concentration must be low as to partially prevent the hydrolytic effects on the cellulose. Figure 1 shows the predicted response surface for the content of glucan in the pulp showing the influences of the more significant variables.

**Figure 1.** Response surface of glucan in pulp at a constant value of 0.05% HA.

In order to achieve a more precise information about the severity of the treatment over cellulose, the glucose content of the black liquors was analyzed, as well as the viscosity of the pulps, which is a measure of the chain length of the cellulose. The maximum value of monomeric glucose solubilized from glucan in the liquor was 7.2% (the average was 4.0%), indicating a probable hydrolysis of cellulose to some extent. On the other hand, the viscosities of the pulps, under conditions in which the highest delignification was achieved, were around 600 mL/g, a relatively low value compared with other pulps with the same KN. It was confirmed that the cellulose suffered a certain attack resulting in a reduction of the chain length.

It should be noted that the analysis of the formic acid content of the pulps (after quantitative acid hydrolysis) reached a significant mass percentage (~5%), probably due to the incorporation of formates in the lignin molecule. This fact was determined by several researchers in different treatments with formic acid [11,29] and is confirmed, in this work, with the spectroscopic data shown below.

A mathematical optimization allowed the definition of the optimal conditions of fractionation introducing, with the same weight, the following constraints for the composition of the pulp: the highest possible content in glucose, the lowest KN and the lowest content in hemicelluloses. These optimal conditions are a time of 60 min, FA concentration of 95%, and a HA concentration of 0.05% (experiment 11).
