silica-supported hclo4 and khso4 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 (nahso3) in the presence of reusable green heterogeneous sio2/hclo4 and sio2/khso4 (silica-supported hclo4 and khso4 catalysts) under conventional and solvent-free microwave irradiation. the reactions afforded very good yields of products within 3 to 5 hour under conventional conditions. however, the reaction times in microwave-assisted protocol are drastically reduced to 3 to 5 minutes (from 3 to 5 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.