# Pretreatments Applied to Wheat Straw to Obtain Bioethanol
## 3 Pretreatments Applied to Wheat Straw
### Chemical Pretreatments Applied to Wheat Straw
#### 3.2.4 Ionic Liquid Pretreatment  
By pretreating wheat straw with acidic ionic liquids, a high yield of carbohydrates and sugars was obtained, enzymatic delignification was improved by maintaining the activity and stability of cellulase, while ether bonds were broken to extract lignin, as they have an effect on lignin depolymerization [68; 69; 70; 71]. Sugars were reduced by 80% in the case of wheat straw treated with ionic liquid compared to untreated ones [71]. The following substances are used in this pretreatment: pyrrolidinium-based, imidazolium-based, ammonium-based, phosphonium-based, sulfonium-based, 1-ethyl-3-methylimidazolium acetate [70; 71; 72].  
The ionic liquid is a good solvent. It is a salt that melts at room temperature; it is non-volatile; difficult to oxidize; it can replace volatile organic solvents; it accelerates enzymatic hydrolysis; it can be recycled [31; 40; 65].  
One of the main advantages of using ionic liquids in the biomass conversion process is their ability to dissolve a large amount of cellulose and lignin, and the low volatility allows the reaction to take place at high temperatures [64].  
The biomass of wheat straw pretreated for 0.5 h and 3 h decreased, being 27.6 and 56.96%, respectively [40].  
Untreated wheat straws had a higher content of hemicellulose and lignin compared to the treated ones [40]. The hemicellulose was reduced by 64.45% and 18.6% and the total delignification of the biomass pretreated for 3 hours and 0.5 hours was obtained in the proportion of 80.16% and 45.37% respectively [40]. The pretreatment with ionic liquid favored the disintegration of the amorphous fractions of lignin and hemicellulose from the sample matrix [67].  
The morphological aspect of the samples was treated with ionic liquid for 5 h (Figure 6b) and 3 h (Figure 6c) and untreated (Figure 6a) at 3000\(\times\) magnification, as highlighted by SEM microscopy (Figure 6). It can be observed that, in the case of untreated wheat straw, the surface is orderly, rigid and smooth, while in the treated ones it is rough, agglomerated, and disordered, with large pores and cracks due to the elimination of lignin and hemicellulose from the wheat straw biomass that disturbed the matrix [67].  
Glucose saccharification production and xylose yields were higher the longer the pretreatment time of wheat straw with ionic liquid [40].  
The ethanol yield was higher in the case of using straw treated for a longer time, namely 3 h, compared to wheat straw pretreated for 0.5 h (these being 84.34% and 52.84%, respectively). In the case of untreated biomass from the maximum theoretical yield, the ethanol yield was only 10.76% [40].  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Raw** & **Acid Type** & **Pretreatment** & **Reducing** & **Delignification** & **Fermentation** & **Bioethanol** & **R** \\
**Material** & & **Condition** & **Sugars** & **Rate (\%)** & **Condition** & **Yield [g/L]** & **R** \\ \hline Wheat straw & H\({}_{2}\)SO\({}_{4}\) (2\%) & 180 \({}^{\circ}\)C; 10 min & 43 g/L & NA & 30 \({}^{\circ}\)C; 72 h & 0.44 & [40] \\ Wheat straw & H\({}_{2}\)SO\({}_{4}\) and Na\({}_{2}\)SO\({}_{3}\) & 180 \({}^{\circ}\)C; 30 min & NA & 21 & 48 h & 17.25 g/g & [40] \\ \hline \hline \end{tabular}
\end{table}
Table 4: Obtaining bioethanol by acid pretreatment of wheat straw (data from [38]).  
Figure 6: Electron micrographs of wheat straw untreated (a), treated for 0.5 h (b) and treated for 3 h (c), at 3000\(\times\) magnification (adapted from [68]).  
The ethanol yield was higher when using wheat straw pretreated for 3 h compared to using wheat straw pretreated for 0.5 h, being 84.3% and 52.84% of the theoretical maximum yield, while only 10.76% was the ethanol yield from the untreated wheat straw [40].  
Pretreatment of wheat straw with ionic liquid before enzymatic hydrolysis determined obtaining a higher yield of ethanol due to successful delignification.