Several acids served as catalysts with [BMIM][Cl] for the hydrolysis of corn stalk: hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, and maleic acid. Overall, hydrochloric acid was the most efficient catalyst. Sulfuric and nitric acids were also efficient, but required a higher loading to achieve the same yield in reducing sugars. At the same temperature (100°C), reactions with phosphoric and maleic acids were much slower than with the other acids, even at high loadings. The combination of hydrochloric acid (7 wt%) and [BMIM][Cl] was efficient in the hydrolysis of corn stalk, rice straw, pine wood, and bagasse [6]. Faster degradation of cellulose and hemicellulose was also observed at higher tem­peratures and for longer pretreatment times for Eucalyptus grandis [32]. The weight loss increased with the amount of hemicellulose, which was higher in softwoods (spruce and pine). More carbohydrates (polysaccharides and lignin) were hydrolyzed as the acid concentration increased [32]. Trifluoroacetic acid (0.2 wt%) also served as an acid catalyst in the dissolution of loblolly pine in [BMIM][Cl] at 120°C. Its effect was similar to sulfuric acid H2SO4 at the same molar concentration. After a 2-h treatment, 62 wt% of the loblolly pine was converted to soluble products. No further increase in the yield was seen after a

4- h treatment [53]. The addition of AlCl3 led to a decrease in pH in a mixture of wood (Metasequoia glyptostroboides) and [BAIM][Cl] and [MAIM][Cl], which accelerated the dissolution of wood at a lower temperature. The amount of insoluble residues in the IL and pH decreased with increasing AlCl3 amount. The selection of the metal chloride affected the pH and the liquefaction efficiency: AlCl3 led to lower pH than SnCl2 and FeCl3. The stronger acidity led to higher liquefaction efficiency [16]. These results were consistent with a previous study in which the initial acid hydrolysis rates of cellobiose increased with increasing acid strength. The conversion of cellobiose to glucose was much faster for acids with negative pKa values, such as methanesulfonic acid (pKa = -1.9) and sulfuric acid (pKa = -3) [26].

From these results, it was argued that biomass does not dissolve in ILs directly, but that it needs to be hydrolyzed first before the dissolution of the hydrolysis products. Pine wood and wheat straw (mesh size smaller than 1 mm) were dissolved in [EMIM][OAc] with acetic acid as catalyst. After dissolution, a drop in pH was observed with formation and accumulation of acetic acid in the IL/biomass solution. The addition of acetic acid to [EMIM][OAc] accel­erated the dissolution of wheat straw. After dissolution and addition of water, the precipitate contained an amount of lignin that increased with the amount of acetic acid added, suggesting that acetic acid also acted as a co-solvent for lignin [47].

Indeed, IL pretreatments with acid may increase the yield of reducing sugars following enzymatic hydrolysis, but they also promote the degradation of cellulose and hemicellulose when conducted at higher temperatures and for longer times [6, 32, 47, 53]. Faster degradation of cellulose and hemicellulose was observed at higher temperatures and for longer pretreatment times for Eucalyptus grandis [32]. For the acid hydrolysis of loblolly pine in [BMIM][Cl], the yield of monosac­charides reached a maximum after 2 and 0.5 h of pretreatment at 120 and 150°C, respectively [53]. Similarly, the yield of reducing sugars after hydrolysis of corn stalk in [BMIM][Cl] with HCl reached a maximum for an incubation time of 30 min at 100°C [6]. High performance liquid chromatography (HPLC) of resi­dues from the acid-catalyzed pretreatment of loblolly pine in [BMIM][Cl] showed that the monosaccharides from biomass reacted by dehydration to form other compounds, such as 5-hydroxymethylfurfural and furfural [53]. 31P NMR spectra of the recycled IL after pretreatment of Eucalyptus grandis exhibited signatures from 5-hydroxymethylfurfural, acetol, 2-methoxy-4-methylphenol, catechol, and acetic acid [32]. Fourier-transform infrared (FTIR) spectroscopy of corn stalk after pretreatment in [BMIM][Cl] with sulfuric acid showed the functionalization of lignin with sulfonic groups [6]. The generation of these by-products reduces the total reducing sugar yield, can affect the enzymatic hydrolysis of the remaining cellulose and complicate the recycling of the IL.