## 2 Results and Discussion  
### Catalyst Characterization  
The silica sulfuric acid (SSA), \(\ce{SiO_{2}}\)-\(\ce{O}\ce{O}\ce{3H}\), prepared by reacting silica gel with chlorosufonic acid in dichloromethane was obtained as a white solid in 98% yield. Table 1 summarizes the physical properties (surface area, pore size, pore volume and acidity amount) as well as chemical compositions of SSA and other four catalysts. The specific surface area was calculated using the BET equation. The total pore volume was determined at 77K for 300 min and also the average pore diameter were was calculated using the Barrett-Joyner-Halenda (BJH) method. The amount of H+ was calculated by titration of catalyst samples in water with standard sodium hydroxide (0.495M). These results show that SSA have a good specific surface area than the three ion exchange resins and high pore volume than Cs2.5/K10. This might be a reason for high catalytic performance of the SSA catalyst in the experimental conditions. Negligible decreases in pore volume, surface area and pore diameter of once used SSA catalyst (Table 1) displayed its good reusability.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Catalysts** & **Cs\({}_{\text{2.5}}\)/K10** & **Amberlyst36** & **Amberlyst15** & **Dowex50X2** & SSA \\ \hline
**Description** & Cs\({}_{\text{2.5}}\)H\({}_{\text{0.5}}\)PW\({}_{\text{12}}\)O\({}_{\text{40}}\) & Macroreticular & Macroreticular & Microreticular & Silica sulfuric \\  & supported on clay & resin & resin & resin & acid \\ \hline
**Chemical** & Cs\({}_{\text{2.5}}\)H\({}_{\text{0.5}}\)PW\({}_{\text{12}}\)O\({}_{\text{40}}\) & Functionalized & Copolymer & Copolymer & \\
**composition** & /aluminosilicate & copolymers & (styrene + 20\% & (styrene + 2\% & \(\ce{SiO_{2}}\)-\(\ce{O}\ce{O}\ce{3H}\) \\  & (lamellar), 30\% & (styrene + DVB) & DVB) & DVB) & \\ \hline
**Acidity type** & Bronsted + Lewis & Bronsted & Bronsted & Bronsted & Bronsted \\ \hline
**Acidity amount** & 0.17 & 5.4 & 4.7 & 4.3 & 2.9 (2.7) \\ (**meqg-1**) & & & & & \\ \hline
**BET surface** & 181 & 35 & 51 & Gel (swells) & 308 (_302_) \\
**area (m2g-1)** & & & & & \\ \hline
**Average pore** & 6.0 & 24 & 40-80 & NA & 2.1 (_2.0_) \\
**diameter (nm)** & & & & & \\ \hline
**pore volume** & **0.29** & 0.20 & **0.40** & NA & **0.509 (_0.498_)** \\ (**cm3-g-1)** & & & & & \\ \hline \hline \end{tabular}  
* All the characteristics of the three resin catalysts were provided by the manufacturer; NA: not available; The corresponding characteristics of once used SSA (washed 3 times with acetone, dried in an oven at 105 \({}^{\circ}\)C for 30 min prior to test) are shown in parentheses as bold italic type.  
\end{table}
Table 1: Characteristics of catalysts a.  
Table 2 summarizes its typical IR absorptions and their assignments. The strong broad absorption bands from 1000 to 1100 cm\({}^{-1}\) correspond to Si-O-Si bridge stretching vibrations (1097 and 1065 cm\({}^{-1}\)) in silica [23]. The peak at _ca._ 971 cm\({}^{-1}\) is associated with Si-OH stretching vibrations in silica. Bands appearing at _ca._ 852 and 886 cm\({}^{-1}\) were assigned to the symmetrical and asymmetrical S-O stretching, respectively [24]. The peak at _ca._ 1178 cm\({}^{-1}\) is the asymmetric S=O stretching vibration, while S=O symmetrical stretching vibrations lies at 1010-1080 cm\({}^{-1}\), overlapped by Si-O stretching bands [24]. The strong broad absorption at about 3200-3500 cm\({}^{-1}\) is due to hydrogen bonded -OH in \(\langle\)SiO\({}_{2}\rangle\)-OSO\({}_{3}\)H. Characteristic IR absorptions of Cs\({}_{2.5}\)/K10 are also summarized in Table 2. The IR bands at _ca._ 1075 cm\({}^{-1}\), 1032 cm\({}^{-1}\) and 982 cm\({}^{-1}\) were due to P-O in the central tetrahedron, K10 clay and terminal W=O, respectively. The peaks at _ca._886 and 790 cm\({}^{-1}\) (asymmetric W-O-W vibrations) are associated with the Keggin polyanion [18].