5.2.1 CATALYST CHARACTERIZATION

The silica sulfuric acid (SSA), prepared by reacting silica gel with chlo — rosufonic 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 Cs25/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.

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 SSA. Characteristic IR absorp­tions of Cs25/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].

a 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 °C for 30 min prior to test) are shown in parentheses as bold italic type.