*3.2. Delignification Treatments*

Table 3 lists data concerning the HS and AS delignification treatments, including SY and the removal percentages of the structural components (lignin, cellulose, and hemicelluloses). For comparative purposes, Table 4 lists results reported for the delignification of hazelnut shells and other biomasses. Little information has been reported on the delignification of native or pretreated HS, with alkaline delignification being the most studied processing method [20,27,28].

**Table 3.** Solid yield and removal percentages of lignin, cellulose, and hemicelluloses achieved in the experiments 1–22 in Table 1. The removal percentages were calculated from their source (HS and AS, respectively).



**Table 4.**Literature reported on the delignification of diverse types of biomass using operational conditions related to the ones used in this work.

Alkaline treatments of HS performed with 2–8% NaOH solutions (experiments 1, 3, and 5) did not exceed 19.2% delignification. These results are in agreement with the data reported by Uzuner et al. [28], who assessed the effects of NaOH concentration on delignification, and achieved a limited delignification degree (20%). The data were justified on the basis of the high recalcitrance of HS lignin to depolymerization. Ho¸sgün and Bozan [20] reported 60% of lignin removal operating at 120 ◦C for 60 min in media containing 2.25% NaOH, conditions that allowed almost complete cellulose recovery.

In this study, as a general trend, the solid dissolution and delignification effects reached in experiments performed with AS (exp. 2, 4, and 6) increased with the NaOH concentration more than they did in assays using HS (exp. 1, 3, and 5). The highest lignin removal on the delignification stage (24.3%) was reached when AS was treated with 8% NaOH at 121 ◦C for 60 min (experiment 6). These results are in accordance with literature using steam-exploded substrates, taking into account both delignification and autohydrolysis (31.6% of lignin removal for the experiment 4) but are below for experiments containing NaOH during longer times or NaBH<sup>4</sup> (42.5 and 48% lignin removal, respectively) [27]. The alkaline delignification of hazelnut tree prunings (before and after hydrothermal processing) was also assessed in media containing 2% NaOH, with a substantially improved delignification when the pretreated biomass was used as a substrate [37].

In the set of experiments performed with aqueous alkaline solutions (exp 1–6), AS led to higher hemicellulose removal than the ones carried out with HS. For example, 79.2% hemicellulose removal was achieved when AS was treated with 8% of NaOH at 121 ◦C for 60 min (exp. 6), conditions under which more than 20% of cellulose was removed (revealing poor selectivity).

The comparison between experiments performed with aqueous alkaline solutions and alkaline-organosolv mixtures (7–12) showed that the presence of ethanol resulted in limited additional delignification (11.3–26.6% of lignin removal). Related results were observed for the solubilization of cellulose and hemicelluloses. The experiments performed at higher temperatures (exp. 13 and 14) led to improved delignification degrees. In related studies, alkaline-organosolv methods were successfully applied to rice husks [35,36]. Native and pretreated sugarcane bagasse were also delignified by alkaline-organosolv delignification, resulting in a significantly higher lignin removal in the native samples [38,39].

Fernández-Rodríguez et al. [34] compared organosolv and alkaline delignification treatments of pre-processed biomass, looking at the manufacture of lignin isolates suitable for valorization alkaline methods led to the best results for almond shell delignification, whereas the opposite behavior was observed for olive tree pruning. In our study, as a general trend, organosolv treatments (exp. 15–17) and acid-catalyzed organosolv assays (exp. 18–22) showed that the amount of cellulose remaining in the solid phase was hardly altered. In assays using HS as a substrate, the delignification increased with respect to the ones performed with AS. This conclusion can be confirmed by comparing the results obtained in assays free from the catalyst (for example, 53.3% lignin removal in exp. 15 in comparison with 35.2% in experiment 16), and also by data analysis of experiments performed in H2SO<sup>4</sup> containing media (65.3% delignification in exp. 18 in comparison with 47.9% in exp. 19).

Coupling stages of autohydrolysis and organosolv delignification has been reported as a suitable strategy for the complete fractionation of biomass [1,15,37], providing an alternative to conventional, single-stage delignification. In a related study, Huijgen et al. [41] compared organosolv and prehydrolysis-organosolv treatments of wheat straw, concluding that the prehydrolysis before organosolv resulted in decreased lignin recovery yields, a fact ascribed to the formation of "pseudo-lignin" and to lignin recondensation during prehydrolysis. El Hage et al. [42] found that the severity of the autohydrolysis modifies the lignin structure, affecting the subsequent organosolv delignification. Obama et al. [40] reported lignin alteration upon autohydrolysis, with the participation of repolymerization reactions (C-C linkages) that negatively affect the further delignification.

In this study, enhanced removal of hemicelluloses from HS and AS was observed in experiments performed in acid-catalyzed organosolv assays. In runs with AS, high degrees of hemicellulose removal (87.7% in exp. 19, performed at 180 ◦C for 60 min; or 93.2% in exp. 20, which lasted 120 min) were achieved in acid-catalyzed organosolv treatments. These results are significantly higher than the ones observed for HS (76% hemicellulose removal in exp. 18). However, it can be noted that the AS hemicellulose content (9.1%) was significantly lower than the one of HS (28.1%). An opposite pattern was observed in experiments performed in the absence ofan acid catalyst, in which HS reached higher hemicellulose removal.

According to the above ideas, SY (which is affected by the contents of lignin and hemicelluloses) were lower for acid-catalyzed experiments. In experiment 18, the limited SY (46.2%) corresponded to 65.3% lignin removal and 76% hemicellulose removal from HS. Using AS as a substrate under the same conditions (exp. 19), the SY (64.9%) corresponded to 47.9% and 87.7% removal of lignin and hemicelluloses, respectively.

A comparative analysis of results confirmed that the conditions of experiments 18 and 19 (dealing with HS and AS, respectively) were the best ones identified in this study. The liquid phase from HS in exp. 18 contained the following hemicellulose-derived compounds: xylooligosaccharides, 10.01 g/L; arabinooligosaccharides, 0.28 g/L; glucooligosaccharides, 0.07 g/L; acetyl groups, 2.25 g/L; xylose, 10.74 g/L; glucose, 0.07 g/L; acetic acid, 2.29 g/L; furfural 1.38 g/L. In comparison, the composition of the liquid resulting from the acid-catalyzed delignification of AS (exp. 19) contained the following hemicellulose-derived compounds: glucooligosaccharides, 2.34 g/L; xylooligosaccharides, 1.25 g/L; acetyl groups, 0.28 g/L; xylose, 4.20 g/L; glucose, 1.98 g/L; acetic acid, 1.06 g/L; furfural 2.06 g/L; hydroxymethylfurfural, 0.24 g/L.
