## 1 Introduction  
In second-generation biorefineries, agricultural by-products or forest biomass are processed to produce energy and a wide variety of precursor chemicals and bio-based materials, similar to the modern petroleum refineries. Among the variety of possible products manufactured in second-generation biorefineries, liquid transportation fuels mainly in the form of ethanol have been rapidly gaining significance [1]. Lignocellulosic materials do not contain monosaccharides readily available for bioconversion. First, they contain polysaccharides--cellulose and hemicelluloses--which have to be hydrolysed, by means of acids or enzymes, to fermentable sugars. Secondly, cellulose in plants is closely associated with hemicelluloses and lignin, preventing the access of hydrolytic agents to cellulose. Thirdly, the crystalline structure of cellulose itself represents an extra obstacle to hydrolysis. Therefore, a pretreatment is required to remove the lignin and hemicelluloses, to reduce the cellulose crystallinity, and to increase the porosity of the cellulose [2]. The yield of cellulose hydrolysis into glucose by the use of acids or enzymes is generally less than 20% when pretreatment is not carried out, whereas the yield after pretreatment often exceeds 90% [3; 4]. In order to increase the yields of sugars and reduce the ethanol production cost, efficient hydrolysis and valorization of hemicellulose sugars have become important [5]. Once the sugars are under their monomeric form, they can be fermented or chemically converted into molecules of interest such as alcohols (ethanol, butanol, xylitol, arabin), aliphatic carboxylic acids (succinic acid, lactic acid, levulinic acid), or other molecules [6; 7]. After the pretreatment leading to the fractionation of the lignocellulosic biomass and before the conversion into molecules of interest, the sugars undergo a purification step. Among the different processes in lignocellulosic biorefineries, the pretreatment and the purification steps usually represent the most expensive stages and high technical challenges [4; 7]. In the literature about lignocellulosic ethanol biorefineries, the notion of pretreatment of the lignocellulosic biomass comes out of the main treatment: the cellulose conversion to ethanol via enzymatic saccharification then fermentation. In this review, instead of pretreatment, the notion of fractionation was favored since the three components of the lignocellulosic are of interest. No cost effective industrial lignocellulosic fractionation process has emerged; they all present some drawbacks such as the formation of fermentation inhibitors, high use of energy or chemicals, waste production, or expensive equipment. The major fractionation processes studied are biological, physical, and chemical, sometimes with some combinations [5]. Among the chemical treatment--hot water, stream explosion, acid, alkaline, organosolv, supercritical fluids, and ionic liquids--mineral acid fractionation is still the method of choice in several model processes [2; 5; 8; 9]. In this review, only mineral acid fractionation was discussed since other acid-like treatments--organic acids, hydrothermal water, supercritical fluids, or acidic ionic liquids--are less or not developed industrially, mainly due to higher cost [10; 11; 12]. For instance, a newly developed process using an acidic ionic liquid showed promising results for lignocellulosic fractionation; however, economic assessments are still to be investigated [13]. These acid-like pretreatments can also produce different molecules from those produced with mineral acid fractionation, which implies different strategies of purification and outlets in other fields of application. For instance, fractionation with organic acids, considered as part of the organosolv processes, such as theprocess developed by the French company CIMV using formic and acetic acids [14], produces sulfur-free and ash-free lignin [15], but modifies the lignin by adding acetyl groups via esterification [16].