## 3 Results
### FT\(-\)IR Analysis of Crude Cellulose and Lignin  
The FT\(-\)IR spectrum of crude cellulose and lignin (Figure 2) showed that the absorption peaks at 3391 cm\({}^{-1}\) and 2895 cm\({}^{-1}\) were attributed to the O\(-\)H stretching vibration of an hydroxyl group and the C\(-\)H stretching vibration of an aromatic methyl group, respectively. After formic acid treatment, the intensity of this absorption peak in the crude cellulose increased significantly, indicating that the cellulose content increased, and the degradation degree of formic acid on cellulose was low [26]. The characteristic peak at 1720 cm\({}^{-1}\) was caused by C\(=\)O stretching vibration of a nonconjugated carbonyl group. The absorption peak became significantly stronger after the formic acid treatment, indicating that formic acid had a strong formylation modification with \(-\)OH on the structure of the crude cellulose [21]. Compared with _ML_ straw, the benzene-skeleton absorption peak intensity of crude cellulose was weakened at 1603 cm\({}^{-1}\), 1513 cm\({}^{-1}\), and 1425 cm\({}^{-1}\) \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Contents (\%)**} \\ \cline{2-5}
**Samples** & **Cellulose** & **Hemicellulose** & **ASL** & **AIL** \\ \hline _ML_ straw & 41.89 \(\pm\) 0.73 \({}^{\mathrm{b}}\) & 18.21 \(\pm\) 0.42 \({}^{\mathrm{a}}\) & 2.53 \(\pm\) 0.04 \({}^{\mathrm{a}}\) & 16.77 \(\pm\) 0.18 \({}^{\mathrm{b}}\) \\ Crude cellulose & 83.60 \(\pm\) 0.15 \({}^{\mathrm{a}}\) & 5.15 \(\pm\) 0.05 \({}^{\mathrm{b}}\) & 0.50 \(\pm\) 0.02 \({}^{\mathrm{c}}\) & 6.35 \(\pm\) 0.27 \({}^{\mathrm{c}}\) \\ Crude lignin & 2.20 \(\pm\) 0.26 \({}^{\mathrm{c}}\) & 3.86 \(\pm\) 0.11 \({}^{\mathrm{c}}\) & 1.40 \(\pm\) 0.01 \({}^{\mathrm{b}}\) & 85.40 \(\pm\) 0.68 \({}^{\mathrm{a}}\) \\ \hline \hline \end{tabular} The same letters show the homogeneous groups.  
\end{table} 
Table 1: The contents of main components of crude cellulose and lignin. indicating that formic acid could effectively separate cellulose and lignin and significantly improve the purity of crude cellulose [27]. The disappearance of the peak at 1251 cm\({}^{-1}\) was attributed to the removal of lignin after formic acid treatment [28]. The C-H symmetrical bending of cellulose at 1325 cm\({}^{-1}\), and the C-O-C and C=O stretching vibrations of cellulose at 1163 cm\({}^{-1}\) and 1060 cm\({}^{-1}\) corresponded to the asymmetric C-O-C stretching vibration and C=O stretching vibration of cellulose, respectively, indicating that the basic results of cellulose remained intact after formic acid treatment [29]. The ratio of absorbance at 1431 cm\({}^{-1}\) and 897 cm\({}^{-1}\) (A\({}_{1431}\)/A\({}_{897}\)) raised suggested that the crystallinity of the crude cellulose separated by formic acid increased compared with that of _ML_ straw. The stretching vibration peak of lignin-OH was located at 3391 cm\({}^{-1}\), and the ratio of A\({}_{3411}\)/A\({}_{1513}\) was relatively low, indicating that the content of hydroxyl in lignin was less. The vibration peaks of the aromatic ring skeleton at 1603 cm\({}^{-1}\), 1513 cm\({}^{-1}\), and 1425 cm\({}^{-1}\) showed strong absorption peaks. Therefore, it was speculated that not only was the purity of the crude lignin treated with formic acid high, but the benzene ring structure was also not damaged [30]. At 1720 cm\({}^{-1}\), there was a stretching vibration of ester group non-conjugated carbonyl C=O, where the peak value was strong because formic acid treatment also leads to the formulation of lignin. The characteristic absorption peak at 1462 cm\({}^{-1}\) was C-H bending vibration connected by benzene rings, the peak at 1121 cm\({}^{-1}\) was C-H plane deformation vibration in the syringyl ring, and the peaks at 1033 cm\({}^{-1}\) and 833 cm\({}^{-1}\) were C-H plane deformation vibration in the guiaacyl ring, respectively.