Silica-supported HClO۴ and KHSO۴ as reusable green catalysts for sulfonation of aromatic compounds under solvent-free conditions

A green protocol is described for sulfonation of aromatic compounds that has been accomplished using sodium bisulfite (NaHSO۳) in the presence of reusable green heterogeneous SiO۲/HClO۴ and SiO۲/KHSO۴ (Silica-supported HClO۴ and KHSO۴ catalysts) under conventional and solvent-free microwave irradiation.  The reactions afforded very good yields of products within ۳ to ۵ hour under conventional conditions. However, the reaction times in microwave-assisted protocol are drastically reduced to ۳ to ۵ minutes (from ۳ to ۵ hour under conventional conditions) followed by increasing product yields. Moreover, the developed silica-supported catalysts could be  recycled for at least three to four times.