**1. Introduction**

Lignocellulosic biomass (LB) is a key resource for the sustainable manufacture of bio-based chemicals and fuels. LB can be processed according to the biorefinery concept, which involves consecutive treatment stages to achieve an integral benefit of the feedstock, with minimal or no waste generation [1]. The success of biorefineries also depends on the right application of technology methodologies for obtaining multi-products [2]. The contribution of biorefineries to a future bio-economy inspires business opportunities based on product diversification while improving environmental performance [3].

In the field of biomass valorization, the biorefinery acts as a platform for chemicals and energy production through the inclusion of diverse conversion technologies [3,4].

The complexity of biomass utilization lies in both the polymeric nature of its main constituents (cellulose, hemicelluloses, and lignin) and their different chemical reactivities. In the last few decades, several fractionation methods have been developed, as summarized in recent literature reviews [2,5–8]. Galbe and Wallberg [6] classified the conventional methods according to their mode of action:

mechanical treatments (e.g., milling or grinding) reduce the particle size of biomass, increasing the surface area of the particles and improving the enzyme accessibility; dilute acid pretreatments (with H2SO4, H3PO<sup>4</sup> or other strong acids), and hydrothermal pretreatments (with hot, compressed water or steam) promote the hydrolysis of hemicelluloses; whereas alkaline and organosolv processing enable the extraction of lignin.

Hydrothermal pretreatments (autohydrolysis, performed with hot, compressed water) causes the selective solubilization of hemicelluloses by depolymerization into soluble compounds of lower molecular weight. Soluble, low molecular weight polymers from hemicelluloses show potential for applications in the manufacture of barrier films or hydrogels [6]; oligosaccharides show prebiotic properties with potential in the pharmaceutical and food markets [9,10]; and monosaccharides can be transformed into value-added chemicals such as xylitol and furfural [11]. Additionally, autohydrolysis increases the biomass surface area and decreases the crystallinity of the cellulosic fraction remaining in the solid phase, facilitating its further hydrolysis.

Solutions of alkalis (such as NaOH, KOH, or ammonia) are suitable for removing lignin by saponification of intermolecular ester bonds, and for increasing the digestibility of cellulose [5]. Alternatively, delignification can be achieved by organosolv treatments with organic solvents (e.g., ethanol) or solvent-water mixtures, in the presence or absence of catalysts. From an environmental point of view, the separation of lignin from the organosolv media by precipitation with water facilitates the wastewater treatment. From an economic perspective, organosolv lignin is a valuable product with potential applications in diverse fields, including the manufacture of adhesives, fibers, films, biodegradable polymers [12], and natural antioxidants [13].

The solids from delignification treatments, with increased cellulose contents, may show increased susceptibility to enzymatic saccharification [1,14,15], or serve as substrates for manufacturing cellulose nanocrystals [16]. In the first case, glucose can be transformed in value-added compounds (e.g., bioethanol or lactic acid) [15,17]; whereas in the second one, cellulose nanocrystals are used in a number of fields, including biological applications.

"Conventional" pretreatments (or their combinations) are expected to play an important role in the future full-scale biorefineries [6]. Combined pretreatments may allow process designs enabling an improved fractionation and an efficient processing into a spectrum of multi-products [8]. For example, consecutive stages of autohydrolysis and delignification allow the separate recovery of hemicellulose-derived saccharides, lignin, and cellulose [18], enabling the integral fractionation of biomass, an aspect of crucial importance for a successful biorefinery approach [19].

Hazelnut (*Corylus avellana* L.) is a commercial crop, whose fruit is widely used in food industries [20]. Turkey is the main producer and exporter of hazelnut, followed by Italy, Azerbaijan, USA, China, Georgia, Iran, France, Chile, and Spain. According to the Food and Agriculture Organization (FAO), 863,888 tons of hazelnuts (with shell) were produced worldwide in 2018 [21]. Hazelnut shells (HS) are the most important byproduct of the hazelnut processing industry, representing more than half of the total nut weight. HS are a low–cost byproduct, usually burned, but could be valorized on the basis of their lignocellulosic nature. Recently, getting value from selected biochemicals in HS has become a motivation for research, due to the compelling economic benefits [3,22]. In terms of composition, HS are mainly made up of lignin: Perez-Armada et al. [23] reported 40.1 wt% of lignin, whereas Demirbas [24] and Surek and Buyukkileci [25] found around 46%, and Ho¸sgün and Bozan [20] near 50%. HS hemicelluloses are mainly made up of xylan, acetyl, and uronic moieties, which accounted jointly for 25–32.5% of the dry weight, whereas the reported cellulose contents are about 26–27% [10,23,26].

Based on the above ideas, this work deals with the development of HS fractionation methods suitable for a multi-product biorefinery. HS and solids resulting from HS autohydrolysis (denoted AS) were subjected to diverse delignification treatments to assess their efficiency. The soluble hemicellulose-derived products were identified, and the solids from selected delignification stages were assayed as substrates for enzymatic hydrolysis. In summary, this work provides an experimental assessment on biorefinery schemes enabling an integral valorization of HS.
