## 3 Results and Discussion
### 3.2. Acidity  
The acid strength of different catalysts is determined by NH\({}_{3}\)-TPD. It can be seen from Figure 5, all the zeolites have two main peaks around 190 and 400 \({}^{\circ}\)C, presenting typical TPD curves, which are attributed to the desorption of NH\({}_{3}\) from the weak and strong acid centers, respectively. Generally, NH\({}_{3}\) has a large absorption force at the strong acid adsorption site, which requires high temperature for desorption. Herein, different samples have almost same desorption temperature for NH\({}_{3}\) molecular. Then, we tested the samples after Pt impregnation for NH\({}_{3}\)-TPD (Figure S2), and the results showed that the NH\({}_{3}\) desorption temperature was basically the same as the samples in the form of protonation. The surface acidity of the catalyst has an important influence on the hydroisomerization of alkane. Figure 6 shows the hydroxyl properties of ZSM-23 zeolites by FT-IR spectra. The vibration region of infrared hydroxyl group is 3800-3300 cm\({}^{-1}\), and the infrared peak at 3745 cm\({}^{-1}\) belongs to the isolated silicon hydroxyl group on the outer surface of the sample [22]. The infrared peak at 3735 cm\({}^{-1}\) belongs to the silicon hydroxyl group in the ZSM-23 micropores [22; 23]. The broad peak at the center of 3500 cm\({}^{-1}\) belongs to the nested silica hydroxyl group, and the hydroxyl group bridged by aluminum silicate appears at 3600 cm\({}^{-1}\)[23], which is B acid. Usually the B acid sites of silica-aluminum zeolites \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Samples**} & \multicolumn{3}{c}{**Surface Area (m\({}^{2}\)/g)**} & \multicolumn{2}{c}{**Volume (cm\({}^{3}\)/g)**} \\ \cline{2-6}  & \(\mathbf{S_{BET}}\) & \(\mathbf{S_{mic}}\) & \(\mathbf{S_{ext}}\) & \(\mathbf{V_{mic}}\) & \(\mathbf{V_{mes}}\) \\ \hline H-ZS-1 & 226.6 & 168.6 & 58.0 & 0.07 & 0.21 \\ H-ZS-2 & 204.2 & 158.3 & 45.9 & 0.05 & 0.20 \\ H-ZN-1 & 243.6 & 169.2 & 74.4 & 0.07 & 0.29 \\ \hline \end{tabular}
\end{table} 
Table 2: Basic properties of the different samples in the form of protonation. Figure 5: NH\({}_{3}\)-TPD profiles of the samples. are supplied by hydroxyl groups bridging (Si-OH-Al) between silica and aluminum, and all samples have the same type of B acid, which is consistent with the same NH\({}_{3}\)-TPD desorption temperature of the three zeolites described above. The IR spectrum of chemisorption of basic molecules has been widely used to determine the acidity of porous materials. The amount of B acid and Lewis acid (L acid) of the catalyst can be further measured by Py-IR. The total acid content of zeolites was calculated by pyridine desorption at 150 \({}^{\circ}\)C, and the strong acid content of the zeolites was calculated by desorption of pyridine at 350 \({}^{\circ}\)C (Figure 7 and Table 3). The bands at 1445-1460 and 1540-1548 cm\({}^{-1}\) are related to adsorption peaks of pyridine on L acid and B acid sites, respectively, while the adsorption peak of 1490 cm\({}^{-1}\) is caused by the interaction between B acid and L acid [24]. The H-ZS-2 contains the least L acid. While, the sample H-ZN-1 has the highest amount of B acid, which is due to its large microporous surface area higher aluminum content on the outer surface. To further clarify the catalytic mechanism, we performed Py-IR characterization of the impregnated catalyst. From Figure S3 and Table S2, we can see the content of the strong acid and weak acid for B acid and L acid was almost the same as the samples in the form of protonation. Figure 6: FT-IR spectra of different zeolites in the region of OH stretching. Figure 7: Infrared spectra of pyridine adsorbed on as-synthesized zeolites at 150 \({}^{\circ}\)C (**a**) and 350 \({}^{\circ}\)C (**b**). Due to the small pore size of ZSM-23 zeolite, the isomerization reaction of long-chain alkanes is mainly carried out on the outer surface and pore mouth of ZSM-23 zeolite. Therefore, the surface acid properties of ZSM-23 zeolite have an important effect on its catalytic performance.