**Laura López, Sandra Rivas \* , Andrés Moure, Carlos Vila and Juan Carlos Parajó**

Department of Chemical Engineering, University of Vigo (Campus Ourense), 32004 As Lagoas, Ourense, Spain; laura.lopez.caamano@uvigo.es (L.L.); amoure@uvigo.es (A.M.); cvila@uvigo.es (C.V.); jcparajo@uvigo.es (J.C.P.) **\*** Correspondence: sandrarivas@uvigo.es; Tel.: +34-988-387075

Received: 22 September 2020; Accepted: 12 October 2020; Published: 14 October 2020

**Abstract:** Hazelnut shells are an important waste from the hazelnut processing industry that could be valorized in a multi-product biorefinery. Individual or combined pretreatments may be integrated in processes enabling the integral fractionation of biomass. In this study, fractionation methods based on alkaline, alkaline-organosolv, organosolv, or acid-catalyzed organosolv treatments were applied to raw or autohydrolyzed hazelnut shells. A comparative analysis of results confirmed that the highest lignin removal was achieved with the acid-catalyzed organosolv delignification, which also allowed limited cellulose losses. When this treatment was applied to raw hazelnut shells, 65.3% of the lignin was removed, valuable hemicellulose-derived products were obtained, and the cellulose content of the processed solids increased up to 54%. Autohydrolysis of hazelnut shells resulted in the partial solubilization of hemicelluloses (mainly in the form of soluble oligosaccharides). Consecutive stages of autohydrolysis and acid-catalyzed organosolv delignification resulted in 47.9% lignin removal, yielding solids of increased cellulose content (55.4%) and very low content of residual hemicelluloses. The suitability of selected delignified and autohydrolyzed-delignified hazelnut shells as substrates for enzymatic hydrolysis was assessed in additional experiments. The most susceptible substrates (from acid-catalyzed organosolv treatments) reached 74.2% cellulose conversion into glucose, with a concentration of 28.52 g glucose/L.

**Keywords:** biorefinery; hazelnut shells; hydrothermal pretreatment; delignification; enzymatic hydrolysis; fractionation; cellulose; hemicelluloses
