A 500 mL suction flask containing nano-silica gel (60.0 g) was
equipped with a constant pressure dropping funnel containing
chlorosulfonic acid (23.3 g, 0.2 mol) and gas inlet tube for conducting
HCl gas over an adsorbing solution i.e., water. Chlorosulfonic
acid was added dropwise over a period of 30 min
at room temperature.