## 3 Results and Discussion
When comparing (Table 4), the contents of C, O, and H in _Salix psammophila_ raw materials were 51.886%, 41.348%, and 6.316%, respectively. The contents of N and S were lower, which were 0.31% and 0.14%, respectively. After the one-step method, the content of N was not obvious, while the content of O and H were significantly decreased, which was caused by the precipitation of O and H on a part of the aromatic ring in the reaction substituted by the sulfonic acid group. The content of S in the catalyst was obviously increased to 26.378% and the measured surface acid amount was 1.28 mmol/g, indicating that the S was introduced into the raw materials of _Salix_, and the sulfonic acid group was formed. As shown, Figure 4 presents that in the infrared spectrum peak of the catalyst, the solid acid catalysts prepared by the one-step method still had a peak of 3400 cm\({}^{-1}\) containing the associated hydroxyl group and the stretching vibration of the hydroxyl group contained in the carboxyl group; the carbonyl peak contained in the carboxyl group at 1600 cm\({}^{-1}\); the stretching vibration peaks of SO\({}_{3}\)H and O = S = O bonds occur at about 1000 cm\({}^{-1}\) and 1100 cm\({}^{-1}\), respectively, demonstrating that concentrated sulfuric acid has played a role in sulfonation and the sulfonyl has been successfully introduced after the one-step method treatment of _Salix psammophila_. In the one-step process of concentrated sulfuric acid, the positively charged sulfonic acid group electrophillically replaced C on the fused ring structure to form a sulfonic acid complex intermediate. After deprotonation, a carbon-based solid acid catalyst with three acidic groups, SO\({}_{3}\)H, OH, and COOH were formed, in which a sulfonic acid group was grafted on a carbon skeleton of a polycyclic aromatic carbon ring arranged in a disorderly manner. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **C** & **H** & **N** & **O** & **S** \\ \hline _Salix psammophila_ & 51.886\% & 6.316\% & 0.31\% & 41.348\% & 0.14\% \\ Catalyst & 50.122\% & 7.583\% & 0.293\% & 15.624\% & 26.378\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Elemental analysis. Figure 4: Infrared spectra and syntactic model of catalysts. Simultaneously, Figure 4 shows that the infrared spectrum peak image of the _Salix_ carboniferous solid acid catalyst prepared under the optimal conditions were reused after washing with hot distilled water. The change of conversion level after four cycles is as follows: 92.35%, 76.23%, 74.22%, 75.73%. The methyl oleate absorption peak could be observed at 3000 cm\({}^{-1}\), while the catalytic activity of catalyst had decreased, to a large extent, caused by the effect of coating of the catalyst with methyl oleate. The conversion level of catalytic biodiesel reaction was obviously improved by changing the washing liquid of the catalyst, soaking in ethanol solution, and repeatedly washing with hot ethanol solution. The change of conversion level after four cycles is as follows: 92.35%, 87.28%, 84.74%, 85.36%. Figure 5 shows the thermogravimetry (TG)-derivative thermogravimetry (DTG) curve of the catalyst under N\({}_{2}\) atmosphere at the heating rate of 20 \({}^{\circ}\)C/min and an ending temperature of 800 \({}^{\circ}\)C. The catalyst dehydration phase occurred at about 105 \({}^{\circ}\)C, and the first weight loss rate peak occurred in the DTG curve, and the TG curve showed that the weight loss ratio was 3.73%. The catalyst thermolysis phase occurred at about 180-280 \({}^{\circ}\)C. The second weight loss rate peak could be observed in the DTG curve, and the TG curve showed that the weight loss ratio was 5.12%. The weight loss rate peak of the DTG curve at 290 \({}^{\circ}\)C was consistent with the peak shape of the experimental initial current instability, which proves that the peak appears to be related to the instrument current instability. The figure presented that the catalyst had been obviously decomposed at 238 \({}^{\circ}\)C, which indicated excellent thermal stability.