The silica gel, Kieselgel 40 (4 nm mean pore diameter, 590 m2g-1), was dried at 120 °C for 3 h in air prior to its use. The following SSA catalyst was prepared by a well-developed procedure [23,30]. A 250 mL suction flask, equipped with a constant pressure dropping funnel containing 5.83 g chlorosulfonic acid and a gas inlet tube for releasing HCl gas, was charged with 10.0 g Kieselgel 40 silica gel and 50 mL CH2Cl2. Chlo­rosulfonic acid was added dropwise over 30 min while stirring at room temperature. HCl gas was immediately evolved and absorbed into wa­ter. The mixture was then stirred for another 30 min. Next, CH2Cl2 was removed by rotary evaporation (50 °C, 20 min). A white solid (SSA, yield, 98%) was obtained and stored in a desiccator until use. K10 clay supported Cs25H05PW12O40 catalyst (hereafter designated Cs25/K10) was prepared by a well-developed route [31]. Typical Fourier Transform In­frared (FTIR) spectra of these two catalysts were recorded on a Thermo Scientific Nicolet 6700 spectrophotometer. Surface area, total pore vol­ume and pore diameter of the catalysts were determined by N2 adsorp­tion at 77 K using a Quantachrome Nova 2000 instrument after evacu­ating at 393 K for 3 h under nitrogen atmospheric. The total amount of acidity (H+) was measured by titration of catalyst samples in water with standardized sodium hydroxide (0.495 M).